app与网站的关系,站长工具网站查询,抚州市住房和城乡建设局网站,互联网营销师考试题库参考#xff1a;http://www.cnblogs.com/skywang12345/p/3496609.html 概要 前面一章#xff0c;我们学习了“公平锁”获取锁的详细流程#xff1b;这里#xff0c;我们再来看看“公平锁”释放锁的过程。内容包括#xff1a; 参考代码 释放公平锁(基于JDK1.7.0_40) “公平…参考http://www.cnblogs.com/skywang12345/p/3496609.html 概要 前面一章我们学习了“公平锁”获取锁的详细流程这里我们再来看看“公平锁”释放锁的过程。内容包括 参考代码 释放公平锁(基于JDK1.7.0_40) “公平锁”的获取过程请参考“Java多线程系列--“JUC锁”03之 公平锁(一)”锁的使用示例请参考“Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock”。 注意 (01) 这里是以“公平锁”来进行说明。 (02) 关于本章的术语如“AQS”“CAS函数”“CLH队列”“公平锁”“非公平锁”“独占锁”“共享锁”等内容请参考Java多线程系列--“JUC锁”03之 公平锁(一)的基本概念。 转载请注明出处http://www.cnblogs.com/skywang12345/p/3496609.html 参考代码 下面给出Java1.7.0_40版本中ReentrantLock和AQS的源码仅供参考 ReentranLock.java 1 /*2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.3 *4 *5 *6 *7 *8 *9 *10 *11 *12 *13 *14 *15 *16 *17 *18 *19 *20 *21 *22 *23 */24 25 /*26 *27 *28 *29 *30 *31 * Written by Doug Lea with assistance from members of JCP JSR-16632 * Expert Group and released to the public domain, as explained at33 * http://creativecommons.org/publicdomain/zero/1.0/34 */35 36 package java.util.concurrent.locks;37 import java.util.*;38 import java.util.concurrent.*;39 import java.util.concurrent.atomic.*;40 41 /**42 * A reentrant mutual exclusion {link Lock} with the same basic43 * behavior and semantics as the implicit monitor lock accessed using44 * {code synchronized} methods and statements, but with extended45 * capabilities.46 *47 * pA {code ReentrantLock} is emowned/em by the thread last48 * successfully locking, but not yet unlocking it. A thread invoking49 * {code lock} will return, successfully acquiring the lock, when50 * the lock is not owned by another thread. The method will return51 * immediately if the current thread already owns the lock. This can52 * be checked using methods {link #isHeldByCurrentThread}, and {link53 * #getHoldCount}.54 *55 * pThe constructor for this class accepts an optional56 * emfairness/em parameter. When set {code true}, under57 * contention, locks favor granting access to the longest-waiting58 * thread. Otherwise this lock does not guarantee any particular59 * access order. Programs using fair locks accessed by many threads60 * may display lower overall throughput (i.e., are slower; often much61 * slower) than those using the default setting, but have smaller62 * variances in times to obtain locks and guarantee lack of63 * starvation. Note however, that fairness of locks does not guarantee64 * fairness of thread scheduling. Thus, one of many threads using a65 * fair lock may obtain it multiple times in succession while other66 * active threads are not progressing and not currently holding the67 * lock.68 * Also note that the untimed {link #tryLock() tryLock} method does not69 * honor the fairness setting. It will succeed if the lock70 * is available even if other threads are waiting.71 *72 * pIt is recommended practice to emalways/em immediately73 * follow a call to {code lock} with a {code try} block, most74 * typically in a before/after construction such as:75 *76 * pre77 * class X {78 * private final ReentrantLock lock new ReentrantLock();79 * // ...80 *81 * public void m() {82 * lock.lock(); // block until condition holds83 * try {84 * // ... method body85 * } finally {86 * lock.unlock()87 * }88 * }89 * }90 * /pre91 *92 * pIn addition to implementing the {link Lock} interface, this93 * class defines methods {code isLocked} and94 * {code getLockQueueLength}, as well as some associated95 * {code protected} access methods that may be useful for96 * instrumentation and monitoring.97 *98 * pSerialization of this class behaves in the same way as built-in99 * locks: a deserialized lock is in the unlocked state, regardless of
100 * its state when serialized.
101 *
102 * pThis lock supports a maximum of 2147483647 recursive locks by
103 * the same thread. Attempts to exceed this limit result in
104 * {link Error} throws from locking methods.
105 *
106 * since 1.5
107 * author Doug Lea
108 */
109 public class ReentrantLock implements Lock, java.io.Serializable {
110 private static final long serialVersionUID 7373984872572414699L;
111 /** Synchronizer providing all implementation mechanics */
112 private final Sync sync;
113
114 /**
115 * Base of synchronization control for this lock. Subclassed
116 * into fair and nonfair versions below. Uses AQS state to
117 * represent the number of holds on the lock.
118 */
119 abstract static class Sync extends AbstractQueuedSynchronizer {
120 private static final long serialVersionUID -5179523762034025860L;
121
122 /**
123 * Performs {link Lock#lock}. The main reason for subclassing
124 * is to allow fast path for nonfair version.
125 */
126 abstract void lock();
127
128 /**
129 * Performs non-fair tryLock. tryAcquire is
130 * implemented in subclasses, but both need nonfair
131 * try for trylock method.
132 */
133 final boolean nonfairTryAcquire(int acquires) {
134 final Thread current Thread.currentThread();
135 int c getState();
136 if (c 0) {
137 if (compareAndSetState(0, acquires)) {
138 setExclusiveOwnerThread(current);
139 return true;
140 }
141 }
142 else if (current getExclusiveOwnerThread()) {
143 int nextc c acquires;
144 if (nextc 0) // overflow
145 throw new Error(Maximum lock count exceeded);
146 setState(nextc);
147 return true;
148 }
149 return false;
150 }
151
152 protected final boolean tryRelease(int releases) {
153 int c getState() - releases;
154 if (Thread.currentThread() ! getExclusiveOwnerThread())
155 throw new IllegalMonitorStateException();
156 boolean free false;
157 if (c 0) {
158 free true;
159 setExclusiveOwnerThread(null);
160 }
161 setState(c);
162 return free;
163 }
164
165 protected final boolean isHeldExclusively() {
166 // While we must in general read state before owner,
167 // we dont need to do so to check if current thread is owner
168 return getExclusiveOwnerThread() Thread.currentThread();
169 }
170
171 final ConditionObject newCondition() {
172 return new ConditionObject();
173 }
174
175 // Methods relayed from outer class
176
177 final Thread getOwner() {
178 return getState() 0 ? null : getExclusiveOwnerThread();
179 }
180
181 final int getHoldCount() {
182 return isHeldExclusively() ? getState() : 0;
183 }
184
185 final boolean isLocked() {
186 return getState() ! 0;
187 }
188
189 /**
190 * Reconstitutes this lock instance from a stream.
191 * param s the stream
192 */
193 private void readObject(java.io.ObjectInputStream s)
194 throws java.io.IOException, ClassNotFoundException {
195 s.defaultReadObject();
196 setState(0); // reset to unlocked state
197 }
198 }
199
200 /**
201 * Sync object for non-fair locks
202 */
203 static final class NonfairSync extends Sync {
204 private static final long serialVersionUID 7316153563782823691L;
205
206 /**
207 * Performs lock. Try immediate barge, backing up to normal
208 * acquire on failure.
209 */
210 final void lock() {
211 if (compareAndSetState(0, 1))
212 setExclusiveOwnerThread(Thread.currentThread());
213 else
214 acquire(1);
215 }
216
217 protected final boolean tryAcquire(int acquires) {
218 return nonfairTryAcquire(acquires);
219 }
220 }
221
222 /**
223 * Sync object for fair locks
224 */
225 static final class FairSync extends Sync {
226 private static final long serialVersionUID -3000897897090466540L;
227
228 final void lock() {
229 acquire(1);
230 }
231
232 /**
233 * Fair version of tryAcquire. Dont grant access unless
234 * recursive call or no waiters or is first.
235 */
236 protected final boolean tryAcquire(int acquires) {
237 final Thread current Thread.currentThread();
238 int c getState();
239 if (c 0) {
240 if (!hasQueuedPredecessors()
241 compareAndSetState(0, acquires)) {
242 setExclusiveOwnerThread(current);
243 return true;
244 }
245 }
246 else if (current getExclusiveOwnerThread()) {
247 int nextc c acquires;
248 if (nextc 0)
249 throw new Error(Maximum lock count exceeded);
250 setState(nextc);
251 return true;
252 }
253 return false;
254 }
255 }
256
257 /**
258 * Creates an instance of {code ReentrantLock}.
259 * This is equivalent to using {code ReentrantLock(false)}.
260 */
261 public ReentrantLock() {
262 sync new NonfairSync();
263 }
264
265 /**
266 * Creates an instance of {code ReentrantLock} with the
267 * given fairness policy.
268 *
269 * param fair {code true} if this lock should use a fair ordering policy
270 */
271 public ReentrantLock(boolean fair) {
272 sync fair ? new FairSync() : new NonfairSync();
273 }
274
275 /**
276 * Acquires the lock.
277 *
278 * pAcquires the lock if it is not held by another thread and returns
279 * immediately, setting the lock hold count to one.
280 *
281 * pIf the current thread already holds the lock then the hold
282 * count is incremented by one and the method returns immediately.
283 *
284 * pIf the lock is held by another thread then the
285 * current thread becomes disabled for thread scheduling
286 * purposes and lies dormant until the lock has been acquired,
287 * at which time the lock hold count is set to one.
288 */
289 public void lock() {
290 sync.lock();
291 }
292
293 /**
294 * Acquires the lock unless the current thread is
295 * {linkplain Thread#interrupt interrupted}.
296 *
297 * pAcquires the lock if it is not held by another thread and returns
298 * immediately, setting the lock hold count to one.
299 *
300 * pIf the current thread already holds this lock then the hold count
301 * is incremented by one and the method returns immediately.
302 *
303 * pIf the lock is held by another thread then the
304 * current thread becomes disabled for thread scheduling
305 * purposes and lies dormant until one of two things happens:
306 *
307 * ul
308 *
309 * liThe lock is acquired by the current thread; or
310 *
311 * liSome other thread {linkplain Thread#interrupt interrupts} the
312 * current thread.
313 *
314 * /ul
315 *
316 * pIf the lock is acquired by the current thread then the lock hold
317 * count is set to one.
318 *
319 * pIf the current thread:
320 *
321 * ul
322 *
323 * lihas its interrupted status set on entry to this method; or
324 *
325 * liis {linkplain Thread#interrupt interrupted} while acquiring
326 * the lock,
327 *
328 * /ul
329 *
330 * then {link InterruptedException} is thrown and the current threads
331 * interrupted status is cleared.
332 *
333 * pIn this implementation, as this method is an explicit
334 * interruption point, preference is given to responding to the
335 * interrupt over normal or reentrant acquisition of the lock.
336 *
337 * throws InterruptedException if the current thread is interrupted
338 */
339 public void lockInterruptibly() throws InterruptedException {
340 sync.acquireInterruptibly(1);
341 }
342
343 /**
344 * Acquires the lock only if it is not held by another thread at the time
345 * of invocation.
346 *
347 * pAcquires the lock if it is not held by another thread and
348 * returns immediately with the value {code true}, setting the
349 * lock hold count to one. Even when this lock has been set to use a
350 * fair ordering policy, a call to {code tryLock()} emwill/em
351 * immediately acquire the lock if it is available, whether or not
352 * other threads are currently waiting for the lock.
353 * This quot;bargingquot; behavior can be useful in certain
354 * circumstances, even though it breaks fairness. If you want to honor
355 * the fairness setting for this lock, then use
356 * {link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
357 * which is almost equivalent (it also detects interruption).
358 *
359 * p If the current thread already holds this lock then the hold
360 * count is incremented by one and the method returns {code true}.
361 *
362 * pIf the lock is held by another thread then this method will return
363 * immediately with the value {code false}.
364 *
365 * return {code true} if the lock was free and was acquired by the
366 * current thread, or the lock was already held by the current
367 * thread; and {code false} otherwise
368 */
369 public boolean tryLock() {
370 return sync.nonfairTryAcquire(1);
371 }
372
373 /**
374 * Acquires the lock if it is not held by another thread within the given
375 * waiting time and the current thread has not been
376 * {linkplain Thread#interrupt interrupted}.
377 *
378 * pAcquires the lock if it is not held by another thread and returns
379 * immediately with the value {code true}, setting the lock hold count
380 * to one. If this lock has been set to use a fair ordering policy then
381 * an available lock emwill not/em be acquired if any other threads
382 * are waiting for the lock. This is in contrast to the {link #tryLock()}
383 * method. If you want a timed {code tryLock} that does permit barging on
384 * a fair lock then combine the timed and un-timed forms together:
385 *
386 * preif (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
387 * /pre
388 *
389 * pIf the current thread
390 * already holds this lock then the hold count is incremented by one and
391 * the method returns {code true}.
392 *
393 * pIf the lock is held by another thread then the
394 * current thread becomes disabled for thread scheduling
395 * purposes and lies dormant until one of three things happens:
396 *
397 * ul
398 *
399 * liThe lock is acquired by the current thread; or
400 *
401 * liSome other thread {linkplain Thread#interrupt interrupts}
402 * the current thread; or
403 *
404 * liThe specified waiting time elapses
405 *
406 * /ul
407 *
408 * pIf the lock is acquired then the value {code true} is returned and
409 * the lock hold count is set to one.
410 *
411 * pIf the current thread:
412 *
413 * ul
414 *
415 * lihas its interrupted status set on entry to this method; or
416 *
417 * liis {linkplain Thread#interrupt interrupted} while
418 * acquiring the lock,
419 *
420 * /ul
421 * then {link InterruptedException} is thrown and the current threads
422 * interrupted status is cleared.
423 *
424 * pIf the specified waiting time elapses then the value {code false}
425 * is returned. If the time is less than or equal to zero, the method
426 * will not wait at all.
427 *
428 * pIn this implementation, as this method is an explicit
429 * interruption point, preference is given to responding to the
430 * interrupt over normal or reentrant acquisition of the lock, and
431 * over reporting the elapse of the waiting time.
432 *
433 * param timeout the time to wait for the lock
434 * param unit the time unit of the timeout argument
435 * return {code true} if the lock was free and was acquired by the
436 * current thread, or the lock was already held by the current
437 * thread; and {code false} if the waiting time elapsed before
438 * the lock could be acquired
439 * throws InterruptedException if the current thread is interrupted
440 * throws NullPointerException if the time unit is null
441 *
442 */
443 public boolean tryLock(long timeout, TimeUnit unit)
444 throws InterruptedException {
445 return sync.tryAcquireNanos(1, unit.toNanos(timeout));
446 }
447
448 /**
449 * Attempts to release this lock.
450 *
451 * pIf the current thread is the holder of this lock then the hold
452 * count is decremented. If the hold count is now zero then the lock
453 * is released. If the current thread is not the holder of this
454 * lock then {link IllegalMonitorStateException} is thrown.
455 *
456 * throws IllegalMonitorStateException if the current thread does not
457 * hold this lock
458 */
459 public void unlock() {
460 sync.release(1);
461 }
462
463 /**
464 * Returns a {link Condition} instance for use with this
465 * {link Lock} instance.
466 *
467 * pThe returned {link Condition} instance supports the same
468 * usages as do the {link Object} monitor methods ({link
469 * Object#wait() wait}, {link Object#notify notify}, and {link
470 * Object#notifyAll notifyAll}) when used with the built-in
471 * monitor lock.
472 *
473 * ul
474 *
475 * liIf this lock is not held when any of the {link Condition}
476 * {linkplain Condition#await() waiting} or {linkplain
477 * Condition#signal signalling} methods are called, then an {link
478 * IllegalMonitorStateException} is thrown.
479 *
480 * liWhen the condition {linkplain Condition#await() waiting}
481 * methods are called the lock is released and, before they
482 * return, the lock is reacquired and the lock hold count restored
483 * to what it was when the method was called.
484 *
485 * liIf a thread is {linkplain Thread#interrupt interrupted}
486 * while waiting then the wait will terminate, an {link
487 * InterruptedException} will be thrown, and the threads
488 * interrupted status will be cleared.
489 *
490 * li Waiting threads are signalled in FIFO order.
491 *
492 * liThe ordering of lock reacquisition for threads returning
493 * from waiting methods is the same as for threads initially
494 * acquiring the lock, which is in the default case not specified,
495 * but for emfair/em locks favors those threads that have been
496 * waiting the longest.
497 *
498 * /ul
499 *
500 * return the Condition object
501 */
502 public Condition newCondition() {
503 return sync.newCondition();
504 }
505
506 /**
507 * Queries the number of holds on this lock by the current thread.
508 *
509 * pA thread has a hold on a lock for each lock action that is not
510 * matched by an unlock action.
511 *
512 * pThe hold count information is typically only used for testing and
513 * debugging purposes. For example, if a certain section of code should
514 * not be entered with the lock already held then we can assert that
515 * fact:
516 *
517 * pre
518 * class X {
519 * ReentrantLock lock new ReentrantLock();
520 * // ...
521 * public void m() {
522 * assert lock.getHoldCount() 0;
523 * lock.lock();
524 * try {
525 * // ... method body
526 * } finally {
527 * lock.unlock();
528 * }
529 * }
530 * }
531 * /pre
532 *
533 * return the number of holds on this lock by the current thread,
534 * or zero if this lock is not held by the current thread
535 */
536 public int getHoldCount() {
537 return sync.getHoldCount();
538 }
539
540 /**
541 * Queries if this lock is held by the current thread.
542 *
543 * pAnalogous to the {link Thread#holdsLock} method for built-in
544 * monitor locks, this method is typically used for debugging and
545 * testing. For example, a method that should only be called while
546 * a lock is held can assert that this is the case:
547 *
548 * pre
549 * class X {
550 * ReentrantLock lock new ReentrantLock();
551 * // ...
552 *
553 * public void m() {
554 * assert lock.isHeldByCurrentThread();
555 * // ... method body
556 * }
557 * }
558 * /pre
559 *
560 * pIt can also be used to ensure that a reentrant lock is used
561 * in a non-reentrant manner, for example:
562 *
563 * pre
564 * class X {
565 * ReentrantLock lock new ReentrantLock();
566 * // ...
567 *
568 * public void m() {
569 * assert !lock.isHeldByCurrentThread();
570 * lock.lock();
571 * try {
572 * // ... method body
573 * } finally {
574 * lock.unlock();
575 * }
576 * }
577 * }
578 * /pre
579 *
580 * return {code true} if current thread holds this lock and
581 * {code false} otherwise
582 */
583 public boolean isHeldByCurrentThread() {
584 return sync.isHeldExclusively();
585 }
586
587 /**
588 * Queries if this lock is held by any thread. This method is
589 * designed for use in monitoring of the system state,
590 * not for synchronization control.
591 *
592 * return {code true} if any thread holds this lock and
593 * {code false} otherwise
594 */
595 public boolean isLocked() {
596 return sync.isLocked();
597 }
598
599 /**
600 * Returns {code true} if this lock has fairness set true.
601 *
602 * return {code true} if this lock has fairness set true
603 */
604 public final boolean isFair() {
605 return sync instanceof FairSync;
606 }
607
608 /**
609 * Returns the thread that currently owns this lock, or
610 * {code null} if not owned. When this method is called by a
611 * thread that is not the owner, the return value reflects a
612 * best-effort approximation of current lock status. For example,
613 * the owner may be momentarily {code null} even if there are
614 * threads trying to acquire the lock but have not yet done so.
615 * This method is designed to facilitate construction of
616 * subclasses that provide more extensive lock monitoring
617 * facilities.
618 *
619 * return the owner, or {code null} if not owned
620 */
621 protected Thread getOwner() {
622 return sync.getOwner();
623 }
624
625 /**
626 * Queries whether any threads are waiting to acquire this lock. Note that
627 * because cancellations may occur at any time, a {code true}
628 * return does not guarantee that any other thread will ever
629 * acquire this lock. This method is designed primarily for use in
630 * monitoring of the system state.
631 *
632 * return {code true} if there may be other threads waiting to
633 * acquire the lock
634 */
635 public final boolean hasQueuedThreads() {
636 return sync.hasQueuedThreads();
637 }
638
639
640 /**
641 * Queries whether the given thread is waiting to acquire this
642 * lock. Note that because cancellations may occur at any time, a
643 * {code true} return does not guarantee that this thread
644 * will ever acquire this lock. This method is designed primarily for use
645 * in monitoring of the system state.
646 *
647 * param thread the thread
648 * return {code true} if the given thread is queued waiting for this lock
649 * throws NullPointerException if the thread is null
650 */
651 public final boolean hasQueuedThread(Thread thread) {
652 return sync.isQueued(thread);
653 }
654
655
656 /**
657 * Returns an estimate of the number of threads waiting to
658 * acquire this lock. The value is only an estimate because the number of
659 * threads may change dynamically while this method traverses
660 * internal data structures. This method is designed for use in
661 * monitoring of the system state, not for synchronization
662 * control.
663 *
664 * return the estimated number of threads waiting for this lock
665 */
666 public final int getQueueLength() {
667 return sync.getQueueLength();
668 }
669
670 /**
671 * Returns a collection containing threads that may be waiting to
672 * acquire this lock. Because the actual set of threads may change
673 * dynamically while constructing this result, the returned
674 * collection is only a best-effort estimate. The elements of the
675 * returned collection are in no particular order. This method is
676 * designed to facilitate construction of subclasses that provide
677 * more extensive monitoring facilities.
678 *
679 * return the collection of threads
680 */
681 protected CollectionThread getQueuedThreads() {
682 return sync.getQueuedThreads();
683 }
684
685 /**
686 * Queries whether any threads are waiting on the given condition
687 * associated with this lock. Note that because timeouts and
688 * interrupts may occur at any time, a {code true} return does
689 * not guarantee that a future {code signal} will awaken any
690 * threads. This method is designed primarily for use in
691 * monitoring of the system state.
692 *
693 * param condition the condition
694 * return {code true} if there are any waiting threads
695 * throws IllegalMonitorStateException if this lock is not held
696 * throws IllegalArgumentException if the given condition is
697 * not associated with this lock
698 * throws NullPointerException if the condition is null
699 */
700 public boolean hasWaiters(Condition condition) {
701 if (condition null)
702 throw new NullPointerException();
703 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
704 throw new IllegalArgumentException(not owner);
705 return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
706 }
707
708 /**
709 * Returns an estimate of the number of threads waiting on the
710 * given condition associated with this lock. Note that because
711 * timeouts and interrupts may occur at any time, the estimate
712 * serves only as an upper bound on the actual number of waiters.
713 * This method is designed for use in monitoring of the system
714 * state, not for synchronization control.
715 *
716 * param condition the condition
717 * return the estimated number of waiting threads
718 * throws IllegalMonitorStateException if this lock is not held
719 * throws IllegalArgumentException if the given condition is
720 * not associated with this lock
721 * throws NullPointerException if the condition is null
722 */
723 public int getWaitQueueLength(Condition condition) {
724 if (condition null)
725 throw new NullPointerException();
726 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
727 throw new IllegalArgumentException(not owner);
728 return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
729 }
730
731 /**
732 * Returns a collection containing those threads that may be
733 * waiting on the given condition associated with this lock.
734 * Because the actual set of threads may change dynamically while
735 * constructing this result, the returned collection is only a
736 * best-effort estimate. The elements of the returned collection
737 * are in no particular order. This method is designed to
738 * facilitate construction of subclasses that provide more
739 * extensive condition monitoring facilities.
740 *
741 * param condition the condition
742 * return the collection of threads
743 * throws IllegalMonitorStateException if this lock is not held
744 * throws IllegalArgumentException if the given condition is
745 * not associated with this lock
746 * throws NullPointerException if the condition is null
747 */
748 protected CollectionThread getWaitingThreads(Condition condition) {
749 if (condition null)
750 throw new NullPointerException();
751 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
752 throw new IllegalArgumentException(not owner);
753 return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
754 }
755
756 /**
757 * Returns a string identifying this lock, as well as its lock state.
758 * The state, in brackets, includes either the String {code Unlocked}
759 * or the String {code Locked by} followed by the
760 * {linkplain Thread#getName name} of the owning thread.
761 *
762 * return a string identifying this lock, as well as its lock state
763 */
764 public String toString() {
765 Thread o sync.getOwner();
766 return super.toString() ((o null) ?
767 [Unlocked] :
768 [Locked by thread o.getName() ]);
769 }
770 } AQS(AbstractQueuedSynchronizer.java) 1 /*2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.3 *4 *5 *6 *7 *8 *9 *10 *11 *12 *13 *14 *15 *16 *17 *18 *19 *20 *21 *22 *23 */24 25 /*26 *27 *28 *29 *30 *31 * Written by Doug Lea with assistance from members of JCP JSR-16632 * Expert Group and released to the public domain, as explained at33 * http://creativecommons.org/publicdomain/zero/1.0/34 */35 36 package java.util.concurrent.locks;37 import java.util.*;38 import java.util.concurrent.*;39 import java.util.concurrent.atomic.*;40 import sun.misc.Unsafe;41 42 /**43 * Provides a framework for implementing blocking locks and related44 * synchronizers (semaphores, events, etc) that rely on45 * first-in-first-out (FIFO) wait queues. This class is designed to46 * be a useful basis for most kinds of synchronizers that rely on a47 * single atomic ttint/tt value to represent state. Subclasses48 * must define the protected methods that change this state, and which49 * define what that state means in terms of this object being acquired50 * or released. Given these, the other methods in this class carry51 * out all queuing and blocking mechanics. Subclasses can maintain52 * other state fields, but only the atomically updated ttint/tt53 * value manipulated using methods {link #getState}, {link54 * #setState} and {link #compareAndSetState} is tracked with respect55 * to synchronization.56 *57 * pSubclasses should be defined as non-public internal helper58 * classes that are used to implement the synchronization properties59 * of their enclosing class. Class60 * ttAbstractQueuedSynchronizer/tt does not implement any61 * synchronization interface. Instead it defines methods such as62 * {link #acquireInterruptibly} that can be invoked as63 * appropriate by concrete locks and related synchronizers to64 * implement their public methods.65 *66 * pThis class supports either or both a default emexclusive/em67 * mode and a emshared/em mode. When acquired in exclusive mode,68 * attempted acquires by other threads cannot succeed. Shared mode69 * acquires by multiple threads may (but need not) succeed. This class70 * does not quot;understandquot; these differences except in the71 * mechanical sense that when a shared mode acquire succeeds, the next72 * waiting thread (if one exists) must also determine whether it can73 * acquire as well. Threads waiting in the different modes share the74 * same FIFO queue. Usually, implementation subclasses support only75 * one of these modes, but both can come into play for example in a76 * {link ReadWriteLock}. Subclasses that support only exclusive or77 * only shared modes need not define the methods supporting the unused mode.78 *79 * pThis class defines a nested {link ConditionObject} class that80 * can be used as a {link Condition} implementation by subclasses81 * supporting exclusive mode for which method {link82 * #isHeldExclusively} reports whether synchronization is exclusively83 * held with respect to the current thread, method {link #release}84 * invoked with the current {link #getState} value fully releases85 * this object, and {link #acquire}, given this saved state value,86 * eventually restores this object to its previous acquired state. No87 * ttAbstractQueuedSynchronizer/tt method otherwise creates such a88 * condition, so if this constraint cannot be met, do not use it. The89 * behavior of {link ConditionObject} depends of course on the90 * semantics of its synchronizer implementation.91 *92 * pThis class provides inspection, instrumentation, and monitoring93 * methods for the internal queue, as well as similar methods for94 * condition objects. These can be exported as desired into classes95 * using an ttAbstractQueuedSynchronizer/tt for their96 * synchronization mechanics.97 *98 * pSerialization of this class stores only the underlying atomic99 * integer maintaining state, so deserialized objects have empty100 * thread queues. Typical subclasses requiring serializability will101 * define a ttreadObject/tt method that restores this to a known102 * initial state upon deserialization.103 *104 * h3Usage/h3105 *106 * pTo use this class as the basis of a synchronizer, redefine the107 * following methods, as applicable, by inspecting and/or modifying108 * the synchronization state using {link #getState}, {link109 * #setState} and/or {link #compareAndSetState}:110 *111 * ul112 * li {link #tryAcquire}113 * li {link #tryRelease}114 * li {link #tryAcquireShared}115 * li {link #tryReleaseShared}116 * li {link #isHeldExclusively}117 */ul118 *119 * Each of these methods by default throws {link120 * UnsupportedOperationException}. Implementations of these methods121 * must be internally thread-safe, and should in general be short and122 * not block. Defining these methods is the emonly/em supported123 * means of using this class. All other methods are declared124 * ttfinal/tt because they cannot be independently varied.125 *126 * pYou may also find the inherited methods from {link127 * AbstractOwnableSynchronizer} useful to keep track of the thread128 * owning an exclusive synchronizer. You are encouraged to use them129 * -- this enables monitoring and diagnostic tools to assist users in130 * determining which threads hold locks.131 *132 * pEven though this class is based on an internal FIFO queue, it133 * does not automatically enforce FIFO acquisition policies. The core134 * of exclusive synchronization takes the form:135 *136 * pre137 * Acquire:138 * while (!tryAcquire(arg)) {139 * emenqueue thread if it is not already queued/em;140 * empossibly block current thread/em;141 * }142 *143 * Release:144 * if (tryRelease(arg))145 * emunblock the first queued thread/em;146 * /pre147 *148 * (Shared mode is similar but may involve cascading signals.)149 *150 * pa namebargingBecause checks in acquire are invoked before151 * enqueuing, a newly acquiring thread may embarge/em ahead of152 * others that are blocked and queued. However, you can, if desired,153 * define tttryAcquire/tt and/or tttryAcquireShared/tt to154 * disable barging by internally invoking one or more of the inspection155 * methods, thereby providing a emfair/em FIFO acquisition order.156 * In particular, most fair synchronizers can define tttryAcquire/tt157 * to return ttfalse/tt if {link #hasQueuedPredecessors} (a method158 * specifically designed to be used by fair synchronizers) returns159 * tttrue/tt. Other variations are possible.160 *161 * pThroughput and scalability are generally highest for the162 * default barging (also known as emgreedy/em,163 * emrenouncement/em, and emconvoy-avoidance/em) strategy.164 * While this is not guaranteed to be fair or starvation-free, earlier165 * queued threads are allowed to recontend before later queued166 * threads, and each recontention has an unbiased chance to succeed167 * against incoming threads. Also, while acquires do not168 * quot;spinquot; in the usual sense, they may perform multiple169 * invocations of tttryAcquire/tt interspersed with other170 * computations before blocking. This gives most of the benefits of171 * spins when exclusive synchronization is only briefly held, without172 * most of the liabilities when it isnt. If so desired, you can173 * augment this by preceding calls to acquire methods with174 * fast-path checks, possibly prechecking {link #hasContended}175 * and/or {link #hasQueuedThreads} to only do so if the synchronizer176 * is likely not to be contended.177 *178 * pThis class provides an efficient and scalable basis for179 * synchronization in part by specializing its range of use to180 * synchronizers that can rely on ttint/tt state, acquire, and181 * release parameters, and an internal FIFO wait queue. When this does182 * not suffice, you can build synchronizers from a lower level using183 * {link java.util.concurrent.atomic atomic} classes, your own custom184 * {link java.util.Queue} classes, and {link LockSupport} blocking185 * support.186 *187 * h3Usage Examples/h3188 *189 * pHere is a non-reentrant mutual exclusion lock class that uses190 * the value zero to represent the unlocked state, and one to191 * represent the locked state. While a non-reentrant lock192 * does not strictly require recording of the current owner193 * thread, this class does so anyway to make usage easier to monitor.194 * It also supports conditions and exposes195 * one of the instrumentation methods:196 *197 * pre198 * class Mutex implements Lock, java.io.Serializable {199 *200 * // Our internal helper class201 * private static class Sync extends AbstractQueuedSynchronizer {202 * // Report whether in locked state203 * protected boolean isHeldExclusively() {204 * return getState() 1;205 * }206 *207 * // Acquire the lock if state is zero208 * public boolean tryAcquire(int acquires) {209 * assert acquires 1; // Otherwise unused210 * if (compareAndSetState(0, 1)) {211 * setExclusiveOwnerThread(Thread.currentThread());212 * return true;213 * }214 * return false;215 * }216 *217 * // Release the lock by setting state to zero218 * protected boolean tryRelease(int releases) {219 * assert releases 1; // Otherwise unused220 * if (getState() 0) throw new IllegalMonitorStateException();221 * setExclusiveOwnerThread(null);222 * setState(0);223 * return true;224 * }225 *226 * // Provide a Condition227 * Condition newCondition() { return new ConditionObject(); }228 *229 * // Deserialize properly230 * private void readObject(ObjectInputStream s)231 * throws IOException, ClassNotFoundException {232 * s.defaultReadObject();233 * setState(0); // reset to unlocked state234 * }235 * }236 *237 * // The sync object does all the hard work. We just forward to it.238 * private final Sync sync new Sync();239 *240 * public void lock() { sync.acquire(1); }241 * public boolean tryLock() { return sync.tryAcquire(1); }242 * public void unlock() { sync.release(1); }243 * public Condition newCondition() { return sync.newCondition(); }244 * public boolean isLocked() { return sync.isHeldExclusively(); }245 * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }246 * public void lockInterruptibly() throws InterruptedException {247 * sync.acquireInterruptibly(1);248 * }249 * public boolean tryLock(long timeout, TimeUnit unit)250 * throws InterruptedException {251 * return sync.tryAcquireNanos(1, unit.toNanos(timeout));252 * }253 * }254 * /pre255 *256 * pHere is a latch class that is like a {link CountDownLatch}257 * except that it only requires a single ttsignal/tt to258 * fire. Because a latch is non-exclusive, it uses the ttshared/tt259 * acquire and release methods.260 *261 * pre262 * class BooleanLatch {263 *264 * private static class Sync extends AbstractQueuedSynchronizer {265 * boolean isSignalled() { return getState() ! 0; }266 *267 * protected int tryAcquireShared(int ignore) {268 * return isSignalled() ? 1 : -1;269 * }270 *271 * protected boolean tryReleaseShared(int ignore) {272 * setState(1);273 * return true;274 * }275 * }276 *277 * private final Sync sync new Sync();278 * public boolean isSignalled() { return sync.isSignalled(); }279 * public void signal() { sync.releaseShared(1); }280 * public void await() throws InterruptedException {281 * sync.acquireSharedInterruptibly(1);282 * }283 * }284 * /pre285 *286 * since 1.5287 * author Doug Lea288 */289 public abstract class AbstractQueuedSynchronizer290 extends AbstractOwnableSynchronizer291 implements java.io.Serializable {292 293 private static final long serialVersionUID 7373984972572414691L;294 295 /**296 * Creates a new ttAbstractQueuedSynchronizer/tt instance297 * with initial synchronization state of zero.298 */299 protected AbstractQueuedSynchronizer() { }300 301 /**302 * Wait queue node class.303 *304 * pThe wait queue is a variant of a CLH (Craig, Landin, and305 * Hagersten) lock queue. CLH locks are normally used for306 * spinlocks. We instead use them for blocking synchronizers, but307 * use the same basic tactic of holding some of the control308 * information about a thread in the predecessor of its node. A309 * status field in each node keeps track of whether a thread310 * should block. A node is signalled when its predecessor311 * releases. Each node of the queue otherwise serves as a312 * specific-notification-style monitor holding a single waiting313 * thread. The status field does NOT control whether threads are314 * granted locks etc though. A thread may try to acquire if it is315 * first in the queue. But being first does not guarantee success;316 * it only gives the right to contend. So the currently released317 * contender thread may need to rewait.318 *319 * pTo enqueue into a CLH lock, you atomically splice it in as new320 * tail. To dequeue, you just set the head field.321 * pre322 * ------ prev ----- -----323 * head | | ---- | | ---- | | tail324 * ------ ----- -----325 * /pre326 *327 * pInsertion into a CLH queue requires only a single atomic328 * operation on tail, so there is a simple atomic point of329 * demarcation from unqueued to queued. Similarly, dequeing330 * involves only updating the head. However, it takes a bit331 * more work for nodes to determine who their successors are,332 * in part to deal with possible cancellation due to timeouts333 * and interrupts.334 *335 * pThe prev links (not used in original CLH locks), are mainly336 * needed to handle cancellation. If a node is cancelled, its337 * successor is (normally) relinked to a non-cancelled338 * predecessor. For explanation of similar mechanics in the case339 * of spin locks, see the papers by Scott and Scherer at340 * http://www.cs.rochester.edu/u/scott/synchronization/341 *342 * pWe also use next links to implement blocking mechanics.343 * The thread id for each node is kept in its own node, so a344 * predecessor signals the next node to wake up by traversing345 * next link to determine which thread it is. Determination of346 * successor must avoid races with newly queued nodes to set347 * the next fields of their predecessors. This is solved348 * when necessary by checking backwards from the atomically349 * updated tail when a nodes successor appears to be null.350 * (Or, said differently, the next-links are an optimization351 * so that we dont usually need a backward scan.)352 *353 * pCancellation introduces some conservatism to the basic354 * algorithms. Since we must poll for cancellation of other355 * nodes, we can miss noticing whether a cancelled node is356 * ahead or behind us. This is dealt with by always unparking357 * successors upon cancellation, allowing them to stabilize on358 * a new predecessor, unless we can identify an uncancelled359 * predecessor who will carry this responsibility.360 *361 * pCLH queues need a dummy header node to get started. But362 * we dont create them on construction, because it would be wasted363 * effort if there is never contention. Instead, the node364 * is constructed and head and tail pointers are set upon first365 * contention.366 *367 * pThreads waiting on Conditions use the same nodes, but368 * use an additional link. Conditions only need to link nodes369 * in simple (non-concurrent) linked queues because they are370 * only accessed when exclusively held. Upon await, a node is371 * inserted into a condition queue. Upon signal, the node is372 * transferred to the main queue. A special value of status373 * field is used to mark which queue a node is on.374 *375 * pThanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill376 * Scherer and Michael Scott, along with members of JSR-166377 * expert group, for helpful ideas, discussions, and critiques378 * on the design of this class.379 */380 static final class Node {381 /** Marker to indicate a node is waiting in shared mode */382 static final Node SHARED new Node();383 /** Marker to indicate a node is waiting in exclusive mode */384 static final Node EXCLUSIVE null;385 386 /** waitStatus value to indicate thread has cancelled */387 static final int CANCELLED 1;388 /** waitStatus value to indicate successors thread needs unparking */389 static final int SIGNAL -1;390 /** waitStatus value to indicate thread is waiting on condition */391 static final int CONDITION -2;392 /**393 * waitStatus value to indicate the next acquireShared should394 * unconditionally propagate395 */396 static final int PROPAGATE -3;397 398 /**399 * Status field, taking on only the values:400 * SIGNAL: The successor of this node is (or will soon be)401 * blocked (via park), so the current node must402 * unpark its successor when it releases or403 * cancels. To avoid races, acquire methods must404 * first indicate they need a signal,405 * then retry the atomic acquire, and then,406 * on failure, block.407 * CANCELLED: This node is cancelled due to timeout or interrupt.408 * Nodes never leave this state. In particular,409 * a thread with cancelled node never again blocks.410 * CONDITION: This node is currently on a condition queue.411 * It will not be used as a sync queue node412 * until transferred, at which time the status413 * will be set to 0. (Use of this value here has414 * nothing to do with the other uses of the415 * field, but simplifies mechanics.)416 * PROPAGATE: A releaseShared should be propagated to other417 * nodes. This is set (for head node only) in418 * doReleaseShared to ensure propagation419 * continues, even if other operations have420 * since intervened.421 * 0: None of the above422 *423 * The values are arranged numerically to simplify use.424 * Non-negative values mean that a node doesnt need to425 * signal. So, most code doesnt need to check for particular426 * values, just for sign.427 *428 * The field is initialized to 0 for normal sync nodes, and429 * CONDITION for condition nodes. It is modified using CAS430 * (or when possible, unconditional volatile writes).431 */432 volatile int waitStatus;433 434 /**435 * Link to predecessor node that current node/thread relies on436 * for checking waitStatus. Assigned during enqueing, and nulled437 * out (for sake of GC) only upon dequeuing. Also, upon438 * cancellation of a predecessor, we short-circuit while439 * finding a non-cancelled one, which will always exist440 * because the head node is never cancelled: A node becomes441 * head only as a result of successful acquire. A442 * cancelled thread never succeeds in acquiring, and a thread only443 * cancels itself, not any other node.444 */445 volatile Node prev;446 447 /**448 * Link to the successor node that the current node/thread449 * unparks upon release. Assigned during enqueuing, adjusted450 * when bypassing cancelled predecessors, and nulled out (for451 * sake of GC) when dequeued. The enq operation does not452 * assign next field of a predecessor until after attachment,453 * so seeing a null next field does not necessarily mean that454 * node is at end of queue. However, if a next field appears455 * to be null, we can scan prevs from the tail to456 * double-check. The next field of cancelled nodes is set to457 * point to the node itself instead of null, to make life458 * easier for isOnSyncQueue.459 */460 volatile Node next;461 462 /**463 * The thread that enqueued this node. Initialized on464 * construction and nulled out after use.465 */466 volatile Thread thread;467 468 /**469 * Link to next node waiting on condition, or the special470 * value SHARED. Because condition queues are accessed only471 * when holding in exclusive mode, we just need a simple472 * linked queue to hold nodes while they are waiting on473 * conditions. They are then transferred to the queue to474 * re-acquire. And because conditions can only be exclusive,475 * we save a field by using special value to indicate shared476 * mode.477 */478 Node nextWaiter;479 480 /**481 * Returns true if node is waiting in shared mode482 */483 final boolean isShared() {484 return nextWaiter SHARED;485 }486 487 /**488 * Returns previous node, or throws NullPointerException if null.489 * Use when predecessor cannot be null. The null check could490 * be elided, but is present to help the VM.491 *492 * return the predecessor of this node493 */494 final Node predecessor() throws NullPointerException {495 Node p prev;496 if (p null)497 throw new NullPointerException();498 else499 return p;500 }501 502 Node() { // Used to establish initial head or SHARED marker503 }504 505 Node(Thread thread, Node mode) { // Used by addWaiter506 this.nextWaiter mode;507 this.thread thread;508 }509 510 Node(Thread thread, int waitStatus) { // Used by Condition511 this.waitStatus waitStatus;512 this.thread thread;513 }514 }515 516 /**517 * Head of the wait queue, lazily initialized. Except for518 * initialization, it is modified only via method setHead. Note:519 * If head exists, its waitStatus is guaranteed not to be520 * CANCELLED.521 */522 private transient volatile Node head;523 524 /**525 * Tail of the wait queue, lazily initialized. Modified only via526 * method enq to add new wait node.527 */528 private transient volatile Node tail;529 530 /**531 * The synchronization state.532 */533 private volatile int state;534 535 /**536 * Returns the current value of synchronization state.537 * This operation has memory semantics of a ttvolatile/tt read.538 * return current state value539 */540 protected final int getState() {541 return state;542 }543 544 /**545 * Sets the value of synchronization state.546 * This operation has memory semantics of a ttvolatile/tt write.547 * param newState the new state value548 */549 protected final void setState(int newState) {550 state newState;551 }552 553 /**554 * Atomically sets synchronization state to the given updated555 * value if the current state value equals the expected value.556 * This operation has memory semantics of a ttvolatile/tt read557 * and write.558 *559 * param expect the expected value560 * param update the new value561 * return true if successful. False return indicates that the actual562 * value was not equal to the expected value.563 */564 protected final boolean compareAndSetState(int expect, int update) {565 // See below for intrinsics setup to support this566 return unsafe.compareAndSwapInt(this, stateOffset, expect, update);567 }568 569 // Queuing utilities570 571 /**572 * The number of nanoseconds for which it is faster to spin573 * rather than to use timed park. A rough estimate suffices574 * to improve responsiveness with very short timeouts.575 */576 static final long spinForTimeoutThreshold 1000L;577 578 /**579 * Inserts node into queue, initializing if necessary. See picture above.580 * param node the node to insert581 * return nodes predecessor582 */583 private Node enq(final Node node) {584 for (;;) {585 Node t tail;586 if (t null) { // Must initialize587 if (compareAndSetHead(new Node()))588 tail head;589 } else {590 node.prev t;591 if (compareAndSetTail(t, node)) {592 t.next node;593 return t;594 }595 }596 }597 }598 599 /**600 * Creates and enqueues node for current thread and given mode.601 *602 * param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared603 * return the new node604 */605 private Node addWaiter(Node mode) {606 Node node new Node(Thread.currentThread(), mode);607 // Try the fast path of enq; backup to full enq on failure608 Node pred tail;609 if (pred ! null) {610 node.prev pred;611 if (compareAndSetTail(pred, node)) {612 pred.next node;613 return node;614 }615 }616 enq(node);617 return node;618 }619 620 /**621 * Sets head of queue to be node, thus dequeuing. Called only by622 * acquire methods. Also nulls out unused fields for sake of GC623 * and to suppress unnecessary signals and traversals.624 *625 * param node the node626 */627 private void setHead(Node node) {628 head node;629 node.thread null;630 node.prev null;631 }632 633 /**634 * Wakes up nodes successor, if one exists.635 *636 * param node the node637 */638 private void unparkSuccessor(Node node) {639 /*640 * If status is negative (i.e., possibly needing signal) try641 * to clear in anticipation of signalling. It is OK if this642 * fails or if status is changed by waiting thread.643 */644 int ws node.waitStatus;645 if (ws 0)646 compareAndSetWaitStatus(node, ws, 0);647 648 /*649 * Thread to unpark is held in successor, which is normally650 * just the next node. But if cancelled or apparently null,651 * traverse backwards from tail to find the actual652 * non-cancelled successor.653 */654 Node s node.next;655 if (s null || s.waitStatus 0) {656 s null;657 for (Node t tail; t ! null t ! node; t t.prev)658 if (t.waitStatus 0)659 s t;660 }661 if (s ! null)662 LockSupport.unpark(s.thread);663 }664 665 /**666 * Release action for shared mode -- signal successor and ensure667 * propagation. (Note: For exclusive mode, release just amounts668 * to calling unparkSuccessor of head if it needs signal.)669 */670 private void doReleaseShared() {671 /*672 * Ensure that a release propagates, even if there are other673 * in-progress acquires/releases. This proceeds in the usual674 * way of trying to unparkSuccessor of head if it needs675 * signal. But if it does not, status is set to PROPAGATE to676 * ensure that upon release, propagation continues.677 * Additionally, we must loop in case a new node is added678 * while we are doing this. Also, unlike other uses of679 * unparkSuccessor, we need to know if CAS to reset status680 * fails, if so rechecking.681 */682 for (;;) {683 Node h head;684 if (h ! null h ! tail) {685 int ws h.waitStatus;686 if (ws Node.SIGNAL) {687 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))688 continue; // loop to recheck cases689 unparkSuccessor(h);690 }691 else if (ws 0 692 !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))693 continue; // loop on failed CAS694 }695 if (h head) // loop if head changed696 break;697 }698 }699 700 /**701 * Sets head of queue, and checks if successor may be waiting702 * in shared mode, if so propagating if either propagate 0 or703 * PROPAGATE status was set.704 *705 * param node the node706 * param propagate the return value from a tryAcquireShared707 */708 private void setHeadAndPropagate(Node node, int propagate) {709 Node h head; // Record old head for check below710 setHead(node);711 /*712 * Try to signal next queued node if:713 * Propagation was indicated by caller,714 * or was recorded (as h.waitStatus) by a previous operation715 * (note: this uses sign-check of waitStatus because716 * PROPAGATE status may transition to SIGNAL.)717 * and718 * The next node is waiting in shared mode,719 * or we dont know, because it appears null720 *721 * The conservatism in both of these checks may cause722 * unnecessary wake-ups, but only when there are multiple723 * racing acquires/releases, so most need signals now or soon724 * anyway.725 */726 if (propagate 0 || h null || h.waitStatus 0) {727 Node s node.next;728 if (s null || s.isShared())729 doReleaseShared();730 }731 }732 733 // Utilities for various versions of acquire734 735 /**736 * Cancels an ongoing attempt to acquire.737 *738 * param node the node739 */740 private void cancelAcquire(Node node) {741 // Ignore if node doesnt exist742 if (node null)743 return;744 745 node.thread null;746 747 // Skip cancelled predecessors748 Node pred node.prev;749 while (pred.waitStatus 0)750 node.prev pred pred.prev;751 752 // predNext is the apparent node to unsplice. CASes below will753 // fail if not, in which case, we lost race vs another cancel754 // or signal, so no further action is necessary.755 Node predNext pred.next;756 757 // Can use unconditional write instead of CAS here.758 // After this atomic step, other Nodes can skip past us.759 // Before, we are free of interference from other threads.760 node.waitStatus Node.CANCELLED;761 762 // If we are the tail, remove ourselves.763 if (node tail compareAndSetTail(node, pred)) {764 compareAndSetNext(pred, predNext, null);765 } else {766 // If successor needs signal, try to set preds next-link767 // so it will get one. Otherwise wake it up to propagate.768 int ws;769 if (pred ! head 770 ((ws pred.waitStatus) Node.SIGNAL ||771 (ws 0 compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) 772 pred.thread ! null) {773 Node next node.next;774 if (next ! null next.waitStatus 0)775 compareAndSetNext(pred, predNext, next);776 } else {777 unparkSuccessor(node);778 }779 780 node.next node; // help GC781 }782 }783 784 /**785 * Checks and updates status for a node that failed to acquire.786 * Returns true if thread should block. This is the main signal787 * control in all acquire loops. Requires that pred node.prev788 *789 * param pred nodes predecessor holding status790 * param node the node791 * return {code true} if thread should block792 */793 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {794 int ws pred.waitStatus;795 if (ws Node.SIGNAL)796 /*797 * This node has already set status asking a release798 * to signal it, so it can safely park.799 */800 return true;801 if (ws 0) {802 /*803 * Predecessor was cancelled. Skip over predecessors and804 * indicate retry.805 */806 do {807 node.prev pred pred.prev;808 } while (pred.waitStatus 0);809 pred.next node;810 } else {811 /*812 * waitStatus must be 0 or PROPAGATE. Indicate that we813 * need a signal, but dont park yet. Caller will need to814 * retry to make sure it cannot acquire before parking.815 */816 compareAndSetWaitStatus(pred, ws, Node.SIGNAL);817 }818 return false;819 }820 821 /**822 * Convenience method to interrupt current thread.823 */824 private static void selfInterrupt() {825 Thread.currentThread().interrupt();826 }827 828 /**829 * Convenience method to park and then check if interrupted830 *831 * return {code true} if interrupted832 */833 private final boolean parkAndCheckInterrupt() {834 LockSupport.park(this);835 return Thread.interrupted();836 }837 838 /*839 * Various flavors of acquire, varying in exclusive/shared and840 * control modes. Each is mostly the same, but annoyingly841 * different. Only a little bit of factoring is possible due to842 * interactions of exception mechanics (including ensuring that we843 * cancel if tryAcquire throws exception) and other control, at844 * least not without hurting performance too much.845 */846 847 /**848 * Acquires in exclusive uninterruptible mode for thread already in849 * queue. Used by condition wait methods as well as acquire.850 *851 * param node the node852 * param arg the acquire argument853 * return {code true} if interrupted while waiting854 */855 final boolean acquireQueued(final Node node, int arg) {856 boolean failed true;857 try {858 boolean interrupted false;859 for (;;) {860 final Node p node.predecessor();861 if (p head tryAcquire(arg)) {862 setHead(node);863 p.next null; // help GC864 failed false;865 return interrupted;866 }867 if (shouldParkAfterFailedAcquire(p, node) 868 parkAndCheckInterrupt())869 interrupted true;870 }871 } finally {872 if (failed)873 cancelAcquire(node);874 }875 }876 877 /**878 * Acquires in exclusive interruptible mode.879 * param arg the acquire argument880 */881 private void doAcquireInterruptibly(int arg)882 throws InterruptedException {883 final Node node addWaiter(Node.EXCLUSIVE);884 boolean failed true;885 try {886 for (;;) {887 final Node p node.predecessor();888 if (p head tryAcquire(arg)) {889 setHead(node);890 p.next null; // help GC891 failed false;892 return;893 }894 if (shouldParkAfterFailedAcquire(p, node) 895 parkAndCheckInterrupt())896 throw new InterruptedException();897 }898 } finally {899 if (failed)900 cancelAcquire(node);901 }902 }903 904 /**905 * Acquires in exclusive timed mode.906 *907 * param arg the acquire argument908 * param nanosTimeout max wait time909 * return {code true} if acquired910 */911 private boolean doAcquireNanos(int arg, long nanosTimeout)912 throws InterruptedException {913 long lastTime System.nanoTime();914 final Node node addWaiter(Node.EXCLUSIVE);915 boolean failed true;916 try {917 for (;;) {918 final Node p node.predecessor();919 if (p head tryAcquire(arg)) {920 setHead(node);921 p.next null; // help GC922 failed false;923 return true;924 }925 if (nanosTimeout 0)926 return false;927 if (shouldParkAfterFailedAcquire(p, node) 928 nanosTimeout spinForTimeoutThreshold)929 LockSupport.parkNanos(this, nanosTimeout);930 long now System.nanoTime();931 nanosTimeout - now - lastTime;932 lastTime now;933 if (Thread.interrupted())934 throw new InterruptedException();935 }936 } finally {937 if (failed)938 cancelAcquire(node);939 }940 }941 942 /**943 * Acquires in shared uninterruptible mode.944 * param arg the acquire argument945 */946 private void doAcquireShared(int arg) {947 final Node node addWaiter(Node.SHARED);948 boolean failed true;949 try {950 boolean interrupted false;951 for (;;) {952 final Node p node.predecessor();953 if (p head) {954 int r tryAcquireShared(arg);955 if (r 0) {956 setHeadAndPropagate(node, r);957 p.next null; // help GC958 if (interrupted)959 selfInterrupt();960 failed false;961 return;962 }963 }964 if (shouldParkAfterFailedAcquire(p, node) 965 parkAndCheckInterrupt())966 interrupted true;967 }968 } finally {969 if (failed)970 cancelAcquire(node);971 }972 }973 974 /**975 * Acquires in shared interruptible mode.976 * param arg the acquire argument977 */978 private void doAcquireSharedInterruptibly(int arg)979 throws InterruptedException {980 final Node node addWaiter(Node.SHARED);981 boolean failed true;982 try {983 for (;;) {984 final Node p node.predecessor();985 if (p head) {986 int r tryAcquireShared(arg);987 if (r 0) {988 setHeadAndPropagate(node, r);989 p.next null; // help GC990 failed false;991 return;992 }993 }994 if (shouldParkAfterFailedAcquire(p, node) 995 parkAndCheckInterrupt())996 throw new InterruptedException();997 }998 } finally {999 if (failed)
1000 cancelAcquire(node);
1001 }
1002 }
1003
1004 /**
1005 * Acquires in shared timed mode.
1006 *
1007 * param arg the acquire argument
1008 * param nanosTimeout max wait time
1009 * return {code true} if acquired
1010 */
1011 private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
1012 throws InterruptedException {
1013
1014 long lastTime System.nanoTime();
1015 final Node node addWaiter(Node.SHARED);
1016 boolean failed true;
1017 try {
1018 for (;;) {
1019 final Node p node.predecessor();
1020 if (p head) {
1021 int r tryAcquireShared(arg);
1022 if (r 0) {
1023 setHeadAndPropagate(node, r);
1024 p.next null; // help GC
1025 failed false;
1026 return true;
1027 }
1028 }
1029 if (nanosTimeout 0)
1030 return false;
1031 if (shouldParkAfterFailedAcquire(p, node)
1032 nanosTimeout spinForTimeoutThreshold)
1033 LockSupport.parkNanos(this, nanosTimeout);
1034 long now System.nanoTime();
1035 nanosTimeout - now - lastTime;
1036 lastTime now;
1037 if (Thread.interrupted())
1038 throw new InterruptedException();
1039 }
1040 } finally {
1041 if (failed)
1042 cancelAcquire(node);
1043 }
1044 }
1045
1046 // Main exported methods
1047
1048 /**
1049 * Attempts to acquire in exclusive mode. This method should query
1050 * if the state of the object permits it to be acquired in the
1051 * exclusive mode, and if so to acquire it.
1052 *
1053 * pThis method is always invoked by the thread performing
1054 * acquire. If this method reports failure, the acquire method
1055 * may queue the thread, if it is not already queued, until it is
1056 * signalled by a release from some other thread. This can be used
1057 * to implement method {link Lock#tryLock()}.
1058 *
1059 * pThe default
1060 * implementation throws {link UnsupportedOperationException}.
1061 *
1062 * param arg the acquire argument. This value is always the one
1063 * passed to an acquire method, or is the value saved on entry
1064 * to a condition wait. The value is otherwise uninterpreted
1065 * and can represent anything you like.
1066 * return {code true} if successful. Upon success, this object has
1067 * been acquired.
1068 * throws IllegalMonitorStateException if acquiring would place this
1069 * synchronizer in an illegal state. This exception must be
1070 * thrown in a consistent fashion for synchronization to work
1071 * correctly.
1072 * throws UnsupportedOperationException if exclusive mode is not supported
1073 */
1074 protected boolean tryAcquire(int arg) {
1075 throw new UnsupportedOperationException();
1076 }
1077
1078 /**
1079 * Attempts to set the state to reflect a release in exclusive
1080 * mode.
1081 *
1082 * pThis method is always invoked by the thread performing release.
1083 *
1084 * pThe default implementation throws
1085 * {link UnsupportedOperationException}.
1086 *
1087 * param arg the release argument. This value is always the one
1088 * passed to a release method, or the current state value upon
1089 * entry to a condition wait. The value is otherwise
1090 * uninterpreted and can represent anything you like.
1091 * return {code true} if this object is now in a fully released
1092 * state, so that any waiting threads may attempt to acquire;
1093 * and {code false} otherwise.
1094 * throws IllegalMonitorStateException if releasing would place this
1095 * synchronizer in an illegal state. This exception must be
1096 * thrown in a consistent fashion for synchronization to work
1097 * correctly.
1098 * throws UnsupportedOperationException if exclusive mode is not supported
1099 */
1100 protected boolean tryRelease(int arg) {
1101 throw new UnsupportedOperationException();
1102 }
1103
1104 /**
1105 * Attempts to acquire in shared mode. This method should query if
1106 * the state of the object permits it to be acquired in the shared
1107 * mode, and if so to acquire it.
1108 *
1109 * pThis method is always invoked by the thread performing
1110 * acquire. If this method reports failure, the acquire method
1111 * may queue the thread, if it is not already queued, until it is
1112 * signalled by a release from some other thread.
1113 *
1114 * pThe default implementation throws {link
1115 * UnsupportedOperationException}.
1116 *
1117 * param arg the acquire argument. This value is always the one
1118 * passed to an acquire method, or is the value saved on entry
1119 * to a condition wait. The value is otherwise uninterpreted
1120 * and can represent anything you like.
1121 * return a negative value on failure; zero if acquisition in shared
1122 * mode succeeded but no subsequent shared-mode acquire can
1123 * succeed; and a positive value if acquisition in shared
1124 * mode succeeded and subsequent shared-mode acquires might
1125 * also succeed, in which case a subsequent waiting thread
1126 * must check availability. (Support for three different
1127 * return values enables this method to be used in contexts
1128 * where acquires only sometimes act exclusively.) Upon
1129 * success, this object has been acquired.
1130 * throws IllegalMonitorStateException if acquiring would place this
1131 * synchronizer in an illegal state. This exception must be
1132 * thrown in a consistent fashion for synchronization to work
1133 * correctly.
1134 * throws UnsupportedOperationException if shared mode is not supported
1135 */
1136 protected int tryAcquireShared(int arg) {
1137 throw new UnsupportedOperationException();
1138 }
1139
1140 /**
1141 * Attempts to set the state to reflect a release in shared mode.
1142 *
1143 * pThis method is always invoked by the thread performing release.
1144 *
1145 * pThe default implementation throws
1146 * {link UnsupportedOperationException}.
1147 *
1148 * param arg the release argument. This value is always the one
1149 * passed to a release method, or the current state value upon
1150 * entry to a condition wait. The value is otherwise
1151 * uninterpreted and can represent anything you like.
1152 * return {code true} if this release of shared mode may permit a
1153 * waiting acquire (shared or exclusive) to succeed; and
1154 * {code false} otherwise
1155 * throws IllegalMonitorStateException if releasing would place this
1156 * synchronizer in an illegal state. This exception must be
1157 * thrown in a consistent fashion for synchronization to work
1158 * correctly.
1159 * throws UnsupportedOperationException if shared mode is not supported
1160 */
1161 protected boolean tryReleaseShared(int arg) {
1162 throw new UnsupportedOperationException();
1163 }
1164
1165 /**
1166 * Returns {code true} if synchronization is held exclusively with
1167 * respect to the current (calling) thread. This method is invoked
1168 * upon each call to a non-waiting {link ConditionObject} method.
1169 * (Waiting methods instead invoke {link #release}.)
1170 *
1171 * pThe default implementation throws {link
1172 * UnsupportedOperationException}. This method is invoked
1173 * internally only within {link ConditionObject} methods, so need
1174 * not be defined if conditions are not used.
1175 *
1176 * return {code true} if synchronization is held exclusively;
1177 * {code false} otherwise
1178 * throws UnsupportedOperationException if conditions are not supported
1179 */
1180 protected boolean isHeldExclusively() {
1181 throw new UnsupportedOperationException();
1182 }
1183
1184 /**
1185 * Acquires in exclusive mode, ignoring interrupts. Implemented
1186 * by invoking at least once {link #tryAcquire},
1187 * returning on success. Otherwise the thread is queued, possibly
1188 * repeatedly blocking and unblocking, invoking {link
1189 * #tryAcquire} until success. This method can be used
1190 * to implement method {link Lock#lock}.
1191 *
1192 * param arg the acquire argument. This value is conveyed to
1193 * {link #tryAcquire} but is otherwise uninterpreted and
1194 * can represent anything you like.
1195 */
1196 public final void acquire(int arg) {
1197 if (!tryAcquire(arg)
1198 acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
1199 selfInterrupt();
1200 }
1201
1202 /**
1203 * Acquires in exclusive mode, aborting if interrupted.
1204 * Implemented by first checking interrupt status, then invoking
1205 * at least once {link #tryAcquire}, returning on
1206 * success. Otherwise the thread is queued, possibly repeatedly
1207 * blocking and unblocking, invoking {link #tryAcquire}
1208 * until success or the thread is interrupted. This method can be
1209 * used to implement method {link Lock#lockInterruptibly}.
1210 *
1211 * param arg the acquire argument. This value is conveyed to
1212 * {link #tryAcquire} but is otherwise uninterpreted and
1213 * can represent anything you like.
1214 * throws InterruptedException if the current thread is interrupted
1215 */
1216 public final void acquireInterruptibly(int arg)
1217 throws InterruptedException {
1218 if (Thread.interrupted())
1219 throw new InterruptedException();
1220 if (!tryAcquire(arg))
1221 doAcquireInterruptibly(arg);
1222 }
1223
1224 /**
1225 * Attempts to acquire in exclusive mode, aborting if interrupted,
1226 * and failing if the given timeout elapses. Implemented by first
1227 * checking interrupt status, then invoking at least once {link
1228 * #tryAcquire}, returning on success. Otherwise, the thread is
1229 * queued, possibly repeatedly blocking and unblocking, invoking
1230 * {link #tryAcquire} until success or the thread is interrupted
1231 * or the timeout elapses. This method can be used to implement
1232 * method {link Lock#tryLock(long, TimeUnit)}.
1233 *
1234 * param arg the acquire argument. This value is conveyed to
1235 * {link #tryAcquire} but is otherwise uninterpreted and
1236 * can represent anything you like.
1237 * param nanosTimeout the maximum number of nanoseconds to wait
1238 * return {code true} if acquired; {code false} if timed out
1239 * throws InterruptedException if the current thread is interrupted
1240 */
1241 public final boolean tryAcquireNanos(int arg, long nanosTimeout)
1242 throws InterruptedException {
1243 if (Thread.interrupted())
1244 throw new InterruptedException();
1245 return tryAcquire(arg) ||
1246 doAcquireNanos(arg, nanosTimeout);
1247 }
1248
1249 /**
1250 * Releases in exclusive mode. Implemented by unblocking one or
1251 * more threads if {link #tryRelease} returns true.
1252 * This method can be used to implement method {link Lock#unlock}.
1253 *
1254 * param arg the release argument. This value is conveyed to
1255 * {link #tryRelease} but is otherwise uninterpreted and
1256 * can represent anything you like.
1257 * return the value returned from {link #tryRelease}
1258 */
1259 public final boolean release(int arg) {
1260 if (tryRelease(arg)) {
1261 Node h head;
1262 if (h ! null h.waitStatus ! 0)
1263 unparkSuccessor(h);
1264 return true;
1265 }
1266 return false;
1267 }
1268
1269 /**
1270 * Acquires in shared mode, ignoring interrupts. Implemented by
1271 * first invoking at least once {link #tryAcquireShared},
1272 * returning on success. Otherwise the thread is queued, possibly
1273 * repeatedly blocking and unblocking, invoking {link
1274 * #tryAcquireShared} until success.
1275 *
1276 * param arg the acquire argument. This value is conveyed to
1277 * {link #tryAcquireShared} but is otherwise uninterpreted
1278 * and can represent anything you like.
1279 */
1280 public final void acquireShared(int arg) {
1281 if (tryAcquireShared(arg) 0)
1282 doAcquireShared(arg);
1283 }
1284
1285 /**
1286 * Acquires in shared mode, aborting if interrupted. Implemented
1287 * by first checking interrupt status, then invoking at least once
1288 * {link #tryAcquireShared}, returning on success. Otherwise the
1289 * thread is queued, possibly repeatedly blocking and unblocking,
1290 * invoking {link #tryAcquireShared} until success or the thread
1291 * is interrupted.
1292 * param arg the acquire argument
1293 * This value is conveyed to {link #tryAcquireShared} but is
1294 * otherwise uninterpreted and can represent anything
1295 * you like.
1296 * throws InterruptedException if the current thread is interrupted
1297 */
1298 public final void acquireSharedInterruptibly(int arg)
1299 throws InterruptedException {
1300 if (Thread.interrupted())
1301 throw new InterruptedException();
1302 if (tryAcquireShared(arg) 0)
1303 doAcquireSharedInterruptibly(arg);
1304 }
1305
1306 /**
1307 * Attempts to acquire in shared mode, aborting if interrupted, and
1308 * failing if the given timeout elapses. Implemented by first
1309 * checking interrupt status, then invoking at least once {link
1310 * #tryAcquireShared}, returning on success. Otherwise, the
1311 * thread is queued, possibly repeatedly blocking and unblocking,
1312 * invoking {link #tryAcquireShared} until success or the thread
1313 * is interrupted or the timeout elapses.
1314 *
1315 * param arg the acquire argument. This value is conveyed to
1316 * {link #tryAcquireShared} but is otherwise uninterpreted
1317 * and can represent anything you like.
1318 * param nanosTimeout the maximum number of nanoseconds to wait
1319 * return {code true} if acquired; {code false} if timed out
1320 * throws InterruptedException if the current thread is interrupted
1321 */
1322 public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
1323 throws InterruptedException {
1324 if (Thread.interrupted())
1325 throw new InterruptedException();
1326 return tryAcquireShared(arg) 0 ||
1327 doAcquireSharedNanos(arg, nanosTimeout);
1328 }
1329
1330 /**
1331 * Releases in shared mode. Implemented by unblocking one or more
1332 * threads if {link #tryReleaseShared} returns true.
1333 *
1334 * param arg the release argument. This value is conveyed to
1335 * {link #tryReleaseShared} but is otherwise uninterpreted
1336 * and can represent anything you like.
1337 * return the value returned from {link #tryReleaseShared}
1338 */
1339 public final boolean releaseShared(int arg) {
1340 if (tryReleaseShared(arg)) {
1341 doReleaseShared();
1342 return true;
1343 }
1344 return false;
1345 }
1346
1347 // Queue inspection methods
1348
1349 /**
1350 * Queries whether any threads are waiting to acquire. Note that
1351 * because cancellations due to interrupts and timeouts may occur
1352 * at any time, a {code true} return does not guarantee that any
1353 * other thread will ever acquire.
1354 *
1355 * pIn this implementation, this operation returns in
1356 * constant time.
1357 *
1358 * return {code true} if there may be other threads waiting to acquire
1359 */
1360 public final boolean hasQueuedThreads() {
1361 return head ! tail;
1362 }
1363
1364 /**
1365 * Queries whether any threads have ever contended to acquire this
1366 * synchronizer; that is if an acquire method has ever blocked.
1367 *
1368 * pIn this implementation, this operation returns in
1369 * constant time.
1370 *
1371 * return {code true} if there has ever been contention
1372 */
1373 public final boolean hasContended() {
1374 return head ! null;
1375 }
1376
1377 /**
1378 * Returns the first (longest-waiting) thread in the queue, or
1379 * {code null} if no threads are currently queued.
1380 *
1381 * pIn this implementation, this operation normally returns in
1382 * constant time, but may iterate upon contention if other threads are
1383 * concurrently modifying the queue.
1384 *
1385 * return the first (longest-waiting) thread in the queue, or
1386 * {code null} if no threads are currently queued
1387 */
1388 public final Thread getFirstQueuedThread() {
1389 // handle only fast path, else relay
1390 return (head tail) ? null : fullGetFirstQueuedThread();
1391 }
1392
1393 /**
1394 * Version of getFirstQueuedThread called when fastpath fails
1395 */
1396 private Thread fullGetFirstQueuedThread() {
1397 /*
1398 * The first node is normally head.next. Try to get its
1399 * thread field, ensuring consistent reads: If thread
1400 * field is nulled out or s.prev is no longer head, then
1401 * some other thread(s) concurrently performed setHead in
1402 * between some of our reads. We try this twice before
1403 * resorting to traversal.
1404 */
1405 Node h, s;
1406 Thread st;
1407 if (((h head) ! null (s h.next) ! null
1408 s.prev head (st s.thread) ! null) ||
1409 ((h head) ! null (s h.next) ! null
1410 s.prev head (st s.thread) ! null))
1411 return st;
1412
1413 /*
1414 * Heads next field might not have been set yet, or may have
1415 * been unset after setHead. So we must check to see if tail
1416 * is actually first node. If not, we continue on, safely
1417 * traversing from tail back to head to find first,
1418 * guaranteeing termination.
1419 */
1420
1421 Node t tail;
1422 Thread firstThread null;
1423 while (t ! null t ! head) {
1424 Thread tt t.thread;
1425 if (tt ! null)
1426 firstThread tt;
1427 t t.prev;
1428 }
1429 return firstThread;
1430 }
1431
1432 /**
1433 * Returns true if the given thread is currently queued.
1434 *
1435 * pThis implementation traverses the queue to determine
1436 * presence of the given thread.
1437 *
1438 * param thread the thread
1439 * return {code true} if the given thread is on the queue
1440 * throws NullPointerException if the thread is null
1441 */
1442 public final boolean isQueued(Thread thread) {
1443 if (thread null)
1444 throw new NullPointerException();
1445 for (Node p tail; p ! null; p p.prev)
1446 if (p.thread thread)
1447 return true;
1448 return false;
1449 }
1450
1451 /**
1452 * Returns {code true} if the apparent first queued thread, if one
1453 * exists, is waiting in exclusive mode. If this method returns
1454 * {code true}, and the current thread is attempting to acquire in
1455 * shared mode (that is, this method is invoked from {link
1456 * #tryAcquireShared}) then it is guaranteed that the current thread
1457 * is not the first queued thread. Used only as a heuristic in
1458 * ReentrantReadWriteLock.
1459 */
1460 final boolean apparentlyFirstQueuedIsExclusive() {
1461 Node h, s;
1462 return (h head) ! null
1463 (s h.next) ! null
1464 !s.isShared()
1465 s.thread ! null;
1466 }
1467
1468 /**
1469 * Queries whether any threads have been waiting to acquire longer
1470 * than the current thread.
1471 *
1472 * pAn invocation of this method is equivalent to (but may be
1473 * more efficient than):
1474 * pre {code
1475 * getFirstQueuedThread() ! Thread.currentThread()
1476 * hasQueuedThreads()}/pre
1477 *
1478 * pNote that because cancellations due to interrupts and
1479 * timeouts may occur at any time, a {code true} return does not
1480 * guarantee that some other thread will acquire before the current
1481 * thread. Likewise, it is possible for another thread to win a
1482 * race to enqueue after this method has returned {code false},
1483 * due to the queue being empty.
1484 *
1485 * pThis method is designed to be used by a fair synchronizer to
1486 * avoid a hrefAbstractQueuedSynchronizer#bargingbarging/a.
1487 * Such a synchronizers {link #tryAcquire} method should return
1488 * {code false}, and its {link #tryAcquireShared} method should
1489 * return a negative value, if this method returns {code true}
1490 * (unless this is a reentrant acquire). For example, the {code
1491 * tryAcquire} method for a fair, reentrant, exclusive mode
1492 * synchronizer might look like this:
1493 *
1494 * pre {code
1495 * protected boolean tryAcquire(int arg) {
1496 * if (isHeldExclusively()) {
1497 * // A reentrant acquire; increment hold count
1498 * return true;
1499 * } else if (hasQueuedPredecessors()) {
1500 * return false;
1501 * } else {
1502 * // try to acquire normally
1503 * }
1504 * }}/pre
1505 *
1506 * return {code true} if there is a queued thread preceding the
1507 * current thread, and {code false} if the current thread
1508 * is at the head of the queue or the queue is empty
1509 * since 1.7
1510 */
1511 public final boolean hasQueuedPredecessors() {
1512 // The correctness of this depends on head being initialized
1513 // before tail and on head.next being accurate if the current
1514 // thread is first in queue.
1515 Node t tail; // Read fields in reverse initialization order
1516 Node h head;
1517 Node s;
1518 return h ! t
1519 ((s h.next) null || s.thread ! Thread.currentThread());
1520 }
1521
1522
1523 // Instrumentation and monitoring methods
1524
1525 /**
1526 * Returns an estimate of the number of threads waiting to
1527 * acquire. The value is only an estimate because the number of
1528 * threads may change dynamically while this method traverses
1529 * internal data structures. This method is designed for use in
1530 * monitoring system state, not for synchronization
1531 * control.
1532 *
1533 * return the estimated number of threads waiting to acquire
1534 */
1535 public final int getQueueLength() {
1536 int n 0;
1537 for (Node p tail; p ! null; p p.prev) {
1538 if (p.thread ! null)
1539 n;
1540 }
1541 return n;
1542 }
1543
1544 /**
1545 * Returns a collection containing threads that may be waiting to
1546 * acquire. Because the actual set of threads may change
1547 * dynamically while constructing this result, the returned
1548 * collection is only a best-effort estimate. The elements of the
1549 * returned collection are in no particular order. This method is
1550 * designed to facilitate construction of subclasses that provide
1551 * more extensive monitoring facilities.
1552 *
1553 * return the collection of threads
1554 */
1555 public final CollectionThread getQueuedThreads() {
1556 ArrayListThread list new ArrayListThread();
1557 for (Node p tail; p ! null; p p.prev) {
1558 Thread t p.thread;
1559 if (t ! null)
1560 list.add(t);
1561 }
1562 return list;
1563 }
1564
1565 /**
1566 * Returns a collection containing threads that may be waiting to
1567 * acquire in exclusive mode. This has the same properties
1568 * as {link #getQueuedThreads} except that it only returns
1569 * those threads waiting due to an exclusive acquire.
1570 *
1571 * return the collection of threads
1572 */
1573 public final CollectionThread getExclusiveQueuedThreads() {
1574 ArrayListThread list new ArrayListThread();
1575 for (Node p tail; p ! null; p p.prev) {
1576 if (!p.isShared()) {
1577 Thread t p.thread;
1578 if (t ! null)
1579 list.add(t);
1580 }
1581 }
1582 return list;
1583 }
1584
1585 /**
1586 * Returns a collection containing threads that may be waiting to
1587 * acquire in shared mode. This has the same properties
1588 * as {link #getQueuedThreads} except that it only returns
1589 * those threads waiting due to a shared acquire.
1590 *
1591 * return the collection of threads
1592 */
1593 public final CollectionThread getSharedQueuedThreads() {
1594 ArrayListThread list new ArrayListThread();
1595 for (Node p tail; p ! null; p p.prev) {
1596 if (p.isShared()) {
1597 Thread t p.thread;
1598 if (t ! null)
1599 list.add(t);
1600 }
1601 }
1602 return list;
1603 }
1604
1605 /**
1606 * Returns a string identifying this synchronizer, as well as its state.
1607 * The state, in brackets, includes the String {code State }
1608 * followed by the current value of {link #getState}, and either
1609 * {code nonempty} or {code empty} depending on whether the
1610 * queue is empty.
1611 *
1612 * return a string identifying this synchronizer, as well as its state
1613 */
1614 public String toString() {
1615 int s getState();
1616 String q hasQueuedThreads() ? non : ;
1617 return super.toString()
1618 [State s , q empty queue];
1619 }
1620
1621
1622 // Internal support methods for Conditions
1623
1624 /**
1625 * Returns true if a node, always one that was initially placed on
1626 * a condition queue, is now waiting to reacquire on sync queue.
1627 * param node the node
1628 * return true if is reacquiring
1629 */
1630 final boolean isOnSyncQueue(Node node) {
1631 if (node.waitStatus Node.CONDITION || node.prev null)
1632 return false;
1633 if (node.next ! null) // If has successor, it must be on queue
1634 return true;
1635 /*
1636 * node.prev can be non-null, but not yet on queue because
1637 * the CAS to place it on queue can fail. So we have to
1638 * traverse from tail to make sure it actually made it. It
1639 * will always be near the tail in calls to this method, and
1640 * unless the CAS failed (which is unlikely), it will be
1641 * there, so we hardly ever traverse much.
1642 */
1643 return findNodeFromTail(node);
1644 }
1645
1646 /**
1647 * Returns true if node is on sync queue by searching backwards from tail.
1648 * Called only when needed by isOnSyncQueue.
1649 * return true if present
1650 */
1651 private boolean findNodeFromTail(Node node) {
1652 Node t tail;
1653 for (;;) {
1654 if (t node)
1655 return true;
1656 if (t null)
1657 return false;
1658 t t.prev;
1659 }
1660 }
1661
1662 /**
1663 * Transfers a node from a condition queue onto sync queue.
1664 * Returns true if successful.
1665 * param node the node
1666 * return true if successfully transferred (else the node was
1667 * cancelled before signal).
1668 */
1669 final boolean transferForSignal(Node node) {
1670 /*
1671 * If cannot change waitStatus, the node has been cancelled.
1672 */
1673 if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
1674 return false;
1675
1676 /*
1677 * Splice onto queue and try to set waitStatus of predecessor to
1678 * indicate that thread is (probably) waiting. If cancelled or
1679 * attempt to set waitStatus fails, wake up to resync (in which
1680 * case the waitStatus can be transiently and harmlessly wrong).
1681 */
1682 Node p enq(node);
1683 int ws p.waitStatus;
1684 if (ws 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
1685 LockSupport.unpark(node.thread);
1686 return true;
1687 }
1688
1689 /**
1690 * Transfers node, if necessary, to sync queue after a cancelled
1691 * wait. Returns true if thread was cancelled before being
1692 * signalled.
1693 * param current the waiting thread
1694 * param node its node
1695 * return true if cancelled before the node was signalled
1696 */
1697 final boolean transferAfterCancelledWait(Node node) {
1698 if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
1699 enq(node);
1700 return true;
1701 }
1702 /*
1703 * If we lost out to a signal(), then we cant proceed
1704 * until it finishes its enq(). Cancelling during an
1705 * incomplete transfer is both rare and transient, so just
1706 * spin.
1707 */
1708 while (!isOnSyncQueue(node))
1709 Thread.yield();
1710 return false;
1711 }
1712
1713 /**
1714 * Invokes release with current state value; returns saved state.
1715 * Cancels node and throws exception on failure.
1716 * param node the condition node for this wait
1717 * return previous sync state
1718 */
1719 final int fullyRelease(Node node) {
1720 boolean failed true;
1721 try {
1722 int savedState getState();
1723 if (release(savedState)) {
1724 failed false;
1725 return savedState;
1726 } else {
1727 throw new IllegalMonitorStateException();
1728 }
1729 } finally {
1730 if (failed)
1731 node.waitStatus Node.CANCELLED;
1732 }
1733 }
1734
1735 // Instrumentation methods for conditions
1736
1737 /**
1738 * Queries whether the given ConditionObject
1739 * uses this synchronizer as its lock.
1740 *
1741 * param condition the condition
1742 * return tttrue/tt if owned
1743 * throws NullPointerException if the condition is null
1744 */
1745 public final boolean owns(ConditionObject condition) {
1746 if (condition null)
1747 throw new NullPointerException();
1748 return condition.isOwnedBy(this);
1749 }
1750
1751 /**
1752 * Queries whether any threads are waiting on the given condition
1753 * associated with this synchronizer. Note that because timeouts
1754 * and interrupts may occur at any time, a tttrue/tt return
1755 * does not guarantee that a future ttsignal/tt will awaken
1756 * any threads. This method is designed primarily for use in
1757 * monitoring of the system state.
1758 *
1759 * param condition the condition
1760 * return tttrue/tt if there are any waiting threads
1761 * throws IllegalMonitorStateException if exclusive synchronization
1762 * is not held
1763 * throws IllegalArgumentException if the given condition is
1764 * not associated with this synchronizer
1765 * throws NullPointerException if the condition is null
1766 */
1767 public final boolean hasWaiters(ConditionObject condition) {
1768 if (!owns(condition))
1769 throw new IllegalArgumentException(Not owner);
1770 return condition.hasWaiters();
1771 }
1772
1773 /**
1774 * Returns an estimate of the number of threads waiting on the
1775 * given condition associated with this synchronizer. Note that
1776 * because timeouts and interrupts may occur at any time, the
1777 * estimate serves only as an upper bound on the actual number of
1778 * waiters. This method is designed for use in monitoring of the
1779 * system state, not for synchronization control.
1780 *
1781 * param condition the condition
1782 * return the estimated number of waiting threads
1783 * throws IllegalMonitorStateException if exclusive synchronization
1784 * is not held
1785 * throws IllegalArgumentException if the given condition is
1786 * not associated with this synchronizer
1787 * throws NullPointerException if the condition is null
1788 */
1789 public final int getWaitQueueLength(ConditionObject condition) {
1790 if (!owns(condition))
1791 throw new IllegalArgumentException(Not owner);
1792 return condition.getWaitQueueLength();
1793 }
1794
1795 /**
1796 * Returns a collection containing those threads that may be
1797 * waiting on the given condition associated with this
1798 * synchronizer. Because the actual set of threads may change
1799 * dynamically while constructing this result, the returned
1800 * collection is only a best-effort estimate. The elements of the
1801 * returned collection are in no particular order.
1802 *
1803 * param condition the condition
1804 * return the collection of threads
1805 * throws IllegalMonitorStateException if exclusive synchronization
1806 * is not held
1807 * throws IllegalArgumentException if the given condition is
1808 * not associated with this synchronizer
1809 * throws NullPointerException if the condition is null
1810 */
1811 public final CollectionThread getWaitingThreads(ConditionObject condition) {
1812 if (!owns(condition))
1813 throw new IllegalArgumentException(Not owner);
1814 return condition.getWaitingThreads();
1815 }
1816
1817 /**
1818 * Condition implementation for a {link
1819 * AbstractQueuedSynchronizer} serving as the basis of a {link
1820 * Lock} implementation.
1821 *
1822 * pMethod documentation for this class describes mechanics,
1823 * not behavioral specifications from the point of view of Lock
1824 * and Condition users. Exported versions of this class will in
1825 * general need to be accompanied by documentation describing
1826 * condition semantics that rely on those of the associated
1827 * ttAbstractQueuedSynchronizer/tt.
1828 *
1829 * pThis class is Serializable, but all fields are transient,
1830 * so deserialized conditions have no waiters.
1831 */
1832 public class ConditionObject implements Condition, java.io.Serializable {
1833 private static final long serialVersionUID 1173984872572414699L;
1834 /** First node of condition queue. */
1835 private transient Node firstWaiter;
1836 /** Last node of condition queue. */
1837 private transient Node lastWaiter;
1838
1839 /**
1840 * Creates a new ttConditionObject/tt instance.
1841 */
1842 public ConditionObject() { }
1843
1844 // Internal methods
1845
1846 /**
1847 * Adds a new waiter to wait queue.
1848 * return its new wait node
1849 */
1850 private Node addConditionWaiter() {
1851 Node t lastWaiter;
1852 // If lastWaiter is cancelled, clean out.
1853 if (t ! null t.waitStatus ! Node.CONDITION) {
1854 unlinkCancelledWaiters();
1855 t lastWaiter;
1856 }
1857 Node node new Node(Thread.currentThread(), Node.CONDITION);
1858 if (t null)
1859 firstWaiter node;
1860 else
1861 t.nextWaiter node;
1862 lastWaiter node;
1863 return node;
1864 }
1865
1866 /**
1867 * Removes and transfers nodes until hit non-cancelled one or
1868 * null. Split out from signal in part to encourage compilers
1869 * to inline the case of no waiters.
1870 * param first (non-null) the first node on condition queue
1871 */
1872 private void doSignal(Node first) {
1873 do {
1874 if ( (firstWaiter first.nextWaiter) null)
1875 lastWaiter null;
1876 first.nextWaiter null;
1877 } while (!transferForSignal(first)
1878 (first firstWaiter) ! null);
1879 }
1880
1881 /**
1882 * Removes and transfers all nodes.
1883 * param first (non-null) the first node on condition queue
1884 */
1885 private void doSignalAll(Node first) {
1886 lastWaiter firstWaiter null;
1887 do {
1888 Node next first.nextWaiter;
1889 first.nextWaiter null;
1890 transferForSignal(first);
1891 first next;
1892 } while (first ! null);
1893 }
1894
1895 /**
1896 * Unlinks cancelled waiter nodes from condition queue.
1897 * Called only while holding lock. This is called when
1898 * cancellation occurred during condition wait, and upon
1899 * insertion of a new waiter when lastWaiter is seen to have
1900 * been cancelled. This method is needed to avoid garbage
1901 * retention in the absence of signals. So even though it may
1902 * require a full traversal, it comes into play only when
1903 * timeouts or cancellations occur in the absence of
1904 * signals. It traverses all nodes rather than stopping at a
1905 * particular target to unlink all pointers to garbage nodes
1906 * without requiring many re-traversals during cancellation
1907 * storms.
1908 */
1909 private void unlinkCancelledWaiters() {
1910 Node t firstWaiter;
1911 Node trail null;
1912 while (t ! null) {
1913 Node next t.nextWaiter;
1914 if (t.waitStatus ! Node.CONDITION) {
1915 t.nextWaiter null;
1916 if (trail null)
1917 firstWaiter next;
1918 else
1919 trail.nextWaiter next;
1920 if (next null)
1921 lastWaiter trail;
1922 }
1923 else
1924 trail t;
1925 t next;
1926 }
1927 }
1928
1929 // public methods
1930
1931 /**
1932 * Moves the longest-waiting thread, if one exists, from the
1933 * wait queue for this condition to the wait queue for the
1934 * owning lock.
1935 *
1936 * throws IllegalMonitorStateException if {link #isHeldExclusively}
1937 * returns {code false}
1938 */
1939 public final void signal() {
1940 if (!isHeldExclusively())
1941 throw new IllegalMonitorStateException();
1942 Node first firstWaiter;
1943 if (first ! null)
1944 doSignal(first);
1945 }
1946
1947 /**
1948 * Moves all threads from the wait queue for this condition to
1949 * the wait queue for the owning lock.
1950 *
1951 * throws IllegalMonitorStateException if {link #isHeldExclusively}
1952 * returns {code false}
1953 */
1954 public final void signalAll() {
1955 if (!isHeldExclusively())
1956 throw new IllegalMonitorStateException();
1957 Node first firstWaiter;
1958 if (first ! null)
1959 doSignalAll(first);
1960 }
1961
1962 /**
1963 * Implements uninterruptible condition wait.
1964 * ol
1965 * li Save lock state returned by {link #getState}.
1966 * li Invoke {link #release} with
1967 * saved state as argument, throwing
1968 * IllegalMonitorStateException if it fails.
1969 * li Block until signalled.
1970 * li Reacquire by invoking specialized version of
1971 * {link #acquire} with saved state as argument.
1972 * /ol
1973 */
1974 public final void awaitUninterruptibly() {
1975 Node node addConditionWaiter();
1976 int savedState fullyRelease(node);
1977 boolean interrupted false;
1978 while (!isOnSyncQueue(node)) {
1979 LockSupport.park(this);
1980 if (Thread.interrupted())
1981 interrupted true;
1982 }
1983 if (acquireQueued(node, savedState) || interrupted)
1984 selfInterrupt();
1985 }
1986
1987 /*
1988 * For interruptible waits, we need to track whether to throw
1989 * InterruptedException, if interrupted while blocked on
1990 * condition, versus reinterrupt current thread, if
1991 * interrupted while blocked waiting to re-acquire.
1992 */
1993
1994 /** Mode meaning to reinterrupt on exit from wait */
1995 private static final int REINTERRUPT 1;
1996 /** Mode meaning to throw InterruptedException on exit from wait */
1997 private static final int THROW_IE -1;
1998
1999 /**
2000 * Checks for interrupt, returning THROW_IE if interrupted
2001 * before signalled, REINTERRUPT if after signalled, or
2002 * 0 if not interrupted.
2003 */
2004 private int checkInterruptWhileWaiting(Node node) {
2005 return Thread.interrupted() ?
2006 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
2007 0;
2008 }
2009
2010 /**
2011 * Throws InterruptedException, reinterrupts current thread, or
2012 * does nothing, depending on mode.
2013 */
2014 private void reportInterruptAfterWait(int interruptMode)
2015 throws InterruptedException {
2016 if (interruptMode THROW_IE)
2017 throw new InterruptedException();
2018 else if (interruptMode REINTERRUPT)
2019 selfInterrupt();
2020 }
2021
2022 /**
2023 * Implements interruptible condition wait.
2024 * ol
2025 * li If current thread is interrupted, throw InterruptedException.
2026 * li Save lock state returned by {link #getState}.
2027 * li Invoke {link #release} with
2028 * saved state as argument, throwing
2029 * IllegalMonitorStateException if it fails.
2030 * li Block until signalled or interrupted.
2031 * li Reacquire by invoking specialized version of
2032 * {link #acquire} with saved state as argument.
2033 * li If interrupted while blocked in step 4, throw InterruptedException.
2034 * /ol
2035 */
2036 public final void await() throws InterruptedException {
2037 if (Thread.interrupted())
2038 throw new InterruptedException();
2039 Node node addConditionWaiter();
2040 int savedState fullyRelease(node);
2041 int interruptMode 0;
2042 while (!isOnSyncQueue(node)) {
2043 LockSupport.park(this);
2044 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2045 break;
2046 }
2047 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2048 interruptMode REINTERRUPT;
2049 if (node.nextWaiter ! null) // clean up if cancelled
2050 unlinkCancelledWaiters();
2051 if (interruptMode ! 0)
2052 reportInterruptAfterWait(interruptMode);
2053 }
2054
2055 /**
2056 * Implements timed condition wait.
2057 * ol
2058 * li If current thread is interrupted, throw InterruptedException.
2059 * li Save lock state returned by {link #getState}.
2060 * li Invoke {link #release} with
2061 * saved state as argument, throwing
2062 * IllegalMonitorStateException if it fails.
2063 * li Block until signalled, interrupted, or timed out.
2064 * li Reacquire by invoking specialized version of
2065 * {link #acquire} with saved state as argument.
2066 * li If interrupted while blocked in step 4, throw InterruptedException.
2067 * /ol
2068 */
2069 public final long awaitNanos(long nanosTimeout)
2070 throws InterruptedException {
2071 if (Thread.interrupted())
2072 throw new InterruptedException();
2073 Node node addConditionWaiter();
2074 int savedState fullyRelease(node);
2075 long lastTime System.nanoTime();
2076 int interruptMode 0;
2077 while (!isOnSyncQueue(node)) {
2078 if (nanosTimeout 0L) {
2079 transferAfterCancelledWait(node);
2080 break;
2081 }
2082 LockSupport.parkNanos(this, nanosTimeout);
2083 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2084 break;
2085
2086 long now System.nanoTime();
2087 nanosTimeout - now - lastTime;
2088 lastTime now;
2089 }
2090 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2091 interruptMode REINTERRUPT;
2092 if (node.nextWaiter ! null)
2093 unlinkCancelledWaiters();
2094 if (interruptMode ! 0)
2095 reportInterruptAfterWait(interruptMode);
2096 return nanosTimeout - (System.nanoTime() - lastTime);
2097 }
2098
2099 /**
2100 * Implements absolute timed condition wait.
2101 * ol
2102 * li If current thread is interrupted, throw InterruptedException.
2103 * li Save lock state returned by {link #getState}.
2104 * li Invoke {link #release} with
2105 * saved state as argument, throwing
2106 * IllegalMonitorStateException if it fails.
2107 * li Block until signalled, interrupted, or timed out.
2108 * li Reacquire by invoking specialized version of
2109 * {link #acquire} with saved state as argument.
2110 * li If interrupted while blocked in step 4, throw InterruptedException.
2111 * li If timed out while blocked in step 4, return false, else true.
2112 * /ol
2113 */
2114 public final boolean awaitUntil(Date deadline)
2115 throws InterruptedException {
2116 if (deadline null)
2117 throw new NullPointerException();
2118 long abstime deadline.getTime();
2119 if (Thread.interrupted())
2120 throw new InterruptedException();
2121 Node node addConditionWaiter();
2122 int savedState fullyRelease(node);
2123 boolean timedout false;
2124 int interruptMode 0;
2125 while (!isOnSyncQueue(node)) {
2126 if (System.currentTimeMillis() abstime) {
2127 timedout transferAfterCancelledWait(node);
2128 break;
2129 }
2130 LockSupport.parkUntil(this, abstime);
2131 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2132 break;
2133 }
2134 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2135 interruptMode REINTERRUPT;
2136 if (node.nextWaiter ! null)
2137 unlinkCancelledWaiters();
2138 if (interruptMode ! 0)
2139 reportInterruptAfterWait(interruptMode);
2140 return !timedout;
2141 }
2142
2143 /**
2144 * Implements timed condition wait.
2145 * ol
2146 * li If current thread is interrupted, throw InterruptedException.
2147 * li Save lock state returned by {link #getState}.
2148 * li Invoke {link #release} with
2149 * saved state as argument, throwing
2150 * IllegalMonitorStateException if it fails.
2151 * li Block until signalled, interrupted, or timed out.
2152 * li Reacquire by invoking specialized version of
2153 * {link #acquire} with saved state as argument.
2154 * li If interrupted while blocked in step 4, throw InterruptedException.
2155 * li If timed out while blocked in step 4, return false, else true.
2156 * /ol
2157 */
2158 public final boolean await(long time, TimeUnit unit)
2159 throws InterruptedException {
2160 if (unit null)
2161 throw new NullPointerException();
2162 long nanosTimeout unit.toNanos(time);
2163 if (Thread.interrupted())
2164 throw new InterruptedException();
2165 Node node addConditionWaiter();
2166 int savedState fullyRelease(node);
2167 long lastTime System.nanoTime();
2168 boolean timedout false;
2169 int interruptMode 0;
2170 while (!isOnSyncQueue(node)) {
2171 if (nanosTimeout 0L) {
2172 timedout transferAfterCancelledWait(node);
2173 break;
2174 }
2175 if (nanosTimeout spinForTimeoutThreshold)
2176 LockSupport.parkNanos(this, nanosTimeout);
2177 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2178 break;
2179 long now System.nanoTime();
2180 nanosTimeout - now - lastTime;
2181 lastTime now;
2182 }
2183 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2184 interruptMode REINTERRUPT;
2185 if (node.nextWaiter ! null)
2186 unlinkCancelledWaiters();
2187 if (interruptMode ! 0)
2188 reportInterruptAfterWait(interruptMode);
2189 return !timedout;
2190 }
2191
2192 // support for instrumentation
2193
2194 /**
2195 * Returns true if this condition was created by the given
2196 * synchronization object.
2197 *
2198 * return {code true} if owned
2199 */
2200 final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
2201 return sync AbstractQueuedSynchronizer.this;
2202 }
2203
2204 /**
2205 * Queries whether any threads are waiting on this condition.
2206 * Implements {link AbstractQueuedSynchronizer#hasWaiters}.
2207 *
2208 * return {code true} if there are any waiting threads
2209 * throws IllegalMonitorStateException if {link #isHeldExclusively}
2210 * returns {code false}
2211 */
2212 protected final boolean hasWaiters() {
2213 if (!isHeldExclusively())
2214 throw new IllegalMonitorStateException();
2215 for (Node w firstWaiter; w ! null; w w.nextWaiter) {
2216 if (w.waitStatus Node.CONDITION)
2217 return true;
2218 }
2219 return false;
2220 }
2221
2222 /**
2223 * Returns an estimate of the number of threads waiting on
2224 * this condition.
2225 * Implements {link AbstractQueuedSynchronizer#getWaitQueueLength}.
2226 *
2227 * return the estimated number of waiting threads
2228 * throws IllegalMonitorStateException if {link #isHeldExclusively}
2229 * returns {code false}
2230 */
2231 protected final int getWaitQueueLength() {
2232 if (!isHeldExclusively())
2233 throw new IllegalMonitorStateException();
2234 int n 0;
2235 for (Node w firstWaiter; w ! null; w w.nextWaiter) {
2236 if (w.waitStatus Node.CONDITION)
2237 n;
2238 }
2239 return n;
2240 }
2241
2242 /**
2243 * Returns a collection containing those threads that may be
2244 * waiting on this Condition.
2245 * Implements {link AbstractQueuedSynchronizer#getWaitingThreads}.
2246 *
2247 * return the collection of threads
2248 * throws IllegalMonitorStateException if {link #isHeldExclusively}
2249 * returns {code false}
2250 */
2251 protected final CollectionThread getWaitingThreads() {
2252 if (!isHeldExclusively())
2253 throw new IllegalMonitorStateException();
2254 ArrayListThread list new ArrayListThread();
2255 for (Node w firstWaiter; w ! null; w w.nextWaiter) {
2256 if (w.waitStatus Node.CONDITION) {
2257 Thread t w.thread;
2258 if (t ! null)
2259 list.add(t);
2260 }
2261 }
2262 return list;
2263 }
2264 }
2265
2266 /**
2267 * Setup to support compareAndSet. We need to natively implement
2268 * this here: For the sake of permitting future enhancements, we
2269 * cannot explicitly subclass AtomicInteger, which would be
2270 * efficient and useful otherwise. So, as the lesser of evils, we
2271 * natively implement using hotspot intrinsics API. And while we
2272 * are at it, we do the same for other CASable fields (which could
2273 * otherwise be done with atomic field updaters).
2274 */
2275 private static final Unsafe unsafe Unsafe.getUnsafe();
2276 private static final long stateOffset;
2277 private static final long headOffset;
2278 private static final long tailOffset;
2279 private static final long waitStatusOffset;
2280 private static final long nextOffset;
2281
2282 static {
2283 try {
2284 stateOffset unsafe.objectFieldOffset
2285 (AbstractQueuedSynchronizer.class.getDeclaredField(state));
2286 headOffset unsafe.objectFieldOffset
2287 (AbstractQueuedSynchronizer.class.getDeclaredField(head));
2288 tailOffset unsafe.objectFieldOffset
2289 (AbstractQueuedSynchronizer.class.getDeclaredField(tail));
2290 waitStatusOffset unsafe.objectFieldOffset
2291 (Node.class.getDeclaredField(waitStatus));
2292 nextOffset unsafe.objectFieldOffset
2293 (Node.class.getDeclaredField(next));
2294
2295 } catch (Exception ex) { throw new Error(ex); }
2296 }
2297
2298 /**
2299 * CAS head field. Used only by enq.
2300 */
2301 private final boolean compareAndSetHead(Node update) {
2302 return unsafe.compareAndSwapObject(this, headOffset, null, update);
2303 }
2304
2305 /**
2306 * CAS tail field. Used only by enq.
2307 */
2308 private final boolean compareAndSetTail(Node expect, Node update) {
2309 return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
2310 }
2311
2312 /**
2313 * CAS waitStatus field of a node.
2314 */
2315 private static final boolean compareAndSetWaitStatus(Node node,
2316 int expect,
2317 int update) {
2318 return unsafe.compareAndSwapInt(node, waitStatusOffset,
2319 expect, update);
2320 }
2321
2322 /**
2323 * CAS next field of a node.
2324 */
2325 private static final boolean compareAndSetNext(Node node,
2326 Node expect,
2327 Node update) {
2328 return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
2329 }
2330 } 释放公平锁(基于JDK1.7.0_40) 1. unlock() unlock()在ReentrantLock.java中实现的源码如下 public void unlock() {sync.release(1);
} 说明 unlock()是解锁函数它是通过AQS的release()函数来实现的。 在这里“1”的含义和“获取锁的函数acquire(1)的含义”一样它是设置“释放锁的状态”的参数。由于“公平锁”是可重入的所以对于同一个线程每释放锁一次锁的状态-1。 关于AQS, ReentrantLock 和 sync的关系如下 public class ReentrantLock implements Lock, java.io.Serializable {private final Sync sync;abstract static class Sync extends AbstractQueuedSynchronizer {...}...
} 从中我们发现sync是ReentrantLock.java中的成员对象而Sync是AQS的子类。 2. release() release()在AQS中实现的源码如下 public final boolean release(int arg) {if (tryRelease(arg)) {Node h head;if (h ! null h.waitStatus ! 0)unparkSuccessor(h);return true;}return false;
} 说明 release()会先调用tryRelease()来尝试释放当前线程锁持有的锁。成功的话则唤醒后继等待线程并返回true。否则直接返回false。 3. tryRelease() tryRelease()在ReentrantLock.java的Sync类中实现源码如下 protected final boolean tryRelease(int releases) {// c是本次释放锁之后的状态int c getState() - releases;// 如果“当前线程”不是“锁的持有者”则抛出异常if (Thread.currentThread() ! getExclusiveOwnerThread())throw new IllegalMonitorStateException();boolean free false;// 如果“锁”已经被当前线程彻底释放则设置“锁”的持有者为null即锁是可获取状态。if (c 0) {free true;setExclusiveOwnerThread(null);}// 设置当前线程的锁的状态。setState(c);return free;
} 说明 tryRelease()的作用是尝试释放锁。 (01) 如果“当前线程”不是“锁的持有者”则抛出异常。 (02) 如果“当前线程”在本次释放锁操作之后对锁的拥有状态是0(即当前线程彻底释放该“锁”)则设置“锁”的持有者为null即锁是可获取状态。同时更新当前线程的锁的状态为0。 getState(), setState()在前一章已经介绍过这里不再说明。 getExclusiveOwnerThread(), setExclusiveOwnerThread()在AQS的父类AbstractOwnableSynchronizer.java中定义源码如下 public abstract class AbstractOwnableSynchronizerimplements java.io.Serializable {// “锁”的持有线程private transient Thread exclusiveOwnerThread;// 设置“锁的持有线程”为tprotected final void setExclusiveOwnerThread(Thread t) {exclusiveOwnerThread t;}// 获取“锁的持有线程”protected final Thread getExclusiveOwnerThread() {return exclusiveOwnerThread;}...
} 4. unparkSuccessor() 在release()中“当前线程”释放锁成功的话会唤醒当前线程的后继线程。 根据CLH队列的FIFO规则“当前线程”(即已经获取锁的线程)肯定是head如果CLH队列非空的话则唤醒锁的下一个等待线程。 下面看看unparkSuccessor()的源码它在AQS中实现。 private void unparkSuccessor(Node node) {// 获取当前线程的状态int ws node.waitStatus;// 如果状态0则设置状态0if (ws 0)compareAndSetWaitStatus(node, ws, 0);//获取当前节点的“有效的后继节点”无效的话则通过for循环进行获取。// 这里的有效是指“后继节点对应的线程状态0”Node s node.next;if (s null || s.waitStatus 0) {s null;for (Node t tail; t ! null t ! node; t t.prev)if (t.waitStatus 0)s t;}// 唤醒“后继节点对应的线程”if (s ! null)LockSupport.unpark(s.thread);
} 说明 unparkSuccessor()的作用是“唤醒当前线程的后继线程”。后继线程被唤醒之后就可以获取该锁并恢复运行了。 关于node.waitStatus的说明请参考“上一章关于Node类的介绍”。 总结 “释放锁”的过程相对“获取锁”的过程比较简单。释放锁时主要进行的操作是更新当前线程对应的锁的状态。如果当前线程对锁已经彻底释放则设置“锁”的持有线程为null设置当前线程的状态为空然后唤醒后继线程。 概要 前面一章我们学习了“公平锁”获取锁的详细流程这里我们再来看看“公平锁”释放锁的过程。内容包括 参考代码 释放公平锁(基于JDK1.7.0_40) “公平锁”的获取过程请参考“Java多线程系列--“JUC锁”03之 公平锁(一)”锁的使用示例请参考“Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock”。 注意 (01) 这里是以“公平锁”来进行说明。 (02) 关于本章的术语如“AQS”“CAS函数”“CLH队列”“公平锁”“非公平锁”“独占锁”“共享锁”等内容请参考Java多线程系列--“JUC锁”03之 公平锁(一)的基本概念。 转载请注明出处http://www.cnblogs.com/skywang12345/p/3496609.html 参考代码 下面给出Java1.7.0_40版本中ReentrantLock和AQS的源码仅供参考 ReentranLock.java 1 /*2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.3 *4 *5 *6 *7 *8 *9 *10 *11 *12 *13 *14 *15 *16 *17 *18 *19 *20 *21 *22 *23 */24 25 /*26 *27 *28 *29 *30 *31 * Written by Doug Lea with assistance from members of JCP JSR-16632 * Expert Group and released to the public domain, as explained at33 * http://creativecommons.org/publicdomain/zero/1.0/34 */35 36 package java.util.concurrent.locks;37 import java.util.*;38 import java.util.concurrent.*;39 import java.util.concurrent.atomic.*;40 41 /**42 * A reentrant mutual exclusion {link Lock} with the same basic43 * behavior and semantics as the implicit monitor lock accessed using44 * {code synchronized} methods and statements, but with extended45 * capabilities.46 *47 * pA {code ReentrantLock} is emowned/em by the thread last48 * successfully locking, but not yet unlocking it. A thread invoking49 * {code lock} will return, successfully acquiring the lock, when50 * the lock is not owned by another thread. The method will return51 * immediately if the current thread already owns the lock. This can52 * be checked using methods {link #isHeldByCurrentThread}, and {link53 * #getHoldCount}.54 *55 * pThe constructor for this class accepts an optional56 * emfairness/em parameter. When set {code true}, under57 * contention, locks favor granting access to the longest-waiting58 * thread. Otherwise this lock does not guarantee any particular59 * access order. Programs using fair locks accessed by many threads60 * may display lower overall throughput (i.e., are slower; often much61 * slower) than those using the default setting, but have smaller62 * variances in times to obtain locks and guarantee lack of63 * starvation. Note however, that fairness of locks does not guarantee64 * fairness of thread scheduling. Thus, one of many threads using a65 * fair lock may obtain it multiple times in succession while other66 * active threads are not progressing and not currently holding the67 * lock.68 * Also note that the untimed {link #tryLock() tryLock} method does not69 * honor the fairness setting. It will succeed if the lock70 * is available even if other threads are waiting.71 *72 * pIt is recommended practice to emalways/em immediately73 * follow a call to {code lock} with a {code try} block, most74 * typically in a before/after construction such as:75 *76 * pre77 * class X {78 * private final ReentrantLock lock new ReentrantLock();79 * // ...80 *81 * public void m() {82 * lock.lock(); // block until condition holds83 * try {84 * // ... method body85 * } finally {86 * lock.unlock()87 * }88 * }89 * }90 * /pre91 *92 * pIn addition to implementing the {link Lock} interface, this93 * class defines methods {code isLocked} and94 * {code getLockQueueLength}, as well as some associated95 * {code protected} access methods that may be useful for96 * instrumentation and monitoring.97 *98 * pSerialization of this class behaves in the same way as built-in99 * locks: a deserialized lock is in the unlocked state, regardless of
100 * its state when serialized.
101 *
102 * pThis lock supports a maximum of 2147483647 recursive locks by
103 * the same thread. Attempts to exceed this limit result in
104 * {link Error} throws from locking methods.
105 *
106 * since 1.5
107 * author Doug Lea
108 */
109 public class ReentrantLock implements Lock, java.io.Serializable {
110 private static final long serialVersionUID 7373984872572414699L;
111 /** Synchronizer providing all implementation mechanics */
112 private final Sync sync;
113
114 /**
115 * Base of synchronization control for this lock. Subclassed
116 * into fair and nonfair versions below. Uses AQS state to
117 * represent the number of holds on the lock.
118 */
119 abstract static class Sync extends AbstractQueuedSynchronizer {
120 private static final long serialVersionUID -5179523762034025860L;
121
122 /**
123 * Performs {link Lock#lock}. The main reason for subclassing
124 * is to allow fast path for nonfair version.
125 */
126 abstract void lock();
127
128 /**
129 * Performs non-fair tryLock. tryAcquire is
130 * implemented in subclasses, but both need nonfair
131 * try for trylock method.
132 */
133 final boolean nonfairTryAcquire(int acquires) {
134 final Thread current Thread.currentThread();
135 int c getState();
136 if (c 0) {
137 if (compareAndSetState(0, acquires)) {
138 setExclusiveOwnerThread(current);
139 return true;
140 }
141 }
142 else if (current getExclusiveOwnerThread()) {
143 int nextc c acquires;
144 if (nextc 0) // overflow
145 throw new Error(Maximum lock count exceeded);
146 setState(nextc);
147 return true;
148 }
149 return false;
150 }
151
152 protected final boolean tryRelease(int releases) {
153 int c getState() - releases;
154 if (Thread.currentThread() ! getExclusiveOwnerThread())
155 throw new IllegalMonitorStateException();
156 boolean free false;
157 if (c 0) {
158 free true;
159 setExclusiveOwnerThread(null);
160 }
161 setState(c);
162 return free;
163 }
164
165 protected final boolean isHeldExclusively() {
166 // While we must in general read state before owner,
167 // we dont need to do so to check if current thread is owner
168 return getExclusiveOwnerThread() Thread.currentThread();
169 }
170
171 final ConditionObject newCondition() {
172 return new ConditionObject();
173 }
174
175 // Methods relayed from outer class
176
177 final Thread getOwner() {
178 return getState() 0 ? null : getExclusiveOwnerThread();
179 }
180
181 final int getHoldCount() {
182 return isHeldExclusively() ? getState() : 0;
183 }
184
185 final boolean isLocked() {
186 return getState() ! 0;
187 }
188
189 /**
190 * Reconstitutes this lock instance from a stream.
191 * param s the stream
192 */
193 private void readObject(java.io.ObjectInputStream s)
194 throws java.io.IOException, ClassNotFoundException {
195 s.defaultReadObject();
196 setState(0); // reset to unlocked state
197 }
198 }
199
200 /**
201 * Sync object for non-fair locks
202 */
203 static final class NonfairSync extends Sync {
204 private static final long serialVersionUID 7316153563782823691L;
205
206 /**
207 * Performs lock. Try immediate barge, backing up to normal
208 * acquire on failure.
209 */
210 final void lock() {
211 if (compareAndSetState(0, 1))
212 setExclusiveOwnerThread(Thread.currentThread());
213 else
214 acquire(1);
215 }
216
217 protected final boolean tryAcquire(int acquires) {
218 return nonfairTryAcquire(acquires);
219 }
220 }
221
222 /**
223 * Sync object for fair locks
224 */
225 static final class FairSync extends Sync {
226 private static final long serialVersionUID -3000897897090466540L;
227
228 final void lock() {
229 acquire(1);
230 }
231
232 /**
233 * Fair version of tryAcquire. Dont grant access unless
234 * recursive call or no waiters or is first.
235 */
236 protected final boolean tryAcquire(int acquires) {
237 final Thread current Thread.currentThread();
238 int c getState();
239 if (c 0) {
240 if (!hasQueuedPredecessors()
241 compareAndSetState(0, acquires)) {
242 setExclusiveOwnerThread(current);
243 return true;
244 }
245 }
246 else if (current getExclusiveOwnerThread()) {
247 int nextc c acquires;
248 if (nextc 0)
249 throw new Error(Maximum lock count exceeded);
250 setState(nextc);
251 return true;
252 }
253 return false;
254 }
255 }
256
257 /**
258 * Creates an instance of {code ReentrantLock}.
259 * This is equivalent to using {code ReentrantLock(false)}.
260 */
261 public ReentrantLock() {
262 sync new NonfairSync();
263 }
264
265 /**
266 * Creates an instance of {code ReentrantLock} with the
267 * given fairness policy.
268 *
269 * param fair {code true} if this lock should use a fair ordering policy
270 */
271 public ReentrantLock(boolean fair) {
272 sync fair ? new FairSync() : new NonfairSync();
273 }
274
275 /**
276 * Acquires the lock.
277 *
278 * pAcquires the lock if it is not held by another thread and returns
279 * immediately, setting the lock hold count to one.
280 *
281 * pIf the current thread already holds the lock then the hold
282 * count is incremented by one and the method returns immediately.
283 *
284 * pIf the lock is held by another thread then the
285 * current thread becomes disabled for thread scheduling
286 * purposes and lies dormant until the lock has been acquired,
287 * at which time the lock hold count is set to one.
288 */
289 public void lock() {
290 sync.lock();
291 }
292
293 /**
294 * Acquires the lock unless the current thread is
295 * {linkplain Thread#interrupt interrupted}.
296 *
297 * pAcquires the lock if it is not held by another thread and returns
298 * immediately, setting the lock hold count to one.
299 *
300 * pIf the current thread already holds this lock then the hold count
301 * is incremented by one and the method returns immediately.
302 *
303 * pIf the lock is held by another thread then the
304 * current thread becomes disabled for thread scheduling
305 * purposes and lies dormant until one of two things happens:
306 *
307 * ul
308 *
309 * liThe lock is acquired by the current thread; or
310 *
311 * liSome other thread {linkplain Thread#interrupt interrupts} the
312 * current thread.
313 *
314 * /ul
315 *
316 * pIf the lock is acquired by the current thread then the lock hold
317 * count is set to one.
318 *
319 * pIf the current thread:
320 *
321 * ul
322 *
323 * lihas its interrupted status set on entry to this method; or
324 *
325 * liis {linkplain Thread#interrupt interrupted} while acquiring
326 * the lock,
327 *
328 * /ul
329 *
330 * then {link InterruptedException} is thrown and the current threads
331 * interrupted status is cleared.
332 *
333 * pIn this implementation, as this method is an explicit
334 * interruption point, preference is given to responding to the
335 * interrupt over normal or reentrant acquisition of the lock.
336 *
337 * throws InterruptedException if the current thread is interrupted
338 */
339 public void lockInterruptibly() throws InterruptedException {
340 sync.acquireInterruptibly(1);
341 }
342
343 /**
344 * Acquires the lock only if it is not held by another thread at the time
345 * of invocation.
346 *
347 * pAcquires the lock if it is not held by another thread and
348 * returns immediately with the value {code true}, setting the
349 * lock hold count to one. Even when this lock has been set to use a
350 * fair ordering policy, a call to {code tryLock()} emwill/em
351 * immediately acquire the lock if it is available, whether or not
352 * other threads are currently waiting for the lock.
353 * This quot;bargingquot; behavior can be useful in certain
354 * circumstances, even though it breaks fairness. If you want to honor
355 * the fairness setting for this lock, then use
356 * {link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
357 * which is almost equivalent (it also detects interruption).
358 *
359 * p If the current thread already holds this lock then the hold
360 * count is incremented by one and the method returns {code true}.
361 *
362 * pIf the lock is held by another thread then this method will return
363 * immediately with the value {code false}.
364 *
365 * return {code true} if the lock was free and was acquired by the
366 * current thread, or the lock was already held by the current
367 * thread; and {code false} otherwise
368 */
369 public boolean tryLock() {
370 return sync.nonfairTryAcquire(1);
371 }
372
373 /**
374 * Acquires the lock if it is not held by another thread within the given
375 * waiting time and the current thread has not been
376 * {linkplain Thread#interrupt interrupted}.
377 *
378 * pAcquires the lock if it is not held by another thread and returns
379 * immediately with the value {code true}, setting the lock hold count
380 * to one. If this lock has been set to use a fair ordering policy then
381 * an available lock emwill not/em be acquired if any other threads
382 * are waiting for the lock. This is in contrast to the {link #tryLock()}
383 * method. If you want a timed {code tryLock} that does permit barging on
384 * a fair lock then combine the timed and un-timed forms together:
385 *
386 * preif (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
387 * /pre
388 *
389 * pIf the current thread
390 * already holds this lock then the hold count is incremented by one and
391 * the method returns {code true}.
392 *
393 * pIf the lock is held by another thread then the
394 * current thread becomes disabled for thread scheduling
395 * purposes and lies dormant until one of three things happens:
396 *
397 * ul
398 *
399 * liThe lock is acquired by the current thread; or
400 *
401 * liSome other thread {linkplain Thread#interrupt interrupts}
402 * the current thread; or
403 *
404 * liThe specified waiting time elapses
405 *
406 * /ul
407 *
408 * pIf the lock is acquired then the value {code true} is returned and
409 * the lock hold count is set to one.
410 *
411 * pIf the current thread:
412 *
413 * ul
414 *
415 * lihas its interrupted status set on entry to this method; or
416 *
417 * liis {linkplain Thread#interrupt interrupted} while
418 * acquiring the lock,
419 *
420 * /ul
421 * then {link InterruptedException} is thrown and the current threads
422 * interrupted status is cleared.
423 *
424 * pIf the specified waiting time elapses then the value {code false}
425 * is returned. If the time is less than or equal to zero, the method
426 * will not wait at all.
427 *
428 * pIn this implementation, as this method is an explicit
429 * interruption point, preference is given to responding to the
430 * interrupt over normal or reentrant acquisition of the lock, and
431 * over reporting the elapse of the waiting time.
432 *
433 * param timeout the time to wait for the lock
434 * param unit the time unit of the timeout argument
435 * return {code true} if the lock was free and was acquired by the
436 * current thread, or the lock was already held by the current
437 * thread; and {code false} if the waiting time elapsed before
438 * the lock could be acquired
439 * throws InterruptedException if the current thread is interrupted
440 * throws NullPointerException if the time unit is null
441 *
442 */
443 public boolean tryLock(long timeout, TimeUnit unit)
444 throws InterruptedException {
445 return sync.tryAcquireNanos(1, unit.toNanos(timeout));
446 }
447
448 /**
449 * Attempts to release this lock.
450 *
451 * pIf the current thread is the holder of this lock then the hold
452 * count is decremented. If the hold count is now zero then the lock
453 * is released. If the current thread is not the holder of this
454 * lock then {link IllegalMonitorStateException} is thrown.
455 *
456 * throws IllegalMonitorStateException if the current thread does not
457 * hold this lock
458 */
459 public void unlock() {
460 sync.release(1);
461 }
462
463 /**
464 * Returns a {link Condition} instance for use with this
465 * {link Lock} instance.
466 *
467 * pThe returned {link Condition} instance supports the same
468 * usages as do the {link Object} monitor methods ({link
469 * Object#wait() wait}, {link Object#notify notify}, and {link
470 * Object#notifyAll notifyAll}) when used with the built-in
471 * monitor lock.
472 *
473 * ul
474 *
475 * liIf this lock is not held when any of the {link Condition}
476 * {linkplain Condition#await() waiting} or {linkplain
477 * Condition#signal signalling} methods are called, then an {link
478 * IllegalMonitorStateException} is thrown.
479 *
480 * liWhen the condition {linkplain Condition#await() waiting}
481 * methods are called the lock is released and, before they
482 * return, the lock is reacquired and the lock hold count restored
483 * to what it was when the method was called.
484 *
485 * liIf a thread is {linkplain Thread#interrupt interrupted}
486 * while waiting then the wait will terminate, an {link
487 * InterruptedException} will be thrown, and the threads
488 * interrupted status will be cleared.
489 *
490 * li Waiting threads are signalled in FIFO order.
491 *
492 * liThe ordering of lock reacquisition for threads returning
493 * from waiting methods is the same as for threads initially
494 * acquiring the lock, which is in the default case not specified,
495 * but for emfair/em locks favors those threads that have been
496 * waiting the longest.
497 *
498 * /ul
499 *
500 * return the Condition object
501 */
502 public Condition newCondition() {
503 return sync.newCondition();
504 }
505
506 /**
507 * Queries the number of holds on this lock by the current thread.
508 *
509 * pA thread has a hold on a lock for each lock action that is not
510 * matched by an unlock action.
511 *
512 * pThe hold count information is typically only used for testing and
513 * debugging purposes. For example, if a certain section of code should
514 * not be entered with the lock already held then we can assert that
515 * fact:
516 *
517 * pre
518 * class X {
519 * ReentrantLock lock new ReentrantLock();
520 * // ...
521 * public void m() {
522 * assert lock.getHoldCount() 0;
523 * lock.lock();
524 * try {
525 * // ... method body
526 * } finally {
527 * lock.unlock();
528 * }
529 * }
530 * }
531 * /pre
532 *
533 * return the number of holds on this lock by the current thread,
534 * or zero if this lock is not held by the current thread
535 */
536 public int getHoldCount() {
537 return sync.getHoldCount();
538 }
539
540 /**
541 * Queries if this lock is held by the current thread.
542 *
543 * pAnalogous to the {link Thread#holdsLock} method for built-in
544 * monitor locks, this method is typically used for debugging and
545 * testing. For example, a method that should only be called while
546 * a lock is held can assert that this is the case:
547 *
548 * pre
549 * class X {
550 * ReentrantLock lock new ReentrantLock();
551 * // ...
552 *
553 * public void m() {
554 * assert lock.isHeldByCurrentThread();
555 * // ... method body
556 * }
557 * }
558 * /pre
559 *
560 * pIt can also be used to ensure that a reentrant lock is used
561 * in a non-reentrant manner, for example:
562 *
563 * pre
564 * class X {
565 * ReentrantLock lock new ReentrantLock();
566 * // ...
567 *
568 * public void m() {
569 * assert !lock.isHeldByCurrentThread();
570 * lock.lock();
571 * try {
572 * // ... method body
573 * } finally {
574 * lock.unlock();
575 * }
576 * }
577 * }
578 * /pre
579 *
580 * return {code true} if current thread holds this lock and
581 * {code false} otherwise
582 */
583 public boolean isHeldByCurrentThread() {
584 return sync.isHeldExclusively();
585 }
586
587 /**
588 * Queries if this lock is held by any thread. This method is
589 * designed for use in monitoring of the system state,
590 * not for synchronization control.
591 *
592 * return {code true} if any thread holds this lock and
593 * {code false} otherwise
594 */
595 public boolean isLocked() {
596 return sync.isLocked();
597 }
598
599 /**
600 * Returns {code true} if this lock has fairness set true.
601 *
602 * return {code true} if this lock has fairness set true
603 */
604 public final boolean isFair() {
605 return sync instanceof FairSync;
606 }
607
608 /**
609 * Returns the thread that currently owns this lock, or
610 * {code null} if not owned. When this method is called by a
611 * thread that is not the owner, the return value reflects a
612 * best-effort approximation of current lock status. For example,
613 * the owner may be momentarily {code null} even if there are
614 * threads trying to acquire the lock but have not yet done so.
615 * This method is designed to facilitate construction of
616 * subclasses that provide more extensive lock monitoring
617 * facilities.
618 *
619 * return the owner, or {code null} if not owned
620 */
621 protected Thread getOwner() {
622 return sync.getOwner();
623 }
624
625 /**
626 * Queries whether any threads are waiting to acquire this lock. Note that
627 * because cancellations may occur at any time, a {code true}
628 * return does not guarantee that any other thread will ever
629 * acquire this lock. This method is designed primarily for use in
630 * monitoring of the system state.
631 *
632 * return {code true} if there may be other threads waiting to
633 * acquire the lock
634 */
635 public final boolean hasQueuedThreads() {
636 return sync.hasQueuedThreads();
637 }
638
639
640 /**
641 * Queries whether the given thread is waiting to acquire this
642 * lock. Note that because cancellations may occur at any time, a
643 * {code true} return does not guarantee that this thread
644 * will ever acquire this lock. This method is designed primarily for use
645 * in monitoring of the system state.
646 *
647 * param thread the thread
648 * return {code true} if the given thread is queued waiting for this lock
649 * throws NullPointerException if the thread is null
650 */
651 public final boolean hasQueuedThread(Thread thread) {
652 return sync.isQueued(thread);
653 }
654
655
656 /**
657 * Returns an estimate of the number of threads waiting to
658 * acquire this lock. The value is only an estimate because the number of
659 * threads may change dynamically while this method traverses
660 * internal data structures. This method is designed for use in
661 * monitoring of the system state, not for synchronization
662 * control.
663 *
664 * return the estimated number of threads waiting for this lock
665 */
666 public final int getQueueLength() {
667 return sync.getQueueLength();
668 }
669
670 /**
671 * Returns a collection containing threads that may be waiting to
672 * acquire this lock. Because the actual set of threads may change
673 * dynamically while constructing this result, the returned
674 * collection is only a best-effort estimate. The elements of the
675 * returned collection are in no particular order. This method is
676 * designed to facilitate construction of subclasses that provide
677 * more extensive monitoring facilities.
678 *
679 * return the collection of threads
680 */
681 protected CollectionThread getQueuedThreads() {
682 return sync.getQueuedThreads();
683 }
684
685 /**
686 * Queries whether any threads are waiting on the given condition
687 * associated with this lock. Note that because timeouts and
688 * interrupts may occur at any time, a {code true} return does
689 * not guarantee that a future {code signal} will awaken any
690 * threads. This method is designed primarily for use in
691 * monitoring of the system state.
692 *
693 * param condition the condition
694 * return {code true} if there are any waiting threads
695 * throws IllegalMonitorStateException if this lock is not held
696 * throws IllegalArgumentException if the given condition is
697 * not associated with this lock
698 * throws NullPointerException if the condition is null
699 */
700 public boolean hasWaiters(Condition condition) {
701 if (condition null)
702 throw new NullPointerException();
703 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
704 throw new IllegalArgumentException(not owner);
705 return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
706 }
707
708 /**
709 * Returns an estimate of the number of threads waiting on the
710 * given condition associated with this lock. Note that because
711 * timeouts and interrupts may occur at any time, the estimate
712 * serves only as an upper bound on the actual number of waiters.
713 * This method is designed for use in monitoring of the system
714 * state, not for synchronization control.
715 *
716 * param condition the condition
717 * return the estimated number of waiting threads
718 * throws IllegalMonitorStateException if this lock is not held
719 * throws IllegalArgumentException if the given condition is
720 * not associated with this lock
721 * throws NullPointerException if the condition is null
722 */
723 public int getWaitQueueLength(Condition condition) {
724 if (condition null)
725 throw new NullPointerException();
726 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
727 throw new IllegalArgumentException(not owner);
728 return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
729 }
730
731 /**
732 * Returns a collection containing those threads that may be
733 * waiting on the given condition associated with this lock.
734 * Because the actual set of threads may change dynamically while
735 * constructing this result, the returned collection is only a
736 * best-effort estimate. The elements of the returned collection
737 * are in no particular order. This method is designed to
738 * facilitate construction of subclasses that provide more
739 * extensive condition monitoring facilities.
740 *
741 * param condition the condition
742 * return the collection of threads
743 * throws IllegalMonitorStateException if this lock is not held
744 * throws IllegalArgumentException if the given condition is
745 * not associated with this lock
746 * throws NullPointerException if the condition is null
747 */
748 protected CollectionThread getWaitingThreads(Condition condition) {
749 if (condition null)
750 throw new NullPointerException();
751 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
752 throw new IllegalArgumentException(not owner);
753 return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
754 }
755
756 /**
757 * Returns a string identifying this lock, as well as its lock state.
758 * The state, in brackets, includes either the String {code Unlocked}
759 * or the String {code Locked by} followed by the
760 * {linkplain Thread#getName name} of the owning thread.
761 *
762 * return a string identifying this lock, as well as its lock state
763 */
764 public String toString() {
765 Thread o sync.getOwner();
766 return super.toString() ((o null) ?
767 [Unlocked] :
768 [Locked by thread o.getName() ]);
769 }
770 } AQS(AbstractQueuedSynchronizer.java) 1 /*2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.3 *4 *5 *6 *7 *8 *9 *10 *11 *12 *13 *14 *15 *16 *17 *18 *19 *20 *21 *22 *23 */24 25 /*26 *27 *28 *29 *30 *31 * Written by Doug Lea with assistance from members of JCP JSR-16632 * Expert Group and released to the public domain, as explained at33 * http://creativecommons.org/publicdomain/zero/1.0/34 */35 36 package java.util.concurrent.locks;37 import java.util.*;38 import java.util.concurrent.*;39 import java.util.concurrent.atomic.*;40 import sun.misc.Unsafe;41 42 /**43 * Provides a framework for implementing blocking locks and related44 * synchronizers (semaphores, events, etc) that rely on45 * first-in-first-out (FIFO) wait queues. This class is designed to46 * be a useful basis for most kinds of synchronizers that rely on a47 * single atomic ttint/tt value to represent state. Subclasses48 * must define the protected methods that change this state, and which49 * define what that state means in terms of this object being acquired50 * or released. Given these, the other methods in this class carry51 * out all queuing and blocking mechanics. Subclasses can maintain52 * other state fields, but only the atomically updated ttint/tt53 * value manipulated using methods {link #getState}, {link54 * #setState} and {link #compareAndSetState} is tracked with respect55 * to synchronization.56 *57 * pSubclasses should be defined as non-public internal helper58 * classes that are used to implement the synchronization properties59 * of their enclosing class. Class60 * ttAbstractQueuedSynchronizer/tt does not implement any61 * synchronization interface. Instead it defines methods such as62 * {link #acquireInterruptibly} that can be invoked as63 * appropriate by concrete locks and related synchronizers to64 * implement their public methods.65 *66 * pThis class supports either or both a default emexclusive/em67 * mode and a emshared/em mode. When acquired in exclusive mode,68 * attempted acquires by other threads cannot succeed. Shared mode69 * acquires by multiple threads may (but need not) succeed. This class70 * does not quot;understandquot; these differences except in the71 * mechanical sense that when a shared mode acquire succeeds, the next72 * waiting thread (if one exists) must also determine whether it can73 * acquire as well. Threads waiting in the different modes share the74 * same FIFO queue. Usually, implementation subclasses support only75 * one of these modes, but both can come into play for example in a76 * {link ReadWriteLock}. Subclasses that support only exclusive or77 * only shared modes need not define the methods supporting the unused mode.78 *79 * pThis class defines a nested {link ConditionObject} class that80 * can be used as a {link Condition} implementation by subclasses81 * supporting exclusive mode for which method {link82 * #isHeldExclusively} reports whether synchronization is exclusively83 * held with respect to the current thread, method {link #release}84 * invoked with the current {link #getState} value fully releases85 * this object, and {link #acquire}, given this saved state value,86 * eventually restores this object to its previous acquired state. No87 * ttAbstractQueuedSynchronizer/tt method otherwise creates such a88 * condition, so if this constraint cannot be met, do not use it. The89 * behavior of {link ConditionObject} depends of course on the90 * semantics of its synchronizer implementation.91 *92 * pThis class provides inspection, instrumentation, and monitoring93 * methods for the internal queue, as well as similar methods for94 * condition objects. These can be exported as desired into classes95 * using an ttAbstractQueuedSynchronizer/tt for their96 * synchronization mechanics.97 *98 * pSerialization of this class stores only the underlying atomic99 * integer maintaining state, so deserialized objects have empty100 * thread queues. Typical subclasses requiring serializability will101 * define a ttreadObject/tt method that restores this to a known102 * initial state upon deserialization.103 *104 * h3Usage/h3105 *106 * pTo use this class as the basis of a synchronizer, redefine the107 * following methods, as applicable, by inspecting and/or modifying108 * the synchronization state using {link #getState}, {link109 * #setState} and/or {link #compareAndSetState}:110 *111 * ul112 * li {link #tryAcquire}113 * li {link #tryRelease}114 * li {link #tryAcquireShared}115 * li {link #tryReleaseShared}116 * li {link #isHeldExclusively}117 */ul118 *119 * Each of these methods by default throws {link120 * UnsupportedOperationException}. Implementations of these methods121 * must be internally thread-safe, and should in general be short and122 * not block. Defining these methods is the emonly/em supported123 * means of using this class. All other methods are declared124 * ttfinal/tt because they cannot be independently varied.125 *126 * pYou may also find the inherited methods from {link127 * AbstractOwnableSynchronizer} useful to keep track of the thread128 * owning an exclusive synchronizer. You are encouraged to use them129 * -- this enables monitoring and diagnostic tools to assist users in130 * determining which threads hold locks.131 *132 * pEven though this class is based on an internal FIFO queue, it133 * does not automatically enforce FIFO acquisition policies. The core134 * of exclusive synchronization takes the form:135 *136 * pre137 * Acquire:138 * while (!tryAcquire(arg)) {139 * emenqueue thread if it is not already queued/em;140 * empossibly block current thread/em;141 * }142 *143 * Release:144 * if (tryRelease(arg))145 * emunblock the first queued thread/em;146 * /pre147 *148 * (Shared mode is similar but may involve cascading signals.)149 *150 * pa namebargingBecause checks in acquire are invoked before151 * enqueuing, a newly acquiring thread may embarge/em ahead of152 * others that are blocked and queued. However, you can, if desired,153 * define tttryAcquire/tt and/or tttryAcquireShared/tt to154 * disable barging by internally invoking one or more of the inspection155 * methods, thereby providing a emfair/em FIFO acquisition order.156 * In particular, most fair synchronizers can define tttryAcquire/tt157 * to return ttfalse/tt if {link #hasQueuedPredecessors} (a method158 * specifically designed to be used by fair synchronizers) returns159 * tttrue/tt. Other variations are possible.160 *161 * pThroughput and scalability are generally highest for the162 * default barging (also known as emgreedy/em,163 * emrenouncement/em, and emconvoy-avoidance/em) strategy.164 * While this is not guaranteed to be fair or starvation-free, earlier165 * queued threads are allowed to recontend before later queued166 * threads, and each recontention has an unbiased chance to succeed167 * against incoming threads. Also, while acquires do not168 * quot;spinquot; in the usual sense, they may perform multiple169 * invocations of tttryAcquire/tt interspersed with other170 * computations before blocking. This gives most of the benefits of171 * spins when exclusive synchronization is only briefly held, without172 * most of the liabilities when it isnt. If so desired, you can173 * augment this by preceding calls to acquire methods with174 * fast-path checks, possibly prechecking {link #hasContended}175 * and/or {link #hasQueuedThreads} to only do so if the synchronizer176 * is likely not to be contended.177 *178 * pThis class provides an efficient and scalable basis for179 * synchronization in part by specializing its range of use to180 * synchronizers that can rely on ttint/tt state, acquire, and181 * release parameters, and an internal FIFO wait queue. When this does182 * not suffice, you can build synchronizers from a lower level using183 * {link java.util.concurrent.atomic atomic} classes, your own custom184 * {link java.util.Queue} classes, and {link LockSupport} blocking185 * support.186 *187 * h3Usage Examples/h3188 *189 * pHere is a non-reentrant mutual exclusion lock class that uses190 * the value zero to represent the unlocked state, and one to191 * represent the locked state. While a non-reentrant lock192 * does not strictly require recording of the current owner193 * thread, this class does so anyway to make usage easier to monitor.194 * It also supports conditions and exposes195 * one of the instrumentation methods:196 *197 * pre198 * class Mutex implements Lock, java.io.Serializable {199 *200 * // Our internal helper class201 * private static class Sync extends AbstractQueuedSynchronizer {202 * // Report whether in locked state203 * protected boolean isHeldExclusively() {204 * return getState() 1;205 * }206 *207 * // Acquire the lock if state is zero208 * public boolean tryAcquire(int acquires) {209 * assert acquires 1; // Otherwise unused210 * if (compareAndSetState(0, 1)) {211 * setExclusiveOwnerThread(Thread.currentThread());212 * return true;213 * }214 * return false;215 * }216 *217 * // Release the lock by setting state to zero218 * protected boolean tryRelease(int releases) {219 * assert releases 1; // Otherwise unused220 * if (getState() 0) throw new IllegalMonitorStateException();221 * setExclusiveOwnerThread(null);222 * setState(0);223 * return true;224 * }225 *226 * // Provide a Condition227 * Condition newCondition() { return new ConditionObject(); }228 *229 * // Deserialize properly230 * private void readObject(ObjectInputStream s)231 * throws IOException, ClassNotFoundException {232 * s.defaultReadObject();233 * setState(0); // reset to unlocked state234 * }235 * }236 *237 * // The sync object does all the hard work. We just forward to it.238 * private final Sync sync new Sync();239 *240 * public void lock() { sync.acquire(1); }241 * public boolean tryLock() { return sync.tryAcquire(1); }242 * public void unlock() { sync.release(1); }243 * public Condition newCondition() { return sync.newCondition(); }244 * public boolean isLocked() { return sync.isHeldExclusively(); }245 * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }246 * public void lockInterruptibly() throws InterruptedException {247 * sync.acquireInterruptibly(1);248 * }249 * public boolean tryLock(long timeout, TimeUnit unit)250 * throws InterruptedException {251 * return sync.tryAcquireNanos(1, unit.toNanos(timeout));252 * }253 * }254 * /pre255 *256 * pHere is a latch class that is like a {link CountDownLatch}257 * except that it only requires a single ttsignal/tt to258 * fire. Because a latch is non-exclusive, it uses the ttshared/tt259 * acquire and release methods.260 *261 * pre262 * class BooleanLatch {263 *264 * private static class Sync extends AbstractQueuedSynchronizer {265 * boolean isSignalled() { return getState() ! 0; }266 *267 * protected int tryAcquireShared(int ignore) {268 * return isSignalled() ? 1 : -1;269 * }270 *271 * protected boolean tryReleaseShared(int ignore) {272 * setState(1);273 * return true;274 * }275 * }276 *277 * private final Sync sync new Sync();278 * public boolean isSignalled() { return sync.isSignalled(); }279 * public void signal() { sync.releaseShared(1); }280 * public void await() throws InterruptedException {281 * sync.acquireSharedInterruptibly(1);282 * }283 * }284 * /pre285 *286 * since 1.5287 * author Doug Lea288 */289 public abstract class AbstractQueuedSynchronizer290 extends AbstractOwnableSynchronizer291 implements java.io.Serializable {292 293 private static final long serialVersionUID 7373984972572414691L;294 295 /**296 * Creates a new ttAbstractQueuedSynchronizer/tt instance297 * with initial synchronization state of zero.298 */299 protected AbstractQueuedSynchronizer() { }300 301 /**302 * Wait queue node class.303 *304 * pThe wait queue is a variant of a CLH (Craig, Landin, and305 * Hagersten) lock queue. CLH locks are normally used for306 * spinlocks. We instead use them for blocking synchronizers, but307 * use the same basic tactic of holding some of the control308 * information about a thread in the predecessor of its node. A309 * status field in each node keeps track of whether a thread310 * should block. A node is signalled when its predecessor311 * releases. Each node of the queue otherwise serves as a312 * specific-notification-style monitor holding a single waiting313 * thread. The status field does NOT control whether threads are314 * granted locks etc though. A thread may try to acquire if it is315 * first in the queue. But being first does not guarantee success;316 * it only gives the right to contend. So the currently released317 * contender thread may need to rewait.318 *319 * pTo enqueue into a CLH lock, you atomically splice it in as new320 * tail. To dequeue, you just set the head field.321 * pre322 * ------ prev ----- -----323 * head | | ---- | | ---- | | tail324 * ------ ----- -----325 * /pre326 *327 * pInsertion into a CLH queue requires only a single atomic328 * operation on tail, so there is a simple atomic point of329 * demarcation from unqueued to queued. Similarly, dequeing330 * involves only updating the head. However, it takes a bit331 * more work for nodes to determine who their successors are,332 * in part to deal with possible cancellation due to timeouts333 * and interrupts.334 *335 * pThe prev links (not used in original CLH locks), are mainly336 * needed to handle cancellation. If a node is cancelled, its337 * successor is (normally) relinked to a non-cancelled338 * predecessor. For explanation of similar mechanics in the case339 * of spin locks, see the papers by Scott and Scherer at340 * http://www.cs.rochester.edu/u/scott/synchronization/341 *342 * pWe also use next links to implement blocking mechanics.343 * The thread id for each node is kept in its own node, so a344 * predecessor signals the next node to wake up by traversing345 * next link to determine which thread it is. Determination of346 * successor must avoid races with newly queued nodes to set347 * the next fields of their predecessors. This is solved348 * when necessary by checking backwards from the atomically349 * updated tail when a nodes successor appears to be null.350 * (Or, said differently, the next-links are an optimization351 * so that we dont usually need a backward scan.)352 *353 * pCancellation introduces some conservatism to the basic354 * algorithms. Since we must poll for cancellation of other355 * nodes, we can miss noticing whether a cancelled node is356 * ahead or behind us. This is dealt with by always unparking357 * successors upon cancellation, allowing them to stabilize on358 * a new predecessor, unless we can identify an uncancelled359 * predecessor who will carry this responsibility.360 *361 * pCLH queues need a dummy header node to get started. But362 * we dont create them on construction, because it would be wasted363 * effort if there is never contention. Instead, the node364 * is constructed and head and tail pointers are set upon first365 * contention.366 *367 * pThreads waiting on Conditions use the same nodes, but368 * use an additional link. Conditions only need to link nodes369 * in simple (non-concurrent) linked queues because they are370 * only accessed when exclusively held. Upon await, a node is371 * inserted into a condition queue. Upon signal, the node is372 * transferred to the main queue. A special value of status373 * field is used to mark which queue a node is on.374 *375 * pThanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill376 * Scherer and Michael Scott, along with members of JSR-166377 * expert group, for helpful ideas, discussions, and critiques378 * on the design of this class.379 */380 static final class Node {381 /** Marker to indicate a node is waiting in shared mode */382 static final Node SHARED new Node();383 /** Marker to indicate a node is waiting in exclusive mode */384 static final Node EXCLUSIVE null;385 386 /** waitStatus value to indicate thread has cancelled */387 static final int CANCELLED 1;388 /** waitStatus value to indicate successors thread needs unparking */389 static final int SIGNAL -1;390 /** waitStatus value to indicate thread is waiting on condition */391 static final int CONDITION -2;392 /**393 * waitStatus value to indicate the next acquireShared should394 * unconditionally propagate395 */396 static final int PROPAGATE -3;397 398 /**399 * Status field, taking on only the values:400 * SIGNAL: The successor of this node is (or will soon be)401 * blocked (via park), so the current node must402 * unpark its successor when it releases or403 * cancels. To avoid races, acquire methods must404 * first indicate they need a signal,405 * then retry the atomic acquire, and then,406 * on failure, block.407 * CANCELLED: This node is cancelled due to timeout or interrupt.408 * Nodes never leave this state. In particular,409 * a thread with cancelled node never again blocks.410 * CONDITION: This node is currently on a condition queue.411 * It will not be used as a sync queue node412 * until transferred, at which time the status413 * will be set to 0. (Use of this value here has414 * nothing to do with the other uses of the415 * field, but simplifies mechanics.)416 * PROPAGATE: A releaseShared should be propagated to other417 * nodes. This is set (for head node only) in418 * doReleaseShared to ensure propagation419 * continues, even if other operations have420 * since intervened.421 * 0: None of the above422 *423 * The values are arranged numerically to simplify use.424 * Non-negative values mean that a node doesnt need to425 * signal. So, most code doesnt need to check for particular426 * values, just for sign.427 *428 * The field is initialized to 0 for normal sync nodes, and429 * CONDITION for condition nodes. It is modified using CAS430 * (or when possible, unconditional volatile writes).431 */432 volatile int waitStatus;433 434 /**435 * Link to predecessor node that current node/thread relies on436 * for checking waitStatus. Assigned during enqueing, and nulled437 * out (for sake of GC) only upon dequeuing. Also, upon438 * cancellation of a predecessor, we short-circuit while439 * finding a non-cancelled one, which will always exist440 * because the head node is never cancelled: A node becomes441 * head only as a result of successful acquire. A442 * cancelled thread never succeeds in acquiring, and a thread only443 * cancels itself, not any other node.444 */445 volatile Node prev;446 447 /**448 * Link to the successor node that the current node/thread449 * unparks upon release. Assigned during enqueuing, adjusted450 * when bypassing cancelled predecessors, and nulled out (for451 * sake of GC) when dequeued. The enq operation does not452 * assign next field of a predecessor until after attachment,453 * so seeing a null next field does not necessarily mean that454 * node is at end of queue. However, if a next field appears455 * to be null, we can scan prevs from the tail to456 * double-check. The next field of cancelled nodes is set to457 * point to the node itself instead of null, to make life458 * easier for isOnSyncQueue.459 */460 volatile Node next;461 462 /**463 * The thread that enqueued this node. Initialized on464 * construction and nulled out after use.465 */466 volatile Thread thread;467 468 /**469 * Link to next node waiting on condition, or the special470 * value SHARED. Because condition queues are accessed only471 * when holding in exclusive mode, we just need a simple472 * linked queue to hold nodes while they are waiting on473 * conditions. They are then transferred to the queue to474 * re-acquire. And because conditions can only be exclusive,475 * we save a field by using special value to indicate shared476 * mode.477 */478 Node nextWaiter;479 480 /**481 * Returns true if node is waiting in shared mode482 */483 final boolean isShared() {484 return nextWaiter SHARED;485 }486 487 /**488 * Returns previous node, or throws NullPointerException if null.489 * Use when predecessor cannot be null. The null check could490 * be elided, but is present to help the VM.491 *492 * return the predecessor of this node493 */494 final Node predecessor() throws NullPointerException {495 Node p prev;496 if (p null)497 throw new NullPointerException();498 else499 return p;500 }501 502 Node() { // Used to establish initial head or SHARED marker503 }504 505 Node(Thread thread, Node mode) { // Used by addWaiter506 this.nextWaiter mode;507 this.thread thread;508 }509 510 Node(Thread thread, int waitStatus) { // Used by Condition511 this.waitStatus waitStatus;512 this.thread thread;513 }514 }515 516 /**517 * Head of the wait queue, lazily initialized. Except for518 * initialization, it is modified only via method setHead. Note:519 * If head exists, its waitStatus is guaranteed not to be520 * CANCELLED.521 */522 private transient volatile Node head;523 524 /**525 * Tail of the wait queue, lazily initialized. Modified only via526 * method enq to add new wait node.527 */528 private transient volatile Node tail;529 530 /**531 * The synchronization state.532 */533 private volatile int state;534 535 /**536 * Returns the current value of synchronization state.537 * This operation has memory semantics of a ttvolatile/tt read.538 * return current state value539 */540 protected final int getState() {541 return state;542 }543 544 /**545 * Sets the value of synchronization state.546 * This operation has memory semantics of a ttvolatile/tt write.547 * param newState the new state value548 */549 protected final void setState(int newState) {550 state newState;551 }552 553 /**554 * Atomically sets synchronization state to the given updated555 * value if the current state value equals the expected value.556 * This operation has memory semantics of a ttvolatile/tt read557 * and write.558 *559 * param expect the expected value560 * param update the new value561 * return true if successful. False return indicates that the actual562 * value was not equal to the expected value.563 */564 protected final boolean compareAndSetState(int expect, int update) {565 // See below for intrinsics setup to support this566 return unsafe.compareAndSwapInt(this, stateOffset, expect, update);567 }568 569 // Queuing utilities570 571 /**572 * The number of nanoseconds for which it is faster to spin573 * rather than to use timed park. A rough estimate suffices574 * to improve responsiveness with very short timeouts.575 */576 static final long spinForTimeoutThreshold 1000L;577 578 /**579 * Inserts node into queue, initializing if necessary. See picture above.580 * param node the node to insert581 * return nodes predecessor582 */583 private Node enq(final Node node) {584 for (;;) {585 Node t tail;586 if (t null) { // Must initialize587 if (compareAndSetHead(new Node()))588 tail head;589 } else {590 node.prev t;591 if (compareAndSetTail(t, node)) {592 t.next node;593 return t;594 }595 }596 }597 }598 599 /**600 * Creates and enqueues node for current thread and given mode.601 *602 * param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared603 * return the new node604 */605 private Node addWaiter(Node mode) {606 Node node new Node(Thread.currentThread(), mode);607 // Try the fast path of enq; backup to full enq on failure608 Node pred tail;609 if (pred ! null) {610 node.prev pred;611 if (compareAndSetTail(pred, node)) {612 pred.next node;613 return node;614 }615 }616 enq(node);617 return node;618 }619 620 /**621 * Sets head of queue to be node, thus dequeuing. Called only by622 * acquire methods. Also nulls out unused fields for sake of GC623 * and to suppress unnecessary signals and traversals.624 *625 * param node the node626 */627 private void setHead(Node node) {628 head node;629 node.thread null;630 node.prev null;631 }632 633 /**634 * Wakes up nodes successor, if one exists.635 *636 * param node the node637 */638 private void unparkSuccessor(Node node) {639 /*640 * If status is negative (i.e., possibly needing signal) try641 * to clear in anticipation of signalling. It is OK if this642 * fails or if status is changed by waiting thread.643 */644 int ws node.waitStatus;645 if (ws 0)646 compareAndSetWaitStatus(node, ws, 0);647 648 /*649 * Thread to unpark is held in successor, which is normally650 * just the next node. But if cancelled or apparently null,651 * traverse backwards from tail to find the actual652 * non-cancelled successor.653 */654 Node s node.next;655 if (s null || s.waitStatus 0) {656 s null;657 for (Node t tail; t ! null t ! node; t t.prev)658 if (t.waitStatus 0)659 s t;660 }661 if (s ! null)662 LockSupport.unpark(s.thread);663 }664 665 /**666 * Release action for shared mode -- signal successor and ensure667 * propagation. (Note: For exclusive mode, release just amounts668 * to calling unparkSuccessor of head if it needs signal.)669 */670 private void doReleaseShared() {671 /*672 * Ensure that a release propagates, even if there are other673 * in-progress acquires/releases. This proceeds in the usual674 * way of trying to unparkSuccessor of head if it needs675 * signal. But if it does not, status is set to PROPAGATE to676 * ensure that upon release, propagation continues.677 * Additionally, we must loop in case a new node is added678 * while we are doing this. Also, unlike other uses of679 * unparkSuccessor, we need to know if CAS to reset status680 * fails, if so rechecking.681 */682 for (;;) {683 Node h head;684 if (h ! null h ! tail) {685 int ws h.waitStatus;686 if (ws Node.SIGNAL) {687 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))688 continue; // loop to recheck cases689 unparkSuccessor(h);690 }691 else if (ws 0 692 !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))693 continue; // loop on failed CAS694 }695 if (h head) // loop if head changed696 break;697 }698 }699 700 /**701 * Sets head of queue, and checks if successor may be waiting702 * in shared mode, if so propagating if either propagate 0 or703 * PROPAGATE status was set.704 *705 * param node the node706 * param propagate the return value from a tryAcquireShared707 */708 private void setHeadAndPropagate(Node node, int propagate) {709 Node h head; // Record old head for check below710 setHead(node);711 /*712 * Try to signal next queued node if:713 * Propagation was indicated by caller,714 * or was recorded (as h.waitStatus) by a previous operation715 * (note: this uses sign-check of waitStatus because716 * PROPAGATE status may transition to SIGNAL.)717 * and718 * The next node is waiting in shared mode,719 * or we dont know, because it appears null720 *721 * The conservatism in both of these checks may cause722 * unnecessary wake-ups, but only when there are multiple723 * racing acquires/releases, so most need signals now or soon724 * anyway.725 */726 if (propagate 0 || h null || h.waitStatus 0) {727 Node s node.next;728 if (s null || s.isShared())729 doReleaseShared();730 }731 }732 733 // Utilities for various versions of acquire734 735 /**736 * Cancels an ongoing attempt to acquire.737 *738 * param node the node739 */740 private void cancelAcquire(Node node) {741 // Ignore if node doesnt exist742 if (node null)743 return;744 745 node.thread null;746 747 // Skip cancelled predecessors748 Node pred node.prev;749 while (pred.waitStatus 0)750 node.prev pred pred.prev;751 752 // predNext is the apparent node to unsplice. CASes below will753 // fail if not, in which case, we lost race vs another cancel754 // or signal, so no further action is necessary.755 Node predNext pred.next;756 757 // Can use unconditional write instead of CAS here.758 // After this atomic step, other Nodes can skip past us.759 // Before, we are free of interference from other threads.760 node.waitStatus Node.CANCELLED;761 762 // If we are the tail, remove ourselves.763 if (node tail compareAndSetTail(node, pred)) {764 compareAndSetNext(pred, predNext, null);765 } else {766 // If successor needs signal, try to set preds next-link767 // so it will get one. Otherwise wake it up to propagate.768 int ws;769 if (pred ! head 770 ((ws pred.waitStatus) Node.SIGNAL ||771 (ws 0 compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) 772 pred.thread ! null) {773 Node next node.next;774 if (next ! null next.waitStatus 0)775 compareAndSetNext(pred, predNext, next);776 } else {777 unparkSuccessor(node);778 }779 780 node.next node; // help GC781 }782 }783 784 /**785 * Checks and updates status for a node that failed to acquire.786 * Returns true if thread should block. This is the main signal787 * control in all acquire loops. Requires that pred node.prev788 *789 * param pred nodes predecessor holding status790 * param node the node791 * return {code true} if thread should block792 */793 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {794 int ws pred.waitStatus;795 if (ws Node.SIGNAL)796 /*797 * This node has already set status asking a release798 * to signal it, so it can safely park.799 */800 return true;801 if (ws 0) {802 /*803 * Predecessor was cancelled. Skip over predecessors and804 * indicate retry.805 */806 do {807 node.prev pred pred.prev;808 } while (pred.waitStatus 0);809 pred.next node;810 } else {811 /*812 * waitStatus must be 0 or PROPAGATE. Indicate that we813 * need a signal, but dont park yet. Caller will need to814 * retry to make sure it cannot acquire before parking.815 */816 compareAndSetWaitStatus(pred, ws, Node.SIGNAL);817 }818 return false;819 }820 821 /**822 * Convenience method to interrupt current thread.823 */824 private static void selfInterrupt() {825 Thread.currentThread().interrupt();826 }827 828 /**829 * Convenience method to park and then check if interrupted830 *831 * return {code true} if interrupted832 */833 private final boolean parkAndCheckInterrupt() {834 LockSupport.park(this);835 return Thread.interrupted();836 }837 838 /*839 * Various flavors of acquire, varying in exclusive/shared and840 * control modes. Each is mostly the same, but annoyingly841 * different. Only a little bit of factoring is possible due to842 * interactions of exception mechanics (including ensuring that we843 * cancel if tryAcquire throws exception) and other control, at844 * least not without hurting performance too much.845 */846 847 /**848 * Acquires in exclusive uninterruptible mode for thread already in849 * queue. Used by condition wait methods as well as acquire.850 *851 * param node the node852 * param arg the acquire argument853 * return {code true} if interrupted while waiting854 */855 final boolean acquireQueued(final Node node, int arg) {856 boolean failed true;857 try {858 boolean interrupted false;859 for (;;) {860 final Node p node.predecessor();861 if (p head tryAcquire(arg)) {862 setHead(node);863 p.next null; // help GC864 failed false;865 return interrupted;866 }867 if (shouldParkAfterFailedAcquire(p, node) 868 parkAndCheckInterrupt())869 interrupted true;870 }871 } finally {872 if (failed)873 cancelAcquire(node);874 }875 }876 877 /**878 * Acquires in exclusive interruptible mode.879 * param arg the acquire argument880 */881 private void doAcquireInterruptibly(int arg)882 throws InterruptedException {883 final Node node addWaiter(Node.EXCLUSIVE);884 boolean failed true;885 try {886 for (;;) {887 final Node p node.predecessor();888 if (p head tryAcquire(arg)) {889 setHead(node);890 p.next null; // help GC891 failed false;892 return;893 }894 if (shouldParkAfterFailedAcquire(p, node) 895 parkAndCheckInterrupt())896 throw new InterruptedException();897 }898 } finally {899 if (failed)900 cancelAcquire(node);901 }902 }903 904 /**905 * Acquires in exclusive timed mode.906 *907 * param arg the acquire argument908 * param nanosTimeout max wait time909 * return {code true} if acquired910 */911 private boolean doAcquireNanos(int arg, long nanosTimeout)912 throws InterruptedException {913 long lastTime System.nanoTime();914 final Node node addWaiter(Node.EXCLUSIVE);915 boolean failed true;916 try {917 for (;;) {918 final Node p node.predecessor();919 if (p head tryAcquire(arg)) {920 setHead(node);921 p.next null; // help GC922 failed false;923 return true;924 }925 if (nanosTimeout 0)926 return false;927 if (shouldParkAfterFailedAcquire(p, node) 928 nanosTimeout spinForTimeoutThreshold)929 LockSupport.parkNanos(this, nanosTimeout);930 long now System.nanoTime();931 nanosTimeout - now - lastTime;932 lastTime now;933 if (Thread.interrupted())934 throw new InterruptedException();935 }936 } finally {937 if (failed)938 cancelAcquire(node);939 }940 }941 942 /**943 * Acquires in shared uninterruptible mode.944 * param arg the acquire argument945 */946 private void doAcquireShared(int arg) {947 final Node node addWaiter(Node.SHARED);948 boolean failed true;949 try {950 boolean interrupted false;951 for (;;) {952 final Node p node.predecessor();953 if (p head) {954 int r tryAcquireShared(arg);955 if (r 0) {956 setHeadAndPropagate(node, r);957 p.next null; // help GC958 if (interrupted)959 selfInterrupt();960 failed false;961 return;962 }963 }964 if (shouldParkAfterFailedAcquire(p, node) 965 parkAndCheckInterrupt())966 interrupted true;967 }968 } finally {969 if (failed)970 cancelAcquire(node);971 }972 }973 974 /**975 * Acquires in shared interruptible mode.976 * param arg the acquire argument977 */978 private void doAcquireSharedInterruptibly(int arg)979 throws InterruptedException {980 final Node node addWaiter(Node.SHARED);981 boolean failed true;982 try {983 for (;;) {984 final Node p node.predecessor();985 if (p head) {986 int r tryAcquireShared(arg);987 if (r 0) {988 setHeadAndPropagate(node, r);989 p.next null; // help GC990 failed false;991 return;992 }993 }994 if (shouldParkAfterFailedAcquire(p, node) 995 parkAndCheckInterrupt())996 throw new InterruptedException();997 }998 } finally {999 if (failed)
1000 cancelAcquire(node);
1001 }
1002 }
1003
1004 /**
1005 * Acquires in shared timed mode.
1006 *
1007 * param arg the acquire argument
1008 * param nanosTimeout max wait time
1009 * return {code true} if acquired
1010 */
1011 private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
1012 throws InterruptedException {
1013
1014 long lastTime System.nanoTime();
1015 final Node node addWaiter(Node.SHARED);
1016 boolean failed true;
1017 try {
1018 for (;;) {
1019 final Node p node.predecessor();
1020 if (p head) {
1021 int r tryAcquireShared(arg);
1022 if (r 0) {
1023 setHeadAndPropagate(node, r);
1024 p.next null; // help GC
1025 failed false;
1026 return true;
1027 }
1028 }
1029 if (nanosTimeout 0)
1030 return false;
1031 if (shouldParkAfterFailedAcquire(p, node)
1032 nanosTimeout spinForTimeoutThreshold)
1033 LockSupport.parkNanos(this, nanosTimeout);
1034 long now System.nanoTime();
1035 nanosTimeout - now - lastTime;
1036 lastTime now;
1037 if (Thread.interrupted())
1038 throw new InterruptedException();
1039 }
1040 } finally {
1041 if (failed)
1042 cancelAcquire(node);
1043 }
1044 }
1045
1046 // Main exported methods
1047
1048 /**
1049 * Attempts to acquire in exclusive mode. This method should query
1050 * if the state of the object permits it to be acquired in the
1051 * exclusive mode, and if so to acquire it.
1052 *
1053 * pThis method is always invoked by the thread performing
1054 * acquire. If this method reports failure, the acquire method
1055 * may queue the thread, if it is not already queued, until it is
1056 * signalled by a release from some other thread. This can be used
1057 * to implement method {link Lock#tryLock()}.
1058 *
1059 * pThe default
1060 * implementation throws {link UnsupportedOperationException}.
1061 *
1062 * param arg the acquire argument. This value is always the one
1063 * passed to an acquire method, or is the value saved on entry
1064 * to a condition wait. The value is otherwise uninterpreted
1065 * and can represent anything you like.
1066 * return {code true} if successful. Upon success, this object has
1067 * been acquired.
1068 * throws IllegalMonitorStateException if acquiring would place this
1069 * synchronizer in an illegal state. This exception must be
1070 * thrown in a consistent fashion for synchronization to work
1071 * correctly.
1072 * throws UnsupportedOperationException if exclusive mode is not supported
1073 */
1074 protected boolean tryAcquire(int arg) {
1075 throw new UnsupportedOperationException();
1076 }
1077
1078 /**
1079 * Attempts to set the state to reflect a release in exclusive
1080 * mode.
1081 *
1082 * pThis method is always invoked by the thread performing release.
1083 *
1084 * pThe default implementation throws
1085 * {link UnsupportedOperationException}.
1086 *
1087 * param arg the release argument. This value is always the one
1088 * passed to a release method, or the current state value upon
1089 * entry to a condition wait. The value is otherwise
1090 * uninterpreted and can represent anything you like.
1091 * return {code true} if this object is now in a fully released
1092 * state, so that any waiting threads may attempt to acquire;
1093 * and {code false} otherwise.
1094 * throws IllegalMonitorStateException if releasing would place this
1095 * synchronizer in an illegal state. This exception must be
1096 * thrown in a consistent fashion for synchronization to work
1097 * correctly.
1098 * throws UnsupportedOperationException if exclusive mode is not supported
1099 */
1100 protected boolean tryRelease(int arg) {
1101 throw new UnsupportedOperationException();
1102 }
1103
1104 /**
1105 * Attempts to acquire in shared mode. This method should query if
1106 * the state of the object permits it to be acquired in the shared
1107 * mode, and if so to acquire it.
1108 *
1109 * pThis method is always invoked by the thread performing
1110 * acquire. If this method reports failure, the acquire method
1111 * may queue the thread, if it is not already queued, until it is
1112 * signalled by a release from some other thread.
1113 *
1114 * pThe default implementation throws {link
1115 * UnsupportedOperationException}.
1116 *
1117 * param arg the acquire argument. This value is always the one
1118 * passed to an acquire method, or is the value saved on entry
1119 * to a condition wait. The value is otherwise uninterpreted
1120 * and can represent anything you like.
1121 * return a negative value on failure; zero if acquisition in shared
1122 * mode succeeded but no subsequent shared-mode acquire can
1123 * succeed; and a positive value if acquisition in shared
1124 * mode succeeded and subsequent shared-mode acquires might
1125 * also succeed, in which case a subsequent waiting thread
1126 * must check availability. (Support for three different
1127 * return values enables this method to be used in contexts
1128 * where acquires only sometimes act exclusively.) Upon
1129 * success, this object has been acquired.
1130 * throws IllegalMonitorStateException if acquiring would place this
1131 * synchronizer in an illegal state. This exception must be
1132 * thrown in a consistent fashion for synchronization to work
1133 * correctly.
1134 * throws UnsupportedOperationException if shared mode is not supported
1135 */
1136 protected int tryAcquireShared(int arg) {
1137 throw new UnsupportedOperationException();
1138 }
1139
1140 /**
1141 * Attempts to set the state to reflect a release in shared mode.
1142 *
1143 * pThis method is always invoked by the thread performing release.
1144 *
1145 * pThe default implementation throws
1146 * {link UnsupportedOperationException}.
1147 *
1148 * param arg the release argument. This value is always the one
1149 * passed to a release method, or the current state value upon
1150 * entry to a condition wait. The value is otherwise
1151 * uninterpreted and can represent anything you like.
1152 * return {code true} if this release of shared mode may permit a
1153 * waiting acquire (shared or exclusive) to succeed; and
1154 * {code false} otherwise
1155 * throws IllegalMonitorStateException if releasing would place this
1156 * synchronizer in an illegal state. This exception must be
1157 * thrown in a consistent fashion for synchronization to work
1158 * correctly.
1159 * throws UnsupportedOperationException if shared mode is not supported
1160 */
1161 protected boolean tryReleaseShared(int arg) {
1162 throw new UnsupportedOperationException();
1163 }
1164
1165 /**
1166 * Returns {code true} if synchronization is held exclusively with
1167 * respect to the current (calling) thread. This method is invoked
1168 * upon each call to a non-waiting {link ConditionObject} method.
1169 * (Waiting methods instead invoke {link #release}.)
1170 *
1171 * pThe default implementation throws {link
1172 * UnsupportedOperationException}. This method is invoked
1173 * internally only within {link ConditionObject} methods, so need
1174 * not be defined if conditions are not used.
1175 *
1176 * return {code true} if synchronization is held exclusively;
1177 * {code false} otherwise
1178 * throws UnsupportedOperationException if conditions are not supported
1179 */
1180 protected boolean isHeldExclusively() {
1181 throw new UnsupportedOperationException();
1182 }
1183
1184 /**
1185 * Acquires in exclusive mode, ignoring interrupts. Implemented
1186 * by invoking at least once {link #tryAcquire},
1187 * returning on success. Otherwise the thread is queued, possibly
1188 * repeatedly blocking and unblocking, invoking {link
1189 * #tryAcquire} until success. This method can be used
1190 * to implement method {link Lock#lock}.
1191 *
1192 * param arg the acquire argument. This value is conveyed to
1193 * {link #tryAcquire} but is otherwise uninterpreted and
1194 * can represent anything you like.
1195 */
1196 public final void acquire(int arg) {
1197 if (!tryAcquire(arg)
1198 acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
1199 selfInterrupt();
1200 }
1201
1202 /**
1203 * Acquires in exclusive mode, aborting if interrupted.
1204 * Implemented by first checking interrupt status, then invoking
1205 * at least once {link #tryAcquire}, returning on
1206 * success. Otherwise the thread is queued, possibly repeatedly
1207 * blocking and unblocking, invoking {link #tryAcquire}
1208 * until success or the thread is interrupted. This method can be
1209 * used to implement method {link Lock#lockInterruptibly}.
1210 *
1211 * param arg the acquire argument. This value is conveyed to
1212 * {link #tryAcquire} but is otherwise uninterpreted and
1213 * can represent anything you like.
1214 * throws InterruptedException if the current thread is interrupted
1215 */
1216 public final void acquireInterruptibly(int arg)
1217 throws InterruptedException {
1218 if (Thread.interrupted())
1219 throw new InterruptedException();
1220 if (!tryAcquire(arg))
1221 doAcquireInterruptibly(arg);
1222 }
1223
1224 /**
1225 * Attempts to acquire in exclusive mode, aborting if interrupted,
1226 * and failing if the given timeout elapses. Implemented by first
1227 * checking interrupt status, then invoking at least once {link
1228 * #tryAcquire}, returning on success. Otherwise, the thread is
1229 * queued, possibly repeatedly blocking and unblocking, invoking
1230 * {link #tryAcquire} until success or the thread is interrupted
1231 * or the timeout elapses. This method can be used to implement
1232 * method {link Lock#tryLock(long, TimeUnit)}.
1233 *
1234 * param arg the acquire argument. This value is conveyed to
1235 * {link #tryAcquire} but is otherwise uninterpreted and
1236 * can represent anything you like.
1237 * param nanosTimeout the maximum number of nanoseconds to wait
1238 * return {code true} if acquired; {code false} if timed out
1239 * throws InterruptedException if the current thread is interrupted
1240 */
1241 public final boolean tryAcquireNanos(int arg, long nanosTimeout)
1242 throws InterruptedException {
1243 if (Thread.interrupted())
1244 throw new InterruptedException();
1245 return tryAcquire(arg) ||
1246 doAcquireNanos(arg, nanosTimeout);
1247 }
1248
1249 /**
1250 * Releases in exclusive mode. Implemented by unblocking one or
1251 * more threads if {link #tryRelease} returns true.
1252 * This method can be used to implement method {link Lock#unlock}.
1253 *
1254 * param arg the release argument. This value is conveyed to
1255 * {link #tryRelease} but is otherwise uninterpreted and
1256 * can represent anything you like.
1257 * return the value returned from {link #tryRelease}
1258 */
1259 public final boolean release(int arg) {
1260 if (tryRelease(arg)) {
1261 Node h head;
1262 if (h ! null h.waitStatus ! 0)
1263 unparkSuccessor(h);
1264 return true;
1265 }
1266 return false;
1267 }
1268
1269 /**
1270 * Acquires in shared mode, ignoring interrupts. Implemented by
1271 * first invoking at least once {link #tryAcquireShared},
1272 * returning on success. Otherwise the thread is queued, possibly
1273 * repeatedly blocking and unblocking, invoking {link
1274 * #tryAcquireShared} until success.
1275 *
1276 * param arg the acquire argument. This value is conveyed to
1277 * {link #tryAcquireShared} but is otherwise uninterpreted
1278 * and can represent anything you like.
1279 */
1280 public final void acquireShared(int arg) {
1281 if (tryAcquireShared(arg) 0)
1282 doAcquireShared(arg);
1283 }
1284
1285 /**
1286 * Acquires in shared mode, aborting if interrupted. Implemented
1287 * by first checking interrupt status, then invoking at least once
1288 * {link #tryAcquireShared}, returning on success. Otherwise the
1289 * thread is queued, possibly repeatedly blocking and unblocking,
1290 * invoking {link #tryAcquireShared} until success or the thread
1291 * is interrupted.
1292 * param arg the acquire argument
1293 * This value is conveyed to {link #tryAcquireShared} but is
1294 * otherwise uninterpreted and can represent anything
1295 * you like.
1296 * throws InterruptedException if the current thread is interrupted
1297 */
1298 public final void acquireSharedInterruptibly(int arg)
1299 throws InterruptedException {
1300 if (Thread.interrupted())
1301 throw new InterruptedException();
1302 if (tryAcquireShared(arg) 0)
1303 doAcquireSharedInterruptibly(arg);
1304 }
1305
1306 /**
1307 * Attempts to acquire in shared mode, aborting if interrupted, and
1308 * failing if the given timeout elapses. Implemented by first
1309 * checking interrupt status, then invoking at least once {link
1310 * #tryAcquireShared}, returning on success. Otherwise, the
1311 * thread is queued, possibly repeatedly blocking and unblocking,
1312 * invoking {link #tryAcquireShared} until success or the thread
1313 * is interrupted or the timeout elapses.
1314 *
1315 * param arg the acquire argument. This value is conveyed to
1316 * {link #tryAcquireShared} but is otherwise uninterpreted
1317 * and can represent anything you like.
1318 * param nanosTimeout the maximum number of nanoseconds to wait
1319 * return {code true} if acquired; {code false} if timed out
1320 * throws InterruptedException if the current thread is interrupted
1321 */
1322 public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
1323 throws InterruptedException {
1324 if (Thread.interrupted())
1325 throw new InterruptedException();
1326 return tryAcquireShared(arg) 0 ||
1327 doAcquireSharedNanos(arg, nanosTimeout);
1328 }
1329
1330 /**
1331 * Releases in shared mode. Implemented by unblocking one or more
1332 * threads if {link #tryReleaseShared} returns true.
1333 *
1334 * param arg the release argument. This value is conveyed to
1335 * {link #tryReleaseShared} but is otherwise uninterpreted
1336 * and can represent anything you like.
1337 * return the value returned from {link #tryReleaseShared}
1338 */
1339 public final boolean releaseShared(int arg) {
1340 if (tryReleaseShared(arg)) {
1341 doReleaseShared();
1342 return true;
1343 }
1344 return false;
1345 }
1346
1347 // Queue inspection methods
1348
1349 /**
1350 * Queries whether any threads are waiting to acquire. Note that
1351 * because cancellations due to interrupts and timeouts may occur
1352 * at any time, a {code true} return does not guarantee that any
1353 * other thread will ever acquire.
1354 *
1355 * pIn this implementation, this operation returns in
1356 * constant time.
1357 *
1358 * return {code true} if there may be other threads waiting to acquire
1359 */
1360 public final boolean hasQueuedThreads() {
1361 return head ! tail;
1362 }
1363
1364 /**
1365 * Queries whether any threads have ever contended to acquire this
1366 * synchronizer; that is if an acquire method has ever blocked.
1367 *
1368 * pIn this implementation, this operation returns in
1369 * constant time.
1370 *
1371 * return {code true} if there has ever been contention
1372 */
1373 public final boolean hasContended() {
1374 return head ! null;
1375 }
1376
1377 /**
1378 * Returns the first (longest-waiting) thread in the queue, or
1379 * {code null} if no threads are currently queued.
1380 *
1381 * pIn this implementation, this operation normally returns in
1382 * constant time, but may iterate upon contention if other threads are
1383 * concurrently modifying the queue.
1384 *
1385 * return the first (longest-waiting) thread in the queue, or
1386 * {code null} if no threads are currently queued
1387 */
1388 public final Thread getFirstQueuedThread() {
1389 // handle only fast path, else relay
1390 return (head tail) ? null : fullGetFirstQueuedThread();
1391 }
1392
1393 /**
1394 * Version of getFirstQueuedThread called when fastpath fails
1395 */
1396 private Thread fullGetFirstQueuedThread() {
1397 /*
1398 * The first node is normally head.next. Try to get its
1399 * thread field, ensuring consistent reads: If thread
1400 * field is nulled out or s.prev is no longer head, then
1401 * some other thread(s) concurrently performed setHead in
1402 * between some of our reads. We try this twice before
1403 * resorting to traversal.
1404 */
1405 Node h, s;
1406 Thread st;
1407 if (((h head) ! null (s h.next) ! null
1408 s.prev head (st s.thread) ! null) ||
1409 ((h head) ! null (s h.next) ! null
1410 s.prev head (st s.thread) ! null))
1411 return st;
1412
1413 /*
1414 * Heads next field might not have been set yet, or may have
1415 * been unset after setHead. So we must check to see if tail
1416 * is actually first node. If not, we continue on, safely
1417 * traversing from tail back to head to find first,
1418 * guaranteeing termination.
1419 */
1420
1421 Node t tail;
1422 Thread firstThread null;
1423 while (t ! null t ! head) {
1424 Thread tt t.thread;
1425 if (tt ! null)
1426 firstThread tt;
1427 t t.prev;
1428 }
1429 return firstThread;
1430 }
1431
1432 /**
1433 * Returns true if the given thread is currently queued.
1434 *
1435 * pThis implementation traverses the queue to determine
1436 * presence of the given thread.
1437 *
1438 * param thread the thread
1439 * return {code true} if the given thread is on the queue
1440 * throws NullPointerException if the thread is null
1441 */
1442 public final boolean isQueued(Thread thread) {
1443 if (thread null)
1444 throw new NullPointerException();
1445 for (Node p tail; p ! null; p p.prev)
1446 if (p.thread thread)
1447 return true;
1448 return false;
1449 }
1450
1451 /**
1452 * Returns {code true} if the apparent first queued thread, if one
1453 * exists, is waiting in exclusive mode. If this method returns
1454 * {code true}, and the current thread is attempting to acquire in
1455 * shared mode (that is, this method is invoked from {link
1456 * #tryAcquireShared}) then it is guaranteed that the current thread
1457 * is not the first queued thread. Used only as a heuristic in
1458 * ReentrantReadWriteLock.
1459 */
1460 final boolean apparentlyFirstQueuedIsExclusive() {
1461 Node h, s;
1462 return (h head) ! null
1463 (s h.next) ! null
1464 !s.isShared()
1465 s.thread ! null;
1466 }
1467
1468 /**
1469 * Queries whether any threads have been waiting to acquire longer
1470 * than the current thread.
1471 *
1472 * pAn invocation of this method is equivalent to (but may be
1473 * more efficient than):
1474 * pre {code
1475 * getFirstQueuedThread() ! Thread.currentThread()
1476 * hasQueuedThreads()}/pre
1477 *
1478 * pNote that because cancellations due to interrupts and
1479 * timeouts may occur at any time, a {code true} return does not
1480 * guarantee that some other thread will acquire before the current
1481 * thread. Likewise, it is possible for another thread to win a
1482 * race to enqueue after this method has returned {code false},
1483 * due to the queue being empty.
1484 *
1485 * pThis method is designed to be used by a fair synchronizer to
1486 * avoid a hrefAbstractQueuedSynchronizer#bargingbarging/a.
1487 * Such a synchronizers {link #tryAcquire} method should return
1488 * {code false}, and its {link #tryAcquireShared} method should
1489 * return a negative value, if this method returns {code true}
1490 * (unless this is a reentrant acquire). For example, the {code
1491 * tryAcquire} method for a fair, reentrant, exclusive mode
1492 * synchronizer might look like this:
1493 *
1494 * pre {code
1495 * protected boolean tryAcquire(int arg) {
1496 * if (isHeldExclusively()) {
1497 * // A reentrant acquire; increment hold count
1498 * return true;
1499 * } else if (hasQueuedPredecessors()) {
1500 * return false;
1501 * } else {
1502 * // try to acquire normally
1503 * }
1504 * }}/pre
1505 *
1506 * return {code true} if there is a queued thread preceding the
1507 * current thread, and {code false} if the current thread
1508 * is at the head of the queue or the queue is empty
1509 * since 1.7
1510 */
1511 public final boolean hasQueuedPredecessors() {
1512 // The correctness of this depends on head being initialized
1513 // before tail and on head.next being accurate if the current
1514 // thread is first in queue.
1515 Node t tail; // Read fields in reverse initialization order
1516 Node h head;
1517 Node s;
1518 return h ! t
1519 ((s h.next) null || s.thread ! Thread.currentThread());
1520 }
1521
1522
1523 // Instrumentation and monitoring methods
1524
1525 /**
1526 * Returns an estimate of the number of threads waiting to
1527 * acquire. The value is only an estimate because the number of
1528 * threads may change dynamically while this method traverses
1529 * internal data structures. This method is designed for use in
1530 * monitoring system state, not for synchronization
1531 * control.
1532 *
1533 * return the estimated number of threads waiting to acquire
1534 */
1535 public final int getQueueLength() {
1536 int n 0;
1537 for (Node p tail; p ! null; p p.prev) {
1538 if (p.thread ! null)
1539 n;
1540 }
1541 return n;
1542 }
1543
1544 /**
1545 * Returns a collection containing threads that may be waiting to
1546 * acquire. Because the actual set of threads may change
1547 * dynamically while constructing this result, the returned
1548 * collection is only a best-effort estimate. The elements of the
1549 * returned collection are in no particular order. This method is
1550 * designed to facilitate construction of subclasses that provide
1551 * more extensive monitoring facilities.
1552 *
1553 * return the collection of threads
1554 */
1555 public final CollectionThread getQueuedThreads() {
1556 ArrayListThread list new ArrayListThread();
1557 for (Node p tail; p ! null; p p.prev) {
1558 Thread t p.thread;
1559 if (t ! null)
1560 list.add(t);
1561 }
1562 return list;
1563 }
1564
1565 /**
1566 * Returns a collection containing threads that may be waiting to
1567 * acquire in exclusive mode. This has the same properties
1568 * as {link #getQueuedThreads} except that it only returns
1569 * those threads waiting due to an exclusive acquire.
1570 *
1571 * return the collection of threads
1572 */
1573 public final CollectionThread getExclusiveQueuedThreads() {
1574 ArrayListThread list new ArrayListThread();
1575 for (Node p tail; p ! null; p p.prev) {
1576 if (!p.isShared()) {
1577 Thread t p.thread;
1578 if (t ! null)
1579 list.add(t);
1580 }
1581 }
1582 return list;
1583 }
1584
1585 /**
1586 * Returns a collection containing threads that may be waiting to
1587 * acquire in shared mode. This has the same properties
1588 * as {link #getQueuedThreads} except that it only returns
1589 * those threads waiting due to a shared acquire.
1590 *
1591 * return the collection of threads
1592 */
1593 public final CollectionThread getSharedQueuedThreads() {
1594 ArrayListThread list new ArrayListThread();
1595 for (Node p tail; p ! null; p p.prev) {
1596 if (p.isShared()) {
1597 Thread t p.thread;
1598 if (t ! null)
1599 list.add(t);
1600 }
1601 }
1602 return list;
1603 }
1604
1605 /**
1606 * Returns a string identifying this synchronizer, as well as its state.
1607 * The state, in brackets, includes the String {code State }
1608 * followed by the current value of {link #getState}, and either
1609 * {code nonempty} or {code empty} depending on whether the
1610 * queue is empty.
1611 *
1612 * return a string identifying this synchronizer, as well as its state
1613 */
1614 public String toString() {
1615 int s getState();
1616 String q hasQueuedThreads() ? non : ;
1617 return super.toString()
1618 [State s , q empty queue];
1619 }
1620
1621
1622 // Internal support methods for Conditions
1623
1624 /**
1625 * Returns true if a node, always one that was initially placed on
1626 * a condition queue, is now waiting to reacquire on sync queue.
1627 * param node the node
1628 * return true if is reacquiring
1629 */
1630 final boolean isOnSyncQueue(Node node) {
1631 if (node.waitStatus Node.CONDITION || node.prev null)
1632 return false;
1633 if (node.next ! null) // If has successor, it must be on queue
1634 return true;
1635 /*
1636 * node.prev can be non-null, but not yet on queue because
1637 * the CAS to place it on queue can fail. So we have to
1638 * traverse from tail to make sure it actually made it. It
1639 * will always be near the tail in calls to this method, and
1640 * unless the CAS failed (which is unlikely), it will be
1641 * there, so we hardly ever traverse much.
1642 */
1643 return findNodeFromTail(node);
1644 }
1645
1646 /**
1647 * Returns true if node is on sync queue by searching backwards from tail.
1648 * Called only when needed by isOnSyncQueue.
1649 * return true if present
1650 */
1651 private boolean findNodeFromTail(Node node) {
1652 Node t tail;
1653 for (;;) {
1654 if (t node)
1655 return true;
1656 if (t null)
1657 return false;
1658 t t.prev;
1659 }
1660 }
1661
1662 /**
1663 * Transfers a node from a condition queue onto sync queue.
1664 * Returns true if successful.
1665 * param node the node
1666 * return true if successfully transferred (else the node was
1667 * cancelled before signal).
1668 */
1669 final boolean transferForSignal(Node node) {
1670 /*
1671 * If cannot change waitStatus, the node has been cancelled.
1672 */
1673 if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
1674 return false;
1675
1676 /*
1677 * Splice onto queue and try to set waitStatus of predecessor to
1678 * indicate that thread is (probably) waiting. If cancelled or
1679 * attempt to set waitStatus fails, wake up to resync (in which
1680 * case the waitStatus can be transiently and harmlessly wrong).
1681 */
1682 Node p enq(node);
1683 int ws p.waitStatus;
1684 if (ws 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
1685 LockSupport.unpark(node.thread);
1686 return true;
1687 }
1688
1689 /**
1690 * Transfers node, if necessary, to sync queue after a cancelled
1691 * wait. Returns true if thread was cancelled before being
1692 * signalled.
1693 * param current the waiting thread
1694 * param node its node
1695 * return true if cancelled before the node was signalled
1696 */
1697 final boolean transferAfterCancelledWait(Node node) {
1698 if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
1699 enq(node);
1700 return true;
1701 }
1702 /*
1703 * If we lost out to a signal(), then we cant proceed
1704 * until it finishes its enq(). Cancelling during an
1705 * incomplete transfer is both rare and transient, so just
1706 * spin.
1707 */
1708 while (!isOnSyncQueue(node))
1709 Thread.yield();
1710 return false;
1711 }
1712
1713 /**
1714 * Invokes release with current state value; returns saved state.
1715 * Cancels node and throws exception on failure.
1716 * param node the condition node for this wait
1717 * return previous sync state
1718 */
1719 final int fullyRelease(Node node) {
1720 boolean failed true;
1721 try {
1722 int savedState getState();
1723 if (release(savedState)) {
1724 failed false;
1725 return savedState;
1726 } else {
1727 throw new IllegalMonitorStateException();
1728 }
1729 } finally {
1730 if (failed)
1731 node.waitStatus Node.CANCELLED;
1732 }
1733 }
1734
1735 // Instrumentation methods for conditions
1736
1737 /**
1738 * Queries whether the given ConditionObject
1739 * uses this synchronizer as its lock.
1740 *
1741 * param condition the condition
1742 * return tttrue/tt if owned
1743 * throws NullPointerException if the condition is null
1744 */
1745 public final boolean owns(ConditionObject condition) {
1746 if (condition null)
1747 throw new NullPointerException();
1748 return condition.isOwnedBy(this);
1749 }
1750
1751 /**
1752 * Queries whether any threads are waiting on the given condition
1753 * associated with this synchronizer. Note that because timeouts
1754 * and interrupts may occur at any time, a tttrue/tt return
1755 * does not guarantee that a future ttsignal/tt will awaken
1756 * any threads. This method is designed primarily for use in
1757 * monitoring of the system state.
1758 *
1759 * param condition the condition
1760 * return tttrue/tt if there are any waiting threads
1761 * throws IllegalMonitorStateException if exclusive synchronization
1762 * is not held
1763 * throws IllegalArgumentException if the given condition is
1764 * not associated with this synchronizer
1765 * throws NullPointerException if the condition is null
1766 */
1767 public final boolean hasWaiters(ConditionObject condition) {
1768 if (!owns(condition))
1769 throw new IllegalArgumentException(Not owner);
1770 return condition.hasWaiters();
1771 }
1772
1773 /**
1774 * Returns an estimate of the number of threads waiting on the
1775 * given condition associated with this synchronizer. Note that
1776 * because timeouts and interrupts may occur at any time, the
1777 * estimate serves only as an upper bound on the actual number of
1778 * waiters. This method is designed for use in monitoring of the
1779 * system state, not for synchronization control.
1780 *
1781 * param condition the condition
1782 * return the estimated number of waiting threads
1783 * throws IllegalMonitorStateException if exclusive synchronization
1784 * is not held
1785 * throws IllegalArgumentException if the given condition is
1786 * not associated with this synchronizer
1787 * throws NullPointerException if the condition is null
1788 */
1789 public final int getWaitQueueLength(ConditionObject condition) {
1790 if (!owns(condition))
1791 throw new IllegalArgumentException(Not owner);
1792 return condition.getWaitQueueLength();
1793 }
1794
1795 /**
1796 * Returns a collection containing those threads that may be
1797 * waiting on the given condition associated with this
1798 * synchronizer. Because the actual set of threads may change
1799 * dynamically while constructing this result, the returned
1800 * collection is only a best-effort estimate. The elements of the
1801 * returned collection are in no particular order.
1802 *
1803 * param condition the condition
1804 * return the collection of threads
1805 * throws IllegalMonitorStateException if exclusive synchronization
1806 * is not held
1807 * throws IllegalArgumentException if the given condition is
1808 * not associated with this synchronizer
1809 * throws NullPointerException if the condition is null
1810 */
1811 public final CollectionThread getWaitingThreads(ConditionObject condition) {
1812 if (!owns(condition))
1813 throw new IllegalArgumentException(Not owner);
1814 return condition.getWaitingThreads();
1815 }
1816
1817 /**
1818 * Condition implementation for a {link
1819 * AbstractQueuedSynchronizer} serving as the basis of a {link
1820 * Lock} implementation.
1821 *
1822 * pMethod documentation for this class describes mechanics,
1823 * not behavioral specifications from the point of view of Lock
1824 * and Condition users. Exported versions of this class will in
1825 * general need to be accompanied by documentation describing
1826 * condition semantics that rely on those of the associated
1827 * ttAbstractQueuedSynchronizer/tt.
1828 *
1829 * pThis class is Serializable, but all fields are transient,
1830 * so deserialized conditions have no waiters.
1831 */
1832 public class ConditionObject implements Condition, java.io.Serializable {
1833 private static final long serialVersionUID 1173984872572414699L;
1834 /** First node of condition queue. */
1835 private transient Node firstWaiter;
1836 /** Last node of condition queue. */
1837 private transient Node lastWaiter;
1838
1839 /**
1840 * Creates a new ttConditionObject/tt instance.
1841 */
1842 public ConditionObject() { }
1843
1844 // Internal methods
1845
1846 /**
1847 * Adds a new waiter to wait queue.
1848 * return its new wait node
1849 */
1850 private Node addConditionWaiter() {
1851 Node t lastWaiter;
1852 // If lastWaiter is cancelled, clean out.
1853 if (t ! null t.waitStatus ! Node.CONDITION) {
1854 unlinkCancelledWaiters();
1855 t lastWaiter;
1856 }
1857 Node node new Node(Thread.currentThread(), Node.CONDITION);
1858 if (t null)
1859 firstWaiter node;
1860 else
1861 t.nextWaiter node;
1862 lastWaiter node;
1863 return node;
1864 }
1865
1866 /**
1867 * Removes and transfers nodes until hit non-cancelled one or
1868 * null. Split out from signal in part to encourage compilers
1869 * to inline the case of no waiters.
1870 * param first (non-null) the first node on condition queue
1871 */
1872 private void doSignal(Node first) {
1873 do {
1874 if ( (firstWaiter first.nextWaiter) null)
1875 lastWaiter null;
1876 first.nextWaiter null;
1877 } while (!transferForSignal(first)
1878 (first firstWaiter) ! null);
1879 }
1880
1881 /**
1882 * Removes and transfers all nodes.
1883 * param first (non-null) the first node on condition queue
1884 */
1885 private void doSignalAll(Node first) {
1886 lastWaiter firstWaiter null;
1887 do {
1888 Node next first.nextWaiter;
1889 first.nextWaiter null;
1890 transferForSignal(first);
1891 first next;
1892 } while (first ! null);
1893 }
1894
1895 /**
1896 * Unlinks cancelled waiter nodes from condition queue.
1897 * Called only while holding lock. This is called when
1898 * cancellation occurred during condition wait, and upon
1899 * insertion of a new waiter when lastWaiter is seen to have
1900 * been cancelled. This method is needed to avoid garbage
1901 * retention in the absence of signals. So even though it may
1902 * require a full traversal, it comes into play only when
1903 * timeouts or cancellations occur in the absence of
1904 * signals. It traverses all nodes rather than stopping at a
1905 * particular target to unlink all pointers to garbage nodes
1906 * without requiring many re-traversals during cancellation
1907 * storms.
1908 */
1909 private void unlinkCancelledWaiters() {
1910 Node t firstWaiter;
1911 Node trail null;
1912 while (t ! null) {
1913 Node next t.nextWaiter;
1914 if (t.waitStatus ! Node.CONDITION) {
1915 t.nextWaiter null;
1916 if (trail null)
1917 firstWaiter next;
1918 else
1919 trail.nextWaiter next;
1920 if (next null)
1921 lastWaiter trail;
1922 }
1923 else
1924 trail t;
1925 t next;
1926 }
1927 }
1928
1929 // public methods
1930
1931 /**
1932 * Moves the longest-waiting thread, if one exists, from the
1933 * wait queue for this condition to the wait queue for the
1934 * owning lock.
1935 *
1936 * throws IllegalMonitorStateException if {link #isHeldExclusively}
1937 * returns {code false}
1938 */
1939 public final void signal() {
1940 if (!isHeldExclusively())
1941 throw new IllegalMonitorStateException();
1942 Node first firstWaiter;
1943 if (first ! null)
1944 doSignal(first);
1945 }
1946
1947 /**
1948 * Moves all threads from the wait queue for this condition to
1949 * the wait queue for the owning lock.
1950 *
1951 * throws IllegalMonitorStateException if {link #isHeldExclusively}
1952 * returns {code false}
1953 */
1954 public final void signalAll() {
1955 if (!isHeldExclusively())
1956 throw new IllegalMonitorStateException();
1957 Node first firstWaiter;
1958 if (first ! null)
1959 doSignalAll(first);
1960 }
1961
1962 /**
1963 * Implements uninterruptible condition wait.
1964 * ol
1965 * li Save lock state returned by {link #getState}.
1966 * li Invoke {link #release} with
1967 * saved state as argument, throwing
1968 * IllegalMonitorStateException if it fails.
1969 * li Block until signalled.
1970 * li Reacquire by invoking specialized version of
1971 * {link #acquire} with saved state as argument.
1972 * /ol
1973 */
1974 public final void awaitUninterruptibly() {
1975 Node node addConditionWaiter();
1976 int savedState fullyRelease(node);
1977 boolean interrupted false;
1978 while (!isOnSyncQueue(node)) {
1979 LockSupport.park(this);
1980 if (Thread.interrupted())
1981 interrupted true;
1982 }
1983 if (acquireQueued(node, savedState) || interrupted)
1984 selfInterrupt();
1985 }
1986
1987 /*
1988 * For interruptible waits, we need to track whether to throw
1989 * InterruptedException, if interrupted while blocked on
1990 * condition, versus reinterrupt current thread, if
1991 * interrupted while blocked waiting to re-acquire.
1992 */
1993
1994 /** Mode meaning to reinterrupt on exit from wait */
1995 private static final int REINTERRUPT 1;
1996 /** Mode meaning to throw InterruptedException on exit from wait */
1997 private static final int THROW_IE -1;
1998
1999 /**
2000 * Checks for interrupt, returning THROW_IE if interrupted
2001 * before signalled, REINTERRUPT if after signalled, or
2002 * 0 if not interrupted.
2003 */
2004 private int checkInterruptWhileWaiting(Node node) {
2005 return Thread.interrupted() ?
2006 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
2007 0;
2008 }
2009
2010 /**
2011 * Throws InterruptedException, reinterrupts current thread, or
2012 * does nothing, depending on mode.
2013 */
2014 private void reportInterruptAfterWait(int interruptMode)
2015 throws InterruptedException {
2016 if (interruptMode THROW_IE)
2017 throw new InterruptedException();
2018 else if (interruptMode REINTERRUPT)
2019 selfInterrupt();
2020 }
2021
2022 /**
2023 * Implements interruptible condition wait.
2024 * ol
2025 * li If current thread is interrupted, throw InterruptedException.
2026 * li Save lock state returned by {link #getState}.
2027 * li Invoke {link #release} with
2028 * saved state as argument, throwing
2029 * IllegalMonitorStateException if it fails.
2030 * li Block until signalled or interrupted.
2031 * li Reacquire by invoking specialized version of
2032 * {link #acquire} with saved state as argument.
2033 * li If interrupted while blocked in step 4, throw InterruptedException.
2034 * /ol
2035 */
2036 public final void await() throws InterruptedException {
2037 if (Thread.interrupted())
2038 throw new InterruptedException();
2039 Node node addConditionWaiter();
2040 int savedState fullyRelease(node);
2041 int interruptMode 0;
2042 while (!isOnSyncQueue(node)) {
2043 LockSupport.park(this);
2044 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2045 break;
2046 }
2047 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2048 interruptMode REINTERRUPT;
2049 if (node.nextWaiter ! null) // clean up if cancelled
2050 unlinkCancelledWaiters();
2051 if (interruptMode ! 0)
2052 reportInterruptAfterWait(interruptMode);
2053 }
2054
2055 /**
2056 * Implements timed condition wait.
2057 * ol
2058 * li If current thread is interrupted, throw InterruptedException.
2059 * li Save lock state returned by {link #getState}.
2060 * li Invoke {link #release} with
2061 * saved state as argument, throwing
2062 * IllegalMonitorStateException if it fails.
2063 * li Block until signalled, interrupted, or timed out.
2064 * li Reacquire by invoking specialized version of
2065 * {link #acquire} with saved state as argument.
2066 * li If interrupted while blocked in step 4, throw InterruptedException.
2067 * /ol
2068 */
2069 public final long awaitNanos(long nanosTimeout)
2070 throws InterruptedException {
2071 if (Thread.interrupted())
2072 throw new InterruptedException();
2073 Node node addConditionWaiter();
2074 int savedState fullyRelease(node);
2075 long lastTime System.nanoTime();
2076 int interruptMode 0;
2077 while (!isOnSyncQueue(node)) {
2078 if (nanosTimeout 0L) {
2079 transferAfterCancelledWait(node);
2080 break;
2081 }
2082 LockSupport.parkNanos(this, nanosTimeout);
2083 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2084 break;
2085
2086 long now System.nanoTime();
2087 nanosTimeout - now - lastTime;
2088 lastTime now;
2089 }
2090 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2091 interruptMode REINTERRUPT;
2092 if (node.nextWaiter ! null)
2093 unlinkCancelledWaiters();
2094 if (interruptMode ! 0)
2095 reportInterruptAfterWait(interruptMode);
2096 return nanosTimeout - (System.nanoTime() - lastTime);
2097 }
2098
2099 /**
2100 * Implements absolute timed condition wait.
2101 * ol
2102 * li If current thread is interrupted, throw InterruptedException.
2103 * li Save lock state returned by {link #getState}.
2104 * li Invoke {link #release} with
2105 * saved state as argument, throwing
2106 * IllegalMonitorStateException if it fails.
2107 * li Block until signalled, interrupted, or timed out.
2108 * li Reacquire by invoking specialized version of
2109 * {link #acquire} with saved state as argument.
2110 * li If interrupted while blocked in step 4, throw InterruptedException.
2111 * li If timed out while blocked in step 4, return false, else true.
2112 * /ol
2113 */
2114 public final boolean awaitUntil(Date deadline)
2115 throws InterruptedException {
2116 if (deadline null)
2117 throw new NullPointerException();
2118 long abstime deadline.getTime();
2119 if (Thread.interrupted())
2120 throw new InterruptedException();
2121 Node node addConditionWaiter();
2122 int savedState fullyRelease(node);
2123 boolean timedout false;
2124 int interruptMode 0;
2125 while (!isOnSyncQueue(node)) {
2126 if (System.currentTimeMillis() abstime) {
2127 timedout transferAfterCancelledWait(node);
2128 break;
2129 }
2130 LockSupport.parkUntil(this, abstime);
2131 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2132 break;
2133 }
2134 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2135 interruptMode REINTERRUPT;
2136 if (node.nextWaiter ! null)
2137 unlinkCancelledWaiters();
2138 if (interruptMode ! 0)
2139 reportInterruptAfterWait(interruptMode);
2140 return !timedout;
2141 }
2142
2143 /**
2144 * Implements timed condition wait.
2145 * ol
2146 * li If current thread is interrupted, throw InterruptedException.
2147 * li Save lock state returned by {link #getState}.
2148 * li Invoke {link #release} with
2149 * saved state as argument, throwing
2150 * IllegalMonitorStateException if it fails.
2151 * li Block until signalled, interrupted, or timed out.
2152 * li Reacquire by invoking specialized version of
2153 * {link #acquire} with saved state as argument.
2154 * li If interrupted while blocked in step 4, throw InterruptedException.
2155 * li If timed out while blocked in step 4, return false, else true.
2156 * /ol
2157 */
2158 public final boolean await(long time, TimeUnit unit)
2159 throws InterruptedException {
2160 if (unit null)
2161 throw new NullPointerException();
2162 long nanosTimeout unit.toNanos(time);
2163 if (Thread.interrupted())
2164 throw new InterruptedException();
2165 Node node addConditionWaiter();
2166 int savedState fullyRelease(node);
2167 long lastTime System.nanoTime();
2168 boolean timedout false;
2169 int interruptMode 0;
2170 while (!isOnSyncQueue(node)) {
2171 if (nanosTimeout 0L) {
2172 timedout transferAfterCancelledWait(node);
2173 break;
2174 }
2175 if (nanosTimeout spinForTimeoutThreshold)
2176 LockSupport.parkNanos(this, nanosTimeout);
2177 if ((interruptMode checkInterruptWhileWaiting(node)) ! 0)
2178 break;
2179 long now System.nanoTime();
2180 nanosTimeout - now - lastTime;
2181 lastTime now;
2182 }
2183 if (acquireQueued(node, savedState) interruptMode ! THROW_IE)
2184 interruptMode REINTERRUPT;
2185 if (node.nextWaiter ! null)
2186 unlinkCancelledWaiters();
2187 if (interruptMode ! 0)
2188 reportInterruptAfterWait(interruptMode);
2189 return !timedout;
2190 }
2191
2192 // support for instrumentation
2193
2194 /**
2195 * Returns true if this condition was created by the given
2196 * synchronization object.
2197 *
2198 * return {code true} if owned
2199 */
2200 final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
2201 return sync AbstractQueuedSynchronizer.this;
2202 }
2203
2204 /**
2205 * Queries whether any threads are waiting on this condition.
2206 * Implements {link AbstractQueuedSynchronizer#hasWaiters}.
2207 *
2208 * return {code true} if there are any waiting threads
2209 * throws IllegalMonitorStateException if {link #isHeldExclusively}
2210 * returns {code false}
2211 */
2212 protected final boolean hasWaiters() {
2213 if (!isHeldExclusively())
2214 throw new IllegalMonitorStateException();
2215 for (Node w firstWaiter; w ! null; w w.nextWaiter) {
2216 if (w.waitStatus Node.CONDITION)
2217 return true;
2218 }
2219 return false;
2220 }
2221
2222 /**
2223 * Returns an estimate of the number of threads waiting on
2224 * this condition.
2225 * Implements {link AbstractQueuedSynchronizer#getWaitQueueLength}.
2226 *
2227 * return the estimated number of waiting threads
2228 * throws IllegalMonitorStateException if {link #isHeldExclusively}
2229 * returns {code false}
2230 */
2231 protected final int getWaitQueueLength() {
2232 if (!isHeldExclusively())
2233 throw new IllegalMonitorStateException();
2234 int n 0;
2235 for (Node w firstWaiter; w ! null; w w.nextWaiter) {
2236 if (w.waitStatus Node.CONDITION)
2237 n;
2238 }
2239 return n;
2240 }
2241
2242 /**
2243 * Returns a collection containing those threads that may be
2244 * waiting on this Condition.
2245 * Implements {link AbstractQueuedSynchronizer#getWaitingThreads}.
2246 *
2247 * return the collection of threads
2248 * throws IllegalMonitorStateException if {link #isHeldExclusively}
2249 * returns {code false}
2250 */
2251 protected final CollectionThread getWaitingThreads() {
2252 if (!isHeldExclusively())
2253 throw new IllegalMonitorStateException();
2254 ArrayListThread list new ArrayListThread();
2255 for (Node w firstWaiter; w ! null; w w.nextWaiter) {
2256 if (w.waitStatus Node.CONDITION) {
2257 Thread t w.thread;
2258 if (t ! null)
2259 list.add(t);
2260 }
2261 }
2262 return list;
2263 }
2264 }
2265
2266 /**
2267 * Setup to support compareAndSet. We need to natively implement
2268 * this here: For the sake of permitting future enhancements, we
2269 * cannot explicitly subclass AtomicInteger, which would be
2270 * efficient and useful otherwise. So, as the lesser of evils, we
2271 * natively implement using hotspot intrinsics API. And while we
2272 * are at it, we do the same for other CASable fields (which could
2273 * otherwise be done with atomic field updaters).
2274 */
2275 private static final Unsafe unsafe Unsafe.getUnsafe();
2276 private static final long stateOffset;
2277 private static final long headOffset;
2278 private static final long tailOffset;
2279 private static final long waitStatusOffset;
2280 private static final long nextOffset;
2281
2282 static {
2283 try {
2284 stateOffset unsafe.objectFieldOffset
2285 (AbstractQueuedSynchronizer.class.getDeclaredField(state));
2286 headOffset unsafe.objectFieldOffset
2287 (AbstractQueuedSynchronizer.class.getDeclaredField(head));
2288 tailOffset unsafe.objectFieldOffset
2289 (AbstractQueuedSynchronizer.class.getDeclaredField(tail));
2290 waitStatusOffset unsafe.objectFieldOffset
2291 (Node.class.getDeclaredField(waitStatus));
2292 nextOffset unsafe.objectFieldOffset
2293 (Node.class.getDeclaredField(next));
2294
2295 } catch (Exception ex) { throw new Error(ex); }
2296 }
2297
2298 /**
2299 * CAS head field. Used only by enq.
2300 */
2301 private final boolean compareAndSetHead(Node update) {
2302 return unsafe.compareAndSwapObject(this, headOffset, null, update);
2303 }
2304
2305 /**
2306 * CAS tail field. Used only by enq.
2307 */
2308 private final boolean compareAndSetTail(Node expect, Node update) {
2309 return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
2310 }
2311
2312 /**
2313 * CAS waitStatus field of a node.
2314 */
2315 private static final boolean compareAndSetWaitStatus(Node node,
2316 int expect,
2317 int update) {
2318 return unsafe.compareAndSwapInt(node, waitStatusOffset,
2319 expect, update);
2320 }
2321
2322 /**
2323 * CAS next field of a node.
2324 */
2325 private static final boolean compareAndSetNext(Node node,
2326 Node expect,
2327 Node update) {
2328 return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
2329 }
2330 } 释放公平锁(基于JDK1.7.0_40) 1. unlock() unlock()在ReentrantLock.java中实现的源码如下 public void unlock() {sync.release(1);
} 说明 unlock()是解锁函数它是通过AQS的release()函数来实现的。 在这里“1”的含义和“获取锁的函数acquire(1)的含义”一样它是设置“释放锁的状态”的参数。由于“公平锁”是可重入的所以对于同一个线程每释放锁一次锁的状态-1。 关于AQS, ReentrantLock 和 sync的关系如下 public class ReentrantLock implements Lock, java.io.Serializable {private final Sync sync;abstract static class Sync extends AbstractQueuedSynchronizer {...}...
} 从中我们发现sync是ReentrantLock.java中的成员对象而Sync是AQS的子类。 2. release() release()在AQS中实现的源码如下 public final boolean release(int arg) {if (tryRelease(arg)) {Node h head;if (h ! null h.waitStatus ! 0)unparkSuccessor(h);return true;}return false;
} 说明 release()会先调用tryRelease()来尝试释放当前线程锁持有的锁。成功的话则唤醒后继等待线程并返回true。否则直接返回false。 3. tryRelease() tryRelease()在ReentrantLock.java的Sync类中实现源码如下 protected final boolean tryRelease(int releases) {// c是本次释放锁之后的状态int c getState() - releases;// 如果“当前线程”不是“锁的持有者”则抛出异常if (Thread.currentThread() ! getExclusiveOwnerThread())throw new IllegalMonitorStateException();boolean free false;// 如果“锁”已经被当前线程彻底释放则设置“锁”的持有者为null即锁是可获取状态。if (c 0) {free true;setExclusiveOwnerThread(null);}// 设置当前线程的锁的状态。setState(c);return free;
} 说明 tryRelease()的作用是尝试释放锁。 (01) 如果“当前线程”不是“锁的持有者”则抛出异常。 (02) 如果“当前线程”在本次释放锁操作之后对锁的拥有状态是0(即当前线程彻底释放该“锁”)则设置“锁”的持有者为null即锁是可获取状态。同时更新当前线程的锁的状态为0。 getState(), setState()在前一章已经介绍过这里不再说明。 getExclusiveOwnerThread(), setExclusiveOwnerThread()在AQS的父类AbstractOwnableSynchronizer.java中定义源码如下 public abstract class AbstractOwnableSynchronizerimplements java.io.Serializable {// “锁”的持有线程private transient Thread exclusiveOwnerThread;// 设置“锁的持有线程”为tprotected final void setExclusiveOwnerThread(Thread t) {exclusiveOwnerThread t;}// 获取“锁的持有线程”protected final Thread getExclusiveOwnerThread() {return exclusiveOwnerThread;}...
} 4. unparkSuccessor() 在release()中“当前线程”释放锁成功的话会唤醒当前线程的后继线程。 根据CLH队列的FIFO规则“当前线程”(即已经获取锁的线程)肯定是head如果CLH队列非空的话则唤醒锁的下一个等待线程。 下面看看unparkSuccessor()的源码它在AQS中实现。 private void unparkSuccessor(Node node) {// 获取当前线程的状态int ws node.waitStatus;// 如果状态0则设置状态0if (ws 0)compareAndSetWaitStatus(node, ws, 0);//获取当前节点的“有效的后继节点”无效的话则通过for循环进行获取。// 这里的有效是指“后继节点对应的线程状态0”Node s node.next;if (s null || s.waitStatus 0) {s null;for (Node t tail; t ! null t ! node; t t.prev)if (t.waitStatus 0)s t;}// 唤醒“后继节点对应的线程”if (s ! null)LockSupport.unpark(s.thread);
} 说明 unparkSuccessor()的作用是“唤醒当前线程的后继线程”。后继线程被唤醒之后就可以获取该锁并恢复运行了。 关于node.waitStatus的说明请参考“上一章关于Node类的介绍”。 总结 “释放锁”的过程相对“获取锁”的过程比较简单。释放锁时主要进行的操作是更新当前线程对应的锁的状态。如果当前线程对锁已经彻底释放则设置“锁”的持有线程为null设置当前线程的状态为空然后唤醒后继线程。
