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AbstractQueuedSynchronizer是抽象同步队列#xff0c;其是实现同步机器的基础组件#xff0c;并发包中的锁的底层就是使用AQS实现的。AQS中 维护了一个volatile int state#xff08;代表共享资源#xff09;和一个FIFO线程等待队列#xff08;多线程争用资源被阻塞…前言
AbstractQueuedSynchronizer是抽象同步队列其是实现同步机器的基础组件并发包中的锁的底层就是使用AQS实现的。AQS中 维护了一个volatile int state代表共享资源和一个FIFO线程等待队列多线程争用资源被阻塞时会进入此队列。这里volatile能够保证多线程下的可见性当state1则代表当前对象锁已经被占有其他线程来加锁时则会失败加锁失败的线程会被放入一个FIFO的等待队列中并且会被UNSAFE.park()操作挂起等待其他获取锁的线程释放锁才能够被唤醒。另外state的操作都是通过CAS来保证其并发修改的安全性。
一、AQS中的关键成员变量 state 在AQS中维护了一个单一变量state对于不同的实现其有不同的意义在ReentrantLock中state表示重入式锁的可重入次数在ReentrantReadWriteLock中state的高16位用于表示读锁的可获取次数低16位用于表示写锁的可重入次数。 exclusiveOwnerThread 继承自AbstractOwnableSynchronizer用于指明当前独占线程。 head、tail 维护了一个队列分别指向首尾节点 Node Node节点内部的SHARED用来标记该线程是在获取共享资源时被阻塞挂起放入AQS队列的EXCLUSIVE用来标识该线程是获取独占资源时被阻塞挂起放入AQS队列的。在Node节点内部有一个成员变量waitStatus记录当前线程等待状态可以为 1:CANCELLED线程被取消了-1:SIGNAL线程需要唤醒-2:CONDITION线程在条件队列里等待-3:PROPAGATE释放资源时需要通知其他节点 ConditionObject ConditionObject和Node一样是AQS的内部类。它用来结合锁实现线程同步其可以访问AQS的内部变量state和AQS阻塞队列。ConditionObject是条件变量每个条件变量对应一个条件队列我们可以看到ConditionObject中有两个指针分别指向条件队列的队尾和队头。条件队列用来存放调用条件变量的await方法后被阻塞的线程。
二、线程中断相关的三个方法 三、Unsafe与LockSupport
Unsafe
CAS的全称是Compare-And-Swap它是一条CPU并发原语。它的功能是判断内存某个位置是否是预期值如果是则更改为新的值这个过程是原子性的CAS并发原语在 java的体现就是sun.mic.Unsafe类个各个方法调用Unsafe类的方法JVM会帮助我们实现CAS汇编指令。这是一个完全依赖于硬件的功能通过它实现原子性操作。由于CAS是一种系统原语由若干指令组成该原语执行必须连续的不许中断。
这里设置了静态代码块提前获取了state、head、tail、waitStatus、next四个参数在对象内存中的偏移量。 private static final Unsafe unsafe Unsafe.getUnsafe();private static final long stateOffset;private static final long headOffset;private static final long tailOffset;private static final long waitStatusOffset;private static final long nextOffset;static {try {stateOffset unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField(state));headOffset unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField(head));tailOffset unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField(tail));waitStatusOffset unsafe.objectFieldOffset(Node.class.getDeclaredField(waitStatus));nextOffset unsafe.objectFieldOffset(Node.class.getDeclaredField(next));} catch (Exception ex) { throw new Error(ex); }}/*** CAS head field. Used only by enq.*/private final boolean compareAndSetHead(Node update) {return unsafe.compareAndSwapObject(this, headOffset, null, update);}/*** CAS tail field. Used only by enq.*/private final boolean compareAndSetTail(Node expect, Node update) {return unsafe.compareAndSwapObject(this, tailOffset, expect, update);}/*** CAS waitStatus field of a node.*/private static final boolean compareAndSetWaitStatus(Node node,int expect,int update) {return unsafe.compareAndSwapInt(node, waitStatusOffset,expect, update);}/*** CAS next field of a node.*/private static final boolean compareAndSetNext(Node node,Node expect,Node update) {return unsafe.compareAndSwapObject(node, nextOffset, expect, update);}LockSupport
LockSupport是用来创建锁和其他同步类的基本线程阻塞原语。简而言之当调用LockSupport.park时表示当前线程将会等待直至获得许可当调用LockSupport.unpark时必须把等待获得许可的线程作为参数进行传递好让此线程继续运行。
park函数阻塞线程并且该线程在下列情况发生之前都会被阻塞: ① 调用unpark函数释放该线程的许可。② 该线程被中断。③ 设置的时间到了。并且当time为绝对时间时isAbsolute为true否则isAbsolute为false。当time为0时表示无限等待直到unpark发生。unpark函数释放线程的许可即激活调用park后阻塞的线程。这个函数不是安全的调用这个函数时要确保线程依旧存活。
public class LockSupportDemo {public static void main(String[] args) {Thread A new Thread(new Runnable() {Overridepublic void run() {System.out.println(线程A被LockSupport.park()阻塞);LockSupport.park();System.out.println(线程A被线程B LockSupport.unpark()唤醒);}},A);A.start();Thread B new Thread(new Runnable() {Overridepublic void run() {System.out.println(线程B唤醒线程A);// 唤醒指定线程t也就是ALockSupport.unpark(A);}},B)B.start();}
}结果
线程A被LockSupport.park()阻塞
线程B唤醒线程A
线程A被线程B LockSupport.unpark()唤醒四、核心源码
以ReentrantLock为例进行讲解AQS是典型的模板方法的实现所以AQS对外暴露了多个个抽象方法tryAcquire、tryRelease等等需要子类进行实现。ReentrantLock的lock方法实际上调用了sync的lock方法而sync继承了AQS同时针对公平策略和非公平策略有不同的实现。这里我们主要看针对非公平锁NonfairSync的实现。
lock() static final class NonfairSync extends Sync {private static final long serialVersionUID 7316153563782823691L;/*** Performs lock. Try immediate barge, backing up to normal* acquire on failure.*/final void lock() {if (compareAndSetState(0, 1))setExclusiveOwnerThread(Thread.currentThread());elseacquire(1);}protected final boolean tryAcquire(int acquires) {return nonfairTryAcquire(acquires);}}public final void acquire(int arg) {if (!tryAcquire(arg) acquireQueued(addWaiter(Node.EXCLUSIVE), arg))selfInterrupt();}final boolean nonfairTryAcquire(int acquires) {final Thread current Thread.currentThread();int c getState();// 第一次加锁if (c 0) {if (compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current);return true;}}// 持有锁的线程重复加锁else if (current getExclusiveOwnerThread()) {int nextc c acquires;if (nextc 0) // overflowthrow new Error(Maximum lock count exceeded);setState(nextc);return true;}return false;}private Node addWaiter(Node mode) {Node node new Node(Thread.currentThread(), mode);// Try the fast path of enq; backup to full enq on failureNode pred tail;// 获取锁失败再次判断队列是否初始化if (pred ! null) {node.prev pred;if (compareAndSetTail(pred, node)) {pred.next node;return node;}}enq(node);return node;}private Node enq(final Node node) {// 第一次执行也就是head和tail两个指针都为null会初始化两个Nodefor (;;) {Node t tail;// 初始化队列设置一个空Node并将head与tail两个指针同时指向该节点if (t null) { // Must initializeif (compareAndSetHead(new Node()))tail head;// 队列已经初始化完成则将该节点插入队列尾部} else {node.prev t;// 注意此时t仍然指向为尾节点的上一个节点if (compareAndSetTail(t, node)) {t.next node;return t;}}}}final boolean acquireQueued(final Node node, int arg) {boolean failed true;try {boolean interrupted false;for (;;) {// 如果当前节点的前置节点是头节点则意味着本次入队操作是第一次final Node p node.predecessor();// 如果是第一次入队则再次尝试获取stateif (p head tryAcquire(arg)) {setHead(node);p.next null; // help GCfailed false;return interrupted;}// 非第一次入队/第一次入对的第二次循环if (shouldParkAfterFailedAcquire(p, node) parkAndCheckInterrupt())interrupted true;}} finally {if (failed)cancelAcquire(node);}}因为lock方法是不可中断的所以从lock方法中进来构建起来的同步队列不会有CANCELLED状态。CONDITION用于条件队列当中。PROPAGETE是用于共享模式下的状态。 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {// Acquire失败以后是否需要挂起true需要-false不需要// 针对ReentrantLock这里指挥判断SIGNALint ws pred.waitStatus;if (ws Node.SIGNAL)/** This node has already set status asking a release* to signal it, so it can safely park.*/return true;// ws 0 CANCELLEDif (ws 0) {/** Predecessor was cancelled. Skip over predecessors and* indicate retry.*/do {node.prev pred pred.prev;} while (pred.waitStatus 0);pred.next node;} else {/** waitStatus must be 0 or PROPAGATE. Indicate that we* need a signal, but dont park yet. Caller will need to* retry to make sure it cannot acquire before parking.*/compareAndSetWaitStatus(pred, ws, Node.SIGNAL);}return false;}private final boolean parkAndCheckInterrupt() {LockSupport.park(this);return Thread.interrupted();}整体流程
unLock() public void unlock() {sync.release(1);}public final boolean release(int arg) {if (tryRelease(arg)) {Node h head;if (h ! null h.waitStatus ! 0)unparkSuccessor(h);return true;}return false;}protected final boolean tryRelease(int releases) {int c getState() - releases;if (Thread.currentThread() ! getExclusiveOwnerThread())throw new IllegalMonitorStateException();// 标识锁是否释放boolean free false;if (c 0) {free true;setExclusiveOwnerThread(null);}setState(c);return free;}private void unparkSuccessor(Node node) {/** If status is negative (i.e., possibly needing signal) try* to clear in anticipation of signalling. It is OK if this* fails or if status is changed by waiting thread.*/int ws node.waitStatus;if (ws 0)compareAndSetWaitStatus(node, ws, 0);/** Thread to unpark is held in successor, which is normally* just the next node. But if cancelled or apparently null,* traverse backwards from tail to find the actual* non-cancelled successor.*/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;}if (s ! null)LockSupport.unpark(s.thread);}这里线程二被唤醒以后将继续执行acquireQueued方法判断线程二的前置节点是否为head如果是则继续使用tryAcquire()方法来尝试获取锁其实就是使用CAS操作来修改state值如果修改成功则代表获取锁成功。接着将线程二设置为head节点然后空置之前的head节点数据被空置的节点数据等着被垃圾回收。在线程二释放锁以后这个时候CLH队列中就只剩下线程三
五、知识拓展
公平锁与非公平锁
非公平锁执行原理公平锁执行原理 参考内容 https://www.bilibili.com/video/BV1vM411r7Bthttps://juejin.cn/post/6844904146127044622
