石家庄网站推广排名,博客网站做外贸可以吗,网络营销有什么方式,wordpress资源管理我们想要的是#xff1a;允许多个线程同时读#xff0c;但只要有一个线程在写#xff0c;其他线程就必须等待。
ReadWriteLock
ReadWriteLock的作用#xff1a;
只允许一个线程写入#xff08;其他线程既不能写入也不能读取#xff09;#xff1b;没有写入时#xf…我们想要的是允许多个线程同时读但只要有一个线程在写其他线程就必须等待。
ReadWriteLock
ReadWriteLock的作用
只允许一个线程写入其他线程既不能写入也不能读取没有写入时多个线程允许同时读提高性能。如何声明读写锁 private final ReentrantReadWriteLock lock new ReentrantReadWriteLock();private final Lock readLock lock.readLock();private final Lock writeLock lock.writeLock();运行下面的代码发现 读写锁并发的时候读的数据并不是写入的终态数据
package org.meituan.javalearn.thread;import lombok.SneakyThrows;import java.util.LinkedList;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;/*** projectName: codebetter* package: org.meituan.javalearn.thread* className: ReadWriteQueue* author: fangjiayueyuan* description: TODO* date: 2023/5/8 下午10:01* version: 1.0*/
public class ReadWriteQueue {public static void main(String[] args) throws InterruptedException {LinkedListThread threadPools new LinkedListThread();final ReadWriteTaskQueue taskQueue new ReadWriteTaskQueue();for(int i0;i199995;i){threadPools.add(new Thread(new Runnable() {SneakyThrowsOverridepublic void run() {System.out.println(taskQueue.getTask());}}));}Thread addThread new Thread(new Runnable() {Overridepublic void run() {for (int i 0; i 1005; i) {taskQueue.addTask(Theith task to be closed);}}});addThread.start(); // 读写锁无法进行wait和notify???for(Thread thread:threadPools){thread.start();}}
}
class ReadWriteTaskQueue{private final ReentrantReadWriteLock lock new ReentrantReadWriteLock();private final Lock readLock lock.readLock();private final Lock writeLock lock.writeLock();LinkedListString taskQueue new LinkedListString();public void addTask(String task){writeLock.lock();try{taskQueue.add(task);}finally {writeLock.unlock();}}public Integer getTask() throws InterruptedException {readLock.lock();try{while(taskQueue.isEmpty()){return 0;}return taskQueue.size();}finally {readLock.unlock();}}}最初写了一个类ReadWriteTaskQueue如下所示。队列可以实现增加任务和读取任务并打印的功能。但实际上这个类实际上两个功能都在写不是读写锁的应用场景。
package org.meituan.javalearn.thread;
import lombok.SneakyThrows;
import java.util.LinkedList;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;/*** projectName: codebetter* package: org.meituan.javalearn.thread* className: ReadWriteQueue* author: fangjiayueyuan* description: TODO* date: 2023/5/8 下午10:01* version: 1.0*/
public class ReadWriteQueue {public static void main(String[] args) throws InterruptedException {LinkedListThread threadPools new LinkedListThread();final ReadWriteTaskQueue taskQueue new ReadWriteTaskQueue();for(int i0;i15;i){threadPools.add(new Thread(new Runnable() {SneakyThrowsOverridepublic void run() {System.out.println(taskQueue.getTask());}}));}Thread addThread new Thread(new Runnable() {Overridepublic void run() {for (int i 0; i 5; i) {taskQueue.addTask(Theith task to be closed);}}});addThread.start(); // 读写锁无法进行wait和notify???for(Thread thread:threadPools){thread.start();}}
}
class ReadWriteTaskQueue{private final ReentrantReadWriteLock lock new ReentrantReadWriteLock();private final Lock readLock lock.readLock();private final Lock writeLock lock.writeLock();LinkedListString taskQueue new LinkedListString();public void addTask(String task){writeLock.lock();try{taskQueue.add(task);}finally {writeLock.unlock();}}public String getTask() throws InterruptedException {readLock.lock();try{while(taskQueue.isEmpty()){return None;}return taskQueue.remove();}finally {readLock.unlock();}}
}注意ReadWriteLock看起来无法实现线程的协调如果需要做类似于发布-订阅这种模式的消息队列则需要通过Condition实现wait和notify来达到多线程协调的目的。
总结
使用ReadWriteLock可以提高读取效率 – ReadWriteLock只允许一个线程写入 – ReadWriteLock允许多个线程在没有写入时同时读取 – ReadWriteLock适合读多写少的场景。缺点 如果有线程正在读写线程需要等待读线程释放锁后才能获取写锁即读的过程中不允许写这是一种悲观的读锁。
StampedLock
乐观锁的意思就是乐观地估计读的过程中大概率不会有写入因此被称为乐观锁。悲观锁就是在读的过程中拒绝写入显然乐观锁的并发效率更高。但有可能造成读写不一致。因此需要增加一些代码来判断读的状态。 声明StampedLockprivate final StampedLock lock new StampedLock; 读锁需要判断读的时候有没有写入通过盖得“章”Stamp来判断
long stamp lock.tryOptimisticRead();// 先尝试使用乐观锁
if(!validate(stamp)) // 通过Stamp来判断顾名思义StampedLock 已经盖过章的lock
stamp lock.readLock(); // 如果在读的时候进行了写入则需要悲观锁来读一遍 由于大概率读的时候不会写所以大概率都是走的乐观锁因此能提高并发
lock.unlockRead(stamp);// 记得关闭读锁写锁
long stamp lock.writeLock(); // 上写锁
lock.unlockWrite(stamp); // 关闭写锁使用StampedLock对上面的ReadWriteTaskQueue进行改造如下
class ReadWriteTaskQueue{private final StampedLock lock new StampedLock();LinkedListString taskQueue new LinkedListString();public void addTask(String task){ // 写锁long stamp lock.writeLock();try{taskQueue.add(task);}finally {lock.unlockWrite(stamp);}}public Integer getTask() throws InterruptedException {long stamp lock.tryOptimisticRead();if(!lock.validate(stamp)){try{stamp lock.readLock();while(taskQueue.isEmpty()){return 0;}return taskQueue.size();}finally {lock.unlockRead(stamp);}}return taskQueue.size();}
}
