JAVA闭锁CountDownLatch详解

闭锁

闭锁是一种同步工具类，可以延迟线程的进度直到其到达终止状态。闭锁的作用相当于一扇门：在闭锁到达结束状态之前，这扇门一直是关闭的，并且没有任何线程能够通过，当到达结束状态时，这扇门会打来并允许所有的线程通过。当闭锁到达结束状态后，将不会再改变状态，因此这扇门将永远保持打开状态。闭锁可以用来确保某些活动直到其他活动都完成后才继续执行，例如：

• 确保某个计算在其需要的所有资源都被初始化之后才继续执行。
• 确保某个服务在其依赖的所有其他服务都已经启动之后才启动。
• 等到直到直到某个操作的所有参与者（例如，在多玩家游戏中的所有玩家）都就绪再继续执行。

CountDownLatch是一种灵活的闭锁实现。闭锁状态包括一个计数器，该计数器被初始化为一个正数，表示需要等待的事件数量。countDown方法递减计数器，表示有一个事件已经发生了，而await方法等待计数器达到零，这表示所有需要等待的事件都已发生。如果计数器的值非零，那么await方法会一直阻塞直到计算器为零，或者等待中的线程中断，或者超时。

下面是CountDownLatch的一个简单例子：运行代码看效果更容易理解
 

package com.joonwhee.imp;
 
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
 
/**
 * CountDownLatch的简单例子
 * @author JoonWhee
 * @Date 2018年1月27日
 */
public class CountDownLatchTest {
    
    static CountDownLatch timeOutCountDownLatch = new CountDownLatch(1);
    
    public static void main(String args[]) {
        try {
            new Driver(10);
            testAwaitTimtOut();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    
    // 测试带超时的await方法
    public static void testAwaitTimtOut() throws InterruptedException {
        System.out.println("before await(long timeout, TimeUnit unit)");
        timeOutCountDownLatch.await(3, TimeUnit.SECONDS);   //等待超时时间为3秒
        System.out.println("after await(long timeout, TimeUnit unit)");
    }
 
}
 
class Driver { 
    public Driver(int N) throws InterruptedException {
        CountDownLatch startSignal = new CountDownLatch(1); // 定义一个CountDownLatch, 计数器值为1, 也就是每次await(), 需要执行1次countDown(), 才能继续执行await()外面的代码
        CountDownLatch doneSignal = new CountDownLatch(N);  // 定义一个CountDownLatch, 计数器值为N, 也就是每次await(), 需要执行N次countDown(), 才能继续执行await()外面的代码
 
        for (int i = 0; i < N; ++i) {
            // 创建并启动线程
            new Thread(new Worker(startSignal, doneSignal)).start();
        }
        Thread.sleep(2000); // 睡眠2秒, 可以看到10个线程都在等待startSignal.countDown()执行
        System.out.println();
        startSignal.countDown(); // 解除所有线程的阻塞
        doneSignal.await(); // 等待所有线程执行doneSignal.countDown(), 才通过
        System.out.println();
        System.out.println("Main thread-after:doneSignal.await()------");
    }
}
 
class Worker implements Runnable {
    private final CountDownLatch startSignal;
    private final CountDownLatch doneSignal;
 
    Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
        this.startSignal = startSignal;
        this.doneSignal = doneSignal;
    }
 
    public void run() {
        try {
            System.out.println(Thread.currentThread().getName() + "-before:startSignal.await()");
            startSignal.await();    // 线程会在此处等待, 直到startSignal.countDown()执行
            System.out.println(Thread.currentThread().getName() + "-after:startSignal.await()");
            doneSignal.countDown();
        } catch (InterruptedException ex) {
        } // return;
    }
}
 
 

注意：

CountDownLatch同步机制Sync也是基于AQS的，但没有公平和非公平模式之分；

    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

常用方法

await()

   /** Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
     *
     * 
If the current count is zero then this method returns immediately.
     *
     * 
If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of two things happen:
     * 

     * 
The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * 
Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * 
     **/

public void await() throws InterruptedException {}

countDown()

   /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     *
     * 
If the current count is greater than zero then it is decremented.
     * If the new count is zero then all waiting threads are re-enabled for
     * thread scheduling purposes.
     *
     * 
If the current count equals zero then nothing happens.
     */
    public void countDown() {}