JUC锁-CountDownLatch(六)

CountDownLatch介绍

CountDownLatch是一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

uml类图

CountDownLatch的数据结构很简单，它是通过”共享锁”实现的。它包含了sync对象，sync是Sync类型。Sync是实例类，它继承于AQS。

CountDownLatch源码分析

1. CountDownLatch(int count)

public CountDownLatch(int count) {if (count < 0) throw new IllegalArgumentException("count < 0");this.sync = new Sync(count);
}

该函数是创建一个Sync对象，而Sync是继承于AQS类。Sync构造函数如下：

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

setState()在AQS中实现，源码如下：

protected final void setState(long newState) {state = newState;
}

说明：在AQS中，state是一个private volatile long类型的对象。对于CountDownLatch而言，state表示的”锁计数器“。CountDownLatch中的getCount()最终是调用AQS中的getState()，返回的state对象，即”锁计数器“。

2.await()

public void await() throws InterruptedException {sync.acquireSharedInterruptibly(1);
}

说明：该函数实际上是调用的AQS的acquireSharedInterruptibly(1);

AQS中的acquireSharedInterruptibly()的源码如下：

public final void acquireSharedInterruptibly(long arg)throws InterruptedException {//如果当前线程是中断状态，则抛出异常InterruptedExceptionif (Thread.interrupted())throw new InterruptedException();//调用tryAcquireShared(arg)尝试获取共享锁,尝试成功则返回//否则就调用doAcquireSharedInterruptibly()if (tryAcquireShared(arg) < 0)doAcquireSharedInterruptibly(arg);
}

tryAcquireShared()在CountDownLatch.java中被重写，它的源码如下：

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

如果”锁计数器=0”，即锁是可获取状态，则返回1；否则，锁是不可获取状态，则返回-1。

doAcquireSharedInterruptibly()方法

private void doAcquireSharedInterruptibly(long arg)throws InterruptedException {// 创建"当前线程"的Node节点，且Node中记录的锁是"共享锁"类型；并将该节点添加到CLH队列末尾。final Node node = addWaiter(Node.SHARED);boolean failed = true;try {for (;;) {// 获取上一个节点。// 如果上一节点是CLH队列的表头，则"尝试获取共享锁"。final Node p = node.predecessor();if (p == head) {long r = tryAcquireShared(arg);if (r >= 0) {setHeadAndPropagate(node, r);p.next = null; // help GCfailed = false;return;}}// (上一节点不是CLH队列的表头) 当前线程一直等待，直到获取到共享锁。// 如果线程在等待过程中被中断过，则再次中断该线程(还原之前的中断状态)。if (shouldParkAfterFailedAcquire(p, node) &&parkAndCheckInterrupt())throw new InterruptedException();}} finally {if (failed)cancelAcquire(node);}
}

doAcquireSharedInterruptibly()会使当前线程一直等待，直到当前线程获取到共享锁(或被中断)才返回。跟共享锁ReadLock的doAcquireShare()方法非常类似，这里就不进行赘述了。

3. countDown()

public void countDown() {sync.releaseShared(1);
}

说明：该函数实际上调用releaseShared(1)释放共享锁。
releaseShared()在AQS中实现，源码如下：

public final boolean releaseShared(int arg) {if (tryReleaseShared(arg)) {doReleaseShared();return true;}return false;
}

说明：releaseShared()的目的是让当前线程释放它所持有的共享锁。
它首先会通过tryReleaseShared()去尝试释放共享锁。尝试成功，则直接返回；尝试失败，则通过doReleaseShared()去释放共享锁。

tryReleaseShared()在CountDownLatch.java中被重写，源码如下：

protected boolean tryReleaseShared(int releases) {// Decrement count; signal when transition to zerofor (;;) {// 获取“锁计数器”的状态int c = getState();if (c == 0)return false;// “锁计数器”-1int nextc = c-1;// 通过CAS函数进行赋值。if (compareAndSetState(c, nextc))return nextc == 0;}
}

说明：tryReleaseShared()的作用是释放共享锁，将“锁计数器”的值-1。

总结：CountDownLatch是通过“共享锁”实现的。在创建CountDownLatch中时，会传递一个int类型参数count，该参数是“锁计数器”的初始状态，表示该“共享锁”最多能被count给线程同时获取。当某线程调用该CountDownLatch对象的await()方法时，该线程会等待“共享锁”可用时，才能获取“共享锁”进而继续运行。而“共享锁”可用的条件，就是“锁计数器”的值为0！而“锁计数器”的初始值为count，每当一个线程调用该CountDownLatch对象的countDown()方法时，才将“锁计数器”-1；通过这种方式，必须有count个线程调用countDown()之后，“锁计数器”才为0，而前面提到的等待线程才能继续运行！

CountDownLatch的使用示例

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;public class CountDownLatchTest1 {private static int LATCH_SIZE = 5;private static CountDownLatch doneSignal;public static void main(String[] args) {try {doneSignal = new CountDownLatch(LATCH_SIZE);// 新建5个任务for(int i=0; i<LATCH_SIZE; i++)new InnerThread().start();System.out.println("main await begin.");// "主线程"等待线程池中5个任务的完成doneSignal.await();System.out.println("main await finished.");} catch (InterruptedException e) {e.printStackTrace();}}static class InnerThread extends Thread{public void run() {try {Thread.sleep(1000);System.out.println(Thread.currentThread().getName() + " sleep 1000ms.");// 将CountDownLatch的数值减1doneSignal.countDown();} catch (InterruptedException e) {e.printStackTrace();}}}
}

运行结果：

main await begin.
Thread-0 sleep 1000ms.
Thread-2 sleep 1000ms.
Thread-1 sleep 1000ms.
Thread-4 sleep 1000ms.
Thread-3 sleep 1000ms.
main await finished.