一、CountDownLatch介绍

CountDownLatch是一种同步手段，允许一个或者更多的线程等待，直到在其他线程正在执行的一组操作完成。给定count数目后CountDownLatch被初始化。await()方法阻塞，直到由于调用countDown()方法，当前count值达到0，之后所有等待线程被释放，而任何后续await()方法的调用会立即返回。这个是只有一次的现场，即count值无法被重设。如果你需要一个能够重设count值的版本，不妨考虑使用CyclicBarrier。

二、CountDownLatch应用

CountDownLatch是一个通用的同步工具，可用于许多目的。一个用count值为1来初始化的CountDownLatch可用作一个开关或者门闩：所有的线程调用await()方法等待一个线程调用countDown()方法后把门打开。一个用count值为N来初始化的CountDownLatch可用作使得一个线程等待，直到N个线程完成各自的事情，或者一些action被完成N次等。CountDownLatch一个有用的特性是：它不需要线程调用countDown()方法等待计数达到零在继续之前，它只是阻止任何线程继续过去一个等待,直到所有线程可以通过。

1、示例应用一

这里有一对类，一组工作线程使用两个countdown latches的示例：

第一个是启动信号，阻止worker线程工作直到driver准备好；

第二个是完成信号，允许driver等到所有的workers工作完成。

代码如下：

package com.pengli.jdk;import java.util.concurrent.CountDownLatch;public class TestCountDownLatch {class Driver {void main() throws InterruptedException {CountDownLatch startSignal = new CountDownLatch(1);CountDownLatch doneSignal = new CountDownLatch(20);for (int i = 0; i < 20; ++i) // create and start threadsnew Thread(new Worker(startSignal, doneSignal)).start();doSomethingElse(); // don't let run yetstartSignal.countDown(); // let all threads proceeddoSomethingElse();doneSignal.await(); // wait for all to finish}void doSomethingElse() {// ...}}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 {startSignal.await();doWork();doneSignal.countDown();} catch (InterruptedException ex) {} // return;}void doWork() {// ...}}
}

2、示例应用二

另一个典型用法是将一个问题分成n份，在一个线程中定义并执行一份，并在latch中count down，然后将所有的线程放入一个队列。当所有的部分完成，协调线程将会通过await()方法，继续处理。当线程必须以这种方式反复count down时，使用CyclicBarrier。

代码如下：

package com.pengli.jdk;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;public class TestCountDownLatch2 {class Driver2 { // ...void main() throws InterruptedException {CountDownLatch doneSignal = new CountDownLatch(20);Executor e = Executors.newFixedThreadPool(20);for (int i = 0; i < 20; ++i) // create and start threadse.execute(new WorkerRunnable(doneSignal, i));doneSignal.await(); // wait for all to finish}}class WorkerRunnable implements Runnable {private final CountDownLatch doneSignal;private final int i;WorkerRunnable(CountDownLatch doneSignal, int i) {this.doneSignal = doneSignal;this.i = i;}public void run() {doWork(i);doneSignal.countDown();}void doWork(int i) {// ...}}
}

三、CountDownLatch实现分析

1、Sync

在CountDownLatch内部，有一个Sync的同步器，它继承自java.util.concurrent包中各种同步工具共用的AbstractQueuedSynchronizer，其实现如下：

    /*** 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 zerofor (;;) {int c = getState();if (c == 0)return false;int nextc = c-1;if (compareAndSetState(c, nextc))return nextc == 0;}}}

关于AbstractQueuedSynchronizer，有其它的文章进行专门的介绍。这里只分析下Sync的实现。其有一个需要入参int count的构造函数，设置AbstractQueuedSynchronizer的state。并覆写了tryAcquireShared()和tryReleaseShared()方法，其中tryReleaseShared()方法用于CountDownLatch的countDown()方法，这个tryReleaseShared()方法的逻辑如下：

在一个for循环内，首先通过getState()获取state值，如果为0，直接返回false，否则取state-1，并尝试CAS操作，修改state状态，并且state等于0，返回true，否则返回false。

tryAcquireShared()方法更简单，判断state（即count值），如果等于0，返回1，否则返回-1.

2、countDown()

countDown()方法的实现很简单，如下：

    /*** Decrements the count of the latch, releasing all waiting threads if* the count reaches zero.** <p>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.** <p>If the current count equals zero then nothing happens.*/public void countDown() {sync.releaseShared(1);}

其核心处理就是减少latch中count值，如果cout值为0，释放所有的等待线程。它调用的是sync的releaseShared()方法，而这个方法是在AbstractQueuedSynchronizer中实现的，如下：

    /*** Releases in shared mode.  Implemented by unblocking one or more* threads if {@link #tryReleaseShared} returns true.** @param arg the release argument.  This value is conveyed to*        {@link #tryReleaseShared} but is otherwise uninterpreted*        and can represent anything you like.* @return the value returned from {@link #tryReleaseShared}*/public final boolean releaseShared(int arg) {if (tryReleaseShared(arg)) {doReleaseShared();return true;}return false;}

先调用tryReleaseShared()方法，即上述Sync的同名方法，并且如果返回true的话，继续调用doReleaseShared()方法，返回true，否则返回false。即如果修改后state（即count）值为正，不做其他处理，否则调用doReleaseShared()方法。

3、await()

await()方法的核心作用是，让当前线程阻塞，直到latch的count值更改为0，或者当前线程被interrupted。如果count值为0，则await()方法直接返回。代码如下：

    /*** Causes the current thread to wait until the latch has counted down to* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.** <p>If the current count is zero then this method returns immediately.** <p>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:* <ul>* <li>The count reaches zero due to invocations of the* {@link #countDown} method; or* <li>Some other thread {@linkplain Thread#interrupt interrupts}* the current thread.* </ul>** <p>If the current thread:* <ul>* <li>has its interrupted status set on entry to this method; or* <li>is {@linkplain Thread#interrupt interrupted} while waiting,* </ul>* then {@link InterruptedException} is thrown and the current thread's* interrupted status is cleared.** @throws InterruptedException if the current thread is interrupted*         while waiting*/public void await() throws InterruptedException {sync.acquireSharedInterruptibly(1);}

还是借助的sync，调用的其acquireSharedInterruptibly()方法，这个方法是在Sync的父类中实现的，代码如下：

    /*** Acquires in shared mode, aborting if interrupted.  Implemented* by first checking interrupt status, then invoking at least once* {@link #tryAcquireShared}, returning on success.  Otherwise the* thread is queued, possibly repeatedly blocking and unblocking,* invoking {@link #tryAcquireShared} until success or the thread* is interrupted.* @param arg the acquire argument* This value is conveyed to {@link #tryAcquireShared} but is* otherwise uninterpreted and can represent anything* you like.* @throws InterruptedException if the current thread is interrupted*/public final void acquireSharedInterruptibly(int arg)throws InterruptedException {if (Thread.interrupted())throw new InterruptedException();if (tryAcquireShared(arg) < 0)doAcquireSharedInterruptibly(arg);}

先调用Sync的tryAcquireShared()方法，如果返回值为负值，则调用doAcquireSharedInterruptibly()方法。上面讲到了，如果state（即count）值为0，则返回1，方法直接返回，否则进入doAcquireSharedInterruptibly()方法，实现阻塞。

doReleaseShared()和doAcquireSharedInterruptibly()方法的介绍参见AbstractQueuedSynchronizer的分析文章。