背景

在java.util.concurrent包中，有一个CountDownLatch的多线程同步器。含义参考javadoc的说明如下：

“A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.”

即它是当前线程用于等待若干子线程结束或超时的通知，用于自身继续处理的同步工具。一个典型使用场景是同步转异步处理：主线程的任务，进行拆分后，由子线程异步处理；处理结束后，主线程进行结果合成返回。

关键方法

a) 构造方法：CountDownLatch的构造函数只有一个

public CountDownLatch(int count);

入参为子线程组中子线程的数量。

b) countDown方法：

public void countDown();

当前线程任务已经就绪时调用，触发倒数计数，表示任务已经完成。

c)await方法：

//等待其它线程倒数结束才返回，否则一直阻塞

public void await() throwsInterruptedException;

//等待其它线程倒数结束，或超过指定的时间public boolean await(long timeout, TimeUnit unit) throws InterruptedException;

当前线程等待其它子线程任务结束，如果当前计数为0，则直接返回，否则表明所关联的其它线程任务并没有结束，当前线程将阻塞，直到计数器归0或超时。

一个示例

一个火箭发射任务的示例源代码如下。假定，main方法是用于火箭发射的整体任务，任务发射需要另外两个人进行相应倒数任务的处理，一个人仅负责偶数序号任务处理，另外一个人仅负责奇数序号任务的处理。

importjava.util.concurrent.CountDownLatch;importjava.util.concurrent.ExecutorService;importjava.util.concurrent.Executors;importjava.util.concurrent.atomic.AtomicInteger;public classRocketFire {private static final CountDownLatch beginSignal = new CountDownLatch(1);private static final CountDownLatch endSignal = new CountDownLatch(2);private static final AtomicInteger counter = new AtomicInteger(10);public static void main(String[] args) throwsInterruptedException {

System.out.println("开始倒数");

ExecutorService fixedThreadPool= Executors.newFixedThreadPool(2);

fixedThreadPool.submit(new NumberPrinter(true, beginSignal, endSignal));

fixedThreadPool.submit(new NumberPrinter(false, beginSignal, endSignal));

beginSignal.countDown();

endSignal.await();

System.out.println("点火！！");

fixedThreadPool.shutdown();

}private static class NumberPrinter implementsRunnable {private final booleanevenNumberPrinter;private finalCountDownLatch begin;private finalCountDownLatch end;public NumberPrinter(booleanevenNumberPrinter, CountDownLatch begin, CountDownLatch end) {this.evenNumberPrinter =evenNumberPrinter;this.begin =begin;this.end =end;

}

@Overridepublic voidrun() {try{

begin.await();

}catch(InterruptedException e) {

e.printStackTrace();

}while (true) {synchronized(counter) {int v =counter.get();if (v < 1) {break;

}if ((evenNumberPrinter && v % 2 == 0) || (!evenNumberPrinter && v % 2 == 1)) {

System.out.println(counter.getAndDecrement());

}

}

}

end.countDown();

}

}

}

输出如下：

开始倒数10

9

8

7

6

5

4

3

2

1点火！！