CountDownLatch 使用

java.util.concurrent.CountDownLatch是一个并发结构，它允许一个或多个线程等待一组给定的操作完成。CountDownLatch用给定的计数进行初始化。该计数通过调用countDown（）方法递减。等待此计数达到零的线程可以调用await（）方法之一。调用await（）会阻塞该线程，直到计数达到零。

下面是一个简单的例子。在Decrementer在CountDownLatch上调用countDown（）3次之后，等待的Waiter将从await（）调用中释放。

CountDownLatch latch = new CountDownLatch(3);

Waiter      waiter      = new Waiter(latch);
Decrementer decrementer = new Decrementer(latch);

new Thread(waiter)     .start();
new Thread(decrementer).start();
Thread.sleep(4000);

public class Waiter implements Runnable{

    CountDownLatch latch = null;

    public Waiter(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Waiter Released");
    }
}

public class Decrementer implements Runnable {

    CountDownLatch latch = null;

    public Decrementer(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {

        try {
            Thread.sleep(1000);
            this.latch.countDown();

            Thread.sleep(1000);
            this.latch.countDown();

            Thread.sleep(1000);
            this.latch.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

[原文]（http://tutorials.jenkov.com/java-util-concurrent/countdownlatch.html）