线程同步工具类CountDownLatch

1.概念

在java1.5被引入，存在于java.util.cucurrent包下

CountDownLatch线程同步计数器：使一个线程等待其他线程各自执行完毕后再执行。

是通过计数器来实现的，计数器的初始值是线程的数量。每当一个线程执行完毕后，计数器的值就-1。当计数器的值为0时，在闭锁上等待的线程就可以恢复工作了。

2.源码

countDownLatch类中只提供了一个构造器：

//参数count为计数值
public CountDownLatch(int count) {  };  

类中有三个方法是最重要的：

//调用await()方法的线程会被挂起，它会等待直到count值为0才继续执行
public void await() throws InterruptedException { };   
//和await()类似，只不过等待一定的时间后count值还没变为0的话就会继续执行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };  
//将count值减1
public void countDown() { };  

3.示例

/**
 * 主线程等待子线程执行完成再执行
 */
public class CountdownLatchTest1 {
    public static void main(String[] args) {
        ExecutorService service = Executors.newFixedThreadPool(3);
        final CountDownLatch latch = new CountDownLatch(3);
        for (int i = 0; i < 3; i++) {
            Runnable runnable = new Runnable() {
                @Override
                public void run() {
                    try {
                        System.out.println("子线程" + Thread.currentThread().getName() + "开始执行");
                        Thread.sleep((long) (Math.random() * 10000));
                        System.out.println("子线程"+Thread.currentThread().getName()+"执行完成");
                        latch.countDown();//当前线程调用此方法，则计数减一
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            };
            service.execute(runnable);
        }

        try {
            System.out.println("主线程"+Thread.currentThread().getName()+"等待子线程执行完成...");
            latch.await();//阻塞当前线程，直到计数器的值为0
            System.out.println("主线程"+Thread.currentThread().getName()+"开始执行...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class CountdownLatchTest2 {
    public static void main(String[] args) {
        ExecutorService service = Executors.newCachedThreadPool();
        final CountDownLatch cdOrder = new CountDownLatch(1);
        final CountDownLatch cdAnswer = new CountDownLatch(4);
        for (int i = 0; i < 4; i++) {
            Runnable runnable = new Runnable() {
                @Override
                public void run() {
                    try {
                        System.out.println("选手" + Thread.currentThread().getName() + "正在等待裁判发布口令");
                        cdOrder.await();
                        System.out.println("选手" + Thread.currentThread().getName() + "已接受裁判口令");
                        Thread.sleep((long) (Math.random() * 10000));
                        System.out.println("选手" + Thread.currentThread().getName() + "到达终点");
                        cdAnswer.countDown();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            };
            service.execute(runnable);
        }
        try {
            Thread.sleep((long) (Math.random() * 10000));
            System.out.println("裁判"+Thread.currentThread().getName()+"即将发布口令");
            cdOrder.countDown();
            System.out.println("裁判"+Thread.currentThread().getName()+"已发送口令，正在等待所有选手到达终点");
            cdAnswer.await();
            System.out.println("所有选手都到达终点");
            System.out.println("裁判"+Thread.currentThread().getName()+"汇总成绩排名");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        service.shutdown();
    }
}