CountDownLatch概念

CountDownLatch又被称为倒计时锁，它是一个同步辅助类，它允许一个或多个线程一直等待直到其他线程执行完毕才开始执行。用给定的计数初始化CountDownLatch，其含义是要被等待执行完的线程个数。每次调用CountDown()，计数减1。
主程序执行到await()函数会阻塞等待线程的执行，直到计数为0
我们来看下它的方法。

public CountDownLatch(int count); //指定计数的次数，只能被设置1次

public void countDown();          //调用此方法则计数减1

public void await() throws InterruptedException   //调用此方法会一直阻塞当前线程，直到计时器的值为0，除非线程被中断。

Public Long getCount();           //得到当前的计数

Public boolean await(long timeout, TimeUnit unit) //调用此方法会一直阻塞当前线程，直到计时器的值为0，除非线程被中断或者计数器超时，返回false代表计数器超时。

CountDownLatch实例

模拟10人赛跑。10人跑完后才喊"Game Over."

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
public class CountDownLatchTest {
    private static final int RUNNER_COUNT = 10;
    public static void main(String[] args) throws InterruptedException {
        final CountDownLatch begin = new CountDownLatch(1);
        final CountDownLatch end = new CountDownLatch(10);
        final ExecutorService exec = Executors.newFixedThreadPool(10);
        for (int i = 0; i < RUNNER_COUNT; i++) {
            final int NO = i + 1;
            //从1号到10号线程都在这里被建立
            Runnable run = new Runnable() {
                @Override
                public void run() {
                    try {
                        //所有线程阻塞在这里，等待枪响
                        begin.await();
                        //每个人跑完的时间
                        Thread.sleep((long)(Math.random() * 10000));
                        System.out.println("No." + NO + " arrived");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } finally {
                        //每跑完一个人，倒计时锁减一
                        end.countDown();//（倒计时锁被减一）
                    }
                }
            };
            exec.submit(run);
        }

        System.out.println("Game Start ...");
        //枪响
        begin.countDown();//倒计时锁被解除减一（begin解除）
        //等待所有人跑完
        end.await();//倒计时锁没有被解除之前，将被阻塞在这里
        end.await(30, TimeUnit.SECONDS);
        System.out.println("Game Over.");

        exec.shutdown();
    }

}

输出

Game Start ...
No.6 arrived
No.4 arrived
No.10 arrived
No.3 arrived
No.9 arrived
No.5 arrived
No.8 arrived
No.7 arrived
No.1 arrived
No.2 arrived
Game Over.