1. 背景

CountDownLatch类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

用给定的计数 初始化 CountDownLatch。由于调用了 countDown() 方法，所以在当前计数到达零之前，await 方法会一直受阻塞。之后，会释放所有等待的线程，await 的所有后续调用都将立即返回。这种现象只出现一次——计数无法被重置。

2. 示范代码

下面代码示范了如果扑克游戏一桌人都到齐了，则翻底牌：

package com.clzhang.sample.thread;import java.util.*;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class SyncCountDownLatch {    class PokerPlayer implements Runnable {        private final String[] POINTS = new String[]{"A", "2", "3", "4", "5", "6", "7", "8", "9",                "10", "J", "Q", "K",};        private String name;        private CountDownLatch countDown;        public PokerPlayer(String name, CountDownLatch countDown) {            this.name = name;            this.countDown = countDown;        }        @Override        public void run() {            try {                Thread.sleep((long) (Math.random() * 5000));                                // 随机抽一张牌                Random random = new Random();                String myPoint = POINTS[random.nextInt(13)];                System.out.println(name + "ready!");                                // 准备就绪，等待其它玩家也就绪                countDown.countDown();                countDown.await();                // 玩家都就绪了，翻底牌                System.out.println(name + ":" + myPoint);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }    public static void main(String[] args) {        final int PER_TABLE_PLAYERS = 4; // 多少人够开一桌的         CountDownLatch countDown = new CountDownLatch(PER_TABLE_PLAYERS);                SyncCountDownLatch ins = new SyncCountDownLatch();        ExecutorService executorPool = Executors.newFixedThreadPool(PER_TABLE_PLAYERS);        for(int i=0; i

输出

3号玩家ready!

0号玩家ready!

1号玩家ready!

2号玩家ready!

3号玩家:5

0号玩家:3

1号玩家:7

2号玩家:6