Java：多线程，CountDownLatch同步器

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<PER_TABLE_PLAYERS; i++) {
            executorPool.execute(ins.new PokerPlayer(i +"号玩家", countDown));
        }

        executorPool.shutdown();
    }
}

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