CountDownLatch 使用（模拟一场比赛）

java.util.concurrency中的CountDownLatch，主要用于等待一个或多个其他线程完成任务。CountDownLatch在初始化时，会被赋一个整数，每次执行countDown()方法，该整数都会减一，直至到0，这一过程不可逆转。其await()方法会在该整数不为0时当前线程阻塞，为0时当前线程进行下去。阻塞时，其他线程得到执行。

下面是一个普通的案例：
Player

import java.util.Random;
import java.util.concurrent.CountDownLatch;
 
public class Player implements Runnable {
 
    private CountDownLatch begin;
    private CountDownLatch end;
    private String playerNO;
 
    public Player(String playerNO, CountDownLatch begin, CountDownLatch end) {
        this.playerNO = playerNO;
        this.begin = begin;
        this.end = end;
    }
 
    @Override
    public void run() {
        // 等待枪响
        try {
            begin.await();
            long timeUsed = new Random().nextInt(10000);
            Thread.sleep(timeUsed);
            System.out.println("运动员" + playerNO + "耗时" + timeUsed + "完成比赛");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            end.countDown();
        }
 
    }
 
}

OlympicsGame

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
 
public class OlympicsGame {
    private static final int PLAYER_SIZE = 5;
    private CountDownLatch begin;
    private CountDownLatch end;
 
    public CountDownLatch getBegin() {
        return begin;
    }
 
    public void setBegin(CountDownLatch begin) {
        this.begin = begin;
    }
 
    public CountDownLatch getEnd() {
        return end;
    }
 
    public void setEnd(CountDownLatch end) {
        this.end = end;
    }
 
    public OlympicsGame() {
        begin = new CountDownLatch(1);
        end = new CountDownLatch(PLAYER_SIZE);
    }
 
    public static void main(String[] args) {
        // 举办一场比赛
        OlympicsGame olympic = new OlympicsGame();
        // 设定比赛开始，枪声
        CountDownLatch begin = olympic.getBegin();
        // 所有运动员结束比赛，才算结束比赛
        CountDownLatch end = olympic.getEnd();
        // 运动员进场，并编号,等待枪响
        Player[] players = new Player[PLAYER_SIZE];
        ExecutorService ex = Executors.newFixedThreadPool(PLAYER_SIZE);
 
        for (int i = 0; i < 5; i++) {
            players[i] = new Player("NO" + (i + 1), begin, end);
            ex.submit(players[i]);
        }
        // 枪响
        begin.countDown();
 
        try {
            // 等待所有运动员到达终点
            end.await();
            System.out.println("比赛结束，所有运动员完成比赛");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
 
}

某次执行结果是：