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：