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：