java.util.concurrency中的CountDownLatch，主要用于等待一個(gè)或多個(gè)其他線程完成任務(wù)。CountDownLatch在初始化時(shí)逊抡，會(huì)被賦一個(gè)整數(shù)派诬，每次執(zhí)行countDown()方法劳淆，該整數(shù)都會(huì)減一，直至到0默赂，這一過程不可逆轉(zhuǎn)沛鸵。其await()方法會(huì)在該整數(shù)不為0時(shí)當(dāng)前線程阻塞，為0時(shí)當(dāng)前線程進(jìn)行下去。阻塞時(shí)曲掰，其他線程得到執(zhí)行疾捍。

下面是一個(gè)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() {
        // 等待槍響
        try {
            begin.await();
            long timeUsed = new Random().nextInt(10000);
            Thread.sleep(timeUsed);
            System.out.println("運(yùn)動(dòng)員" + playerNO + "耗時(shí)" + timeUsed + "完成比賽");
        } 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) {
        // 舉辦一場(chǎng)比賽
        OlympicsGame olympic = new OlympicsGame();
        // 設(shè)定比賽開始，槍聲
        CountDownLatch begin = olympic.getBegin();
        // 所有運(yùn)動(dòng)員結(jié)束比賽栏妖，才算結(jié)束比賽
        CountDownLatch end = olympic.getEnd();
        // 運(yùn)動(dòng)員進(jìn)場(chǎng)乱豆，并編號(hào),等待槍響
        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]);
        }
        // 槍響
        begin.countDown();
 
        try {
            // 等待所有運(yùn)動(dòng)員到達(dá)終點(diǎn)
            end.await();
            System.out.println("比賽結(jié)束，所有運(yùn)動(dòng)員完成比賽");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
 
}

某次執(zhí)行結(jié)果是：

執(zhí)行結(jié)果