CountDownLatch

在java.util.concurrenct这个包下面有很多工具，其中CountDownLatch是一个非常重要的工具类。对于这个类，本文来带大家学习一下。

我们从CountDownLatch这个类的名字就能知道，这个是一个倒计时锁，就是什么意思呢？加入说：我在门上了一把锁，这个锁设定一个数，每达成一个条件则减一，直到这个数字减到了零，则锁打开，可以进入到门里面（程序可以继续执行）。

加锁可以用CountDownLatch.await()方法；减一操作可以用latch.countDown()方法。

闲言碎语不要讲，上代码。

package person.qzq.mutithread.countdownlatch;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;@Slf4j  //这个注释是lombok的注释，请引入lombok包
public class CountDownLatchStudy {public static final int CHANGE_ROUND = 5;public static final int MATCH_ROUND = 6;public static final int TEAM_NUM = 2;public static final String TEAM1 = "AFC Ajax";public static final String TEAM2 = "Machester United";public static final String[] PLAYSINTEAM1 = {"Bertrand Traore", "Frenkie de Jong", "Donny van de Beek", "Davy Klaassen", "Davinson Sanchez", "Joel Veltman"};public static final String[] PLAYSINTEAM2 = {"Wayne Rooney", "Paul Pogba", "Michael Carrick", "Chris Smalling", "Luis Antonio Valencia Mosquera", "Marcos Rojo", "Ander Herrera"};/*** @param args*/public static void main(String[] args) {//设定每次只有一个队员进行主罚ExecutorService executorService = Executors.newFixedThreadPool(1);//当两队十名队员主罚点球之后，进入到规则改变的下一轮CountDownLatch latch = new CountDownLatch(CHANGE_ROUND * TEAM_NUM);for(int i = 0; i < CHANGE_ROUND; i++){executorService.execute(new Worker(TEAM1, PLAYSINTEAM1[i], latch, i));executorService.execute(new Worker(TEAM2, PLAYSINTEAM2[i], latch, i));}try {//一直阻塞，直到两队十名队员点球主罚完毕latch.await();log.info("The team who missed will immediately lose the game after five rounds");} catch (InterruptedException e) {log.error(e.getMessage(), e);}latch = new CountDownLatch((MATCH_ROUND - CHANGE_ROUND) * TEAM_NUM);for(int i = CHANGE_ROUND; i < MATCH_ROUND; i++){executorService.execute(new Worker(TEAM1, PLAYSINTEAM1[i], latch, i));executorService.execute(new Worker(TEAM2, PLAYSINTEAM2[i], latch, i));}try {//主线程阻塞，直到比赛分出胜负latch.await();log.info("The game is over. The winner is {}! Congratuation!!", TEAM2);} catch (InterruptedException e) {log.error(e.getMessage(), e);}executorService.shutdown();}
}@Slf4j
@RequiredArgsConstructor
class Worker implements Runnable{private final String teamName;private final String workerName;private final CountDownLatch latch;private final long index;@Overridepublic void run() {log.info("{}-{}-{}:\"I am penalty kicker.\"", teamName, workerName, index);try {log.info("{}-{}-{}:\"I am thinking which direction to shoot at.\"", teamName, workerName, index);//越到后来主罚点球的队员思考时间越长，压力很大Thread.sleep(index*1000);log.info("{}-{}-{}:\"OK. Decided\"", teamName, workerName, index);if(index >= CountDownLatchStudy.CHANGE_ROUND && teamName.compareTo(CountDownLatchStudy.TEAM1) == 0){//设定阿贾克斯的第六名队员射失点球，有点黑哨的嫌疑log.info("{}-{}-{}:\"Oh. No! I missed\"", teamName, workerName, index);}else {log.info("{}-{}-{}:\"Goal!Goal!Goal!Alei!Alei!Alei!\"", teamName, workerName, index);}} catch (InterruptedException e) {log.error(e.getMessage(), e);} finally{//主罚完毕，计数器减一latch.countDown();  }}
}

执行结果

17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Bertrand Traore-0:"I am penalty kicker."
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Bertrand Traore-0:"I am thinking which direction to shoot at."
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Bertrand Traore-0:"OK. Decided"
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Bertrand Traore-0:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Wayne Rooney-0:"I am penalty kicker."
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Wayne Rooney-0:"I am thinking which direction to shoot at."
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Wayne Rooney-0:"OK. Decided"
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Wayne Rooney-0:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Frenkie de Jong-1:"I am penalty kicker."
17:24:06.789 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Frenkie de Jong-1:"I am thinking which direction to shoot at."
17:24:07.780 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Frenkie de Jong-1:"OK. Decided"
17:24:07.780 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Frenkie de Jong-1:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:07.780 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Paul Pogba-1:"I am penalty kicker."
17:24:07.780 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Paul Pogba-1:"I am thinking which direction to shoot at."
17:24:08.781 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Paul Pogba-1:"OK. Decided"
17:24:08.781 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Paul Pogba-1:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:08.781 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Donny van de Beek-2:"I am penalty kicker."
17:24:08.781 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Donny van de Beek-2:"I am thinking which direction to shoot at."
17:24:10.781 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Donny van de Beek-2:"OK. Decided"
17:24:10.781 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Donny van de Beek-2:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:10.782 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Michael Carrick-2:"I am penalty kicker."
17:24:10.782 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Michael Carrick-2:"I am thinking which direction to shoot at."
17:24:12.782 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Michael Carrick-2:"OK. Decided"
17:24:12.783 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Michael Carrick-2:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:12.783 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davy Klaassen-3:"I am penalty kicker."
17:24:12.783 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davy Klaassen-3:"I am thinking which direction to shoot at."
17:24:15.783 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davy Klaassen-3:"OK. Decided"
17:24:15.783 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davy Klaassen-3:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:15.784 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Chris Smalling-3:"I am penalty kicker."
17:24:15.784 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Chris Smalling-3:"I am thinking which direction to shoot at."
17:24:18.791 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Chris Smalling-3:"OK. Decided"
17:24:18.791 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Chris Smalling-3:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:18.792 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davinson Sanchez-4:"I am penalty kicker."
17:24:18.792 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davinson Sanchez-4:"I am thinking which direction to shoot at."
17:24:22.798 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davinson Sanchez-4:"OK. Decided"
17:24:22.798 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Davinson Sanchez-4:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:22.798 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Luis Antonio Valencia Mosquera-4:"I am penalty kicker."
17:24:22.799 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Luis Antonio Valencia Mosquera-4:"I am thinking which direction to shoot at."
17:24:26.799 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Luis Antonio Valencia Mosquera-4:"OK. Decided"
17:24:26.799 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Luis Antonio Valencia Mosquera-4:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:26.799 [main] INFO person.qzq.mutithread.countdownlatch.CountDownLatchStudy - The team who missed will immediately lose the game after five rounds
17:24:26.799 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Joel Veltman-5:"I am penalty kicker."
17:24:26.799 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Joel Veltman-5:"I am thinking which direction to shoot at."
17:24:31.800 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Joel Veltman-5:"OK. Decided"
17:24:31.800 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - AFC Ajax-Joel Veltman-5:"Oh. No! I missed"
17:24:31.800 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Marcos Rojo-5:"I am penalty kicker."
17:24:31.800 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Marcos Rojo-5:"I am thinking which direction to shoot at."
17:24:36.801 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Marcos Rojo-5:"OK. Decided"
17:24:36.801 [pool-1-thread-1] INFO person.qzq.mutithread.countdownlatch.Worker - Machester United-Marcos Rojo-5:"Goal!Goal!Goal!Alei!Alei!Alei!"
17:24:36.801 [main] INFO person.qzq.mutithread.countdownlatch.CountDownLatchStudy - The game is over. The winner is Machester United! Congratuation!!

代码说明：
这个模拟了足球的点球大战程序，马上不久就要进行欧联杯决赛了，对阵双方是曼联和阿贾克斯两只球队。点球大战的规则是：双方互射5轮点球，如果双方分出胜负，则终止比赛；如果双方射中的数字一致，则进入下一轮，如果有一方射失点球，而另一方射中，则比赛立刻终止。

程序中，在第五轮点球互射完成后，双方比分一致。则打印出进入规则转换的第六轮的日志；这时计数器已经随着比赛的进行，十名球员已经踢完，第一个计数器解锁成功，继续往下执行。

进入第六轮之后，阿贾克斯球员罚失点球，曼联球员罚进点球，第二个计数器解锁成功，打印出获胜方是曼联，恭喜他们的日志。

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