CountDownLatch 闭锁

1.简述

从名字可以看出，CountDownLatch是一个倒数计数的锁， 当倒数到0时触发事件，也就是开锁，其他人就可以进入了。

2.相关类

java.util.concurrent.CountDownLatch

类。

java.util.concurrent.CountDownLatch.CountDownLatch(int count)

构造函数，设置计数器的初始值。

void java.util.concurrent.CountDownLatch.countDown()

每调用一次该方法，计数器减1。

void java.util.concurrent.CountDownLatch.await() 

当前线程被阻塞，直到计数为0.

3.示例代码

package com.yichudu;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/*
下面的例子简单的说明了CountDownLatch的使用方法，模拟了100米赛跑，10名选手已经准备就绪，只等裁判一声令下。当所有人都到达终点时，比赛结束。
*/
public class CountDownLatchDemo {

private static final int PLAY_AMOUNT = 10;

public static void main(String[] args) {

/*
* 比赛开始：只要裁判说开始，那么所有跑步选手就可以开始跑了
*/
CountDownLatch begin = new CountDownLatch(1);

/*
* 每个队员跑到末尾时，则报告一个到达，所有人员都到达时，则比赛结束
*/
CountDownLatch end = new CountDownLatch(PLAY_AMOUNT);
Player[] plays = new Player[PLAY_AMOUNT];
for (int i = 0; i < PLAY_AMOUNT; i++) {
plays[i] = new Player(i + 1, begin, end);
}
ExecutorService exe = Executors.newFixedThreadPool(PLAY_AMOUNT);
for (Player p : plays) {// 各就各位
exe.execute(p);
}
System.out.println("比赛开始");
begin.countDown();// 宣布开始
try {
end.await();// 等待结束
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
System.out.println("比赛结束");
}

// 注意：此时main线程已经要结束了，但是exe线程如果不关闭是不会结束的
exe.shutdown();
}

}

class Player implements Runnable {
private int id;
private CountDownLatch begin;
private CountDownLatch end;

public Player(int id, CountDownLatch begin, CountDownLatch end) {
super();
this.id = id;
this.begin = begin;
this.end = end;
}

public void run() {
try {
begin.await();// 必须等到裁判countdown到0的时候才开始
Thread.sleep((long) (Math.random() * 100));// 模拟跑步需要的时间
System.out.println("Play " + id + " has arrived. ");

} catch (InterruptedException e) {
e.printStackTrace();
} finally {
end.countDown();// 向评委报告跑到终点了
}
}
}
/*
比赛开始
Play 3 has arrived.
Play 4 has arrived.
Play 5 has arrived.
Play 6 has arrived.
Play 1 has arrived.
Play 10 has arrived.
Play 8 has arrived.
Play 9 has arrived.
Play 2 has arrived.
Play 7 has arrived.
比赛结束
*/