CountDownLatch实现多线程等待后共同执行

01 CountDownLatch

CountDownLatch 通过 count 计数实现屏障设定（阻塞），当满足设定的条件后一个或者多个线程时候一起执行。这里的屏障指的就是一个线程会被阻塞，不再继续向下执行。

CountDownLatch 中的计数器会做减法。当执行到它的 await（）方法之后，如果计数器不为 0，则阻塞，知道其他线程调用 countDown（）之后扣减计数为 0 之后，原本阻塞的线程才会继续向下执行。

02 缺点

计数不能被重置。这就导致实现过程中不能动态的修改计数，只能按照设定好的计数来使用。

03 常用方法

await() 阻塞并等待计数为0再往下执行。

await(long timeout, TimeUnit unit) 阻塞并等待指定的时间，如果在指定的时间内计数未能到 0，则自动唤醒，向下执行。

countDown() 计数器中的计数减一

getCount() 获得当前计数的值

04 案例

Game.java

package com.page.concurrent.countdownlatch;

import java.util.ArrayList;
import java.util.concurrent.CountDownLatch;

public class Game {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(1);

        ArrayList<Runner> runners = new ArrayList<>();
        for (int i = 0; i < 3; i++) {
            Runner runner = new Runner(countDownLatch);
            runner.start();
            runners.add(runner);
        }

        System.out.println("ready?");
        countDownLatch.await();
        System.out.println("Go!");
    }
}

Runner.java

package com.page.concurrent.countdownlatch;


import java.util.concurrent.CountDownLatch;

public class Runner extends Thread {

    private final CountDownLatch countDownLatch;

    public Runner(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(1000 * 5);
        } catch (InterruptedException e) {
            System.out.println("Error " + e.getMessage());
        }
        System.out.println(Thread.currentThread().getName() + " had ready! Waiting other runners.");
        countDownLatch.countDown();
        System.out.println(Thread.currentThread().getName() + " GO!");
    }
}