CountDownLatch 使用

CountDownLatch 倒计数器，是多线程并发时使用的类。

主要有2个应用场景：协调子线程结束和子线程开始

场景1：协调子线程结束，等待所有子线程结束

countDown() 执行多次 -》 await() 执行一次

例如：组织4个人开会，所有人员都到齐就开始

public class CountDownLatchTest1 {
    public static void main(String[] args) throws InterruptedException {
        //倒计数4个
        CountDownLatch countDownLatch = new CountDownLatch(4);
        Thread f1 = new Thread(()->{
            countDownLatch.countDown();
            System.out.println("f1 ok");
        });
        Thread f2 = new Thread(()->{
            countDownLatch.countDown();
            System.out.println("f2 ok");

        });
        Thread f3 = new Thread(()->{
            countDownLatch.countDown();
            System.out.println("f3 ok");
        });
        Thread f4 = new Thread(()->{
            try {
                System.out.println("f4在上卫生间，马上到");
                Thread.sleep(1500);
                System.out.println("f4上完卫生间了");
                countDownLatch.countDown();
                System.out.println("f4 ok");
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        });

        f1.start();
        f2.start();
        f3.start();
        f4.start();
        System.out.println("所有人都通知了");
        countDownLatch.await();
        System.out.println("人员到齐开会");
    }
}

 

场景2：协调子线程开始，统一各线程的开始时机

await() 执行多次 -》countDown() 执行一次

例如：比赛时，发令枪响了，就开始

public class CountDownLatchTest2 {
    public static void main(String[] args) throws InterruptedException {
        //这里设置1个计数，所有线程等待这1个计数
        CountDownLatch countDownLatch = new CountDownLatch(1);
        Thread f1 = new Thread(()->{
            System.out.println("f1 等待");
            waitToFight(countDownLatch);
            System.out.println("f1 出发");
        });

        Thread f2 = new Thread(()->{
            System.out.println("f2 等待");
            waitToFight(countDownLatch);
            System.out.println("f2 出发");

        });

        Thread f3 = new Thread(()->{
            System.out.println("f3 等待");
            waitToFight(countDownLatch);
            System.out.println("f3 出发");

        });

        Thread f4 = new Thread(()->{
            System.out.println("f4 等待");
            waitToFight(countDownLatch);
            System.out.println("f4 出发");

        });

        f1.start();
        f2.start();
        f3.start();
        f4.start();
        System.out.println("倒计时5秒");
        Thread.sleep(5000);
        countDownLatch.countDown();
        System.out.println("比赛开始");
    }

    //调用countDownLatch等待
    private static void waitToFight(CountDownLatch countDownLatch){
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }
}

 

核心方法：

方法	功能
await()	等待latch为0
await(long timeout, TimeUnit unit)	等待latch为0，或者超时
countDown()	latch数减1
getCount()	获取当前latch数