CountDownLatch的使用方式和场景

一.描述
　 CountDownLatch是一个用来线程同步的工具,CountDownLatch会初始化一个计时器,通过countdown()方法进行计数递减,
并在在计数器归零之前,执行到await()的线程就会进入等待,等计数器归零,等待线程就会继续执行.

二.应用场景
1.在N个线程执行完毕再开始执行某一线程

public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        new Thread(() -> {
            System.out.println("执行线程" + Thread.currentThread().getName());
            countDownLatch.countDown();
        }, "线程A").start();
        new Thread(() -> {
            System.out.println("执行线程" + Thread.currentThread().getName());
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            countDownLatch.countDown();
        }, "线程B").start();
        System.out.println(Thread.currentThread().getName() + "等待 A, B 线程");
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "线程执行结束");
    }

执行效果:

执行线程线程A
main等待 A, B 线程
执行线程线程B
main线程执行结束

　　2.让N个线程在某个条件并行执行

public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(1);
        //AtomicInteger计数,确认两个线程都准备好后开始并行执行
        AtomicInteger atomicInteger = new AtomicInteger();
        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + "线程准备完成");
            atomicInteger.incrementAndGet();
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("执行线程" + Thread.currentThread().getName());
        }, "线程A").start();
        
        new Thread(() -> {
            //本线程延迟两秒
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "线程准备完成");
            atomicInteger.incrementAndGet();
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("执行线程" + Thread.currentThread().getName());
        }, "线程B").start();
        
        while(atomicInteger.get() < 2) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        countDownLatch.countDown();
        System.out.println("A, B线程开始并行执行");
    }

执行效果:

线程A线程准备完成
线程B线程准备完成
A, B线程开始并行执行
执行线程线程A
执行线程线程B