概述

允许一个或者多个线程等待其他的线程执行

场景1：A线程必须在B线程或者更多线程执行完才能执行（线程先后执行）
场景2：多个线程并行（不是并发）执行

场景1

    /**
     * 阻塞一个主线程 其他线程 countDown后 唤醒主线程
     */
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(3);
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        for (int i = 0; i < 3; i++) {
            executorService.execute(() -> {
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "线程执行完毕");
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();  //程序运行到此处，主线程开始阻塞，等countDown()三次后，继续执行
        System.out.println("主线程执行完毕");
        executorService.shutdown();
    }

执行结果：

场景2

    /**
     * 阻塞多个子线程 主线程 countDown后 唤醒所有子线程
     */
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(1);
        ExecutorService executorService = Executors.newFixedThreadPool(5);
        for (int i = 0; i < 5; i++) {
            executorService.execute(() -> {
                try {
                    System.err.println("运动员："+Thread.currentThread().getName() + " 准备就绪");
                    countDownLatch.await();  //程序执行到此处，进入阻塞状态
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.err.println("运动员："+Thread.currentThread().getName() + " 开始跑");
            });
        }
        Thread.sleep(1000);
        System.out.println("枪响！！！");
        Thread.sleep(10);
        countDownLatch.countDown(); //countDown() 执行完成后，所有子线程开始 并行执行

        executorService.shutdown();
    }

执行结果：