单个线程等待：多个线程(任务)完成后，进行汇总合并

public class TestCountDownLatch1 {
     public static void main(String[] args) throws InterruptedException {
          int count = 3;
          CountDownLatch countDownLatch = new CountDownLatch(count);
          for (int i = 0; i < count; i++) {
               final int index = i;
               new Thread(() -> {
                    try {
                         Thread.sleep(1000 + ThreadLocalRandom.current().nextInt(1000));
                         System.out.println("finish" + index + Thread.currentThread().getName());
                    } catch (InterruptedException e) {
                         e.printStackTrace();
                    }finally{
                        countDownLatch.countDown();
                    }
               }).start();
          }
          countDownLatch.await();// 主线程在阻塞，当计数器==0，就唤醒主线程往下执行。
          System.out.println("主线程:在所有任务运行完成后，进行结果汇总");
     }
}

　　这种场景应该是用的最多了,比如我们打开一个电商的个人中心页面，我们需要调用，用户信息接口、用户订单接口、用户会员信息等接口，然后合并后一起给到前端，假设每个接口最长耗时为1s，如果我们同步调用的话最大耗时时间是3s，如果我们采用异步调用然后合并结果，所以最大的耗时时间是3s。每个接口调用返回数据后调用countDown方法，让计数器进行减1，当把计数器减为0时的这个线程会去唤醒主线程，让它继续往下走。

个人理解：

 long start = System.currentTimeMillis();
        int count = 3;
        CountDownLatch countDownLatch = new CountDownLatch(count);
        for (int i = 0; i < count; i++) {
            final int index = i;
            int finalI = i;
            new Thread(() -> {
                try {
//                    Thread.sleep(1000 + ThreadLocalRandom.current().nextInt(1000));
                    if(0 == finalI){
                        T1();
                    }
                    if(1 == finalI){
                        T2();
                    }
                    if(2 == finalI){
                        T3();
                    }
                    System.out.println("finish" + index + Thread.currentThread().getName());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }finally{
                    countDownLatch.countDown();
                }
            }).start();
        }
        countDownLatch.await();// 主线程在阻塞，当计数器==0，就唤醒主线程往下执行。
        System.out.println("主线程:在所有任务运行完成后，进行结果汇总");
        System.out.println(System.currentTimeMillis()-start);
    }

    public void T1() throws InterruptedException {
        Thread.sleep(1000);
        System.out.println("T1");
    }

    public void T2() throws InterruptedException {
        Thread.sleep(1000);
        System.out.println("T2");
    }

    public void T3() throws InterruptedException {
        Thread.sleep(1000);
        System.out.println("T3");
    }

　　执行结果

地址https://mp.weixin.qq.com/s/ga3x8LYxDMgCzdfn6UUT_w