CountDownLatch

1. 简单介绍CountDownLatch的用法

public class TestCountDownLatch {
    public static void main(String[] args) {
        usingJoin();
        usingCountDownLatch();
    }

    private static void usingCountDownLatch() {
        Thread[] threads = new Thread[100];
        CountDownLatch latch = new CountDownLatch(threads.length);//设置为100

        for(int i=0; i<threads.length; i++) {
            threads[i] = new Thread(()->{
                int result = 0;
                for(int j=0; j<10000; j++) result += j;
                latch.countDown(); //每调用一次减一
            });
        }

        for (int i = 0; i < threads.length; i++) {
            threads[i].start();
        }

        try {
            latch.await(); //这边用来阻塞，当100减为0时，就是当100个线程都运行结束，这边才会继续执行下去
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("end latch");
    }

    private static void usingJoin() {
        Thread[] threads = new Thread[100];

        for(int i=0; i<threads.length; i++) {
            threads[i] = new Thread(()->{
                int result = 0;
                for(int j=0; j<10000; j++) result += j;
            });
        }

        for (int i = 0; i < threads.length; i++) {
            threads[i].start();
        }

        for (int i = 0; i < threads.length; i++) {
            try {
                threads[i].join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        System.out.println("end join");
    }
}

上面意思是，当其他线程执行完到latch -> 0,主线程才会在latch.await() 继续执行下去，不然就会等待。
也可以用join()来实现。