使用场景：

      一个或N个线程，等待其它线程完成某项操作之后才能继续往下执行。CountDownLatch描述的是，一个或N个线程等待其他线程的关系。

使用方法：

1. 设CountDownLatch个数：CountDownLatch countDownLatch=new CountDownLatch(3);
2. 在等待线程中await：countDownLatch.await();
3. 在其他线程中减少count值：countDownLatch.getCount();
4. 一旦其他线程中的countDownLatch.getCount()的次数为实例化时的count值，就唤醒等待线程

public class T06_TestCountDownLatch {
    public static void main(String[] args) {
        usingJoin();
        usingCountDownLatch();
    }

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

        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();
            });
        }

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

        try {
            latch.await();
        } 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");
    }
}