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");

    }

}