CycliBarriar和CountdownLatch(计数器)

CyclicBarrier可以重复使用，而CountdownLatch不能重复使用。

countDownLatch这个类使一个线程等待其他线程各自执行完毕再执行。

是通过一个计数器来实现的，计数器的初始值是线程的数量。通过countDown()来减一，等减到0，await()方法就进入等待run状态。

例子:

   public static CountDownLatch countDownLatch = new CountDownLatch(3);

    public static void doCountDownLatch(){

        Thread t1 = new Thread(() -> {
            try {
                System.out.println("进入T1 await中!");
                countDownLatch.await();
                System.out.println("T1 Complete!");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 3; i++) {
                System.out.println("进入T2!");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
                System.out.println("T2 Complete --: " + (3 - i));
            }
        });

        t1.start();
        t2.start();
    }