CountDownLatch允许一个或多个线程等待其他线程完成操作。
    CountDownLatch的构造函数接收一个int类型的参数作为计数器，当我们调用countDown方法时，计数器会减1，await方法会阻塞当前线程，直到计数器变成0。

    1. //countDown最后的执行逻辑
    2. protected boolean tryReleaseShared(int releases) {
    3.     // Decrement count; signal when transition to zero
    4.     for (;;) {
    5.         int c = getState();
    6.         if (c == 0)
    7.             return false;
    8.         int nextc = c-1;
    9.         if (compareAndSetState(c, nextc))
    10.             return nextc == 0;
    11.     }
    12. }
    1. public final void acquireSharedInterruptibly(int arg)
    2.         throws InterruptedException {
    3.     //相应中断
    4.     if (Thread.interrupted())
    5.         throw new InterruptedException();
    6.     if (tryAcquireShared(arg) < 0)
    7.         doAcquireSharedInterruptibly(arg);
    8. }
    9. protected int tryAcquireShared(int acquires) {
    10.     return (getState() == 0) ? 1 : -1;
    11. }