58.CountdownLatch

用来进行线程同步协作，等待所有线程完成倒计时。

构造参数用来初始化等待计数值，await方法用来等待计数归零，countDown方法用来让计数减一。

CountdownLatch普通使用

@Slf4j
public class CountdownLatchDemo {

    public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(3);
        new Thread(() -> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            log.debug("t1 ready");
            countDownLatch.countDown();

        }, "t1").start();

        new Thread(() -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            log.debug("t2 ready");
            countDownLatch.countDown();

        }, "t2").start();

        new Thread(() -> {
            try {
                Th