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