JUC 是学习 Java 并发编程的小伙伴不可避免的一个 pkg，JUC提供了对并发编程的底层支持，比如我们熟悉的线程池、MQ、线程同步... 都有JUC的影子，下面我们一起来看看JUC下比较重要的几个class。
CountdownLatch
先看一下 latch 是什么意思：
你们都知道门闩。假设我们现在有一扇铁门。每条线跑到铁门时都会脱下门闩。当所有锁闩都从门上取下时，门打开，线程可以继续执行。
[En]
You all know the latch. Suppose we have an iron door now. Each thread takes off a latch when it runs to the iron door. When all the latches are taken off the door, the door opens and the thread can continue to execute.
```java
public class CountdownLatchTest {
public static void main(String[] args) throws InterruptedException {
// 传入放多少个门闩
CountDownLatch cdl = new CountDownLatch(10);
// 启 10 个线程
for (int i = 0; i < 10; i++) {
new Thread(() -> {
System.out.println(Thread.currentThread().getName() + " 取下一把门闩");
// 每个线程运行完就取下一把门闩
cdl.countDown();