CountDownLatch是Java concurrent包下的一个同步工具。它可以让一个（或多个）线程等待，直到其他线程中的某些操作完成。

本质上是一个信号量，我们把它比作一个有N个插销的大门，它把等待（调用await）的线程挡住了, 直到其他线程把插销去完了（调用countDown减到0），这批线程才能执行。

下面是根据oracle官方文档改的一个例子：

/** *  * Sample usage: Here is a pair of classes in which a group of worker threads use two countdown latches: The first is a start signal that prevents any worker from proceeding until the driver is ready for them to proceed; The second is a completion signal that allows the driver to wait until all workers have completed. */public class CountDownLatchDemo {    public static void main(String[] args) throws InterruptedException {        new Driver().run();    }}class Driver {    static final int N = 5;    public void run() throws InterruptedException {        CountDownLatch startSignal = new CountDownLatch(1);        CountDownLatch doneSignal = new CountDownLatch(N);        for (int i=0; i

把Driver看作是一个监工，Worker看作是工人，有5个。

• 监工发号施令后，工人们才可以干活。所以工人干活前调用startSignal.await()， 直到监工调用startSignal.countDown() 后Worker后续doWork才开始执行。 
• 工人们干完活后通知监工，监工才可以接着发号。所以监工调用doneSignal.await()在哪儿等着，直到工人们都干完活（调用5次doneSignal.countDown()）后，才能接着执行。

看下执行结果，有助于理解整个工作过程和latch机制：

doSomething

worker work.

worker work.

worker work.

worker work.

worker work.

doSomething