构造函数内部，初始化一个Sync(count)

//java.util.concurrent.CountDownLatch

public CountDownLatch(int count) {

this.sync = new Sync(count);

}

private static final class Sync extends AbstractQueuedSynchronizer {

Sync(int count) {

//AQS中的state值,充当计数器

setState(count);

}

}

1.图解AQS框架

2.AQS内部类Node属性介绍

3.countDown()方法里面做了什么？

//java.util.concurrent.CountDownLatch

public void countDown() {

sync.releaseShared(1);

}

//java.util.concurrent.locks.AbstractQueuedSynchronizer

public final boolean releaseShared(int arg) {

//AQS里面的tryReleaseShared需要子类覆写

if (tryReleaseShared(arg)) {

//state为0的时候，去唤醒等待队列中的线程

doReleaseShared();

return true;

}

return false;

}

//java.util.concurrent.CountDownLatch.Sync

protected boolean tryReleaseShared(int releases) {

for (;