Constructor

public CountDownLatch(int count) {
    if (count < 0) throw new IllegalArgumentException("count < 0");
    // 可以看到CountDownLatch的资源数其实就是Sync的state而已，之后的操作就是围绕这个计数展开
    this.sync = new Sync(count);
}

Sync(int count) {
    setState(count);
}

countDown

public void countDown() {
    sync.releaseShared(1);
}

public final boolean releaseShared(int arg) {
    // 释放共享锁成功
    if (tryReleaseShared(arg)) {
        // 唤醒后继的所有节点
        doReleaseShared();
        return true;
    }
    return false;
}

protected boolean tryReleaseShared(int releases) {
    // 其实就是state自减，一直归零为止
    for (;;) {
        int c = getState();
        if (c == 0)
            return false;
        int nextc = c-1;
        if (compareAndSetState(c, nextc))
            return nextc == 0;
    }
}

await

public void await() throws InterruptedException {
    // AQS托管，也就是当state归零，那么await就结束返回
    sync.acquireSharedInterruptibly(1);
}

public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    // 如果当前线程是中断状态，那么抛出中断异常
    if (Thread.interrupted())
        throw new InterruptedException();
    // 尝试获取共享锁，如果失败，下面开始挂起
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}

protected int tryAcquireShared(int acquires) {
    // 取得共享锁的条件是state归零
    // 否则说明释放的资源还没有回收完毕，那么会，等待上面countdown来唤醒
    return (getState() == 0) ? 1 : -1;
}