CountDownLatch

跟Semaphore一样，也是通过AQS的共享锁机制实现的，用于一个或多个线程等待其他线程完成的一组操作。原理如下：

1、用AQS的state变量表示操作个数

2、用AQS的共享锁机制完成唤醒

3、等待锁的线程使用acquireShared方法获取共享锁等待

4、操作线程使用releaseShared方法用于唤醒等待共享锁的线程

public class CountDownLatch {
   
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

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

        int getCount() {
            return getState();
        }
		//获取共享锁子类实现
        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;  //AQS state为0时，获取成功，其他失败
        }
		//释放共享锁
        protected boolean tryReleaseShared(int releases) {
            //count递减
            for (;;) {
                int c = getState();
                if (c == 0)  //已经为0了，没有可释放的，返回false
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))  //CAS更新state，为0时唤醒AQS阻塞队列节点
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

  
    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

   
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

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

    public long getCount() {
        return sync.getCount();
    }

}
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