CountDownLatch源码分析

CountDownLatch、Semaphore（信号量）和ReentrantReadWriteLock.ReadLock（读锁）都采用AbstractOwnableSynchronizer共享排队的方式实现。

关于AbstractQueuedSynchronizer中的独占锁和共享锁，请参考ReentrantLock（http://www.cnblogs.com/bjorney/p/8040085.html）和ReentrantReadWriteLock（http://www.cnblogs.com/bjorney/p/8064268.html）

1. CountDownLatch

public class CountDownLatch {
    private final Sync sync;

    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count); // sync的锁状态（锁计数）state = count
    }

    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);  // 参数1并未使用
    }

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

    public void countDown() {
        sync.releaseShared(1); // sync的锁状态（锁计数）state--，见Sync.tryReleaseShared
    }

    ... ...

}

2. CountDownLatch.sync

private static final class Sync extends AbstractQueuedSynchronizer {
    Sync(int count) {
        setState(count); // sync的锁状态（锁计数）state = count
    }

    protected int tryAcquireShared(int acquires) {
        return (getState() == 0) ? 1 : -1; // 若state > 0（尝试取锁失败），则当前线程进入SyncQueue排队等锁
    }

    protected boolean tryReleaseShared(int releases) {
        for (;;) {
            // CAS(state)失败将回到此处
            int c = getState();                 /*记录state*/
            if (c == 0)
                return false;
            int nextc = c - 1;
            if (compareAndSetState(c, nextc))   /*CAS设置state -= 1*/
                return nextc == 0; // 若state为0（尝试释放锁成功），则唤醒所有在SyncQueue排队等锁的节点（线程）
        }
    }
}