CountDownLatch、Semaphore（信号量）和ReentrantReadWriteLock.ReadLock（读锁）都采用AbstractOwnableSynchronizer中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
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; // sync的锁状态（锁计数）state不为0则尝试取锁失败，进入SyncQueue排队等锁 } protected boolean tryReleaseShared(int releases) { for (;;) { 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; // sync的锁状态（锁计数）state为0则尝试释放锁成功，唤醒在SyncQueue排队等锁的所有节点（线程） } } }
2. CountDownLatch.sync
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 } ... ... }
