CountDownLatch源碼分析

CountDownLatch、Semaphore（信號量）和ReentrantReadWriteLock.ReadLock（讀鎖）都採用AbstractOwnableSynchronizer共享排隊的方式實現。

關於AbstractQueuedSynchronizer中的獨佔鎖和共享鎖，請參考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的鎖狀態（鎖計數）state = count
    }

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

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

    public void countDown() {
        sync.releaseShared(1); // sync的鎖狀態（鎖計數）state--，見Sync.tryReleaseShared
    }

    ... ...

}

2. CountDownLatch.sync

private static final class Sync extends AbstractQueuedSynchronizer {
    Sync(int count) {
        setState(count); // sync的鎖狀態（鎖計數）state = count
    }

    protected int tryAcquireShared(int acquires) {
        return (getState() == 0) ? 1 : -1; // 若state > 0（嘗試取鎖失敗），則當前線程進入SyncQueue排隊等鎖
    }

    protected boolean tryReleaseShared(int releases) {
        for (;;) {
            // CAS(state)失敗將回到此處
            int c = getState();                 /*記錄state*/
            if (c == 0)
                return false;
            int nextc = c - 1;
            if (compareAndSetState(c, nextc))   /*CAS設置state -= 1*/ 
                return nextc == 0; // 若state爲0（嘗試釋放鎖成功），則喚醒全部在SyncQueue排隊等鎖的節點（線程）
        }
    }
}