CountDownLatch

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http://ifeve.com/talk-concurrency-countdownlatch/

实现原理

CountDownLatch内部实现了一个AQS的实例Sync，CountDownLatch的初始化容量为state状态值，调用countdown时，会转而调用sync.releaseShared(1)，state状态减一，而调用await()时，则转而调用sync.acquireSharedInterruptibly(1)，Sync的tryAcquireShared()实现如下所示。由代码可知，只有state状态为0时，才会获取锁成功。

private static final class Sync extends AbstractQueuedSynchronizer {    private static final long serialVersionUID = 4982264981922014374L;    Sync(int count) {        setState(count);    }    int getCount() {        return getState();    }    protected int tryAcquireShared(int acquires) {        return (getState() == 0) ? 1 : -1;    }    protected boolean tryReleaseShared(int releases) {        // Decrement count; signal when transition to zero        for (;;) {            int c = getState();            if (c == 0)                return false;            int nextc = c-1;            if (compareAndSetState(c, nextc))                return nextc == 0;        }    }}

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