What is the advantage of using a QueudSynchronizer to implement CountLatch

Question

CountLatch is a thread control mechanism whereby a thread (or many threads) can block by calling await() on a CountLatch object, which will release when its countDown() method has been called some number of times.

Since I'm familiar with the concept of thread control with wait() and notify() , there is a (to me) obvious implementation of CountLatch, like this:

private volatile int count; // initialised in constructor

public synchronized void countDown() {
   count--;
   if(count <= 0) {
       notifyAll();
   }
}

public synchronized void await() throws InterruptedException {
    while(count > 0) {
        wait();
    }
}

However, Java 5 provides its own implementation, java.util.concurrent.CountLatch which uses an internal class extended from AbstractQueuedSynchronizer .

private static final class Sync extends AbstractQueuedSynchronizer {
    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;
            }
        }
    }

Java 5 CountLatch is essentially a wrapper around this Sync object:

• countDown() calls sync.releaseShared(1)
• await() calls sync.acquireSharedInterruptibly(1)

What is the advantage of this more complex approach?

Answer 1

您提出的方法与JDK之间的主要区别在于您正在使用锁，而AbstractQueuedSynchronizer是无锁的，并且在内部使用Compare-And-Swap，这在中等争用下提供了更好的性能。