by shihang.mai

使用

CountDownLatch cd = new CountDownLatch(2);
//在需要等待的地方调用，那么该线程阻塞
cd.await();
//其他线程完成任务后调用此就会-1，直到为0，await的地方就会唤醒
cd.countDown();

源码分析

先来看一下构造

public class CountDownLatch {
  public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

  private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

    }
}

调用AQS

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

      protected final void setState(int newState) {
        state = newState;
    }
}

其实就设置了一下state=传入的值

await

public class CountDownLatch {
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
}

继续调用AQS

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
}

tryAcquireShared调用CountDownLatch内部类Sync

private static final class Sync extends AbstractQueuedSynchronizer {

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }
    }

当调用await，getState当然!=0，返回-1,那么继续调用AQS的doAcquireSharedInterruptibly

private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

1. 封装为share Node放入CLH队列
2. 获取前驱节点，当然是head，而tryAcquireShared，state!=0当然返回-1
3. 那么挂起当前线程
4. 当其他线程countDown，知道state==0，那么这个线程唤醒

countDown

public class CountDownLatch {
  public void countDown() {
        sync.releaseShared(1);
    }
}

继续调用AQS

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
}

调用CountDownLatch的内部类Sync

private static final class Sync extends AbstractQueuedSynchronizer {
        protected boolean tryReleaseShared(int releases) {
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

释放的话调用AQS的doReleaseShared

private void doReleaseShared() {
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

每次countDown,会将state-1，知道为0调用AQS的doReleaseShared释放阻塞的线程

总结

1. 初始化时设置state的值
2. 调用await()方法，会加入到等待队列中，线程会被挂起，等待被唤醒。它会等待直到state值为0才继续执行
3. 每次调用countDown()方法时，state减1，当state等于0时，会唤醒AQS等待队列中的线程。