基于AQS分析CountDownLatch的原理

示例代码

先贴一段示例代码

public class TestCountDownLatch {

    public static CountDownLatch countDownLatch = new CountDownLatch(3);

    public static void main(String[] args) throws InterruptedException {
        for (int i=0;i<3;i++){
            new Thread(new Task()).start();
        }
        System.out.println("线程启动结束，主线程进入等待状态");
        countDownLatch.await();
        System.out.println("主线程结束");
    }
}

class Task implements Runnable{
    @Override
    public void run() {
        try {
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName()+" task ");
            TestCountDownLatch.countDownLatch.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

主线程会在countDownLatch.await()这里阻塞，三个Task线程执行完各自的TestCountDownLatch.countDownLatch.countDown()后，主线程继续向下执行。
这个是主线程阻塞等待子线程的例子，当前这个阻塞等待并不拘泥于主线程，可以让任意一个线程进行await，当CountDownLatch计数器归零时线程才会继续

原理

new CountDownLatch(3)

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);
        }

        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;
            }
        }
    }

可以看到CountDownLatch是通过一个继承AQS的内部锁实现的，构造器设定了锁的状态值。

await

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

public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

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

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. 主线程调用await时会尝试去获取share锁，如果此时state！=0，就会进入doAcquireSharedInterruptibly中，这个方法如果看过AQS的实现不会陌生，就是AQS中构造同步队列节点的方法。
2. AQS将当前线程（主线程）构造成一个节点（如果是第一个节点则会构建一个空的头节点），然后主线程会自旋再去尝试获得一次share锁
3. 还是没有获取到，这时候将节点阻塞

countDown

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

public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

 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;
            }
        }

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;
        }
    }

private void unparkSuccessor(Node node) {
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

1. 工作线程每次调用countDown，都会调用releaseShared(1);，即释放了一把共享锁。该方法会CAS更改state值，只有当state减到0时才会返回true，即示例代码中的前两个线程调用该方法只会让state自减1，当第三个线程将state减为0了才会发生下面的动作
2. 调用doReleaseShared，将头节点的WaitStatus更改后进入unparkSuccessor(h)，这个函数会删掉所有取消获取锁的线程节点，同时调用LockSupport.unpark方法将头节点的下个节点（主线程）唤醒
3. 主线程唤醒后又回到了doAcquireSharedInterruptibly中的for(;;)自旋中，这时候由于state=0因此是可以获取到共享锁的，然后会进入到if条件中将自己设置为头节点，并尝试唤醒后面的节点
4. 这时候主线程会返回，也就是从countDownLatch.await();返回可以继续往下执行了

总结

总结下来就是，创建CountDownLatch时会让其自己持有数量n的共享锁，每次countDown就是在释放这个共享锁，await的线程要等到这个共享锁完全被释放了才会返回