Java - CountDownLatch学习和使用

CountDownLatch常用于多线程开发中，某些线程需要等待一部分线程执行完毕之后再进行。最常见的场景就是一部调用多个接口，然后将返回值汇聚成一个汇总结果。下面将介绍CountDownLatch的使用及原理解析。

1. 使用演示

public class CountDownLatchTest {

    private static CountDownLatch countDownLatch = new CountDownLatch(2);

    public static void main(String[] args) throws InterruptedException {
        new Thread(new MThread()).start();
        new Thread(new MThread()).start();
        Thread.sleep(2000);
        countDownLatch.countDown();
        countDownLatch.countDown();
    }

    public static class MThread implements Runnable{

        @Override
        public void run() {
            try {
                countDownLatch.await();
                System.out.println(Thread.currentThread().getName() + "run结束");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

• 输出

out => 
Thread-0run结束
Thread-1run结束

2. 底层逻辑展示

CountDownLatch底层依赖AQS实现。

1. 初始化时通过构造方法将state赋值。
2. 当有线程(可以是多个)执行await时，进入到等待队列中，并执行LockSupport.park挂起线程。
3. 当有线程执行countDown时，进行CAS的state-1操作。
4. 当线程执行countDown，执行完state-1操作发现state == 0时，执行LockSupport.unpark唤醒等待队列中的全部线程。

下面是流程图:

3. 重点源码说明

• 初始化

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

Sync(int count) {
    setState(count); // 初始化state
}

• 执行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; // 如果state == 0 则直接放行，如果state != 0则阻塞线程
}

private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED); // 创建共享锁节点
    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()) // 执行LockSupport.park阻塞线程
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private final boolean parkAndCheckInterrupt() {
    LockSupport.park(this);// 执行LockSupport.park阻塞线程
    return Thread.interrupted();
}

• 执行coutDown

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

public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) { // 尝试释放共享锁
        doReleaseShared(); // 释放共享锁
        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)) // CAS 执行state - 1
            return nextc == 0; // 如果state == 0，则表示可以释放共享锁
    }
}

private void doReleaseShared() {
    for (;;) { // 唤醒等待队列中的全部阻塞线程
        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;
    }
}