CountDownLatch源码分析

实例代码

public static void main(String[] args) throws InterruptedException {
    ExecutorService executorService = Executors.newFixedThreadPool(3);
    //规定几步
    CountDownLatch countDownLatch = new CountDownLatch(3);//构造函数
    executorService.submit(()->{
        System.out.println("饭煮好了");
        countDownLatch.countDown();
    });
    executorService.submit(()->{
        System.out.println("菜炒好了");
        countDownLatch.countDown();
    });
    executorService.submit(()->{
        System.out.println("汤煮好了");
        countDownLatch.countDown();
    });
    countDownLatch.await();
    //只有上面规定几步都执行完成了才会执行下面语句，否则继续等待
    System.out.println("出来吃饭了");
}

构造函数new CountDownLatch(3)

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

Sync代码继承AQS
    private static final class Sync extends AbstractQueuedSynchronizer {
//将AQS的state设置为count
        Sync(int count) {
            setState(count);
        }
}

countDownLatch.countDown()方法

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

    public final boolean releaseShared(int arg) {
//判断是否达到临界点，能否释放锁
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

//主要是利用cas获取
        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;
            }
        }

/*
* 释放锁
*/

    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;
        }
    }

countDownLatch.await();

    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
//判断state是否大于0，如果其他的countdown方法已经执行完了这边会返回1，这个时候这边不会进入该方法，也不会中断线程，直接执行下面的业务逻辑
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

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

    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
//添加一个waiter，设置模式为SHARED(包含初始化waiter队列，可参考AQS文章)
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
//找到头结点
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
//如果state为0则返回1(getState() == 0) ? 1 : -1
                    if (r >= 0) {
//当state为0，则表示waiter可以执行了
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
//停止该线程,唤醒线程也是这边唤醒的
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }