java源码 - CountDownLatch

开篇

CountDownLatch是一个同步工具类，用来协调多个线程之间的同步，或者说起到线程之间的通信（而不是用作互斥的作用）。
CountDownLatch能够使一个线程在等待另外一些线程完成各自工作之后，再继续执行。使用一个计数器进行实现。计数器初始值为线程的数量。当每一个线程完成自己任务后，计数器的值就会减一。当计数器的值为0时，表示所有的线程都已经完成了任务，然后在CountDownLatch上等待的线程就可以恢复执行任务。
CountDownLatch是一次性的，计数器的值只能在构造方法中初始化一次，之后没有任何机制再次对其设置值，当CountDownLatch使用完毕后，它不能再次被使用。

CountDownLatch的用法

CountDownLatch典型用法1：某一线程在开始运行前等待n个线程执行完毕。将CountDownLatch的计数器初始化为n new CountDownLatch(n) ，每当一个任务线程执行完毕，就将计数器减1 countdownlatch.countDown()，当计数器的值变为0时，在CountDownLatch上 await() 的线程就会被唤醒。一个典型应用场景就是启动一个服务时，主线程需要等待多个组件加载完毕，之后再继续执行。
CountDownLatch典型用法2：实现多个线程开始执行任务的最大并行性。注意是并行性，不是并发，强调的是多个线程在某一时刻同时开始执行。类似于赛跑，将多个线程放到起点，等待发令枪响，然后同时开跑。做法是初始化一个共享的CountDownLatch(1)，将其计数器初始化为1，多个线程在开始执行任务前首先 coundownlatch.await()，当主线程调用 countDown() 时，计数器变为0，多个线程同时被唤醒。

CountDownLatch的demo

public class CountDownLatchDemo {
    public static void main(String[] args) throws InterruptedException{
        CountDownLatch countDownLatch = new CountDownLatch(2){
            @Override
            public void await() throws InterruptedException {
                super.await();
                System.out.println(Thread.currentThread().getName() +  " count down is ok");
            }
        };
        
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                //do something
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + " is done");
                countDownLatch.countDown();
            }
        }, "thread1");
        
        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + " is done");
                countDownLatch.countDown();
            }
        }, "thread2");
        
        thread1.start();
        thread2.start();
        
        countDownLatch.await();
    }

CountDownLatch的类定义

CountDownLatch内部包含Sync类。
CountDownLatch内部包含Sync类的对象sync。
Sync类继承自AQS(神奇的AQS)，构造函数设置AQS的state值为等待值。

public class CountDownLatch {

    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;
            }
        }
    }

    private final Sync sync;

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

CountDownLatch的等待过程

CountDownLatch通过await()进入等待。
CountDownLatch通过await(long timeout, TimeUnit unit)进入超时等待。

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

    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

CountDownLatch的await()过程

await()通过sync.acquireSharedInterruptibly()获锁。
acquireSharedInterruptibly通过tryAcquireShared()尝试获锁。
tryAcquireShared()判断获锁成功与否的依据是AQS的state的值是否为零。
获锁失败后通过doAcquireSharedInterruptibly()进入锁等待队列CLH。

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


    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        // 尝试获锁失败
        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);
        }
    }

CountDownLatch的await(long timeout, TimeUnit unit)过程

await(long timeout, TimeUnit unit)通过sync.tryAcquireSharedNanos()获锁。
tryAcquireSharedNanos()通过doAcquireSharedNanos()尝试获锁。
tryAcquireShared()判断获锁成功与否的依据是AQS的state的值是否为零。
获锁失败后通过doAcquireSharedNanos()进入锁等待队列CLH，和doAcquireSharedInterruptibly()方法相比增加了超时检测机制，通过LockSupport.parkNanos()实现超时。

    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }



    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 ||
            doAcquireSharedNanos(arg, nanosTimeout);
    }



    private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        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 true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

CountDownLatch的唤醒过程

CountDownLatch通过sync.releaseShared(1)释放锁实现state的递减
tryReleaseShared()方法判断锁状态state==0，递减后值为0说明锁已经被释放。
releaseShared()释放锁成功后通过doReleaseShared()方法唤醒所有等待线程。
doReleaseShared()唤醒锁的过程是一个传播性的唤醒，通过线程A唤醒线程B，然后由线程B唤醒线程C的传播性依次唤醒所有等待线程。

    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) {
        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;
        }
    }

总结

CountDownLatch的工作原理，总结起来就两点（基于AQS实现）：

初始化锁状态的值为需要等待的线程数。
判断锁状态是否已经释放，如果锁未释放所有等待锁的线程就会进入等待的CLH队列。
如果锁状态已经释放，那么就会通过传播性唤醒所有的等待线程。

开篇

CountDownLatch的用法

CountDownLatch的demo

CountDownLatch的类定义

CountDownLatch的等待过程

CountDownLatch的唤醒过程

总结

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