CountDownLatch1.8源码

雨点打透心脏的1/2处 2022-02-13 03:51 194阅读 0赞

CountDownLatch这个类能够使一个线程等待其他线程完成各自的工作后再执行。例如，应用程序的主线程希望在负责启动框架服务的线程已经启动所有的框架服务之后再执行。

CountDownLatch如何工作？  
CountDownLatch是通过一个计数器来实现的，计数器的初始值为线程的数量。每当一个线程完成了自己的任务后，计数器的值就会减1。当计数器值到达0时，它表示所有的线程已经完成了任务，然后在闭锁上等待的线程就可以恢复执行任务。

API  
countDownLatch.countDown()  
countDownLatch.await();

使用

private static Random random = new Random();
        private static int num = 3;
    
        public static void main(String[] args) {
            CountDownLatch latch = new CountDownLatch(num);
            System.out.println("begin " + new Date());
            System.out.println(latch.getCount());
    
            for (int i = 0; i < num; i++) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            Thread.sleep(1000_1000 + random.nextInt(5000));
                            System.out.println(Thread.currentThread().getName() + " " + new Date());
                            latch.countDown();
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                }).start();
            }
    
            try {
                latch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " " + new Date());
            System.out.println(latch.getCount());
        }

3个线程1000秒后，才结束任务，await等待1000秒

## CountDownLatch构造函数 ##

count赋值给aqs的state同步状态

public class CountDownLatch {
        /**
         * Synchronization control For CountDownLatch.
         * Uses AQS state to represent count.
         */
        private static final class Sync extends AbstractQueuedSynchronizer {
            private static final long serialVersionUID = 4982264981922014374L;
    
            Sync(int count) {
                setState(count);
            }
    	...
    	}
    	
        public CountDownLatch(int count) {
            if (count < 0) throw new IllegalArgumentException("count < 0");
            this.sync = new Sync(count);
        }

AbstractQueuedSynchronizer

/**
         * Sets the value of synchronization state.
         * This operation has memory semantics of a {@code volatile} write.
         * @param newState the new state value
         */
        protected final void setState(int newState) {
            state = newState;
        }

## await ##

CountDownLatch等待同步状态减小到0

/**
         * Causes the current thread to wait until the latch has counted down to
         * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
         */
        public void await() throws InterruptedException {
            sync.acquireSharedInterruptibly(1);
        }

AbstractQueuedSynchronizer

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

CountDownLatch.Sync如果同步状态为0，获取共享锁成功，返回1。否则失败。

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

AbstractQueuedSynchronizer

private volatile int state;
    
        protected final int getState() {
            return state;
        }
    
        /**
         * Acquires in shared interruptible mode.
         * @param arg the acquire argument
         */
        private void doAcquireSharedInterruptibly(int arg)
            throws InterruptedException {
            final Node node = addWaiter(Node.SHARED);//共享模式创建节点，队尾入队
            boolean failed = true;
            try {
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head) {
                        int r = tryAcquireShared(arg);//因为等待1000秒，所以state=3，r=-1，不执行下面代码
                        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);
            }
        }

当前线程创建节点，入队

/**
         * Creates and enqueues node for current thread and given mode.
         *
         * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
         * @return the new node
         */
        private Node addWaiter(Node mode) {
            Node node = new Node(Thread.currentThread(), mode);
            // Try the fast path of enq; backup to full enq on failure
            Node pred = tail;
            if (pred != null) {//这段功能，enq方法中都有的
                node.prev = pred;
                if (compareAndSetTail(pred, node)) {//队列非空，当前节点设为尾节点，链接在前尾节点后面
                    pred.next = node;
                    return node;
                }
            }
            enq(node);
            return node;
        }
    
        /**
         * Inserts node into queue, initializing if necessary. See picture above.
         * @param node the node to insert
         * @return node's predecessor
         */
        private Node enq(final Node node) {
            for (;;) {
                Node t = tail;
                if (t == null) { // Must initialize
                    if (compareAndSetHead(new Node()))//队列为空，创建空的节点，同时设为头结点和尾节点
                        tail = head;
                } else {
                    node.prev = t;
                    if (compareAndSetTail(t, node)) {//队列非空，当前节点设为尾节点，链接在前尾节点后面
                        t.next = node;
                        return t;
                    }
                }
            }
        }

上面r返回-1，获取共享锁失败，就把前节点设为signal值，然后阻塞当前线程

/**
         * Checks and updates status for a node that failed to acquire.
         * Returns true if thread should block. This is the main signal
         * control in all acquire loops.  Requires that pred == node.prev.
         *
         * @param pred node's predecessor holding status
         * @param node the node
         * @return {@code true} if thread should block
         */
        private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
            int ws = pred.waitStatus;//ws=0
            if (ws == Node.SIGNAL)//下一次循环到执行此方法，就返回true
                /*
                 * This node has already set status asking a release
                 * to signal it, so it can safely park.
                 */
                return true;
            if (ws > 0) {
                /*
                 * Predecessor was cancelled. Skip over predecessors and
                 * indicate retry.
                 */
                do {
                    node.prev = pred = pred.prev;
                } while (pred.waitStatus > 0);
                pred.next = node;
            } else {
                /*
                 * waitStatus must be 0 or PROPAGATE.  Indicate that we
                 * need a signal, but don't park yet.  Caller will need to
                 * retry to make sure it cannot acquire before parking.
                 */
                compareAndSetWaitStatus(pred, ws, Node.SIGNAL);//前节点waitStatus自旋锁设为-1，
            }
            return false;
        }

阻塞当前方法，知道其他现场唤醒当前线程，然后检测是否是中断唤醒，然是正常唤醒

/**
         * Convenience method to park and then check if interrupted
         *
         * @return {@code true} if interrupted
         */
        private final boolean parkAndCheckInterrupt() {
            LockSupport.park(this);
            return Thread.interrupted();
        }

当前线程就阻塞在此处，知道1000秒后，state=0

## latch.countDown() ##

aqs的state减1

CountDownLatch

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

AbstractQueuedSynchronizer

/**
         * Releases in shared mode.  Implemented by unblocking one or more
         * threads if {@link #tryReleaseShared} returns true.
         *
         * @param arg the release argument.  This value is conveyed to
         *        {@link #tryReleaseShared} but is otherwise uninterpreted
         *        and can represent anything you like.
         * @return the value returned from {@link #tryReleaseShared}
         */
        public final boolean releaseShared(int arg) {
            if (tryReleaseShared(arg)) {
                doReleaseShared();
                return true;
            }
            return false;
        }

CountDownLatch.Sync从1变成0时，才返回true，否则返回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;
                }
            }
        }

AbstractQueuedSynchronizer

/**
         * Release action for shared mode -- signals successor and ensures
         * propagation. (Note: For exclusive mode, release just amounts
         * to calling unparkSuccessor of head if it needs signal.)
         */
        private void doReleaseShared() {
            /*
             * Ensure that a release propagates, even if there are other
             * in-progress acquires/releases.  This proceeds in the usual
             * way of trying to unparkSuccessor of head if it needs
             * signal. But if it does not, status is set to PROPAGATE to
             * ensure that upon release, propagation continues.
             * Additionally, we must loop in case a new node is added
             * while we are doing this. Also, unlike other uses of
             * unparkSuccessor, we need to know if CAS to reset status
             * fails, if so rechecking.
             */
            for (;;) {
                Node h = head;
                if (h != null && h != tail) {
                    int ws = h.waitStatus;
                    if (ws == Node.SIGNAL) {//头结点是唤醒状态值
                        if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))//把头结点状态值改为0，返回true，！后为false，不执行continue，执行执行下面的唤醒线程方法
                            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;
            }
        }
    
    
        /**
         * Wakes up node's successor, if one exists.
         *
         * @param node the node
         */
        private void unparkSuccessor(Node node) {
            /*
             * If status is negative (i.e., possibly needing signal) try
             * to clear in anticipation of signalling.  It is OK if this
             * fails or if status is changed by waiting thread.
             */
            int ws = node.waitStatus;//-1
            if (ws < 0)
                compareAndSetWaitStatus(node, ws, 0);
    
            /*
             * Thread to unpark is held in successor, which is normally
             * just the next node.  But if cancelled or apparently null,
             * traverse backwards from tail to find the actual
             * non-cancelled successor.
             */
            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);//唤醒await阻塞的线程
        }

## 继续await ##

/**
         * Acquires in shared interruptible mode.
         * @param arg the acquire argument
         */
        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);//主线程唤醒，说明state=0，r=1
                        if (r >= 0) {
                            setHeadAndPropagate(node, r);
                            p.next = null; // help GC
                            failed = false;
                            return;
                        }
                    }
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())//false
                        throw new InterruptedException();
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }
    
        private final boolean parkAndCheckInterrupt() {
            LockSupport.park(this);//唤醒线程，继续执行
            return Thread.interrupted();//非中断，返回false
        }
    
        /**
         * Sets head of queue, and checks if successor may be waiting
         * in shared mode, if so propagating if either propagate > 0 or
         * PROPAGATE status was set.
         *
         * @param node the node
         * @param propagate the return value from a tryAcquireShared
         */
        private void setHeadAndPropagate(Node node, int propagate) {
            Node h = head; // Record old head for check below
            setHead(node);//主线程节点设为头结点
            /*
             * Try to signal next queued node if:
             *   Propagation was indicated by caller,
             *     or was recorded (as h.waitStatus either before
             *     or after setHead) by a previous operation
             *     (note: this uses sign-check of waitStatus because
             *      PROPAGATE status may transition to SIGNAL.)
             * and
             *   The next node is waiting in shared mode,
             *     or we don't know, because it appears null
             *
             * The conservatism in both of these checks may cause
             * unnecessary wake-ups, but only when there are multiple
             * racing acquires/releases, so most need signals now or soon
             * anyway.
             */
            if (propagate > 0 || h == null || h.waitStatus < 0 ||
                (h = head) == null || h.waitStatus < 0) {//propagate =1
                Node s = node.next;
                if (s == null || s.isShared())
                    doReleaseShared();
            }
        }
    
    
        /**
         * Release action for shared mode -- signals successor and ensures
         * propagation. (Note: For exclusive mode, release just amounts
         * to calling unparkSuccessor of head if it needs signal.)
         */
        private void doReleaseShared() {
            /*
             * Ensure that a release propagates, even if there are other
             * in-progress acquires/releases.  This proceeds in the usual
             * way of trying to unparkSuccessor of head if it needs
             * signal. But if it does not, status is set to PROPAGATE to
             * ensure that upon release, propagation continues.
             * Additionally, we must loop in case a new node is added
             * while we are doing this. Also, unlike other uses of
             * unparkSuccessor, we need to know if CAS to reset status
             * fails, if so rechecking.
             */
            for (;;) {
                Node h = head;
                if (h != null && h != tail) {//主线程节点变成头结点，head != null，但是尾节点始终是主线程节点，所以h == tail，此处不成立
                    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 //直接到这里
                    break;
            }
        }