一个关于wait/notify与锁关系的探究

  wait/notify 机制是解决生产者消费者问题的良药。它的核心逻辑是基于条件变量的锁机制处理。所以,它们到底是什么关系?wait()时是否需要持有锁? notify()是否需要持有锁?先说答案:都需要持有锁。

  wait需要持有锁的原因是,你肯定需要知道在哪个对象上进行等待,如果不持有锁,将无法做到对象变更时进行实时感知通知的作用。与此同时,为了让其他线程可以操作该值的变化,它必须要先释放掉锁,然后在该节点上进行等待。不持有锁而进行wait,可能会导致长眠不起。而且,如果不持有锁,则当wait之后的操作,都可能是错的,因为可能这个数据已经过时,其实也叫线程不安全了。总之,一切为了安全,单独的wait做不成这事。

  notify需要持有锁的原因是,它要保证线程的安全,只有它知道数据变化了,所以它有权力去通知其他线程数据变化。而且通知完之后,不能立即释放锁,即必须在持有锁的情况下进行通知,否则notify后续的工作的线程安全性将无法保证,尽量它是在lock的范围内,但却因为锁释放,将导致不可预期的结果。而且在notify的时候,并不能真正地将对应的线程唤醒,即不能从操作系统层面唤醒线程,因为此时当前通知线程持有锁,而此时如果将其他等待线程唤醒,它们将立即参与到锁的竞争中来,而这时的竞争是一定会失败的,这可能会导致被唤醒的线程立即又进入等待队列,更糟糕的是它可能再也不会被唤醒 了。所以不能将在持有锁的时,将对应的线程真正唤醒,我们看到的notify只是从语言上下文级别,将它从等待队列转移到同步队列而已,对此操作系统一无所知。

 

1. 实验验证

  我们通过一个实验来看一下,wait/和notify是否会在持有锁的情况下进行。

    private ReentrantLock mainLock = new ReentrantLock();

    @Test
    public void testWaitNotify() throws InterruptedException {
        Condition c1 = mainLock.newCondition();
        Condition c3 = mainLock.newCondition();

        CountDownLatch t1StartLatch = new CountDownLatch(2);
        Thread t1 = new Thread(() -> {
            mainLock.lock();
            try {
                System.out.println(LocalDateTime.now() + " - t1 start");
                c1.await();
                System.out.println(LocalDateTime.now() + " - t1 c1 await out");
                // 过早通知问题,导致无法测试下一步
//                c3.await();
//                System.out.println(LocalDateTime.now() + " - t1 c2 await out");
                t1StartLatch.await();
                System.out.println(LocalDateTime.now() + " - t1 sleeping");
                SleepUtil.sleepMillis(10_000L);
                c1.signalAll();
                c3.signalAll();
                System.out.println(LocalDateTime.now() + " - t1 notified, sleeping again");
                SleepUtil.sleepMillis(10_000L);
                System.out.println(LocalDateTime.now() + " - t1 out");
            }
            catch (Exception e) {
                System.err.println("t1 exception ");
                e.printStackTrace();
            }
            finally {
                mainLock.unlock();
            }
        }, "t1");
        Thread t2 = new Thread(() -> {
            mainLock.lock();
            try {
                t1StartLatch.countDown();
                System.out.println(LocalDateTime.now() + " - t2 c1 signal");
                c1.signalAll();
                System.out.println(LocalDateTime.now() + " - t2 wait");
                c1.await();
                System.out.println(LocalDateTime.now() + " - t2 out");
            }
            catch (Exception e) {
                System.err.println("t2 exception ");
                e.printStackTrace();
            }
            finally {
                mainLock.unlock();
            }
        }, "t2");
        Thread t3 = new Thread(() -> {
            mainLock.lock();
            try {
                t1StartLatch.countDown();
                System.out.println(LocalDateTime.now() + " - t3 c3 signal");
                c3.signalAll();
                System.out.println(LocalDateTime.now() + " - t3 wait");
                c3.await();
                System.out.println(LocalDateTime.now() + " - t3 out");
            }
            catch (Exception e) {
                System.err.println("t2 exception ");
                e.printStackTrace();
            }
            finally {
                mainLock.unlock();
            }
        }, "t3");
        t1.start();
        t2.start();
        t3.start();
        t1.join();
        System.out.println(LocalDateTime.now() + " - main t1 out");
        t2.join();
        System.out.println(LocalDateTime.now() + " - main t2 out");
        t3.join();
        System.out.println(LocalDateTime.now() + " - main t3 out");
    }

  大概意思是,针对同一个锁,wait之后,是否可以被其他线程进入临界区?如果wait之前不通知进入,wait之后能进入,说明wait依赖于锁,而且会释放当前锁。notify之后,wait()是否会立即执行,如果必须等到notify的模块完成后,才执行,说明notify是必须要依赖于锁的。

  结果如下:

2022-03-27T20:09:43.588 - t1 start
2022-03-27T20:09:43.603 - t2 c1 signal
2022-03-27T20:09:43.603 - t2 wait
2022-03-27T20:09:43.603 - t3 c3 signal
2022-03-27T20:09:43.603 - t3 wait
2022-03-27T20:09:43.603 - t1 c1 await out
2022-03-27T20:09:43.603 - t1 sleeping
2022-03-27T20:09:53.605 - t1 notified, sleeping again
2022-03-27T20:10:03.612 - t1 out
2022-03-27T20:10:03.612 - t2 out
2022-03-27T20:10:03.612 - main t1 out
2022-03-27T20:10:03.612 - t3 out
2022-03-27T20:10:03.612 - main t2 out
2022-03-27T20:10:03.612 - main t3 out


2022-03-27T20:11:39.982 - t1 start
2022-03-27T20:11:39.982 - t2 c1 signal
2022-03-27T20:11:39.982 - t2 wait
2022-03-27T20:11:39.982 - t3 c3 signal
2022-03-27T20:11:39.982 - t3 wait
2022-03-27T20:11:39.982 - t1 c1 await out
2022-03-27T20:11:39.982 - t1 sleeping
2022-03-27T20:11:49.989 - t1 notified, sleeping again
2022-03-27T20:11:59.990 - t1 out
2022-03-27T20:11:59.990 - t2 out
2022-03-27T20:11:59.990 - main t1 out
2022-03-27T20:11:59.990 - t3 out
2022-03-27T20:11:59.990 - main t2 out
2022-03-27T20:11:59.990 - main t3 out

  

2. wait/notify 的实现机制

  我们以AQS的实现机制为线索,探索wait/notify机制。它在唤醒操作队列时,设置状态为 SIGNAL , 但它实际不执行操作系统唤醒。

        //     java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#signalAll
        /**
         * Moves all threads from the wait queue for this condition to
         * the wait queue for the owning lock.
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }

        // java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#doSignalAll
        /**
         * Removes and transfers all nodes.
         * @param first (non-null) the first node on condition queue
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }
    // java.util.concurrent.locks.AbstractQueuedSynchronizer#transferForSignal
    /**
     * Transfers a node from a condition queue onto sync queue.
     * Returns true if successful.
     * @param node the node
     * @return true if successfully transferred (else the node was
     * cancelled before signal)
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        // 不到万不得已,不会真正唤醒等待中的队列,从而满足notify无法将线程唤醒的作用,或者说线程仍然在操作系统的等待队列上
        // 它只是将当前线程移动到本语文的同步队列中,以下线程下次运行过来时可以通过该限制
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }
    
    /**
     * 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()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

        // java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#await()
        /**
         * Implements interruptible condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled or interrupted.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            // 进来等待队列,先释放锁,此时进入线程不安全状态
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            // 此判断只是本语文级别的等待队列限制
            // notify 时只能满足这个条件,而不会将线程从操作系统挂起队列中唤醒,即不会进行 LockSupport.unpark()
            while (!isOnSyncQueue(node)) {
                // 交由操作系统进行线程挂起
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            // 重新进行锁的获取,尝试
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }
    // java.util.concurrent.locks.AbstractQueuedSynchronizer#acquireQueued
    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     *
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                // 获取当锁,则替换head后返回
                // 而 tryAcquire() 则由各自策略实现
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 如果获取不到锁,则重新进入操作系统等待队列
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

  所以,总结:

1. wait将会释放持有的锁;
2. wait将会加入到语言级别的等待队列,同时也会提交给操作系统的等待队列,做到真正的线程挂起;
3. wait将会在被操作系统唤醒后,重新进行新一轮的锁获取尝试,返回时已携带回原有的锁,从外部看起来就像锁一直都在一样;
4. notify不会真正的唤醒等待的线程,而只是将各等待线程从语言级别的等待队列移出,到语言级别的同步队列;
5. notify只有在极端情况下,才会做到线程的真正唤醒作用,比如中断,但这被唤醒的线程将无法正常进行业务操作,所以也是安全的;
6. 只有在整体的锁在进行 unlock() 的时候,才会唤醒线程,使其重新参与锁的竞争;

  

3. lock/unlock 流程

  同样的AQS的实现为线索,lock/unlock 流程如下:

    // java.util.concurrent.locks.ReentrantLock#lock
    /**
     * Acquires the lock.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds the lock then the hold
     * count is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until the lock has been acquired,
     * at which time the lock hold count is set to one.
     */
    public void lock() {
        sync.lock();
    }
    
        // java.util.concurrent.locks.ReentrantLock.NonfairSync#lock
        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }
    // java.util.concurrent.locks.AbstractQueuedSynchronizer#acquire
    /**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            // 同上wait时的锁争抢操作
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
    
    // java.util.concurrent.locks.ReentrantLock#unlock
    /**
     * Attempts to release this lock.
     *
     * <p>If the current thread is the holder of this lock then the hold
     * count is decremented.  If the hold count is now zero then the lock
     * is released.  If the current thread is not the holder of this
     * lock then {@link IllegalMonitorStateException} is thrown.
     *
     * @throws IllegalMonitorStateException if the current thread does not
     *         hold this lock
     */
    public void unlock() {
        sync.release(1);
    }
    
    // java.util.concurrent.locks.AbstractQueuedSynchronizer#release
    /**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryRelease} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            // 直接唤醒头节点(真正的唤醒)
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
    
    // java.util.concurrent.locks.AbstractQueuedSynchronizer#unparkSuccessor
    /**
     * 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;
        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;
        }
        // 真正唤醒线程,只有一个线程将被唤醒
        if (s != null)
            LockSupport.unpark(s.thread);
    }

  总结: lock/unlock 是一个真正的上锁解锁操作,上锁时如未成功,则进行park()进行操作系统挂起,解锁时将头节点unpark()交由操作系统调度。

 

4. 唤醒多个等待线程

  如何唤醒多个等待线程?共享锁有这个需求,其实也是notifyAll 的表面语义所在。

    // java.util.concurrent.locks.AbstractQueuedSynchronizer#releaseShared
    /**
     * 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;
    }

    // java.util.concurrent.locks.AbstractQueuedSynchronizer#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) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    // 唤醒头节点
                    unparkSuccessor(h);
                }
                // 因为上一头节点刚刚被设置为0,说明正在执行中,设置当前head为 PROPAGATE
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            // 即尽量只设置一个 head 节点即可
            // 除非在这期间发生变更
            if (h == head)                   // loop if head changed
                break;
        }
    }


    // java.util.concurrent.locks.AbstractQueuedSynchronizer#acquireSharedInterruptibly
    /**
     * Acquires in shared mode, aborting if interrupted.  Implemented
     * by first checking interrupt status, then invoking at least once
     * {@link #tryAcquireShared}, returning on success.  Otherwise the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted.
     * @param arg the acquire argument.
     * This value is conveyed to {@link #tryAcquireShared} but is
     * otherwise uninterpreted and can represent anything
     * you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
    // java.util.concurrent.locks.AbstractQueuedSynchronizer#doAcquireSharedInterruptibly
    /**
     * 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);
                    if (r >= 0) {
                        // 共享式锁的传播性质实现
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    // java.util.concurrent.locks.AbstractQueuedSynchronizer#setHeadAndPropagate
    /**
     * 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);
        /*
         * 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) {
            Node s = node.next;
            // 递归进行唤醒下一线程节点,从而级联唤醒
            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) {
                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: 多个线程的唤醒,主要是使用了级联唤醒的机制,在做共享锁时,根据现有的情况,进行唤醒下一线程。而当线程调度很快或算法不确定时,就会给人一种所有线程一起被唤醒工作的效果。

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