同步锁基本原理与实现

　　为充分利用机器性能，人们发明了多线程。但同时带来了线程安全问题，于是人们又发明了同步锁。

　　这个问题自然人人知道，但你真的了解同步锁吗？还是说你会用其中的上锁与解锁功能？

　　今天我们就一起来深入看同步锁的原理和实现吧！

 

一、同步锁的职责

　　同步锁的职责可以说就一个，限制资源的使用（线程安全从属）。

　　它一般至少会包含两个功能: 1. 给资源加锁； 2. 给资源解锁；另外，它一般还有 等待/通知 即 wait/notify 的功能；

　　同步锁的应用场景：多个线程同时操作一个事务必须保证正确性；一个资源只能同时由一线程访问操作；一个资源最多只能接入k的并发访问；保证访问的顺序性；

　　同步锁的实现方式：操作系统调度实现；应用自行实现；CAS自旋；

　　同步锁的几个问题：

　　　　为什么它能保证线程安全？

　　　　锁等待耗CPU吗？

　　　　使用锁后性能下降严重的原因是啥？

 

二、同步锁的实现一：lock/unlock

　　其实对于应用层来说，非常多就是 lock/unlock , 这也是锁的核心。

　　AQS 是java中很多锁实现的基础，因为它屏蔽了很多繁杂而底层的阻塞操作，为上层抽象出易用的接口。

　　我们就以AQS作为跳板，先来看一下上锁的过程。为不至于陷入具体锁的业务逻辑中，我们先以最简单的 CountDownLatch 看看。

    // 先看看 CountDownLatch 的基础数据结构，可以说是不能再简单了，就继承了 AQS，然后简单覆写了几个必要方法。
    // java.util.concurrent.CountDownLatch.Sync
    /**
     * 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);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            // 只有一种情况会获取锁成功，即 state == 0 的时候
            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;
                // 原始的锁数量是在初始化时指定的不可变的，每次释放一个锁标识
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    // 只有一情况会释放锁成功，即本次释放后 state == 0
                    return nextc == 0;
            }
        }
    }
    private final Sync sync;

 

重点1，我们看看上锁过程，即 await() 的调用。

    public void await() throws InterruptedException {
        // 调用 AQS 的接口，由AQS实现了锁的骨架逻辑
        sync.acquireSharedInterruptibly(1);
    }
    
    // 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();
        // 首先尝试获取锁，如果成功就不用阻塞了
        // 而从上面的逻辑我们看到，获取锁相当之简单，所以，获取锁本身并没有太多的性能消耗哟
        // 如果获取锁失败，则会进行稍后尝试，这应该是复杂而精巧的
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
    
    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        // 首先将当前线程添加排队队尾，此处会保证线程安全，稍后我们可以看到
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                // 获取其上一节点，如果上一节点是头节点，就代表当前线程可以再次尝试获取锁了
                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;
                    }
                }
                // 先检测是否需要阻塞，然后再进行阻塞等待，阻塞由 LockSupport 底层支持
                // 如果阻塞后，将不会主动唤醒，只会由 unlock 时，主动被通知
                // 因此，此处即是获取锁的最终等待点
                // 操作系统将不会再次调度到本线程，直到获取到锁
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    // 如此线程安全地添加当前线程到队尾？ CAS 保证
    /**
     * 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) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                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()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }
    
    // 检测是否需要进行阻塞
    /**
     * 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;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
             // 只有前置节点是 SIGNAL 状态的节点，才需要进行 阻塞等待，当然前置节点会在下一次循环中被设置好
            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);
        }
        return false;
    }
    
    // park 阻塞实现
    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        // 将当前 AQS 实例作为锁对象 blocker, 进行操作系统调用阻塞, 所以所有等待锁的线程将会在同一个锁前提下执行
        LockSupport.park(this);
        return Thread.interrupted();
    }

　　如上，上锁过程是比较简单明了的。加入一队列，然后由操作系统将线程调出。（那么操作系统是如何把线程调出的呢？有兴趣自行研究）

 

重点2. 解锁过程，即 countDown() 调用

    public void countDown() {
        // 同样直接调用 AQS 的接口，由AQS实现了锁的释放骨架逻辑
        sync.releaseShared(1);
    }
    // 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) {
        // 调用业务实现的释放逻辑，如果成功，再执行底层的释放，如队列移除，线程通知等等
        // 在 CountDownLatch 的实现中，只有 state == 0 时才会成功，所以它只会执行一次底层释放
        // 这也是我们认为 CountDownLatch 能够做到多线程同时执行的效果的原因之一
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    
    /**
     * 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;
            // 队列不为空才进行释放
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                // 看过上面的 lock 逻辑，我们知道只要在阻塞状态，一定是 Node.SIGNAL 
                if (ws == Node.SIGNAL) {
                    // 状态改变成功，才进行后续的唤醒逻辑
                    // 因为先改变状态成功，才算是线程安全的，再进行唤醒，否则进入下一次循环再检查
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    // 将头节点的下一节点唤醒，如有必要
                    unparkSuccessor(h);
                }
                // 这里的 propagates, 是要传播啥呢？？
                // 为什么只唤醒了一个线程，其他线程也可以动了？
                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;
        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.
         */
        // 唤醒下一个节点
        // 但如果下一节点已经取消等待了，那么就找下一个没最近的没被取消的线程进行唤醒
        // 唤醒只是针对一个线程的哟
        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);
    }

 

重要3. 线程解锁的传播性？

　　因为从上一节的讲解中，我们看到，当用户调用 countDown 时，仅仅是让操作系统唤醒了 head 的下一个节点线程或者最近未取消的节点。那么，从哪里来的所有线程都获取了锁从而运行呢？

　　其实是在 获取锁的过程中，还有一点我们未看清：

    // java.util.concurrent.locks.AbstractQueuedSynchronizer#doAcquireShared
    /**
     * Acquires in shared uninterruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    // 当countDown被调用后，head节点被唤醒，执行
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        // 获取到锁后，设置node为下一个头节点，并把唤醒状态传播下去，而这里面肯定会做一些唤醒其他线程的操作，请看下文
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    /**
     * 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) {
            // 如果有必要，则做一次唤醒下一线程的操作
            // 在 countDown() 不会触发此操作，所以这里只是一个内部调用传播
            Node s = node.next;
            if (s == null || s.isShared())
                // 此处锁释放逻辑如上，总之，又是另一次的唤醒触发
                doReleaseShared();
        }
    }

　　到此，我们明白了它是怎么做到一个锁释放，所有线程可通行的。也从根本上回答了我们猜想，所有线程同时并发运行。然而并没有，它只是通过唤醒传播性来依次唤醒各个等待线程的。从绝对时间性上来讲，都是有先后关系的。以后可别再浅显说是同时执行了哟。

 

三、 锁的切换：wait/notify

　　上面看出，针对一个lock/unlock 的过程还是很简单的，由操作系统负责大头，实现代码也并不多。

　　但是针对稍微有点要求的场景，就会进行条件式的操作。比如：持有某个锁运行一段代码，但是，运行时发现某条件不满足，需要进行等待而不能直接结束，直到条件成立。即所谓的 wait 操作。

　　乍一看，wait/notify 与 lock/unlock 很像，其实不然。区分主要是 lock/unlock 是针对整个代码段的，而 wait/notify 则是针对某个条件的，即获取了锁不代表条件成立了，但是条件成立了一定要在锁的前提下才能进行安全操作。

　　那么，是否 wait/notify 也一样的实现简单呢？比如java的最基础类 Object 类就提供了 wait/notify 功能。

　　我们既然想一探究竟，还是以并发包下的实现作为基础吧，毕竟 java 才是我们的强项。

　　本次，咱们以  ArrayBlockingQueue#put/take 作为基础看下这种场景的使用先。

　　ArrayBlockingQueue 的put/take 特性就是，put当队列满时，一直阻塞，直到有可用位置才继续运行下一步。而take当队列为空时一样阻塞，直到队列里有数据才运行下一步。这种场景使用锁主不好搞了，因为这是一个条件判断。put/take 如下：

    // java.util.concurrent.ArrayBlockingQueue#put
    /**
     * Inserts the specified element at the tail of this queue, waiting
     * for space to become available if the queue is full.
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            // 当队列满时，一直等待
            while (count == items.length)
                notFull.await();
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }
    
    // java.util.concurrent.ArrayBlockingQueue#take
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            // 当队列为空时一直等待
            while (count == 0)
                notEmpty.await();
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

　　看起来相当简单，完全符合人类思维。只是，这里使用的两个变量进行控制流程 notFull,notEmpty. 这两个变量是如何进行关联的呢？

　　在这之前，我们还需要补充下上面的例子，即 notFull.await(), notEmpty.await(); 被阻塞了，何时才能运行呢？如上代码在各自的入队和出队完成之后进行通知就可以了。

    // 与 put 对应，入队完成后，队列自然就不为空了，通知下 notEmpty 就好了
    /**
     * Inserts element at current put position, advances, and signals.
     * Call only when holding lock.
     */
    private void enqueue(E x) {
        // assert lock.getHoldCount() == 1;
        // assert items[putIndex] == null;
        final Object[] items = this.items;
        items[putIndex] = x;
        if (++putIndex == items.length)
            putIndex = 0;
        count++;
        // 我已放入一个元素，不为空了
        notEmpty.signal();
    }
    // 与 take 对应，出队完成后，自然就不可能是满的了，至少一个空余空间。
    /**
     * Extracts element at current take position, advances, and signals.
     * Call only when holding lock.
     */
    private E dequeue() {
        // assert lock.getHoldCount() == 1;
        // assert items[takeIndex] != null;
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        E x = (E) items[takeIndex];
        items[takeIndex] = null;
        if (++takeIndex == items.length)
            takeIndex = 0;
        count--;
        if (itrs != null)
            itrs.elementDequeued();
        // 我已移除一个元素，肯定没有满了，你们继续放入吧
        notFull.signal();
        return x;
    }

　　是不是超级好理解。是的。不过，我们不是想看 ArrayBlockingQueue 是如何实现的，我们是要论清 wait/notify 是如何实现的。因为毕竟，他们不是一个锁那么简单。

    // 三个锁的关系，即 notEmpty, notFull 都是 ReentrantLock 的条件锁，相当于是其子集吧
    /** Main lock guarding all access */
    final ReentrantLock lock;

    /** Condition for waiting takes */
    private final Condition notEmpty;

    /** Condition for waiting puts */
    private final Condition notFull;
    
    public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }
    // lock.newCondition() 是什么鬼？它是 AQS 中实现的 ConditionObject
    // java.util.concurrent.locks.ReentrantLock#newCondition
    public Condition newCondition() {
        return sync.newCondition();
    }
        // java.util.concurrent.locks.ReentrantLock.Sync#newCondition
        final ConditionObject newCondition() {
            // AQS 中定义
            return new ConditionObject();
        }

　　接下来，我们要带着几个疑问来看这个 Condition 的对象：

　　　　1. 它的 wait/notify 是如何实现的？
　　　　2. 它是如何与互相进行联系的？
　　　　3. 为什么 wait/notify 必须要在外面的lock获取之后才能执行？
　　　　4. 它与Object的wait/notify 有什么相同和不同点？

　　能够回答了上面的问题，基本上对其原理与实现也就理解得差不多了。

 

重点1. wait/notify 是如何实现的？

　　我们从上面可以看到，它是通过调用 await()/signal() 实现的，到底做事如何，且看下面。

        // java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#await()
        /**
         * Implements interruptible condition wait.
         * 

         * 
 If current thread is interrupted, throw InterruptedException.
         * 
 Save lock state returned by {@link #getState}.
         * 
 Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * 
 Block until signalled or interrupted.
         * 
 Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * 
 If interrupted while blocked in step 4, throw InterruptedException.
         * 
         */
        public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            // 添加当前线程到 等待线程队列中，有 lastWaiter/firstWaiter 维护
            Node node = addConditionWaiter();
            // 释放当前lock中持有的锁，详情且看下文
            int savedState = fullyRelease(node);
            // 从以下开始，将不再保证线程安全性，因为当前的锁已经释放，其他线程将会重新竞争锁使用
            int interruptMode = 0;
            // 循环判定，如果当前节点不在 sync 同步队列中，那么就反复阻塞自己
            // 所以判断是否在 同步队列上，是很重要的
            while (!isOnSyncQueue(node)) {
                // 没有在同步队列，阻塞
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            // 当条件被满足后，需要重新竞争锁，详情看下文
            // 竞争到锁后，原样返回到 wait 的原点，继续执行业务逻辑
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            // 下面是异常处理，忽略
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }
    /**
     * Invokes release with current state value; returns saved state.
     * Cancels node and throws exception on failure.
     * @param node the condition node for this wait
     * @return previous sync state
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            int savedState = getState();
            // 预期的，都是释放锁成功，如果失败，说明当前线程并并未获取到锁，引发异常
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }
    /**
     * 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) {
        // tryRelease 由客户端自定义实现
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
    
    // 如何判定当前线程是否在同步队列中或者可以进行同步队列？
    /**
     * Returns true if a node, always one that was initially placed on
     * a condition queue, is now waiting to reacquire on sync queue.
     * @param node the node
     * @return true if is reacquiring
     */
    final boolean isOnSyncQueue(Node node) {
        // 如果上一节点还没有被移除，当前节点就不能被加入到同步队列
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        // 如果当前节点的下游节点已经存在，则它自身必定已经被移到同步队列中
        if (node.next != null) // If has successor, it must be on queue
            return true;
        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
         // 最终直接从同步队列中查找，如果找到，则自身已经在同步队列中
        return findNodeFromTail(node);
    }

    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * @return true if present
     */
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }
    
    // 当条件被满足后，需要重新竞争锁，以保证外部的锁语义，因为之前自己已经将锁主动释放
    // 这个锁与 lock/unlock 时的一毛一样，没啥可讲的
    // 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();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

　　总结一下 wait 的逻辑:

　　　　1. 前提：自身已获取到外部锁；
　　　　2. 将当前线程添加到 ConditionQueue 等待队列中；
　　　　3. 释放已获取到的锁；
　　　　4. 反复检查进入等待，直到当前节点被移动到同步队列中；
　　　　5. 条件满足被唤醒，重新竞争外部锁，成功则返回，否则继续阻塞；(外部锁是同一个，这也是要求两个对象必须存在依赖关系的原因)
　　　　6. wait前线程持有锁，wait后线程持有锁，没有一点外部锁变化；

 

重点2. 厘清了 wait, 接下来，我们看 signal() 通知唤醒的实现：

        // java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#signal
        /**
         * Moves the longest-waiting thread, if one exists, 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 signal() {
            // 只有获取锁的实例，才可以进行signal，否则你拿什么去保证线程安全呢
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            // 通知 firstWaiter 
            if (first != null)
                doSignal(first);
        }
        
        /**
         * Removes and transfers nodes until hit non-cancelled one or
         * null. Split out from signal in part to encourage compilers
         * to inline the case of no waiters.
         * @param first (non-null) the first node on condition queue
         */
        private void doSignal(Node first) {
            // 最多只转移一个 节点
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }
    // 将一个节点从 等待队列 移动到 同步队列中，即可参与下一轮竞争
    // 只有确实移动成功才会返回 true
    // 说明：当前线程是持有锁的线程
    // 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).
         */
        // 同步队列由 head/tail 指针维护
        Node p = enq(node);
        int ws = p.waitStatus;
        // 注意，此处正常情况下并不会唤醒等待线程，仅是将队列转移。 
        // 因为当前线程的锁保护区域并未完成,完成后自然会唤醒其他等待线程
        // 否则将会存在当前线程任务还未执行完成，却被其他线程抢了先去，那接下来的任务当如何？？
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

　　总结一下，notify 的功能原理如下：

　　　　1. 前提：自身已获取到外部锁；
　　　　2. 转移下一个等待队列的节点到同步队列中；
　　　　3. 如果遇到下一节点被取消情况，顺延到再下一节点直到为空，至多转移一个节点；
　　　　4. 正常情况下不做线程的唤醒操作；

　　所以，实现 wait/notify, 最关键的就是维护两个队列，等待队列与同步队列，而且都要求是在有外部锁保证的情况下执行。

　　到此，我们也能回答一个问题：为什么wait/notify一定要在锁模式下才能运行？

　　因为wait是等待条件成立，此时必定存在竞争需要做保护，而它自身又必须释放锁以使外部条件可成立，且后续需要做恢复动作；而notify之后可能还有后续工作必须保障安全，notify只是锁的一个子集。。。

 

四、通知所有线程的实现：notifyAll

　　有时条件成立后，可以允许所有线程通行，这时就可以进行 notifyAll, 那么如果达到通知所有的目的呢？是一起通知还是？？

　　以下是 AQS 中的实现：

        // java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#signalAll
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }
        /**
         * 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);
        }

　　可以看到，它是通过遍历所有节点，依次转移等待队列到同步队列（通知）的，原本就没有人能同时干几件事的！

　　本文从java实现的角度去解析同步锁的原理与实现，但并不局限于java。道理总是相通的，只是像操作系统这样的大佬，能干的活更纯粹：比如让cpu根本不用调度一个线程。