同步鎖基本原理與實現

　　爲充分利用機器性能，人們發明了多線程。但同時帶來了線程安全問題，因而人們又發明了同步鎖。

　　這個問題天然人人知道，但你真的瞭解同步鎖嗎？仍是說你會用其中的上鎖與解鎖功能？

　　今天咱們就一塊兒來深刻看同步鎖的原理和實現吧！

 

1、同步鎖的職責

　　同步鎖的職責能夠說就一個，限制資源的使用（線程安全從屬）。

　　它通常至少會包含兩個功能: 1. 給資源加鎖； 2. 給資源解鎖；另外，它通常還有 等待/通知 即 wait/notify 的功能；

　　同步鎖的應用場景：多個線程同時操做一個事務必須保證正確性；一個資源只能同時由一線程訪問操做；一個資源最多隻能接入k的併發訪問；保證訪問的順序性；

　　同步鎖的實現方式：操做系統調度實現；應用自行實現；CAS自旋；

　　同步鎖的幾個問題：

　　　　爲何它能保證線程安全？

　　　　鎖等待耗CPU嗎？

　　　　使用鎖後性能降低嚴重的緣由是啥？

 

2、同步鎖的實現一：lock/unlock

　　其實對於應用層來講，很是多就是 lock/unlock , 這也是鎖的核心。

　　AQS 是java中不少鎖實現的基礎，由於它屏蔽了不少繁雜而底層的阻塞操做，爲上層抽象出易用的接口。

　　咱們就以AQS做爲跳板，先來看一下上鎖的過程。爲不至於陷入具體鎖的業務邏輯中，咱們先以最簡單的 CountDownLatch 看看。

    // 先看看 CountDownLatch 的基礎數據結構，能夠說是不能再簡單了，就繼承了 AQS，而後簡單覆寫了幾個必要方法。
    // 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) {
            // 只有一種狀況會獲取鎖成功，即 state == 0 的時候
            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;
                // 原始的鎖數量是在初始化時指定的不可變的，每次釋放一個鎖標識
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    // 只有一狀況會釋放鎖成功，即本次釋放後 state == 0
                    return nextc == 0;
            }
        }
    }
    private final Sync sync;

 

重點1，咱們看看上鎖過程，即 await() 的調用。

    public void await() throws InterruptedException {
        // 調用 AQS 的接口，由AQS實現了鎖的骨架邏輯
        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();
        // 首先嚐試獲取鎖，若是成功就不用阻塞了
        // 而從上面的邏輯咱們看到，獲取鎖至關之簡單，因此，獲取鎖自己並無太多的性能消耗喲
        // 若是獲取鎖失敗，則會進行稍後嘗試，這應該是複雜而精巧的
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
    
    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        // 首先將當前線程添加排隊隊尾，此處會保證線程安全，稍後咱們能夠看到
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                // 獲取其上一節點，若是上一節點是頭節點，就表明當前線程能夠再次嘗試獲取鎖了
                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;
                    }
                }
                // 先檢測是否須要阻塞，而後再進行阻塞等待，阻塞由 LockSupport 底層支持
                // 若是阻塞後，將不會主動喚醒，只會由 unlock 時，主動被通知
                // 所以，此處便是獲取鎖的最終等待點
                // 操做系統將不會再次調度到本線程，直到獲取到鎖
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    // 如此線程安全地添加當前線程到隊尾？ CAS 保證
    /**
     * 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;
                }
            }
        }
    }
    
    // 檢測是否須要進行阻塞
    /**
     * 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.
             */
             // 只有前置節點是 SIGNAL 狀態的節點，才須要進行 阻塞等待，固然前置節點會在下一次循環中被設置好
            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 阻塞實現
    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        // 將當前 AQS 實例做爲鎖對象 blocker, 進行操做系統調用阻塞, 因此全部等待鎖的線程將會在同一個鎖前提下執行
        LockSupport.park(this);
        return Thread.interrupted();
    }

　　如上，上鎖過程是比較簡單明瞭的。加入一隊列，而後由操做系統將線程調出。（那麼操做系統是如何把線程調出的呢？有興趣自行研究）

 

重點2. 解鎖過程，即 countDown() 調用

    public void countDown() {
        // 一樣直接調用 AQS 的接口，由AQS實現了鎖的釋放骨架邏輯
        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) {
        // 調用業務實現的釋放邏輯，若是成功，再執行底層的釋放，如隊列移除，線程通知等等
        // 在 CountDownLatch 的實現中，只有 state == 0 時纔會成功，因此它只會執行一次底層釋放
        // 這也是咱們認爲 CountDownLatch 可以作到多線程同時執行的效果的緣由之一
        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;
            // 隊列不爲空才進行釋放
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                // 看過上面的 lock 邏輯，咱們知道只要在阻塞狀態，必定是 Node.SIGNAL 
                if (ws == Node.SIGNAL) {
                    // 狀態改變成功，才進行後續的喚醒邏輯
                    // 由於先改變狀態成功，纔算是線程安全的，再進行喚醒，不然進入下一次循環再檢查
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    // 將頭節點的下一節點喚醒，若有必要
                    unparkSuccessor(h);
                }
                // 這裏的 propagates, 是要傳播啥呢？？
                // 爲何只喚醒了一個線程，其餘線程也能夠動了？
                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.
         */
        // 喚醒下一個節點
        // 但若是下一節點已經取消等待了，那麼就找下一個沒最近的沒被取消的線程進行喚醒
        // 喚醒只是針對一個線程的喲
        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. 線程解鎖的傳播性？

　　由於從上一節的講解中，咱們看到，當用戶調用 countDown 時，僅僅是讓操做系統喚醒了 head 的下一個節點線程或者最近未取消的節點。那麼，從哪裏來的全部線程都獲取了鎖從而運行呢？

　　實際上是在 獲取鎖的過程當中，還有一點咱們未看清：

    // 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) {
                    // 當countDown被調用後，head節點被喚醒，執行
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        // 獲取到鎖後，設置node爲下一個頭節點，並把喚醒狀態傳播下去，而這裏面確定會作一些喚醒其餘線程的操做，請看下文
                        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) {
            // 若是有必要，則作一次喚醒下一線程的操做
            // 在 countDown() 不會觸發此操做，因此這裏只是一個內部調用傳播
            Node s = node.next;
            if (s == null || s.isShared())
                // 此處鎖釋放邏輯如上，總之，又是另外一次的喚醒觸發
                doReleaseShared();
        }
    }

　　到此，咱們明白了它是怎麼作到一個鎖釋放，全部線程可通行的。也從根本上回答了咱們猜測，全部線程同時併發運行。然而並無，它只是經過喚醒傳播性來依次喚醒各個等待線程的。從絕對時間性上來說，都是有前後關係的。之後可別再淺顯說是同時執行了喲。

 

3、 鎖的切換：wait/notify

　　上面看出，針對一個lock/unlock 的過程仍是很簡單的，由操做系統負責大頭，實現代碼也並很少。

　　可是針對稍微有點要求的場景，就會進行條件式的操做。好比：持有某個鎖運行一段代碼，可是，運行時發現某條件不知足，須要進行等待而不能直接結束，直到條件成立。即所謂的 wait 操做。

　　乍一看，wait/notify 與 lock/unlock 很像，其實否則。區分主要是 lock/unlock 是針對整個代碼段的，而 wait/notify 則是針對某個條件的，即獲取了鎖不表明條件成立了，可是條件成立了必定要在鎖的前提下才能進行安全操做。

　　那麼，是否 wait/notify 也同樣的實現簡單呢？好比java的最基礎類 Object 類就提供了 wait/notify 功能。

　　咱們既然想一探究竟，仍是以併發包下的實現做爲基礎吧，畢竟 java 纔是咱們的強項。

　　本次，我們以  ArrayBlockingQueue#put/take 做爲基礎看下這種場景的使用先。

　　ArrayBlockingQueue 的put/take 特性就是，put當隊列滿時，一直阻塞，直到有可用位置才繼續運行下一步。而take當隊列爲空時同樣阻塞，直到隊列裏有數據才運行下一步。這種場景使用鎖主很差搞了，由於這是一個條件判斷。put/take 以下：

    // 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 {
            // 當隊列滿時，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 {
            // 當隊列爲空時一直等待
            while (count == 0)
                notEmpty.await();
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

　　看起來至關簡單，徹底符合人類思惟。只是，這裏使用的兩個變量進行控制流程 notFull,notEmpty. 這兩個變量是如何進行關聯的呢？

　　在這以前，咱們還須要補充下上面的例子，即 notFull.await(), notEmpty.await(); 被阻塞了，什麼時候才能運行呢？如上代碼在各自的入隊和出隊完成以後進行通知就能夠了。

    // 與 put 對應，入隊完成後，隊列天然就不爲空了，通知下 notEmpty 就行了
    /**
     * 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++;
        // 我已放入一個元素，不爲空了
        notEmpty.signal();
    }
    // 與 take 對應，出隊完成後，天然就不多是滿的了，至少一個空餘空間。
    /**
     * 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();
        // 我已移除一個元素，確定沒有滿了，大家繼續放入吧
        notFull.signal();
        return x;
    }

　　是否是超級好理解。是的。不過，咱們不是想看 ArrayBlockingQueue 是如何實現的，咱們是要論清 wait/notify 是如何實現的。由於畢竟，他們不是一個鎖那麼簡單。

    // 三個鎖的關係，即 notEmpty, notFull 都是 ReentrantLock 的條件鎖，至關因而其子集吧
    /** 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() 是什麼鬼？它是 AQS 中實現的 ConditionObject
    // java.util.concurrent.locks.ReentrantLock#newCondition
    public Condition newCondition() {
        return sync.newCondition();
    }
        // java.util.concurrent.locks.ReentrantLock.Sync#newCondition
        final ConditionObject newCondition() {
            // AQS 中定義
            return new ConditionObject();
        }

　　接下來，咱們要帶着幾個疑問來看這個 Condition 的對象：

　　　　1. 它的 wait/notify 是如何實現的？
　　　　2. 它是如何與互相進行聯繫的？
　　　　3. 爲何 wait/notify 必需要在外面的lock獲取以後才能執行？
　　　　4. 它與Object的wait/notify 有什麼相同和不一樣點？

　　可以回答了上面的問題，基本上對其原理與實現也就理解得差很少了。

 

重點1. wait/notify 是如何實現的？

　　咱們從上面能夠看到，它是經過調用 await()/signal() 實現的，到底作事如何，且看下面。

        // 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();
            // 添加當前線程到 等待線程隊列中，有 lastWaiter/firstWaiter 維護
            Node node = addConditionWaiter();
            // 釋放當前lock中持有的鎖，詳情且看下文
            int savedState = fullyRelease(node);
            // 從如下開始，將再也不保證線程安全性，由於當前的鎖已經釋放，其餘線程將會從新競爭鎖使用
            int interruptMode = 0;
            // 循環斷定，若是當前節點不在 sync 同步隊列中，那麼就反覆阻塞本身
            // 因此判斷是否在 同步隊列上，是很重要的
            while (!isOnSyncQueue(node)) {
                // 沒有在同步隊列，阻塞
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            // 當條件被知足後，須要從新競爭鎖，詳情看下文
            // 競爭到鎖後，原樣返回到 wait 的原點，繼續執行業務邏輯
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            // 下面是異常處理，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();
            // 預期的，都是釋放鎖成功，若是失敗，說明當前線程並並未獲取到鎖，引起異常
            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 由客戶端自定義實現
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
    
    // 如何斷定當前線程是否在同步隊列中或者能夠進行同步隊列？
    /**
     * 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) {
        // 若是上一節點尚未被移除，當前節點就不能被加入到同步隊列
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        // 若是當前節點的下游節點已經存在，則它自身一定已經被移到同步隊列中
        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.
         */
         // 最終直接從同步隊列中查找，若是找到，則自身已經在同步隊列中
        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;
        }
    }
    
    // 當條件被知足後，須要從新競爭鎖，以保證外部的鎖語義，由於以前本身已經將鎖主動釋放
    // 這個鎖與 lock/unlock 時的一毛同樣，沒啥可講的
    // 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);
        }
    }

　　總結一下 wait 的邏輯:

　　　　1. 前提：自身已獲取到外部鎖；
　　　　2. 將當前線程添加到 ConditionQueue 等待隊列中；
　　　　3. 釋放已獲取到的鎖；
　　　　4. 反覆檢查進入等待，直到當前節點被移動到同步隊列中；
　　　　5. 條件知足被喚醒，從新競爭外部鎖，成功則返回，不然繼續阻塞；(外部鎖是同一個，這也是要求兩個對象必須存在依賴關係的緣由)
　　　　6. wait前線程持有鎖，wait後線程持有鎖，沒有一點外部鎖變化；

 

重點2. 釐清了 wait, 接下來，咱們看 signal() 通知喚醒的實現：

        // 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() {
            // 只有獲取鎖的實例，才能夠進行signal，不然你拿什麼去保證線程安全呢
            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) {
            // 最多隻轉移一個 節點
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }
    // 將一個節點從 等待隊列 移動到 同步隊列中，便可參與下一輪競爭
    // 只有確實移動成功纔會返回 true
    // 說明：當前線程是持有鎖的線程
    // 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).
         */
        // 同步隊列由 head/tail 指針維護
        Node p = enq(node);
        int ws = p.waitStatus;
        // 注意，此處正常狀況下並不會喚醒等待線程，僅是將隊列轉移。 
        // 由於當前線程的鎖保護區域並未完成,完成後天然會喚醒其餘等待線程
        // 不然將會存在當前線程任務還未執行完成，卻被其餘線程搶了先去，那接下來的任務當如何？？
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

　　總結一下，notify 的功能原理以下：

　　　　1. 前提：自身已獲取到外部鎖；
　　　　2. 轉移下一個等待隊列的節點到同步隊列中；
　　　　3. 若是遇到下一節點被取消狀況，順延到再下一節點直到爲空，至多轉移一個節點；
　　　　4. 正常狀況下不作線程的喚醒操做；

　　因此，實現 wait/notify, 最關鍵的就是維護兩個隊列，等待隊列與同步隊列，並且都要求是在有外部鎖保證的狀況下執行。

　　到此，咱們也能回答一個問題：爲何wait/notify必定要在鎖模式下才能運行？

　　由於wait是等待條件成立，此時一定存在競爭須要作保護，而它自身又必須釋放鎖以使外部條件可成立，且後續須要作恢復動做；而notify以後可能還有後續工做必須保障安全，notify只是鎖的一個子集。。。

 

4、通知全部線程的實現：notifyAll

　　有時條件成立後，能夠容許全部線程通行，這時就能夠進行 notifyAll, 那麼若是達到通知全部的目的呢？是一塊兒通知仍是？？

　　如下是 AQS 中的實現：

        // 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);
        }

　　能夠看到，它是經過遍歷全部節點，依次轉移等待隊列到同步隊列（通知）的，本來就沒有人能同時幹幾件事的！

　　本文從java實現的角度去解析同步鎖的原理與實現，但並不侷限於java。道理老是相通的，只是像操做系統這樣的大佬，能幹的活更純粹：好比讓cpu根本不用調度一個線程。