同步锁基本原理与实现

    // 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;

    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();    }

    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);    }

    // 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();        }    }

    // 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();        }    }

    // 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;    }

    // 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();        }

        // 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);        }    }

        // 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;    }

        // 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);        }