Java AQS 解析

AbstractQueuedSynchronizer（AQS）

Java并发包（JUC）中提供了很多并发工具，这其中，很多我们耳熟能详的并发工具，譬如ReentrangLock、Semaphore，它们的实现都用到了一个共同的基类–AbstractQueuedSynchronizer,简称AQS。AQS是一个用来构建锁和同步器的框架，使用AQS能简单且高效地构造出应用广泛的大量的同步器，比如我们提到的ReentrantLock，Semaphore，其他的诸如ReentrantReadWriteLock，SynchronousQueue，FutureTask等等皆是基于AQS的。当然，我们自己也能利用AQS非常轻松容易地构造出符合我们自己需求的同步器。

基本原理

AQS使用一个int成员变量来表示同步状态，通过内置的FIFO队列来完成获取资源线程的排队工作。

private volatile int state;//共享变量，使用volatile修饰保证线程可见性

状态信息通过protected类型的getState，setState，compareAndSetState进行操作

protected final int getState()
protected final void setState(int newState)
protected final boolean compareAndSetState(int expect, int update)

AQS支持两种同步方式：

• 独占式
• 共享式

这样方便使用者实现不同类型的同步组件，独占式如ReentrantLock，共享式如Semaphore，CountDownLatch，组合式的如ReentrantReadWriteLock。总之，AQS为使用提供了底层支撑，如何组装实现，使用者可以自由发挥。

同步器的设计是基于模板方法模式的，一般的使用方式是这样：

　　1.使用者继承AbstractQueuedSynchronizer并重写指定的方法。（这些重写方法很简单，无非是对于共享资源state的获取和释放）

　　2.将AQS组合在自定义同步组件的实现中，并调用其模板方法，而这些模板方法会调用使用者重写的方法。

可能需要重新定义的protected method

//独占模式
protected boolean tryAcquire(int arg);
protected boolean tryRelease(int arg);
//共享模式
protected int tryAcquireShared(int arg);
protected boolean tryReleaseShared(int arg);
//判断当前线程是否排它持有state
protected boolean isHeldExclusively()；

//以上方法在AQS中默认实现为throw new UnsupportedOperationException();

子类可以维护其他状态字段，但是只有使用方法getState、setState和compareAndSetState操纵的原子更新的{int state}值在同步方面被跟踪。AbstractQueuedSynchronizer的子类应该定义为非公共的内部助手类，用于实现其封闭类的同步属性。

思想

对于使用者来讲，我们无需关心获取资源失败，线程排队，线程阻塞/唤醒等一系列复杂的实现，这些都在AQS中为我们处理好了。我们只需要负责好自己的那个环节就好，也就是获取/释放共享资源state的姿势T_T。很经典的模板方法设计模式的应用，AQS为我们定义好顶级逻辑的骨架，并提取出公用的线程入队列/出队列，阻塞/唤醒等一系列复杂逻辑的实现，将部分简单的可由使用者决定的操作逻辑延迟到子类中去实现即可。

使用

首先，我们需要去继承AbstractQueuedSynchronizer这个类，然后我们根据我们的需求去重写相应的方法，比如要实现一个独占锁，那就去重写tryAcquire，tryRelease方法，要实现共享锁，就去重写tryAcquireShared，tryReleaseShared；最后，在我们的组件中调用AQS中的模板方法就可以了，而这些模板方法是会调用到我们之前重写的那些方法的。也就是说，我们只需要很小的工作量就可以实现自己的同步组件，重写的那些方法，仅仅是一些简单的对于共享资源state的获取和释放操作，至于像是获取资源失败，线程需要阻塞之类的操作，自然是AQS帮我们完成了。

源码

AQS维护了一个共享资源state，通过内置的FIFO队列完成排队的工作。这个队列是一个由Node节点组成的双向量表。AQS分别维护了它的一头一尾两个指针。

独占式

Node节点

Node节点是AQS里面的一个静态内部类

static final class Node {
        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;

        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3;

        /**
         * Status field, taking on only the values:
         *   SIGNAL:     The successor of this node is (or will soon be)
         *               blocked (via park), so the current node must
         *               unpark its successor when it releases or
         *               cancels. To avoid races, acquire methods must
         *               first indicate they need a signal,
         *               then retry the atomic acquire, and then,
         *               on failure, block.
         *   CANCELLED:  This node is cancelled due to timeout or interrupt.
         *               Nodes never leave this state. In particular,
         *               a thread with cancelled node never again blocks.
         *   CONDITION:  This node is currently on a condition queue.
         *               It will not be used as a sync queue node
         *               until transferred, at which time the status
         *               will be set to 0. (Use of this value here has
         *               nothing to do with the other uses of the
         *               field, but simplifies mechanics.)
         *   PROPAGATE:  A releaseShared should be propagated to other
         *               nodes. This is set (for head node only) in
         *               doReleaseShared to ensure propagation
         *               continues, even if other operations have
         *               since intervened.
         *   0:          None of the above
         *
         * The values are arranged numerically to simplify use.
         * Non-negative values mean that a node doesn't need to
         * signal. So, most code doesn't need to check for particular
         * values, just for sign.
         *
         * The field is initialized to 0 for normal sync nodes, and
         * CONDITION for condition nodes.  It is modified using CAS
         * (or when possible, unconditional volatile writes).
         */
        volatile int waitStatus;

        /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueuing, and nulled
         * out (for sake of GC) only upon dequeuing.  Also, upon
         * cancellation of a predecessor, we short-circuit while
         * finding a non-cancelled one, which will always exist
         * because the head node is never cancelled: A node becomes
         * head only as a result of successful acquire. A
         * cancelled thread never succeeds in acquiring, and a thread only
         * cancels itself, not any other node.
         */
        volatile Node prev;

        /**
         * Link to the successor node that the current node/thread
         * unparks upon release. Assigned during enqueuing, adjusted
         * when bypassing cancelled predecessors, and nulled out (for
         * sake of GC) when dequeued.  The enq operation does not
         * assign next field of a predecessor until after attachment,
         * so seeing a null next field does not necessarily mean that
         * node is at end of queue. However, if a next field appears
         * to be null, we can scan prev's from the tail to
         * double-check.  The next field of cancelled nodes is set to
         * point to the node itself instead of null, to make life
         * easier for isOnSyncQueue.
         */
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on
         * construction and nulled out after use.
         */
        volatile Thread thread;

        /**
         * Link to next node waiting on condition, or the special
         * value SHARED.  Because condition queues are accessed only
         * when holding in exclusive mode, we just need a simple
         * linked queue to hold nodes while they are waiting on
         * conditions. They are then transferred to the queue to
         * re-acquire. And because conditions can only be exclusive,
         * we save a field by using special value to indicate shared
         * mode.
         */
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode.
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * Returns previous node, or throws NullPointerException if null.
         * Use when predecessor cannot be null.  The null check could
         * be elided, but is present to help the VM.
         *
         * @return the predecessor of this node
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

        Node() {    // Used to establish initial head or SHARED marker
        }

        Node(Thread thread, Node mode) {     // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }

        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

里面这几个字段是比较重要的

字段	类型	含义
CANCELLED	waitStatus取值范围之一	因为超时或者中断，节点会被设置为取消状态，被取消状态的节点不应该去竞争锁，只能保持取消状态不变，不能转为其他状态。处于这种状态的节点会被踢出队列，被GC回收。
SIGNAL	waitStatus取值范围之一	表示这个节点的继任节点被阻塞了，到时候需要通知它。
CONDITION	waitStatus取值范围之一	表示这个节点因为等待某个条件而被阻塞
PROPAGATE	waitStatus取值范围之一	使用在共享模式头结点有可能处于这种状态，表示锁的下一次获取可以无条件传播；
waitStatus	int	初始值为0，新节点的默认状态
prev	Node	当前节点的前一个节点
next	Node	当前节点的继任节点
thread	Thread	与节点关联的排队中的线程

获取锁

接下来我们看看，acquire方法是怎么获取锁的

/**
     * 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) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

上述方法主要做以下操作：

1. 调用使用者重写的tryAcquire方法，如果返回true，表示获取同步状态成功，直接返回
2. 如果获取同步状态失败，就构造独占式同步节点，通过addWaiter方法将此节点添加到同步队列的尾部
3. 该节点在队列中尝试获取同步状态，如果获取不到，则阻塞节点线程，知道被前驱节点唤醒或者被中断

看一下addWaiter方法

/**
     * 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;
    }

以CAS方式将当前节点加入到队列的尾部，如果失败了，就进入enq方法。

enq方法

/**
     * 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;
                }
            }
        }
    }

enq中有一个死循环，在这个循环中，一直用CAS方法取设置节点。直到成功为止。

接着我们在看看acquireQueued方法

/**
     * 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);
        }
    }

acquireQueued内部也是一个死循环，只有前驱结点是头结点的结点，也就是老二结点，才有机会去tryAcquire；若tryAcquire成功，表示获取同步状态成功，将此结点设置为头结点；若是非老二结点，或者tryAcquire失败，则进入shouldParkAfterFailedAcquire去判断判断当前线程是否应该阻塞，若可以，调用parkAndCheckInterrupt阻塞当前线程，直到被中断或者被前驱结点唤醒。若还不能休息，继续循环。

shouldParkAfterFailedAcquire

/**
     * 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.
             */
            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;
    }

若shouldParkAfterFailedAcquire返回true，也就是当前结点的前驱结点为SIGNAL状态，则意味着当前结点可以放心休息，进入parking状态了。parkAncCheckInterrupt阻塞线程并处理中断。

parkAndCheckInterrupt

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

总的来说就是：

1. 首先tryAcquire获取同步状态，成功则直接返回；否则，进入下一环节；
2. 线程获取同步状态失败，就构造一个结点，加入同步队列中，这个过程要保证线程安全；
3. 加入队列中的结点线程进入自旋状态，若是老二结点（即前驱结点为头结点），才有机会尝试去获取同步状态；否则，当其前驱结点的状态为SIGNAL，线程便可安心休息，进入阻塞状态，直到被中断或者被前驱结点唤醒。

释放锁

release方法

/**
     * 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;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

调用实现类的tryRelease方法，如果成功了，就唤醒继任节点。

/**
     * 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;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

如果继任节点为空或者状态为CANCEL，则从尾部往前遍历找到一个处于正常阻塞状态的节点，进行唤醒。

共享式

共享式：共享式地获取同步状态。对于独占式同步组件来讲，同一时刻只有一个线程能获取到同步状态，其他线程都得去排队等待，其待重写的尝试获取同步状态的方法tryAcquire返回值为boolean，这很容易理解；对于共享式同步组件来讲，同一时刻可以有多个线程同时获取到同步状态，这也是“共享”的意义所在。其待重写的尝试获取同步状态的方法tryAcquireShared返回值为int。

获取锁

/**
     * Attempts to acquire in shared mode. This method should query if
     * the state of the object permits it to be acquired in the shared
     * mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return a negative value on failure; zero if acquisition in shared
     *         mode succeeded but no subsequent shared-mode acquire can
     *         succeed; and a positive value if acquisition in shared
     *         mode succeeded and subsequent shared-mode acquires might
     *         also succeed, in which case a subsequent waiting thread
     *         must check availability. (Support for three different
     *         return values enables this method to be used in contexts
     *         where acquires only sometimes act exclusively.)  Upon
     *         success, this object has been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

1.当返回值大于0时，表示获取同步状态成功，同时还有剩余同步状态可供其他线程获取；

2.当返回值等于0时，表示获取同步状态成功，但没有可用同步状态了；

3.当返回值小于0时，表示获取同步状态失败。

如果获取同步状态失败，则调用doAcquireShared方法

/**
     * Acquires in shared uninterruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);//构造一个共享结点，添加到同步队列尾部。若队列初始为空，先添加一个无意义的傀儡结点，再将新节点添加到队列尾部。
        boolean failed = true;//是否获取成功
        try {
            boolean interrupted = false;//线程parking过程中是否被中断过
            for (;;) {//死循环
                final Node p = node.predecessor();//找到前驱结点
                if (p == head) {//头结点持有同步状态，只有前驱是头结点，才有机会尝试获取同步状态
                    int r = tryAcquireShared(arg);//尝试获取同步装填
                    if (r >= 0) {//r>=0,获取成功
                        setHeadAndPropagate(node, r);//获取成功就将当前结点设置为头结点，若还有可用资源，传播下去，也就是继续唤醒后继结点
                        p.next = null; // 方便GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&//是否能安心进入parking状态
                    parkAndCheckInterrupt())//阻塞线程
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

大体逻辑与独占式的acquireQueued差距不大，只不过由于是共享式，会有多个线程同时获取到线程，也可能同时释放线程，空出很多同步状态，所以当排队中的老二获取到同步状态，如果还有可用资源，会继续传播下去。

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;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

释放锁

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;
    }

---

/**
    * 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

AQS是JUC中很多同步组件的构建基础，简单来讲，它内部实现主要是状态变量state和一个FIFO队列来完成，同步队列的头结点是当前获取到同步状态的结点，获取同步状态state失败的线程，会被构造成一个结点（或共享式或独占式）加入到同步队列尾部（采用自旋CAS来保证此操作的线程安全），随后线程会阻塞；释放时唤醒头结点的后继结点，使其加入对同步状态的争夺中。

AQS为我们定义好了顶层的处理实现逻辑，我们在使用AQS构建符合我们需求的同步组件时，只需重写tryAcquire，tryAcquireShared，tryRelease，tryReleaseShared几个方法，来决定同步状态的释放和获取即可，至于背后复杂的线程排队，线程阻塞/唤醒，如何保证线程安全，都由AQS为我们完成了，这也是非常典型的模板方法的应用。AQS定义好顶级逻辑的骨架，并提取出公用的线程入队列/出队列，阻塞/唤醒等一系列复杂逻辑的实现，将部分简单的可由使用者决定的操作逻辑延迟到子类中去实现。　

参考资料

队列同步器AQS-AbstractQueuedSynchronizer 原理分析

Java并发包基石-AQS详解