AQS源码解析

AQS

    AQS 内部维护了一个FIFO双向队列，Node内部类就是队列的节点；（关键: FIFO、双向）
    head，此队列头结点
    tail，此队列尾节点
    state,表示同步状态，共享模式下，一个同步锁可能被多个线程持有，所以用int表示
    /**
     * Head of the wait queue, lazily initialized.  Except for
     * initialization, it is modified only via method setHead.  Note:
     * If head exists, its waitStatus is guaranteed not to be
     * CANCELLED.
     */
    private transient volatile Node head;
    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
    private transient volatile Node tail;
    /**
     * The synchronization state.
     */
    private volatile int state;

        /** 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 节点状态
CANCELLED  1 被取消的线程
SIGNAL    -1 需要被唤醒的线程
CONDITION -2 表示线程正在等待条件
PROPAGATE -3 表示下一个acquisitionShared应 无条件地传播
0 默认值，无意义
        /** 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;

• 独占模式 EXCLUSIVE
• 共享模式 SHARED

获取锁的两种模式

1. 尝试获取锁，但实际是否获得锁并不重要。获取到锁直接返回true；没有获取到锁直接返回false。

不会阻塞

1. 没有获得锁的话就会一直阻塞，直到获得锁

这两种模式在AQS中都有实现，分别是 tryAcquire() 和 acquire()

tryAcquire()

tryAcquire()用于尝试获取锁，获取结果会立即返回。
AQS中的tryAcquire()方法并没有提供具体的实现，要求AQS的继承者必须自己实现此方法，例如ReentryLock、CountDownLacth，都自己实现了tryAcquire()方法。如果没有实现电话，会抛出UnsupportedOperationException() 异常

    /**
     * Attempts to acquire in exclusive mode. This method should query
     * if the state of the object permits it to be acquired in the
     * exclusive 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. This can be used
     * to implement method {@link Lock#tryLock()}.
     *
     * <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 {@code true} if successful. 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 exclusive mode is not supported
     */
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

acquire()

用于在独占模式下获取锁，直到成功
执行步骤：

1. 执行tryAcquire()，如果获取到锁（返回ture），直接执行selfInterrupt()
2. tryAcquire()没有获取到锁（返回false)，执行 acquireQueued(addWaiter(Node.EXCLUSIVE), arg)，

节点入队

1. addWaiter(Node.EXCLUSIVE)
   1. 将当前线程封装成Node，并加入等待队列
2. acquireQueued（node,arg）
3. 如果获取锁失败，会执行 if 的代码 selfInterrupt()，将当前线程中断

    /**
     * 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();
    }

1. 将当前线程，封装成node
2. 如果尾节点不为空，则用cas将当前节点设为尾节点
3. 如果cas失败，则进入完整入队方法 enq()
    /**
     * 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
        // 先尝试快速入队，失败的话就调用完整入队方法
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

入队方法，将节点node加入队列
1. 如果尾节点为空（说明队列还未初始化），则用cas初始化一个队列
2. 如果尾节点不为空，则用cas将当前节点设为尾节点
    /**
     * 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 (;;) { // 自旋操作
            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;
                }
            }
        }
    }

配合release方法，对线程进行挂起和响应。
对已经在队列中的线程以不间断的方式进行获取。被条件等待方法和获取方法所调用。
1. 判断
cancelAcquire(node)
    /**
     * 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 (;;) {
                // 判断当前节点的前一节点是否是头结点
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 判断当前线程是否需要被挂起，如果成功挂起，则interrupted置为true
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

判断当前节点的线程是否需要挂起
    /**
     * 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.
             */
             // 前置节点状态为signal，前置节点也在等待拿锁，那当前节点就可以挂起等待。
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
             // 前置节点状态为cancel，则删除前面所有waitStatus为CANCEL的节点
            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.
             */
             // 其他状态，将前置节点waitStatus置为SIGNAL
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

将当前节点线程挂起
    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

只有Head的后一个节点有资格拿锁，且需要保证同时有且只有这一个节点在拿锁。

被挂起的线程何时被唤醒呢？

释放锁

tryRelease()由继承子类自行定义逻辑

    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

1. 调用tryRealease()方法尝试释放锁
2. 如果tryRealease()成功，返回true，则唤醒队列的下一节点
    /**
     * 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;
    }

node 为头结点（也就是刚刚执行结束的线程）
1. 将node的waitStatus 通过cas置为0
2. 寻找node的继任者，通常及时node的next节点，但有时也不是，这是就要查找
3. 查找逻辑：
    从队列尾节点开始向前寻找 最靠近node的waitStatus<=0 的节点


    /**
     * 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);


 被唤醒的线程被保留在后继者中，通常就是下一个节点。
 但如果被取消或显然是空的，就从尾部向前遍历，以找到实际的未被取消的继任者。
        /*
         * 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);
    }

！为什么寻找successor时要从后往前查找？

首先要明确，从后往前寻找时，并不是找到第一个符合s == null || s.waitStatus > 0就行，而是一直找到离node最近的那一个。所以就有了这个疑问，既然要找的是离node最近的一个，为什么不直接从node开始向后寻找呢？
原因在入队方法enq()逻辑里
重点关注入队语句，将一个节点node入队，即放在尾节点Tail后，有两个步骤：

// 
tail.next = node;

node.prev = tail;

从逻辑上讲，单线程环境下，这两个步骤谁先谁后无所谓。而多线程下，就有问题了（因为这两个操作不是原子操作），回到AQS的入队操作：

if (compareAndSetTail(t, node)) {
    t.next = node;
    return t;
}

上面逻辑中，compareAndSetTail(t, node)，利用cas将node的prev指针设置为tail，成功后再执行if括号里的内容；但是括号里的操作不是同步操作。这个时刻，node的prev指针已经指向了tail，但是tail.next
指针还没建立。若此时其他线程调用了unpark操作，那tail的next指针就无法成功建立。
所以从头往后就无法完整遍历队列，从后往前就可以

    private Node enq(final Node node) {
        for (;;) { // 自旋操作
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
// 入队语句在这里
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

Head头结点

头结点是个虚节点，不存储实际线程;一个线程的节点被设为head节点，意外该线程节点完成，出队。

    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }