并发编程系列之深入理解JUC中AQS（AbstractQueuedSynchronizer）

  AQS通过内部实现的FIFO等待队列来完成资源获取线程的等待工作，如果当前线程获取资源事变，AQS则会将当前线程以及等待状态等信息构造成一个Node节点，并将其加入等待队列中，同时会阻塞当前线程；当其它获取到资源的线程释放持有的资源时，则会把等待队列中的线程唤醒，使其再次尝试获取对应的资源。

AbstractQueuedSynchronizer同步器

队列

  首先从源码看一下同步队列的结构

    /**
     * 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.
     */
    // 等待队列头结点
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
    // 等待队列的尾结点
    private transient volatile Node tail;

    /**
     * The synchronization state.
     */
    // 同步状态
    private volatile int state;

  同步队列的结构如下图：

  AQS同步器类中的其他方法主要是用于插入节点、释放节点：

同步状态

  AQS类使用单个int（32位）来保存同步状态，并暴露出getState、setState以及compareAndSet操作来读取和更新这个同步状态。state被声明为volatile，并且通过CAS操作来实现compareAndSetState，使得当且仅当同步状态拥有一个一致的期望值时，才会被原子的设置成新值，这样就达到了同步状态的原子性管理，确保了同步状态的原子性、可见性和有序性。
  基于AQS实现的类必须根据暴露出的状态相关方法定义tryAcquire和tryRelease方法，以控制acquire和release操作。当同步状态满足时，tryAcquire方法必须返回true，而当新的同步状态允许后续acquire时，tryRelease方法也必须返回true。

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a {@code volatile} read.
     * @return current state value
     */
    // 获取变量的读取状态
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a {@code volatile} write.
     * @param newState the new state value
     */
    // 写入变量的同步状态
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a {@code volatile} read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
    // 使用CAS设置同步状态，确保线程安全
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

Node节点

    static final class Node {
        /** Marker to indicate a node is waiting in shared mode */
        // 声明共享模式下的等待节点
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode */
        // 声明独占模式下的等待节点
        static final Node EXCLUSIVE = null;

        /** waitStatus value to indicate thread has cancelled */
        // waitStatus的常量值--表示线程已取消
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking */
        // waitStatus的常量值--表示后继线程需要取消挂起
        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        // waitStatus的常量值--表示线程正在等待条件
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
         // waitStatus的常量值--表示下一个acquireShared应无条件传播
        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).
         */
        // 其值只能为CANCELLED、SIGNAL、CONDITION、PROPAGATE或0
        // 初始值为0
        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.
         */
        // 前驱节点
        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.
         */
        // 后继节点
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on
         * construction and nulled out after use.
         */
        // 当前节点的线程，在节点初始化时赋值，使用后为null
        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.
         */
        // 下一个等待节点
        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;
        }
    }

  Node节点的结构如下图：