java AQS（AbstractQueuedSynchronizer）同步器詳解

　　除了內置鎖（synchronized）外，java AQS（AbstractQueuedSynchronizer）同步器幾乎是全部同步容器，同步工具類的基礎。ReentrantLock、ReentrantReadWriteLock就是經過內部類繼承並實現AQS的接口來實現相關功能的。

　　查看AQS的源碼，包含如下幾個內容：

• Node數據結構定義，用於存放被阻塞（自旋或者掛起）的線程信息，或者存放等待condition的線程，經過標記區別，共用統一數據結構
• node隊列通用操做，包括隊列操做和release操做（釋放資源），node隊列擴展於CLH隊列，
• 各類acquire操做（獲取資源）的通用方法
• 暴露的方法，包括tryAcquire/tryRelease等非阻塞接口方法，以及acquire/release等阻塞方法
• 隊列檢查方法，包括隊列中是否還有node、線程是否在隊列中等
• Condition相關數據結構以及方法
• node節點字段以及同步器字段field的CAS輔助方法，用於實現原子CAS操做

　　Node節點的數據結構以下所示:

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

        /** waitStatus value to indicate thread has cancelled */
       // 線程已取消
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking */
       // 後續節點線程須要調度
        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        // 線程等待某一條件
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        // 下次acquireShared無條件傳遞
        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 enqueing, 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;
        }
    }