AbstractQueuedSynchronizer 源碼分析

AQS是經過CHL隊列來實現鎖請求阻塞列表的。能夠經過acquire(int arg)來分析，當前線程競爭鎖時的流程，而後再經過release(int arg)方法來分析，當前線程釋放一個鎖時的流程。這兩個方法都是獨佔鎖的流程。相對應的acquireShared(int arg) ，releaseShared(int arg)是共享鎖的獲取釋放流程。

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic <tt>int</tt> value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated <tt>int</tt>
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 * 這個類根據fifo策略，實現一個阻塞鎖同步器。他設計的基礎，是依靠一個int 型的原子變量表明同步器的狀態。
 * <p>Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * <tt>AbstractQueuedSynchronizer</tt> does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their public methods.
 *
 * <p>This class supports either or both a default <em>exclusive</em>
 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not &quot;understand&quot; these differences except in the
 * mechanical sense that when a shared mode acquire succeeds, the next
 * waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 * 能夠根據這個類，構造共享或者獨佔模式的鎖。
 * <p>This class defines a nested {@link ConditionObject} class that
 * can be used as a {@link Condition} implementation by subclasses
 * supporting exclusive mode for which method {@link
 * #isHeldExclusively} reports whether synchronization is exclusively
 * held with respect to the current thread, method {@link #release}
 * invoked with the current {@link #getState} value fully releases
 * this object, and {@link #acquire}, given this saved state value,
 * eventually restores this object to its previous acquired state.  No
 * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *  類中有個內部類ConditionObject，用於實現條件隊列。
 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an <tt>AbstractQueuedSynchronizer</tt> for their
 * synchronization mechanics.
 *
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a <tt>readObject</tt> method that restores this to a known
 * initial state upon deserialization.
 *  。。。。。。。省略。。
 *
 * @since 1.5
 * @author Doug Lea
 */
public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

    private static final long serialVersionUID = 7373984972572414691L;

    /**
     * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance
     * with initial synchronization state of zero.
     */
    protected AbstractQueuedSynchronizer() { }

    /**
     * 等待隊列的Node類：
     * Wait queue node class.
     *  等待隊列用的是CLH隊列。CLH 隊列通常用於自旋鎖隊列。
     * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
     * Hagersten) lock queue. CLH locks are normally used for
     * spinlocks.  We instead use them for blocking synchronizers, but
     * use the same basic tactic of holding some of the control
     * information about a thread in the predecessor of its node.
     * 這裏，咱們使用相同的控制前置節點策略，把它用在阻塞同步器上。
     * A "status" field in each node keeps track of whether a thread
     * should block.  
     * 每一個節點的status 字段，代表這個節點是否應該阻塞。
     * A node is signalled when its predecessor
     * releases.  
     * 當一個節點的前置節點釋放鎖，它會獲得通知。
     * Each node of the queue otherwise serves as a
     * specific-notification-style monitor holding a single waiting
     * thread. 
     * 每一個節點，以明確通知交互方式，保存着一個等待線程。
     * The status field does NOT control whether threads are
     * granted locks etc though. 
     * status字段，不表明一個線程是否已經得到鎖。
     * A thread may try to acquire if it is
     * first in the queue. But being first does not guarantee success;
     * it only gives the right to contend.  So the currently released
     * contender thread may need to rewait.
     * 一個狀態只代表，某個線程是否有權利參與鎖競爭了。
     * <p>To enqueue into a CLH lock, you atomically splice it in as new
     * tail. To dequeue, you just set the head field.
     * 一個線程入隊，就是把線程放入隊列尾部。數據結構以下圖：CLH隊列實際上是一個個Node類組成的鏈表，Node 中的waitStatus字段值表示節點的阻塞策略
     * <pre>
     *      +------+  prev +-----+       +-----+
     * head |      | <---- |     | <---- |     |  tail
     *      +------+       +-----+       +-----+
     * </pre>
     *
     * <p>Insertion into a CLH queue requires only a single atomic
     * operation on "tail", so there is a simple atomic point of
     * demarcation from unqueued to queued.
     * 入隊在tail節點，原子操做
     * Similarly, dequeing involves only updating the "head". However, it takes a bit
     * more work for nodes to determine who their successors are,
     * in part to deal with possible cancellation due to timeouts
     * and interrupts.
     * 相似地，出隊在head節點上操做，取消，還要考慮當前節點的下一個節點線程。
     * <p>The "prev" links (not used in original CLH locks), are mainly
     * needed to handle cancellation. If a node is cancelled, its
     * successor is (normally) relinked to a non-cancelled
     * predecessor.
     * 節點的prev字段（在原始的 CLH locks是沒用到的）主要用於，一個節點被取消了，它的下一個節點，從新指向，它的前一個節點。 
     * For explanation of similar mechanics in the case
     * of spin locks, see the papers by Scott and Scherer at
     * http://www.cs.rochester.edu/u/scott/synchronization/
     * 這裏的一篇論文，解釋在自旋鎖中類似的機制。
     * <p>We also use "next" links to implement blocking mechanics.
     * The thread id for each node is kept in its own node, so a
     * predecessor signals the next node to wake up by traversing
     * next link to determine which thread it is. 
     * 用next字段實現阻塞機制，每一個節點保存有它本身的的線程id
     * 前置節點能夠經過next 字段找到它的下一個節點線程去喚醒。
     * Determination of successor must avoid races with newly queued nodes to set
     * the "next" fields of their predecessors.  This is solved
     * when necessary by checking backwards from the atomically
     * updated "tail" when a node's successor appears to be null.
     * (Or, said differently, the next-links are an optimization
     * so that we don't usually need a backward scan.)
     *  以隊列下一個節點的方式，能夠很方便的解決，在決定誰是下一個節點時，會產生的競爭問題。
     * <p>Cancellation introduces some conservatism to the basic
     * algorithms.  Since we must poll for cancellation of other
     * nodes, we can miss noticing whether a cancelled node is
     * ahead or behind us. This is dealt with by always unparking
     * successors upon cancellation, allowing them to stabilize on
     * a new predecessor, unless we can identify an uncancelled
     * predecessor who will carry this responsibility.
     *  取消一個節點，就是它的下一個節點掛到一個新的前置節點。
     * <p>CLH queues need a dummy header node to get started. But
     * we don't create them on construction, because it would be wasted
     * effort if there is never contention. Instead, the node
     * is constructed and head and tail pointers are set upon first
     * contention.
     *  CLH隊列須要一個崗哨節點。爲了節約性能，這個節點只有在有競爭者節點時才建立，
     *  此時，頭結點，尾節點都同時指向這個節點。
     * <p>Threads waiting on Conditions use the same nodes, but
     * use an additional link. Conditions only need to link nodes
     * in simple (non-concurrent) linked queues because they are
     * only accessed when exclusively held.  Upon await, a node is
     * inserted into a condition queue.  Upon signal, the node is
     * transferred to the main queue.  A special value of status
     * field is used to mark which queue a node is on.
     * 條件隊列，使用相同的node節點。可是使用另外的連接（nextWaiter），條件隊列是非併發的隊列，由於，只用獲取排他鎖，後才能操做它。
     * 當await的時候，加入條件隊列，當signal時候，把一個節點放到主隊列（鎖隊列）。
     * 一個status字段值（CONDITION = -2），能夠指定當前節點時在哪一個隊列。
     * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
     * Scherer and Michael Scott, along with members of JSR-166
     * expert group, for helpful ideas, discussions, and critiques
     * on the design of this class.
     */
    static final class Node {
         
        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();//說明節點是共享模式
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;//說明節點是獨佔模式

        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1;//說明節點線程被取消
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL    = -1;//說明當前線程的下一個節點是須要被調度喚醒的節點
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;//說明節點線程在條件隊列上等待
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate//說明，下一個共享獲取，能夠無條件傳播（被喚醒）。
         */
        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.
	 *               當前線程的，下一個節點（即將）已經被阻塞。因此當前線程，在釋放鎖時（取消時），要喚醒它的繼任者（下一個節點）
         *   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;

        /**
	 *這個節點連接是條件隊列用的。
         * 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;
        }
    }

    /**
     * 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;

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a <tt>volatile</tt> read.
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a <tt>volatile</tt> write.
     * @param newState the new state value
     */
    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 <tt>volatile</tt> read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     * @return true if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

    // Queuing utilities

    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     * 把一個節點插入到等待隊列。若是有須要，初始化。
     */
    private Node enq(final Node node) {
        for (;;) {//不停的嘗試。
            Node t = tail;
            if (t == null) { // Must initialize 尾節點爲空，就建立一個空節點，做爲頭節點
                if (compareAndSetHead(new Node()))
                    tail = head;//而且尾節點指向頭節點
            } else {
                node.prev = t;//頭節點已經建立好，把當前節點的前置節點，指向頭節點。
                if (compareAndSetTail(t, node)) {//原子排他方式，把當前節點設置爲尾節點。
                    t.next = node;//設置成功把當，原來的尾節點的next指向當前節點。
                    return t;
                }
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     * 把當前線程，以指定的模式（獨佔或者分享）的方式，放入隊列
     * @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) {//當前隊列不空
            node.prev = pred;//當前隊列的前置隊列，指向尾節點。
            if (compareAndSetTail(pred, node)) {//原子排他方式，把當前節點設置爲尾節點。
                pred.next = node;//設置成功把當，原來的尾節點的next指向當前節點。
                return node;//返回最新節點。
            }
        }
        enq(node);//若是當前登臺隊列爲空,把一個節點插入到等待隊列。
        return node;//返回當前節點。
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     * Wakes up node's successor, if one exists.
     * 喚醒當前節點的繼任者節點線程。
     * @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.
	 * 若是，繼任者已被取消或者爲null，就從尾部遍歷，找到最近的一個非取消節點。
         */
        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);//native方法喚醒繼任者節點線程。	
    }




    // Utilities for various versions of acquire

    /**
     * Cancels an ongoing attempt to acquire.
     *  取消一個節點
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;

        node.thread = null;

        // Skip cancelled predecessors
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary.
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves.
        if (node == tail && compareAndSetTail(node, pred)) {//若是是尾節點
            compareAndSetNext(pred, predNext, null);//前置節點下一個節點爲null.刪除當前節點。
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
	    // 設置 當前節點的node.next爲下一個節點。
            int ws;
            if (pred != head &&
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                unparkSuccessor(node);//喚醒繼任者節點
            }

            node.next = node; // help GC
        }
    }

    /**
     * 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
     * 把node節點前置（沒取消）節點，waitStatus設置爲-1
     * @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.前置節點已經被設置爲可喚醒狀態。能夠放心阻塞當前節點。
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry. 若是前置節點被取消，直接拋棄，直到找到沒取消的前置節點。
             */
            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.
	     * 給前置節點信號爲-1。
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

 
    /*
     * Various flavors of acquire, varying in exclusive/shared and
     * control modes.  Each is mostly the same, but annoyingly
     * different.  Only a little bit of factoring is possible due to
     * interactions of exception mechanics (including ensuring that we
     * cancel if tryAcquire throws exception) and other control, at
     * least not without hurting performance too much.
     */

    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     * 一個已經在隊列中的線程，嘗試獲取一個排他鎖。
     * @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();//獲取當前節點的前置節點
                if (p == head && tryAcquire(arg)) {//若是前置節點是頭節點,不停的嘗試，知道成功。說明沒有線程在等待。調用tryAcquire(arg)嘗試改變狀態變量值。
                    setHead(node);//若是成功了，就把當前節點設置爲頭節點，而後出列。
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
		//不是頭節點，表示前面還有等待的節點。調用shouldParkAfterFailedAcquire(p, node)設置當前節點前置節點waitStatus=-1，
		//若是前置節點waitStatus值已被設置爲-1(返回true)，接着調用parkAndCheckInterrupt()阻塞當前節點，安全放心的阻塞。
		//若是前置節點waitStatus不爲-1（>0）,設置前置節點的waitStatus爲-1 返回false,不阻塞當前節點。
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)//這個正常執行不到，異常時，取消進程。
                cancelAcquire(node);
        }
    }



    /**
     * 先從獲取一個排他鎖，提及，
     * 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))
	    //tryAcquire(arg)嘗試修改state狀態值，失敗返回flase,
	    //執行acquireQueued(addWaiter(Node.EXCLUSIVE), arg)方法。把當前請求線程放入隊列
	    
            selfInterrupt();//既不能修改state值，也不能入隊列的，就自我中斷。
    }



    /**
     * 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}.
     * 釋放一個獨佔鎖。
     * @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)) {//若是子類實現的tryRelase返回爲true
            Node h = head;//把頭節點賦給h
            if (h != null && h.waitStatus != 0)//頭節點不爲null，而且waitStatus不爲0，爲0時下一個節點已經被喚醒
                unparkSuccessor(h);//喚醒h的下個節點（即當前隊列頭第一個等待節點）若是waitStatus爲負數，就把waitStatus設置爲0
            return true;
        }
        return false;
    }

 
}