abstractqueuedsynchronizer.java

JAVA的一些源码 JAVA2 STANDARD EDITION DEVELOPMENT KIT 5.0

        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;
        /** 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;

        /**
         * 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:  Node is cancelled due to timeout or interrupt
         *               Nodes never leave this state. In particular,
         *               a thread with cancelled node never again blocks.
         *   CONDITION:  Node is currently on a condition queue
         *               It will not be used as a sync queue node until
         *               transferred. (Use of this value here
         *               has nothing to do with the other uses
         *               of the field, but simplifies mechanics.)
         *   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 only using
         * CAS.
         */
        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 once during enqueuing, and
         * nulled out (for sake of GC) when no longer needed.  Upon
         * cancellation, we cannot adjust this field, but can notice
         * status and bypass the node if cancelled.  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.
         */
        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.
         * @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

    /**
     * Insert 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
                Node h = new Node(); // Dummy header
                h.next = node;
                node.prev = h;
                if (compareAndSetHead(h)) {
                    tail = node;
                    return h;
                }
            }
            else {
                node.prev = t;     
                if (compareAndSetTail(t, node)) {
                    t.next = node; 
                    return t; 
                }
            }
        }
    }

    /**
     * Create and enq node for given thread and mode
     * @param current the thread
     * @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;
    }

    /**
     * Set 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; 
    }

    /**
     * Wake up node's successor, if one exists.
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * Try to clear status in anticipation of signalling.  It is
         * OK if this fails or if status is changed by waiting thread.
         */
        compareAndSetWaitStatus(node, Node.SIGNAL, 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.
         */
        Thread thread;
        Node s = node.next;
        if (s != null && s.waitStatus <= 0)
            thread = s.thread;
        else {
            thread = null;
            for (s = tail; s != null && s != node; s = s.prev) 
                if (s.waitStatus <= 0)
                    thread = s.thread;
        }
        LockSupport.unpark(thread);
    }

    /**
     * Set head of queue, and check if successor may be waiting
     * in shared mode, if so propagating if propagate > 0.
     * @param pred the node holding waitStatus for node
     * @param node the node 
     * @param propagate the return value from a tryAcquireShared
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        setHead(node);
        if (propagate > 0 && node.waitStatus != 0) {
            /*
             * Don't bother fully figuring out successor.  If it
             * looks null, call unparkSuccessor anyway to be safe.
             */
            Node s = node.next; 
            if (s == null || s.isShared())
                unparkSuccessor(node);
        }
    }

    // Utilities for various versions of acquire

    /**
     * Cancel an ongoing attempt to acquire.
     * @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;

	// Getting this before setting waitStatus ensures staleness
	Node predNext = pred.next;

	// Can use unconditional write instead of CAS here
	node.waitStatus = Node.CANCELLED;

	// If we are the tail, remove ourselves
	if (node == tail && compareAndSetTail(node, pred)) {
	    compareAndSetNext(pred, predNext, null);
	} else {
	    // If "active" predecessor found...
	    if (pred != head
		&& (pred.waitStatus == Node.SIGNAL
		    || compareAndSetWaitStatus(pred, 0, Node.SIGNAL))
		&& pred.thread != null) {

		// If successor is active, set predecessor's next link
		Node next = node.next;
		if (next != null && next.waitStatus <= 0)
		    compareAndSetNext(pred, predNext, next);
	    } else {
		unparkSuccessor(node);
	    }

	    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
     * @param pred node's predecessor holding status
     * @param node the node 
     * @return true if thread should block
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int s = pred.waitStatus;
        if (s < 0)
            /*
             * This node has already set status asking a release