abstractqueuedsynchronizer.java

JAVA的一些源码 JAVA2 STANDARD EDITION DEVELOPMENT KIT 5.0

     * @return the collection of threads
     */
    public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in exclusive mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to an exclusive acquire.
     * @return the collection of threads
     */
    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (!p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in shared mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to a shared acquire.
     * @return the collection of threads
     */
    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a string identifying this synchronizer, as well as its
     * state.  The state, in brackets, includes the String &quot;State
     * =&quot; followed by the current value of {@link #getState}, and
     * either &quot;nonempty&quot; or &quot;empty&quot; depending on
     * whether the queue is empty.
     *
     * @return a string identifying this synchronizer, as well as its state.
     */
    public String toString() {
        int s = getState();
        String q  = hasQueuedThreads()? "non" : "";
        return super.toString() + 
            "[State = " + s + ", " + q + "empty queue]";
    }


    // Internal support methods for Conditions

    /**
     * Returns true if a node, always one that was initially placed on
     * a condition queue, is now waiting to reacquire on sync queue.
     * @param node the node
     * @return true if is reacquiring
     */
    final boolean isOnSyncQueue(Node node) {
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        if (node.next != null) // If has successor, it must be on queue
            return true;
        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
        return findNodeFromTail(node);
    } 

    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * @return true if present
     */
    private boolean findNodeFromTail(Node node) {
        Node t = tail; 
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }

    /**
     * Transfers a node from a condition queue onto sync queue. 
     * Returns true if successful.
     * @param node the node
     * @return true if successfully transferred (else the node was
     * cancelled before signal).
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
        Node p = enq(node);
        int c = p.waitStatus;
        if (c > 0 || !compareAndSetWaitStatus(p, c, Node.SIGNAL)) 
            LockSupport.unpark(node.thread);
        return true;
    }

    /**
     * Transfers node, if necessary, to sync queue after a cancelled
     * wait. Returns true if thread was cancelled before being
     * signalled.
     * @param current the waiting thread
     * @param node its node
     * @return true if cancelled before the node was signalled.
     */
    final boolean transferAfterCancelledWait(Node node) {
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        while (!isOnSyncQueue(node)) 
            Thread.yield();
        return false;
    }

    /**
     * Invoke release with current state value; return saved state.
     * Cancel node and throw exception on failure.
     * @param node the condition node for this wait
     * @return previous sync state
     */
    final int fullyRelease(Node node) {
        try {
            int savedState = getState();
            if (release(savedState))
                return savedState;
        } catch(RuntimeException ex) {
            node.waitStatus = Node.CANCELLED;
            throw ex;
        }
        // reach here if release fails
        node.waitStatus = Node.CANCELLED;
        throw new IllegalMonitorStateException();
    }

    // Instrumentation methods for conditions

    /**
     * Queries whether the given ConditionObject 
     * uses this synchronizer as its lock.
     * @param condition the condition
     * @return <tt>true</tt> if owned
     * @throws NullPointerException if condition null
     */
    public final boolean owns(ConditionObject condition) {
        if (condition == null)
            throw new NullPointerException();
        return condition.isOwnedBy(this);
    }

    /**
     * Queries whether any threads are waiting on the given condition
     * associated with this synchronizer. Note that because timeouts
     * and interrupts may occur at any time, a <tt>true</tt> return
     * does not guarantee that a future <tt>signal</tt> will awaken
     * any threads.  This method is designed primarily for use in
     * monitoring of the system state.
     * @param condition the condition
     * @return <tt>true</tt> if there are any waiting threads.
     * @throws IllegalMonitorStateException if exclusive synchronization 
     * is not held
     * @throws IllegalArgumentException if the given condition is
     * not associated with this synchronizer
     * @throws NullPointerException if condition null
     */ 
    public final boolean hasWaiters(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.hasWaiters();
    }

    /**
     * Returns an estimate of the number of threads waiting on the
     * given condition associated with this synchronizer. Note that
     * because timeouts and interrupts may occur at any time, the
     * estimate serves only as an upper bound on the actual number of
     * waiters.  This method is designed for use in monitoring of the
     * system state, not for synchronization control.
     * @param condition the condition
     * @return the estimated number of waiting threads.
     * @throws IllegalMonitorStateException if exclusive synchronization 
     * is not held
     * @throws IllegalArgumentException if the given condition is
     * not associated with this synchronizer
     * @throws NullPointerException if condition null
     */ 
    public final int getWaitQueueLength(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitQueueLength();
    }

    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this
     * synchronizer.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate. The elements of the
     * returned collection are in no particular order.  
     * @param condition the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException if exclusive synchronization 
     * is not held
     * @throws IllegalArgumentException if the given condition is
     * not associated with this synchronizer
     * @throws NullPointerException if condition null
     */
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitingThreads();
    }

    /**
     * Condition implementation for a {@link
     * AbstractQueuedSynchronizer} serving as the basis of a {@link
     * Lock} implementation.
     *
     * <p> Method documentation for this class describes mechanics,
     * not behavioral specifications from the point of view of Lock
     * and Condition users. Exported versions of this class will in
     * general need to be accompanied by documentation describing
     * condition semantics that rely on those of the associated
     * <tt>AbstractQueuedSynchronizer</tt>.
     * 
     * <p> This class is Serializable, but all fields are transient,
     * so deserialized conditions have no waiters.
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        private transient Node lastWaiter;

        /**
         * Creates a new <tt>ConditionObject</tt> instance.
         */
        public ConditionObject() { }

        // Internal methods

        /**
         * Add a new waiter to wait queue
         * @return its new wait node
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }

        /**
         * Remove and transfer nodes until hit non-cancelled one or
         * null. Split out from signal in part to encourage compilers
         * to inline the case of no waiters.
         * @param first (non-null) the first node on condition queue
         */
        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null) 
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

        /**
         * Remove and transfer all nodes.
         * @param first (non-null) the first node on condition queue
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter  = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }

        // public methods

        /**
         * Moves the longest-waiting thread, i