Java 类java.util.concurrent.locks.AbstractQueuedSynchronizer.Node 实例源码

/**
 * Creates and enqueues node for current thread and given mode.
 *
 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 * @return the new node
 */
private Node addWaiter(Node mode) {
    Node node = new Node(mode);

    for (;;) {
        Node oldTail = tail;
        if (oldTail != null) {
            node.setPrevRelaxed(oldTail);
            if (compareAndSetTail(oldTail, node)) {
                oldTail.next = node;
                return node;
            }
        } else {
            initializeSyncQueue();
        }
    }
}

/**
 * Acquires in exclusive uninterruptible mode for thread already in
 * queue. Used by condition wait methods as well as acquire.
 *
 * @param node the node
 * @param arg the acquire argument
 * @return {@code true} if interrupted while waiting
 */
final boolean acquireQueued(final Node node, long arg) {
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Acquires in exclusive interruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireInterruptibly(long arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.EXCLUSIVE);
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                return;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Acquires in shared uninterruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireShared(long arg) {
    final Node node = addWaiter(Node.SHARED);
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {
                long r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    if (interrupted)
                        selfInterrupt();
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Acquires in shared interruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireSharedInterruptibly(long arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {
                long r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * 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);
}

/**
 * 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 (!node.compareAndSetWaitStatus(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 ws = p.waitStatus;
    if (ws > 0 || !p.compareAndSetWaitStatus(ws, 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 node the node
 * @return true if cancelled before the node was signalled
 */
final boolean transferAfterCancelledWait(Node node) {
    if (node.compareAndSetWaitStatus(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;
}

/**
 * Adds 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(Node.CONDITION);

    if (t == null)
        firstWaiter = node;
    else
        t.nextWaiter = node;
    lastWaiter = node;
    return node;
}

/**
 * Unlinks cancelled waiter nodes from condition queue.
 * Called only while holding lock. This is called when
 * cancellation occurred during condition wait, and upon
 * insertion of a new waiter when lastWaiter is seen to have
 * been cancelled. This method is needed to avoid garbage
 * retention in the absence of signals. So even though it may
 * require a full traversal, it comes into play only when
 * timeouts or cancellations occur in the absence of
 * signals. It traverses all nodes rather than stopping at a
 * particular target to unlink all pointers to garbage nodes
 * without requiring many re-traversals during cancellation
 * storms.
 */
private void unlinkCancelledWaiters() {
    Node t = firstWaiter;
    Node trail = null;
    while (t != null) {
        Node next = t.nextWaiter;
        if (t.waitStatus != Node.CONDITION) {
            t.nextWaiter = null;
            if (trail == null)
                firstWaiter = next;
            else
                trail.nextWaiter = next;
            if (next == null)
                lastWaiter = trail;
        }
        else
            trail = t;
        t = next;
    }
}

/**
 * Implements interruptible condition wait.
 * <ol>
 * <li>If current thread is interrupted, throw InterruptedException.
 * <li>Save lock state returned by {@link #getState}.
 * <li>Invoke {@link #release} with saved state as argument,
 *     throwing IllegalMonitorStateException if it fails.
 * <li>Block until signalled or interrupted.
 * <li>Reacquire by invoking specialized version of
 *     {@link #acquire} with saved state as argument.
 * <li>If interrupted while blocked in step 4, throw InterruptedException.
 * </ol>
 */
public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();
    long savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
}

/**
 * Creates and enqueues node for current thread and given mode.
 *
 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 * @return the new node
 */
private Node addWaiter(Node mode) {
    Node node = new Node(mode);

    for (;;) {
        Node oldTail = tail;
        if (oldTail != null) {
            U.putObject(node, Node.PREV, oldTail);
            if (compareAndSetTail(oldTail, node)) {
                oldTail.next = node;
                return node;
            }
        } else {
            initializeSyncQueue();
        }
    }
}

/**
 * Acquires in exclusive uninterruptible mode for thread already in
 * queue. Used by condition wait methods as well as acquire.
 *
 * @param node the node
 * @param arg the acquire argument
 * @return {@code true} if interrupted while waiting
 */
final boolean acquireQueued(final Node node, long arg) {
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Acquires in exclusive interruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireInterruptibly(long arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.EXCLUSIVE);
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                return;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Acquires in shared uninterruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireShared(long arg) {
    final Node node = addWaiter(Node.SHARED);
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {
                long r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    if (interrupted)
                        selfInterrupt();
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Acquires in shared interruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireSharedInterruptibly(long arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {
                long r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * 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);
}

/**
 * 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 (!node.compareAndSetWaitStatus(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 ws = p.waitStatus;
    if (ws > 0 || !p.compareAndSetWaitStatus(ws, 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 node the node
 * @return true if cancelled before the node was signalled
 */
final boolean transferAfterCancelledWait(Node node) {
    if (node.compareAndSetWaitStatus(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;
}

/**
 * Adds 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(Node.CONDITION);

    if (t == null)
        firstWaiter = node;
    else
        t.nextWaiter = node;
    lastWaiter = node;
    return node;
}

/**
 * Unlinks cancelled waiter nodes from condition queue.
 * Called only while holding lock. This is called when
 * cancellation occurred during condition wait, and upon
 * insertion of a new waiter when lastWaiter is seen to have
 * been cancelled. This method is needed to avoid garbage
 * retention in the absence of signals. So even though it may
 * require a full traversal, it comes into play only when
 * timeouts or cancellations occur in the absence of
 * signals. It traverses all nodes rather than stopping at a
 * particular target to unlink all pointers to garbage nodes
 * without requiring many re-traversals during cancellation
 * storms.
 */
private void unlinkCancelledWaiters() {
    Node t = firstWaiter;
    Node trail = null;
    while (t != null) {
        Node next = t.nextWaiter;
        if (t.waitStatus != Node.CONDITION) {
            t.nextWaiter = null;
            if (trail == null)
                firstWaiter = next;
            else
                trail.nextWaiter = next;
            if (next == null)
                lastWaiter = trail;
        }
        else
            trail = t;
        t = next;
    }
}

/**
 * Implements interruptible condition wait.
 * <ol>
 * <li>If current thread is interrupted, throw InterruptedException.
 * <li>Save lock state returned by {@link #getState}.
 * <li>Invoke {@link #release} with saved state as argument,
 *     throwing IllegalMonitorStateException if it fails.
 * <li>Block until signalled or interrupted.
 * <li>Reacquire by invoking specialized version of
 *     {@link #acquire} with saved state as argument.
 * <li>If interrupted while blocked in step 4, throw InterruptedException.
 * </ol>
 */
public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();
    long savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
}

/**
 * Inserts node into queue, initializing if necessary. See picture above.
 * @param node the node to insert
 * @return node's predecessor
 */
private Node enq(Node node) {
    for (;;) {
        Node oldTail = tail;
        if (oldTail != null) {
            node.setPrevRelaxed(oldTail);
            if (compareAndSetTail(oldTail, node)) {
                oldTail.next = node;
                return oldTail;
            }
        } else {
            initializeSyncQueue();
        }
    }
}

/**
 * Wakes up node's successor, if one exists.
 *
 * @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)
        node.compareAndSetWaitStatus(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.
     */
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {
        s = null;
        for (Node p = tail; p != node && p != null; p = p.prev)
            if (p.waitStatus <= 0)
                s = p;
    }
    if (s != null)
        LockSupport.unpark(s.thread);
}

/**
 * Release action for shared mode -- signals successor and ensures
 * propagation. (Note: For exclusive mode, release just amounts
 * to calling unparkSuccessor of head if it needs signal.)
 */
private void doReleaseShared() {
    /*
     * Ensure that a release propagates, even if there are other
     * in-progress acquires/releases.  This proceeds in the usual
     * way of trying to unparkSuccessor of head if it needs
     * signal. But if it does not, status is set to PROPAGATE to
     * ensure that upon release, propagation continues.
     * Additionally, we must loop in case a new node is added
     * while we are doing this. Also, unlike other uses of
     * unparkSuccessor, we need to know if CAS to reset status
     * fails, if so rechecking.
     */
    for (;;) {
        Node h = head;
        if (h != null && h != tail) {
            int ws = h.waitStatus;
            if (ws == Node.SIGNAL) {
                if (!h.compareAndSetWaitStatus(Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                unparkSuccessor(h);
            }
            else if (ws == 0 &&
                     !h.compareAndSetWaitStatus(0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed
            break;
    }
}

/**
 * Sets head of queue, and checks if successor may be waiting
 * in shared mode, if so propagating if either propagate > 0 or
 * PROPAGATE status was set.
 *
 * @param node the node
 * @param propagate the return value from a tryAcquireShared
 */
private void setHeadAndPropagate(Node node, long propagate) {
    Node h = head; // Record old head for check below
    setHead(node);
    /*
     * Try to signal next queued node if:
     *   Propagation was indicated by caller,
     *     or was recorded (as h.waitStatus either before
     *     or after setHead) by a previous operation
     *     (note: this uses sign-check of waitStatus because
     *      PROPAGATE status may transition to SIGNAL.)
     * and
     *   The next node is waiting in shared mode,
     *     or we don't know, because it appears null
     *
     * The conservatism in both of these checks may cause
     * unnecessary wake-ups, but only when there are multiple
     * racing acquires/releases, so most need signals now or soon
     * anyway.
     */
    if (propagate > 0 || h == null || h.waitStatus < 0 ||
        (h = head) == null || h.waitStatus < 0) {
        Node s = node.next;
        if (s == null || s.isShared())
            doReleaseShared();
    }
}

/**
 * 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 {@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.
         */
        return true;
    if (ws > 0) {
        /*
         * Predecessor was cancelled. Skip over predecessors and
         * indicate retry.
         */
        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.
         */
        pred.compareAndSetWaitStatus(ws, Node.SIGNAL);
    }
    return false;
}

/**
 * Acquires in exclusive timed mode.
 *
 * @param arg the acquire argument
 * @param nanosTimeout max wait time
 * @return {@code true} if acquired
 */
private boolean doAcquireNanos(long arg, long nanosTimeout)
        throws InterruptedException {
    if (nanosTimeout <= 0L)
        return false;
    final long deadline = System.nanoTime() + nanosTimeout;
    final Node node = addWaiter(Node.EXCLUSIVE);
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                return true;
            }
            nanosTimeout = deadline - System.nanoTime();
            if (nanosTimeout <= 0L) {
                cancelAcquire(node);
                return false;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
                LockSupport.parkNanos(this, nanosTimeout);
            if (Thread.interrupted())
                throw new InterruptedException();
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Acquires in shared timed mode.
 *
 * @param arg the acquire argument
 * @param nanosTimeout max wait time
 * @return {@code true} if acquired
 */
private boolean doAcquireSharedNanos(long arg, long nanosTimeout)
        throws InterruptedException {
    if (nanosTimeout <= 0L)
        return false;
    final long deadline = System.nanoTime() + nanosTimeout;
    final Node node = addWaiter(Node.SHARED);
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {
                long r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    return true;
                }
            }
            nanosTimeout = deadline - System.nanoTime();
            if (nanosTimeout <= 0L) {
                cancelAcquire(node);
                return false;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
                LockSupport.parkNanos(this, nanosTimeout);
            if (Thread.interrupted())
                throw new InterruptedException();
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

/**
 * Version of getFirstQueuedThread called when fastpath fails.
 */
private Thread fullGetFirstQueuedThread() {
    /*
     * The first node is normally head.next. Try to get its
     * thread field, ensuring consistent reads: If thread
     * field is nulled out or s.prev is no longer head, then
     * some other thread(s) concurrently performed setHead in
     * between some of our reads. We try this twice before
     * resorting to traversal.
     */
    Node h, s;
    Thread st;
    if (((h = head) != null && (s = h.next) != null &&
         s.prev == head && (st = s.thread) != null) ||
        ((h = head) != null && (s = h.next) != null &&
         s.prev == head && (st = s.thread) != null))
        return st;

    /*
     * Head's next field might not have been set yet, or may have
     * been unset after setHead. So we must check to see if tail
     * is actually first node. If not, we continue on, safely
     * traversing from tail back to head to find first,
     * guaranteeing termination.
     */

    Thread firstThread = null;
    for (Node p = tail; p != null && p != head; p = p.prev) {
        Thread t = p.thread;
        if (t != null)
            firstThread = t;
    }
    return firstThread;
}

/**
 * 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<>();
    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<>();
    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 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) {
    // We check for node first, since it's likely to be at or near tail.
    // tail is known to be non-null, so we could re-order to "save"
    // one null check, but we leave it this way to help the VM.
    for (Node p = tail;;) {
        if (p == node)
            return true;
        if (p == null)
            return false;
        p = p.prev;
    }
}

/**
 * Invokes release with current state value; returns saved state.
 * Cancels node and throws exception on failure.
 * @param node the condition node for this wait
 * @return previous sync state
 */
final long fullyRelease(Node node) {
    try {
        long savedState = getState();
        if (release(savedState))
            return savedState;
        throw new IllegalMonitorStateException();
    } catch (Throwable t) {
        node.waitStatus = Node.CANCELLED;
        throw t;
    }
}

/**
 * Removes and transfers 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);
}

/**
 * Removes and transfers 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);
}

/**
 * Moves all threads from the wait queue for this condition to
 * the wait queue for the owning lock.
 *
 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
 *         returns {@code false}
 */
public final void signalAll() {
    if (!isHeldExclusively())
        throw new IllegalMonitorStateException();
    Node first = firstWaiter;
    if (first != null)
        doSignalAll(first);
}

/**
 * Implements uninterruptible condition wait.
 * <ol>
 * <li>Save lock state returned by {@link #getState}.
 * <li>Invoke {@link #release} with saved state as argument,
 *     throwing IllegalMonitorStateException if it fails.
 * <li>Block until signalled.
 * <li>Reacquire by invoking specialized version of
 *     {@link #acquire} with saved state as argument.
 * </ol>
 */
public final void awaitUninterruptibly() {
    Node node = addConditionWaiter();
    long savedState = fullyRelease(node);
    boolean interrupted = false;
    while (!isOnSyncQueue(node)) {
        LockSupport.park(this);
        if (Thread.interrupted())
            interrupted = true;
    }
    if (acquireQueued(node, savedState) || interrupted)
        selfInterrupt();
}

/**
 * Implements timed condition wait.
 * <ol>
 * <li>If current thread is interrupted, throw InterruptedException.
 * <li>Save lock state returned by {@link #getState}.
 * <li>Invoke {@link #release} with saved state as argument,
 *     throwing IllegalMonitorStateException if it fails.
 * <li>Block until signalled, interrupted, or timed out.
 * <li>Reacquire by invoking specialized version of
 *     {@link #acquire} with saved state as argument.
 * <li>If interrupted while blocked in step 4, throw InterruptedException.
 * </ol>
 */
public final long awaitNanos(long nanosTimeout)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    // We don't check for nanosTimeout <= 0L here, to allow
    // awaitNanos(0) as a way to "yield the lock".
    final long deadline = System.nanoTime() + nanosTimeout;
    long initialNanos = nanosTimeout;
    Node node = addConditionWaiter();
    long savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        if (nanosTimeout <= 0L) {
            transferAfterCancelledWait(node);
            break;
        }
        if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
            LockSupport.parkNanos(this, nanosTimeout);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
        nanosTimeout = deadline - System.nanoTime();
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null)
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
    long remaining = deadline - System.nanoTime(); // avoid overflow
    return (remaining <= initialNanos) ? remaining : Long.MIN_VALUE;
}

/**
 * Implements absolute timed condition wait.
 * <ol>
 * <li>If current thread is interrupted, throw InterruptedException.
 * <li>Save lock state returned by {@link #getState}.
 * <li>Invoke {@link #release} with saved state as argument,
 *     throwing IllegalMonitorStateException if it fails.
 * <li>Block until signalled, interrupted, or timed out.
 * <li>Reacquire by invoking specialized version of
 *     {@link #acquire} with saved state as argument.
 * <li>If interrupted while blocked in step 4, throw InterruptedException.
 * <li>If timed out while blocked in step 4, return false, else true.
 * </ol>
 */
public final boolean awaitUntil(Date deadline)
        throws InterruptedException {
    long abstime = deadline.getTime();
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();
    long savedState = fullyRelease(node);
    boolean timedout = false;
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        if (System.currentTimeMillis() >= abstime) {
            timedout = transferAfterCancelledWait(node);
            break;
        }
        LockSupport.parkUntil(this, abstime);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null)
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
    return !timedout;
}