jaroslav@1890: /*
jaroslav@1890:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@1890:  *
jaroslav@1890:  * This code is free software; you can redistribute it and/or modify it
jaroslav@1890:  * under the terms of the GNU General Public License version 2 only, as
jaroslav@1890:  * published by the Free Software Foundation.  Oracle designates this
jaroslav@1890:  * particular file as subject to the "Classpath" exception as provided
jaroslav@1890:  * by Oracle in the LICENSE file that accompanied this code.
jaroslav@1890:  *
jaroslav@1890:  * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@1890:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@1890:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
jaroslav@1890:  * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@1890:  * accompanied this code).
jaroslav@1890:  *
jaroslav@1890:  * You should have received a copy of the GNU General Public License version
jaroslav@1890:  * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@1890:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@1890:  *
jaroslav@1890:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@1890:  * or visit www.oracle.com if you need additional information or have any
jaroslav@1890:  * questions.
jaroslav@1890:  */
jaroslav@1890: 
jaroslav@1890: /*
jaroslav@1890:  * This file is available under and governed by the GNU General Public
jaroslav@1890:  * License version 2 only, as published by the Free Software Foundation.
jaroslav@1890:  * However, the following notice accompanied the original version of this
jaroslav@1890:  * file:
jaroslav@1890:  *
jaroslav@1890:  * Written by Doug Lea, Bill Scherer, and Michael Scott with
jaroslav@1890:  * assistance from members of JCP JSR-166 Expert Group and released to
jaroslav@1890:  * the public domain, as explained at
jaroslav@1890:  * http://creativecommons.org/publicdomain/zero/1.0/
jaroslav@1890:  */
jaroslav@1890: 
jaroslav@1890: package java.util.concurrent;
jaroslav@1890: import java.util.concurrent.locks.*;
jaroslav@1890: import java.util.*;
jaroslav@1890: 
jaroslav@1890: /**
jaroslav@1890:  * A {@linkplain BlockingQueue blocking queue} in which each insert
jaroslav@1890:  * operation must wait for a corresponding remove operation by another
jaroslav@1890:  * thread, and vice versa.  A synchronous queue does not have any
jaroslav@1890:  * internal capacity, not even a capacity of one.  You cannot
jaroslav@1890:  * <tt>peek</tt> at a synchronous queue because an element is only
jaroslav@1890:  * present when you try to remove it; you cannot insert an element
jaroslav@1890:  * (using any method) unless another thread is trying to remove it;
jaroslav@1890:  * you cannot iterate as there is nothing to iterate.  The
jaroslav@1890:  * <em>head</em> of the queue is the element that the first queued
jaroslav@1890:  * inserting thread is trying to add to the queue; if there is no such
jaroslav@1890:  * queued thread then no element is available for removal and
jaroslav@1890:  * <tt>poll()</tt> will return <tt>null</tt>.  For purposes of other
jaroslav@1890:  * <tt>Collection</tt> methods (for example <tt>contains</tt>), a
jaroslav@1890:  * <tt>SynchronousQueue</tt> acts as an empty collection.  This queue
jaroslav@1890:  * does not permit <tt>null</tt> elements.
jaroslav@1890:  *
jaroslav@1890:  * <p>Synchronous queues are similar to rendezvous channels used in
jaroslav@1890:  * CSP and Ada. They are well suited for handoff designs, in which an
jaroslav@1890:  * object running in one thread must sync up with an object running
jaroslav@1890:  * in another thread in order to hand it some information, event, or
jaroslav@1890:  * task.
jaroslav@1890:  *
jaroslav@1890:  * <p> This class supports an optional fairness policy for ordering
jaroslav@1890:  * waiting producer and consumer threads.  By default, this ordering
jaroslav@1890:  * is not guaranteed. However, a queue constructed with fairness set
jaroslav@1890:  * to <tt>true</tt> grants threads access in FIFO order.
jaroslav@1890:  *
jaroslav@1890:  * <p>This class and its iterator implement all of the
jaroslav@1890:  * <em>optional</em> methods of the {@link Collection} and {@link
jaroslav@1890:  * Iterator} interfaces.
jaroslav@1890:  *
jaroslav@1890:  * <p>This class is a member of the
jaroslav@1890:  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
jaroslav@1890:  * Java Collections Framework</a>.
jaroslav@1890:  *
jaroslav@1890:  * @since 1.5
jaroslav@1890:  * @author Doug Lea and Bill Scherer and Michael Scott
jaroslav@1890:  * @param <E> the type of elements held in this collection
jaroslav@1890:  */
jaroslav@1890: public class SynchronousQueue<E> extends AbstractQueue<E>
jaroslav@1890:     implements BlockingQueue<E>, java.io.Serializable {
jaroslav@1890:     private static final long serialVersionUID = -3223113410248163686L;
jaroslav@1890: 
jaroslav@1890:     /*
jaroslav@1890:      * This class implements extensions of the dual stack and dual
jaroslav@1890:      * queue algorithms described in "Nonblocking Concurrent Objects
jaroslav@1890:      * with Condition Synchronization", by W. N. Scherer III and
jaroslav@1890:      * M. L. Scott.  18th Annual Conf. on Distributed Computing,
jaroslav@1890:      * Oct. 2004 (see also
jaroslav@1890:      * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
jaroslav@1890:      * The (Lifo) stack is used for non-fair mode, and the (Fifo)
jaroslav@1890:      * queue for fair mode. The performance of the two is generally
jaroslav@1890:      * similar. Fifo usually supports higher throughput under
jaroslav@1890:      * contention but Lifo maintains higher thread locality in common
jaroslav@1890:      * applications.
jaroslav@1890:      *
jaroslav@1890:      * A dual queue (and similarly stack) is one that at any given
jaroslav@1890:      * time either holds "data" -- items provided by put operations,
jaroslav@1890:      * or "requests" -- slots representing take operations, or is
jaroslav@1890:      * empty. A call to "fulfill" (i.e., a call requesting an item
jaroslav@1890:      * from a queue holding data or vice versa) dequeues a
jaroslav@1890:      * complementary node.  The most interesting feature of these
jaroslav@1890:      * queues is that any operation can figure out which mode the
jaroslav@1890:      * queue is in, and act accordingly without needing locks.
jaroslav@1890:      *
jaroslav@1890:      * Both the queue and stack extend abstract class Transferer
jaroslav@1890:      * defining the single method transfer that does a put or a
jaroslav@1890:      * take. These are unified into a single method because in dual
jaroslav@1890:      * data structures, the put and take operations are symmetrical,
jaroslav@1890:      * so nearly all code can be combined. The resulting transfer
jaroslav@1890:      * methods are on the long side, but are easier to follow than
jaroslav@1890:      * they would be if broken up into nearly-duplicated parts.
jaroslav@1890:      *
jaroslav@1890:      * The queue and stack data structures share many conceptual
jaroslav@1890:      * similarities but very few concrete details. For simplicity,
jaroslav@1890:      * they are kept distinct so that they can later evolve
jaroslav@1890:      * separately.
jaroslav@1890:      *
jaroslav@1890:      * The algorithms here differ from the versions in the above paper
jaroslav@1890:      * in extending them for use in synchronous queues, as well as
jaroslav@1890:      * dealing with cancellation. The main differences include:
jaroslav@1890:      *
jaroslav@1890:      *  1. The original algorithms used bit-marked pointers, but
jaroslav@1890:      *     the ones here use mode bits in nodes, leading to a number
jaroslav@1890:      *     of further adaptations.
jaroslav@1890:      *  2. SynchronousQueues must block threads waiting to become
jaroslav@1890:      *     fulfilled.
jaroslav@1890:      *  3. Support for cancellation via timeout and interrupts,
jaroslav@1890:      *     including cleaning out cancelled nodes/threads
jaroslav@1890:      *     from lists to avoid garbage retention and memory depletion.
jaroslav@1890:      *
jaroslav@1890:      * Blocking is mainly accomplished using LockSupport park/unpark,
jaroslav@1890:      * except that nodes that appear to be the next ones to become
jaroslav@1890:      * fulfilled first spin a bit (on multiprocessors only). On very
jaroslav@1890:      * busy synchronous queues, spinning can dramatically improve
jaroslav@1890:      * throughput. And on less busy ones, the amount of spinning is
jaroslav@1890:      * small enough not to be noticeable.
jaroslav@1890:      *
jaroslav@1890:      * Cleaning is done in different ways in queues vs stacks.  For
jaroslav@1890:      * queues, we can almost always remove a node immediately in O(1)
jaroslav@1890:      * time (modulo retries for consistency checks) when it is
jaroslav@1890:      * cancelled. But if it may be pinned as the current tail, it must
jaroslav@1890:      * wait until some subsequent cancellation. For stacks, we need a
jaroslav@1890:      * potentially O(n) traversal to be sure that we can remove the
jaroslav@1890:      * node, but this can run concurrently with other threads
jaroslav@1890:      * accessing the stack.
jaroslav@1890:      *
jaroslav@1890:      * While garbage collection takes care of most node reclamation
jaroslav@1890:      * issues that otherwise complicate nonblocking algorithms, care
jaroslav@1890:      * is taken to "forget" references to data, other nodes, and
jaroslav@1890:      * threads that might be held on to long-term by blocked
jaroslav@1890:      * threads. In cases where setting to null would otherwise
jaroslav@1890:      * conflict with main algorithms, this is done by changing a
jaroslav@1890:      * node's link to now point to the node itself. This doesn't arise
jaroslav@1890:      * much for Stack nodes (because blocked threads do not hang on to
jaroslav@1890:      * old head pointers), but references in Queue nodes must be
jaroslav@1890:      * aggressively forgotten to avoid reachability of everything any
jaroslav@1890:      * node has ever referred to since arrival.
jaroslav@1890:      */
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Shared internal API for dual stacks and queues.
jaroslav@1890:      */
jaroslav@1890:     abstract static class Transferer {
jaroslav@1890:         /**
jaroslav@1890:          * Performs a put or take.
jaroslav@1890:          *
jaroslav@1890:          * @param e if non-null, the item to be handed to a consumer;
jaroslav@1890:          *          if null, requests that transfer return an item
jaroslav@1890:          *          offered by producer.
jaroslav@1890:          * @param timed if this operation should timeout
jaroslav@1890:          * @param nanos the timeout, in nanoseconds
jaroslav@1890:          * @return if non-null, the item provided or received; if null,
jaroslav@1890:          *         the operation failed due to timeout or interrupt --
jaroslav@1890:          *         the caller can distinguish which of these occurred
jaroslav@1890:          *         by checking Thread.interrupted.
jaroslav@1890:          */
jaroslav@1890:         abstract Object transfer(Object e, boolean timed, long nanos);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /** The number of CPUs, for spin control */
jaroslav@1895:     static final int NCPUS = 1;
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * The number of times to spin before blocking in timed waits.
jaroslav@1890:      * The value is empirically derived -- it works well across a
jaroslav@1890:      * variety of processors and OSes. Empirically, the best value
jaroslav@1890:      * seems not to vary with number of CPUs (beyond 2) so is just
jaroslav@1890:      * a constant.
jaroslav@1890:      */
jaroslav@1890:     static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * The number of times to spin before blocking in untimed waits.
jaroslav@1890:      * This is greater than timed value because untimed waits spin
jaroslav@1890:      * faster since they don't need to check times on each spin.
jaroslav@1890:      */
jaroslav@1890:     static final int maxUntimedSpins = maxTimedSpins * 16;
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * The number of nanoseconds for which it is faster to spin
jaroslav@1890:      * rather than to use timed park. A rough estimate suffices.
jaroslav@1890:      */
jaroslav@1890:     static final long spinForTimeoutThreshold = 1000L;
jaroslav@1890: 
jaroslav@1890:     /** Dual stack */
jaroslav@1890:     static final class TransferStack extends Transferer {
jaroslav@1890:         /*
jaroslav@1890:          * This extends Scherer-Scott dual stack algorithm, differing,
jaroslav@1890:          * among other ways, by using "covering" nodes rather than
jaroslav@1890:          * bit-marked pointers: Fulfilling operations push on marker
jaroslav@1890:          * nodes (with FULFILLING bit set in mode) to reserve a spot
jaroslav@1890:          * to match a waiting node.
jaroslav@1890:          */
jaroslav@1890: 
jaroslav@1890:         /* Modes for SNodes, ORed together in node fields */
jaroslav@1890:         /** Node represents an unfulfilled consumer */
jaroslav@1890:         static final int REQUEST    = 0;
jaroslav@1890:         /** Node represents an unfulfilled producer */
jaroslav@1890:         static final int DATA       = 1;
jaroslav@1890:         /** Node is fulfilling another unfulfilled DATA or REQUEST */
jaroslav@1890:         static final int FULFILLING = 2;
jaroslav@1890: 
jaroslav@1890:         /** Return true if m has fulfilling bit set */
jaroslav@1890:         static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
jaroslav@1890: 
jaroslav@1890:         /** Node class for TransferStacks. */
jaroslav@1890:         static final class SNode {
jaroslav@1890:             volatile SNode next;        // next node in stack
jaroslav@1890:             volatile SNode match;       // the node matched to this
jaroslav@1890:             volatile Thread waiter;     // to control park/unpark
jaroslav@1890:             Object item;                // data; or null for REQUESTs
jaroslav@1890:             int mode;
jaroslav@1890:             // Note: item and mode fields don't need to be volatile
jaroslav@1890:             // since they are always written before, and read after,
jaroslav@1890:             // other volatile/atomic operations.
jaroslav@1890: 
jaroslav@1890:             SNode(Object item) {
jaroslav@1890:                 this.item = item;
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             boolean casNext(SNode cmp, SNode val) {
jaroslav@1895:                 if (next == cmp) {
jaroslav@1895:                     next = val;
jaroslav@1895:                     return true;
jaroslav@1895:                 }
jaroslav@1895:                 return false;
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             /**
jaroslav@1890:              * Tries to match node s to this node, if so, waking up thread.
jaroslav@1890:              * Fulfillers call tryMatch to identify their waiters.
jaroslav@1890:              * Waiters block until they have been matched.
jaroslav@1890:              *
jaroslav@1890:              * @param s the node to match
jaroslav@1890:              * @return true if successfully matched to s
jaroslav@1890:              */
jaroslav@1890:             boolean tryMatch(SNode s) {
jaroslav@1895:                 if (match == null) {
jaroslav@1895:                     match = s;
jaroslav@1890:                     Thread w = waiter;
jaroslav@1890:                     if (w != null) {    // waiters need at most one unpark
jaroslav@1890:                         waiter = null;
jaroslav@1890:                         LockSupport.unpark(w);
jaroslav@1890:                     }
jaroslav@1890:                     return true;
jaroslav@1890:                 }
jaroslav@1890:                 return match == s;
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             /**
jaroslav@1890:              * Tries to cancel a wait by matching node to itself.
jaroslav@1890:              */
jaroslav@1890:             void tryCancel() {
jaroslav@1895:                 if (match == null) {
jaroslav@1895:                     match = this;
jaroslav@1895:                 }
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             boolean isCancelled() {
jaroslav@1890:                 return match == this;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /** The head (top) of the stack */
jaroslav@1890:         volatile SNode head;
jaroslav@1890: 
jaroslav@1890:         boolean casHead(SNode h, SNode nh) {
jaroslav@1895:             if (head == h) {
jaroslav@1895:                 head = nh;
jaroslav@1895:                 return true;
jaroslav@1895:             }
jaroslav@1895:             return false;
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Creates or resets fields of a node. Called only from transfer
jaroslav@1890:          * where the node to push on stack is lazily created and
jaroslav@1890:          * reused when possible to help reduce intervals between reads
jaroslav@1890:          * and CASes of head and to avoid surges of garbage when CASes
jaroslav@1890:          * to push nodes fail due to contention.
jaroslav@1890:          */
jaroslav@1890:         static SNode snode(SNode s, Object e, SNode next, int mode) {
jaroslav@1890:             if (s == null) s = new SNode(e);
jaroslav@1890:             s.mode = mode;
jaroslav@1890:             s.next = next;
jaroslav@1890:             return s;
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Puts or takes an item.
jaroslav@1890:          */
jaroslav@1890:         Object transfer(Object e, boolean timed, long nanos) {
jaroslav@1890:             /*
jaroslav@1890:              * Basic algorithm is to loop trying one of three actions:
jaroslav@1890:              *
jaroslav@1890:              * 1. If apparently empty or already containing nodes of same
jaroslav@1890:              *    mode, try to push node on stack and wait for a match,
jaroslav@1890:              *    returning it, or null if cancelled.
jaroslav@1890:              *
jaroslav@1890:              * 2. If apparently containing node of complementary mode,
jaroslav@1890:              *    try to push a fulfilling node on to stack, match
jaroslav@1890:              *    with corresponding waiting node, pop both from
jaroslav@1890:              *    stack, and return matched item. The matching or
jaroslav@1890:              *    unlinking might not actually be necessary because of
jaroslav@1890:              *    other threads performing action 3:
jaroslav@1890:              *
jaroslav@1890:              * 3. If top of stack already holds another fulfilling node,
jaroslav@1890:              *    help it out by doing its match and/or pop
jaroslav@1890:              *    operations, and then continue. The code for helping
jaroslav@1890:              *    is essentially the same as for fulfilling, except
jaroslav@1890:              *    that it doesn't return the item.
jaroslav@1890:              */
jaroslav@1890: 
jaroslav@1890:             SNode s = null; // constructed/reused as needed
jaroslav@1890:             int mode = (e == null) ? REQUEST : DATA;
jaroslav@1890: 
jaroslav@1890:             for (;;) {
jaroslav@1890:                 SNode h = head;
jaroslav@1890:                 if (h == null || h.mode == mode) {  // empty or same-mode
jaroslav@1890:                     if (timed && nanos <= 0) {      // can't wait
jaroslav@1890:                         if (h != null && h.isCancelled())
jaroslav@1890:                             casHead(h, h.next);     // pop cancelled node
jaroslav@1890:                         else
jaroslav@1890:                             return null;
jaroslav@1890:                     } else if (casHead(h, s = snode(s, e, h, mode))) {
jaroslav@1890:                         SNode m = awaitFulfill(s, timed, nanos);
jaroslav@1890:                         if (m == s) {               // wait was cancelled
jaroslav@1890:                             clean(s);
jaroslav@1890:                             return null;
jaroslav@1890:                         }
jaroslav@1890:                         if ((h = head) != null && h.next == s)
jaroslav@1890:                             casHead(h, s.next);     // help s's fulfiller
jaroslav@1890:                         return (mode == REQUEST) ? m.item : s.item;
jaroslav@1890:                     }
jaroslav@1890:                 } else if (!isFulfilling(h.mode)) { // try to fulfill
jaroslav@1890:                     if (h.isCancelled())            // already cancelled
jaroslav@1890:                         casHead(h, h.next);         // pop and retry
jaroslav@1890:                     else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
jaroslav@1890:                         for (;;) { // loop until matched or waiters disappear
jaroslav@1890:                             SNode m = s.next;       // m is s's match
jaroslav@1890:                             if (m == null) {        // all waiters are gone
jaroslav@1890:                                 casHead(s, null);   // pop fulfill node
jaroslav@1890:                                 s = null;           // use new node next time
jaroslav@1890:                                 break;              // restart main loop
jaroslav@1890:                             }
jaroslav@1890:                             SNode mn = m.next;
jaroslav@1890:                             if (m.tryMatch(s)) {
jaroslav@1890:                                 casHead(s, mn);     // pop both s and m
jaroslav@1890:                                 return (mode == REQUEST) ? m.item : s.item;
jaroslav@1890:                             } else                  // lost match
jaroslav@1890:                                 s.casNext(m, mn);   // help unlink
jaroslav@1890:                         }
jaroslav@1890:                     }
jaroslav@1890:                 } else {                            // help a fulfiller
jaroslav@1890:                     SNode m = h.next;               // m is h's match
jaroslav@1890:                     if (m == null)                  // waiter is gone
jaroslav@1890:                         casHead(h, null);           // pop fulfilling node
jaroslav@1890:                     else {
jaroslav@1890:                         SNode mn = m.next;
jaroslav@1890:                         if (m.tryMatch(h))          // help match
jaroslav@1890:                             casHead(h, mn);         // pop both h and m
jaroslav@1890:                         else                        // lost match
jaroslav@1890:                             h.casNext(m, mn);       // help unlink
jaroslav@1890:                     }
jaroslav@1890:                 }
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Spins/blocks until node s is matched by a fulfill operation.
jaroslav@1890:          *
jaroslav@1890:          * @param s the waiting node
jaroslav@1890:          * @param timed true if timed wait
jaroslav@1890:          * @param nanos timeout value
jaroslav@1890:          * @return matched node, or s if cancelled
jaroslav@1890:          */
jaroslav@1890:         SNode awaitFulfill(SNode s, boolean timed, long nanos) {
jaroslav@1890:             /*
jaroslav@1890:              * When a node/thread is about to block, it sets its waiter
jaroslav@1890:              * field and then rechecks state at least one more time
jaroslav@1890:              * before actually parking, thus covering race vs
jaroslav@1890:              * fulfiller noticing that waiter is non-null so should be
jaroslav@1890:              * woken.
jaroslav@1890:              *
jaroslav@1890:              * When invoked by nodes that appear at the point of call
jaroslav@1890:              * to be at the head of the stack, calls to park are
jaroslav@1890:              * preceded by spins to avoid blocking when producers and
jaroslav@1890:              * consumers are arriving very close in time.  This can
jaroslav@1890:              * happen enough to bother only on multiprocessors.
jaroslav@1890:              *
jaroslav@1890:              * The order of checks for returning out of main loop
jaroslav@1890:              * reflects fact that interrupts have precedence over
jaroslav@1890:              * normal returns, which have precedence over
jaroslav@1890:              * timeouts. (So, on timeout, one last check for match is
jaroslav@1890:              * done before giving up.) Except that calls from untimed
jaroslav@1890:              * SynchronousQueue.{poll/offer} don't check interrupts
jaroslav@1890:              * and don't wait at all, so are trapped in transfer
jaroslav@1890:              * method rather than calling awaitFulfill.
jaroslav@1890:              */
jaroslav@1890:             long lastTime = timed ? System.nanoTime() : 0;
jaroslav@1890:             Thread w = Thread.currentThread();
jaroslav@1890:             SNode h = head;
jaroslav@1890:             int spins = (shouldSpin(s) ?
jaroslav@1890:                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);
jaroslav@1890:             for (;;) {
jaroslav@1890:                 if (w.isInterrupted())
jaroslav@1890:                     s.tryCancel();
jaroslav@1890:                 SNode m = s.match;
jaroslav@1890:                 if (m != null)
jaroslav@1890:                     return m;
jaroslav@1890:                 if (timed) {
jaroslav@1890:                     long now = System.nanoTime();
jaroslav@1890:                     nanos -= now - lastTime;
jaroslav@1890:                     lastTime = now;
jaroslav@1890:                     if (nanos <= 0) {
jaroslav@1890:                         s.tryCancel();
jaroslav@1890:                         continue;
jaroslav@1890:                     }
jaroslav@1890:                 }
jaroslav@1890:                 if (spins > 0)
jaroslav@1890:                     spins = shouldSpin(s) ? (spins-1) : 0;
jaroslav@1890:                 else if (s.waiter == null)
jaroslav@1890:                     s.waiter = w; // establish waiter so can park next iter
jaroslav@1890:                 else if (!timed)
jaroslav@1890:                     LockSupport.park(this);
jaroslav@1890:                 else if (nanos > spinForTimeoutThreshold)
jaroslav@1890:                     LockSupport.parkNanos(this, nanos);
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Returns true if node s is at head or there is an active
jaroslav@1890:          * fulfiller.
jaroslav@1890:          */
jaroslav@1890:         boolean shouldSpin(SNode s) {
jaroslav@1890:             SNode h = head;
jaroslav@1890:             return (h == s || h == null || isFulfilling(h.mode));
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Unlinks s from the stack.
jaroslav@1890:          */
jaroslav@1890:         void clean(SNode s) {
jaroslav@1890:             s.item = null;   // forget item
jaroslav@1890:             s.waiter = null; // forget thread
jaroslav@1890: 
jaroslav@1890:             /*
jaroslav@1890:              * At worst we may need to traverse entire stack to unlink
jaroslav@1890:              * s. If there are multiple concurrent calls to clean, we
jaroslav@1890:              * might not see s if another thread has already removed
jaroslav@1890:              * it. But we can stop when we see any node known to
jaroslav@1890:              * follow s. We use s.next unless it too is cancelled, in
jaroslav@1890:              * which case we try the node one past. We don't check any
jaroslav@1890:              * further because we don't want to doubly traverse just to
jaroslav@1890:              * find sentinel.
jaroslav@1890:              */
jaroslav@1890: 
jaroslav@1890:             SNode past = s.next;
jaroslav@1890:             if (past != null && past.isCancelled())
jaroslav@1890:                 past = past.next;
jaroslav@1890: 
jaroslav@1890:             // Absorb cancelled nodes at head
jaroslav@1890:             SNode p;
jaroslav@1890:             while ((p = head) != null && p != past && p.isCancelled())
jaroslav@1890:                 casHead(p, p.next);
jaroslav@1890: 
jaroslav@1890:             // Unsplice embedded nodes
jaroslav@1890:             while (p != null && p != past) {
jaroslav@1890:                 SNode n = p.next;
jaroslav@1890:                 if (n != null && n.isCancelled())
jaroslav@1890:                     p.casNext(n, n.next);
jaroslav@1890:                 else
jaroslav@1890:                     p = n;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /** Dual Queue */
jaroslav@1890:     static final class TransferQueue extends Transferer {
jaroslav@1890:         /*
jaroslav@1890:          * This extends Scherer-Scott dual queue algorithm, differing,
jaroslav@1890:          * among other ways, by using modes within nodes rather than
jaroslav@1890:          * marked pointers. The algorithm is a little simpler than
jaroslav@1890:          * that for stacks because fulfillers do not need explicit
jaroslav@1890:          * nodes, and matching is done by CAS'ing QNode.item field
jaroslav@1890:          * from non-null to null (for put) or vice versa (for take).
jaroslav@1890:          */
jaroslav@1890: 
jaroslav@1890:         /** Node class for TransferQueue. */
jaroslav@1890:         static final class QNode {
jaroslav@1890:             volatile QNode next;          // next node in queue
jaroslav@1890:             volatile Object item;         // CAS'ed to or from null
jaroslav@1890:             volatile Thread waiter;       // to control park/unpark
jaroslav@1890:             final boolean isData;
jaroslav@1890: 
jaroslav@1890:             QNode(Object item, boolean isData) {
jaroslav@1890:                 this.item = item;
jaroslav@1890:                 this.isData = isData;
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             boolean casNext(QNode cmp, QNode val) {
jaroslav@1895:                 if (next == cmp) {
jaroslav@1895:                     next = val;
jaroslav@1895:                     return true;
jaroslav@1895:                 }
jaroslav@1895:                 return false;
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             boolean casItem(Object cmp, Object val) {
jaroslav@1895:                 if (item == cmp) {
jaroslav@1895:                     item = val;
jaroslav@1895:                     return true;
jaroslav@1895:                 }
jaroslav@1895:                 return false;
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             /**
jaroslav@1890:              * Tries to cancel by CAS'ing ref to this as item.
jaroslav@1890:              */
jaroslav@1890:             void tryCancel(Object cmp) {
jaroslav@1895:                 if (item == cmp) {
jaroslav@1895:                     item = this;
jaroslav@1895:                 }
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             boolean isCancelled() {
jaroslav@1890:                 return item == this;
jaroslav@1890:             }
jaroslav@1890: 
jaroslav@1890:             /**
jaroslav@1890:              * Returns true if this node is known to be off the queue
jaroslav@1890:              * because its next pointer has been forgotten due to
jaroslav@1890:              * an advanceHead operation.
jaroslav@1890:              */
jaroslav@1890:             boolean isOffList() {
jaroslav@1890:                 return next == this;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /** Head of queue */
jaroslav@1890:         transient volatile QNode head;
jaroslav@1890:         /** Tail of queue */
jaroslav@1890:         transient volatile QNode tail;
jaroslav@1890:         /**
jaroslav@1890:          * Reference to a cancelled node that might not yet have been
jaroslav@1890:          * unlinked from queue because it was the last inserted node
jaroslav@1890:          * when it cancelled.
jaroslav@1890:          */
jaroslav@1890:         transient volatile QNode cleanMe;
jaroslav@1890: 
jaroslav@1890:         TransferQueue() {
jaroslav@1890:             QNode h = new QNode(null, false); // initialize to dummy node.
jaroslav@1890:             head = h;
jaroslav@1890:             tail = h;
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Tries to cas nh as new head; if successful, unlink
jaroslav@1890:          * old head's next node to avoid garbage retention.
jaroslav@1890:          */
jaroslav@1890:         void advanceHead(QNode h, QNode nh) {
jaroslav@1895:             if (head == h) {
jaroslav@1895:                 head = nh;
jaroslav@1890:                 h.next = h; // forget old next
jaroslav@1895:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Tries to cas nt as new tail.
jaroslav@1890:          */
jaroslav@1890:         void advanceTail(QNode t, QNode nt) {
jaroslav@1895:             if (tail == t) {
jaroslav@1895:                 tail = nt;
jaroslav@1895:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Tries to CAS cleanMe slot.
jaroslav@1890:          */
jaroslav@1890:         boolean casCleanMe(QNode cmp, QNode val) {
jaroslav@1895:             if (cleanMe == cmp) {
jaroslav@1895:                 cleanMe = val;
jaroslav@1895:                 return true;
jaroslav@1895:             }
jaroslav@1895:             return false;
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Puts or takes an item.
jaroslav@1890:          */
jaroslav@1890:         Object transfer(Object e, boolean timed, long nanos) {
jaroslav@1890:             /* Basic algorithm is to loop trying to take either of
jaroslav@1890:              * two actions:
jaroslav@1890:              *
jaroslav@1890:              * 1. If queue apparently empty or holding same-mode nodes,
jaroslav@1890:              *    try to add node to queue of waiters, wait to be
jaroslav@1890:              *    fulfilled (or cancelled) and return matching item.
jaroslav@1890:              *
jaroslav@1890:              * 2. If queue apparently contains waiting items, and this
jaroslav@1890:              *    call is of complementary mode, try to fulfill by CAS'ing
jaroslav@1890:              *    item field of waiting node and dequeuing it, and then
jaroslav@1890:              *    returning matching item.
jaroslav@1890:              *
jaroslav@1890:              * In each case, along the way, check for and try to help
jaroslav@1890:              * advance head and tail on behalf of other stalled/slow
jaroslav@1890:              * threads.
jaroslav@1890:              *
jaroslav@1890:              * The loop starts off with a null check guarding against
jaroslav@1890:              * seeing uninitialized head or tail values. This never
jaroslav@1890:              * happens in current SynchronousQueue, but could if
jaroslav@1890:              * callers held non-volatile/final ref to the
jaroslav@1890:              * transferer. The check is here anyway because it places
jaroslav@1890:              * null checks at top of loop, which is usually faster
jaroslav@1890:              * than having them implicitly interspersed.
jaroslav@1890:              */
jaroslav@1890: 
jaroslav@1890:             QNode s = null; // constructed/reused as needed
jaroslav@1890:             boolean isData = (e != null);
jaroslav@1890: 
jaroslav@1890:             for (;;) {
jaroslav@1890:                 QNode t = tail;
jaroslav@1890:                 QNode h = head;
jaroslav@1890:                 if (t == null || h == null)         // saw uninitialized value
jaroslav@1890:                     continue;                       // spin
jaroslav@1890: 
jaroslav@1890:                 if (h == t || t.isData == isData) { // empty or same-mode
jaroslav@1890:                     QNode tn = t.next;
jaroslav@1890:                     if (t != tail)                  // inconsistent read
jaroslav@1890:                         continue;
jaroslav@1890:                     if (tn != null) {               // lagging tail
jaroslav@1890:                         advanceTail(t, tn);
jaroslav@1890:                         continue;
jaroslav@1890:                     }
jaroslav@1890:                     if (timed && nanos <= 0)        // can't wait
jaroslav@1890:                         return null;
jaroslav@1890:                     if (s == null)
jaroslav@1890:                         s = new QNode(e, isData);
jaroslav@1890:                     if (!t.casNext(null, s))        // failed to link in
jaroslav@1890:                         continue;
jaroslav@1890: 
jaroslav@1890:                     advanceTail(t, s);              // swing tail and wait
jaroslav@1890:                     Object x = awaitFulfill(s, e, timed, nanos);
jaroslav@1890:                     if (x == s) {                   // wait was cancelled
jaroslav@1890:                         clean(t, s);
jaroslav@1890:                         return null;
jaroslav@1890:                     }
jaroslav@1890: 
jaroslav@1890:                     if (!s.isOffList()) {           // not already unlinked
jaroslav@1890:                         advanceHead(t, s);          // unlink if head
jaroslav@1890:                         if (x != null)              // and forget fields
jaroslav@1890:                             s.item = s;
jaroslav@1890:                         s.waiter = null;
jaroslav@1890:                     }
jaroslav@1890:                     return (x != null) ? x : e;
jaroslav@1890: 
jaroslav@1890:                 } else {                            // complementary-mode
jaroslav@1890:                     QNode m = h.next;               // node to fulfill
jaroslav@1890:                     if (t != tail || m == null || h != head)
jaroslav@1890:                         continue;                   // inconsistent read
jaroslav@1890: 
jaroslav@1890:                     Object x = m.item;
jaroslav@1890:                     if (isData == (x != null) ||    // m already fulfilled
jaroslav@1890:                         x == m ||                   // m cancelled
jaroslav@1890:                         !m.casItem(x, e)) {         // lost CAS
jaroslav@1890:                         advanceHead(h, m);          // dequeue and retry
jaroslav@1890:                         continue;
jaroslav@1890:                     }
jaroslav@1890: 
jaroslav@1890:                     advanceHead(h, m);              // successfully fulfilled
jaroslav@1890:                     LockSupport.unpark(m.waiter);
jaroslav@1890:                     return (x != null) ? x : e;
jaroslav@1890:                 }
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Spins/blocks until node s is fulfilled.
jaroslav@1890:          *
jaroslav@1890:          * @param s the waiting node
jaroslav@1890:          * @param e the comparison value for checking match
jaroslav@1890:          * @param timed true if timed wait
jaroslav@1890:          * @param nanos timeout value
jaroslav@1890:          * @return matched item, or s if cancelled
jaroslav@1890:          */
jaroslav@1890:         Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
jaroslav@1890:             /* Same idea as TransferStack.awaitFulfill */
jaroslav@1890:             long lastTime = timed ? System.nanoTime() : 0;
jaroslav@1890:             Thread w = Thread.currentThread();
jaroslav@1890:             int spins = ((head.next == s) ?
jaroslav@1890:                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);
jaroslav@1890:             for (;;) {
jaroslav@1890:                 if (w.isInterrupted())
jaroslav@1890:                     s.tryCancel(e);
jaroslav@1890:                 Object x = s.item;
jaroslav@1890:                 if (x != e)
jaroslav@1890:                     return x;
jaroslav@1890:                 if (timed) {
jaroslav@1890:                     long now = System.nanoTime();
jaroslav@1890:                     nanos -= now - lastTime;
jaroslav@1890:                     lastTime = now;
jaroslav@1890:                     if (nanos <= 0) {
jaroslav@1890:                         s.tryCancel(e);
jaroslav@1890:                         continue;
jaroslav@1890:                     }
jaroslav@1890:                 }
jaroslav@1890:                 if (spins > 0)
jaroslav@1890:                     --spins;
jaroslav@1890:                 else if (s.waiter == null)
jaroslav@1890:                     s.waiter = w;
jaroslav@1890:                 else if (!timed)
jaroslav@1890:                     LockSupport.park(this);
jaroslav@1890:                 else if (nanos > spinForTimeoutThreshold)
jaroslav@1890:                     LockSupport.parkNanos(this, nanos);
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         /**
jaroslav@1890:          * Gets rid of cancelled node s with original predecessor pred.
jaroslav@1890:          */
jaroslav@1890:         void clean(QNode pred, QNode s) {
jaroslav@1890:             s.waiter = null; // forget thread
jaroslav@1890:             /*
jaroslav@1890:              * At any given time, exactly one node on list cannot be
jaroslav@1890:              * deleted -- the last inserted node. To accommodate this,
jaroslav@1890:              * if we cannot delete s, we save its predecessor as
jaroslav@1890:              * "cleanMe", deleting the previously saved version
jaroslav@1890:              * first. At least one of node s or the node previously
jaroslav@1890:              * saved can always be deleted, so this always terminates.
jaroslav@1890:              */
jaroslav@1890:             while (pred.next == s) { // Return early if already unlinked
jaroslav@1890:                 QNode h = head;
jaroslav@1890:                 QNode hn = h.next;   // Absorb cancelled first node as head
jaroslav@1890:                 if (hn != null && hn.isCancelled()) {
jaroslav@1890:                     advanceHead(h, hn);
jaroslav@1890:                     continue;
jaroslav@1890:                 }
jaroslav@1890:                 QNode t = tail;      // Ensure consistent read for tail
jaroslav@1890:                 if (t == h)
jaroslav@1890:                     return;
jaroslav@1890:                 QNode tn = t.next;
jaroslav@1890:                 if (t != tail)
jaroslav@1890:                     continue;
jaroslav@1890:                 if (tn != null) {
jaroslav@1890:                     advanceTail(t, tn);
jaroslav@1890:                     continue;
jaroslav@1890:                 }
jaroslav@1890:                 if (s != t) {        // If not tail, try to unsplice
jaroslav@1890:                     QNode sn = s.next;
jaroslav@1890:                     if (sn == s || pred.casNext(s, sn))
jaroslav@1890:                         return;
jaroslav@1890:                 }
jaroslav@1890:                 QNode dp = cleanMe;
jaroslav@1890:                 if (dp != null) {    // Try unlinking previous cancelled node
jaroslav@1890:                     QNode d = dp.next;
jaroslav@1890:                     QNode dn;
jaroslav@1890:                     if (d == null ||               // d is gone or
jaroslav@1890:                         d == dp ||                 // d is off list or
jaroslav@1890:                         !d.isCancelled() ||        // d not cancelled or
jaroslav@1890:                         (d != t &&                 // d not tail and
jaroslav@1890:                          (dn = d.next) != null &&  //   has successor
jaroslav@1890:                          dn != d &&                //   that is on list
jaroslav@1890:                          dp.casNext(d, dn)))       // d unspliced
jaroslav@1890:                         casCleanMe(dp, null);
jaroslav@1890:                     if (dp == pred)
jaroslav@1890:                         return;      // s is already saved node
jaroslav@1890:                 } else if (casCleanMe(null, pred))
jaroslav@1890:                     return;          // Postpone cleaning s
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * The transferer. Set only in constructor, but cannot be declared
jaroslav@1890:      * as final without further complicating serialization.  Since
jaroslav@1890:      * this is accessed only at most once per public method, there
jaroslav@1890:      * isn't a noticeable performance penalty for using volatile
jaroslav@1890:      * instead of final here.
jaroslav@1890:      */
jaroslav@1890:     private transient volatile Transferer transferer;
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Creates a <tt>SynchronousQueue</tt> with nonfair access policy.
jaroslav@1890:      */
jaroslav@1890:     public SynchronousQueue() {
jaroslav@1890:         this(false);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.
jaroslav@1890:      *
jaroslav@1890:      * @param fair if true, waiting threads contend in FIFO order for
jaroslav@1890:      *        access; otherwise the order is unspecified.
jaroslav@1890:      */
jaroslav@1890:     public SynchronousQueue(boolean fair) {
jaroslav@1890:         transferer = fair ? new TransferQueue() : new TransferStack();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Adds the specified element to this queue, waiting if necessary for
jaroslav@1890:      * another thread to receive it.
jaroslav@1890:      *
jaroslav@1890:      * @throws InterruptedException {@inheritDoc}
jaroslav@1890:      * @throws NullPointerException {@inheritDoc}
jaroslav@1890:      */
jaroslav@1890:     public void put(E o) throws InterruptedException {
jaroslav@1890:         if (o == null) throw new NullPointerException();
jaroslav@1890:         if (transferer.transfer(o, false, 0) == null) {
jaroslav@1890:             Thread.interrupted();
jaroslav@1890:             throw new InterruptedException();
jaroslav@1890:         }
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Inserts the specified element into this queue, waiting if necessary
jaroslav@1890:      * up to the specified wait time for another thread to receive it.
jaroslav@1890:      *
jaroslav@1890:      * @return <tt>true</tt> if successful, or <tt>false</tt> if the
jaroslav@1890:      *         specified waiting time elapses before a consumer appears.
jaroslav@1890:      * @throws InterruptedException {@inheritDoc}
jaroslav@1890:      * @throws NullPointerException {@inheritDoc}
jaroslav@1890:      */
jaroslav@1890:     public boolean offer(E o, long timeout, TimeUnit unit)
jaroslav@1890:         throws InterruptedException {
jaroslav@1890:         if (o == null) throw new NullPointerException();
jaroslav@1890:         if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
jaroslav@1890:             return true;
jaroslav@1890:         if (!Thread.interrupted())
jaroslav@1890:             return false;
jaroslav@1890:         throw new InterruptedException();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Inserts the specified element into this queue, if another thread is
jaroslav@1890:      * waiting to receive it.
jaroslav@1890:      *
jaroslav@1890:      * @param e the element to add
jaroslav@1890:      * @return <tt>true</tt> if the element was added to this queue, else
jaroslav@1890:      *         <tt>false</tt>
jaroslav@1890:      * @throws NullPointerException if the specified element is null
jaroslav@1890:      */
jaroslav@1890:     public boolean offer(E e) {
jaroslav@1890:         if (e == null) throw new NullPointerException();
jaroslav@1890:         return transferer.transfer(e, true, 0) != null;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Retrieves and removes the head of this queue, waiting if necessary
jaroslav@1890:      * for another thread to insert it.
jaroslav@1890:      *
jaroslav@1890:      * @return the head of this queue
jaroslav@1890:      * @throws InterruptedException {@inheritDoc}
jaroslav@1890:      */
jaroslav@1890:     public E take() throws InterruptedException {
jaroslav@1890:         Object e = transferer.transfer(null, false, 0);
jaroslav@1890:         if (e != null)
jaroslav@1890:             return (E)e;
jaroslav@1890:         Thread.interrupted();
jaroslav@1890:         throw new InterruptedException();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Retrieves and removes the head of this queue, waiting
jaroslav@1890:      * if necessary up to the specified wait time, for another thread
jaroslav@1890:      * to insert it.
jaroslav@1890:      *
jaroslav@1890:      * @return the head of this queue, or <tt>null</tt> if the
jaroslav@1890:      *         specified waiting time elapses before an element is present.
jaroslav@1890:      * @throws InterruptedException {@inheritDoc}
jaroslav@1890:      */
jaroslav@1890:     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
jaroslav@1890:         Object e = transferer.transfer(null, true, unit.toNanos(timeout));
jaroslav@1890:         if (e != null || !Thread.interrupted())
jaroslav@1890:             return (E)e;
jaroslav@1890:         throw new InterruptedException();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Retrieves and removes the head of this queue, if another thread
jaroslav@1890:      * is currently making an element available.
jaroslav@1890:      *
jaroslav@1890:      * @return the head of this queue, or <tt>null</tt> if no
jaroslav@1890:      *         element is available.
jaroslav@1890:      */
jaroslav@1890:     public E poll() {
jaroslav@1890:         return (E)transferer.transfer(null, true, 0);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns <tt>true</tt>.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @return <tt>true</tt>
jaroslav@1890:      */
jaroslav@1890:     public boolean isEmpty() {
jaroslav@1890:         return true;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns zero.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @return zero.
jaroslav@1890:      */
jaroslav@1890:     public int size() {
jaroslav@1890:         return 0;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns zero.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @return zero.
jaroslav@1890:      */
jaroslav@1890:     public int remainingCapacity() {
jaroslav@1890:         return 0;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Does nothing.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      */
jaroslav@1890:     public void clear() {
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns <tt>false</tt>.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @param o the element
jaroslav@1890:      * @return <tt>false</tt>
jaroslav@1890:      */
jaroslav@1890:     public boolean contains(Object o) {
jaroslav@1890:         return false;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns <tt>false</tt>.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @param o the element to remove
jaroslav@1890:      * @return <tt>false</tt>
jaroslav@1890:      */
jaroslav@1890:     public boolean remove(Object o) {
jaroslav@1890:         return false;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns <tt>false</tt> unless the given collection is empty.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @param c the collection
jaroslav@1890:      * @return <tt>false</tt> unless given collection is empty
jaroslav@1890:      */
jaroslav@1890:     public boolean containsAll(Collection<?> c) {
jaroslav@1890:         return c.isEmpty();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns <tt>false</tt>.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @param c the collection
jaroslav@1890:      * @return <tt>false</tt>
jaroslav@1890:      */
jaroslav@1890:     public boolean removeAll(Collection<?> c) {
jaroslav@1890:         return false;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns <tt>false</tt>.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> has no internal capacity.
jaroslav@1890:      *
jaroslav@1890:      * @param c the collection
jaroslav@1890:      * @return <tt>false</tt>
jaroslav@1890:      */
jaroslav@1890:     public boolean retainAll(Collection<?> c) {
jaroslav@1890:         return false;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Always returns <tt>null</tt>.
jaroslav@1890:      * A <tt>SynchronousQueue</tt> does not return elements
jaroslav@1890:      * unless actively waited on.
jaroslav@1890:      *
jaroslav@1890:      * @return <tt>null</tt>
jaroslav@1890:      */
jaroslav@1890:     public E peek() {
jaroslav@1890:         return null;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns an empty iterator in which <tt>hasNext</tt> always returns
jaroslav@1890:      * <tt>false</tt>.
jaroslav@1890:      *
jaroslav@1890:      * @return an empty iterator
jaroslav@1890:      */
jaroslav@1890:     public Iterator<E> iterator() {
jaroslav@1890:         return Collections.emptyIterator();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns a zero-length array.
jaroslav@1890:      * @return a zero-length array
jaroslav@1890:      */
jaroslav@1890:     public Object[] toArray() {
jaroslav@1890:         return new Object[0];
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Sets the zeroeth element of the specified array to <tt>null</tt>
jaroslav@1890:      * (if the array has non-zero length) and returns it.
jaroslav@1890:      *
jaroslav@1890:      * @param a the array
jaroslav@1890:      * @return the specified array
jaroslav@1890:      * @throws NullPointerException if the specified array is null
jaroslav@1890:      */
jaroslav@1890:     public <T> T[] toArray(T[] a) {
jaroslav@1890:         if (a.length > 0)
jaroslav@1890:             a[0] = null;
jaroslav@1890:         return a;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * @throws UnsupportedOperationException {@inheritDoc}
jaroslav@1890:      * @throws ClassCastException            {@inheritDoc}
jaroslav@1890:      * @throws NullPointerException          {@inheritDoc}
jaroslav@1890:      * @throws IllegalArgumentException      {@inheritDoc}
jaroslav@1890:      */
jaroslav@1890:     public int drainTo(Collection<? super E> c) {
jaroslav@1890:         if (c == null)
jaroslav@1890:             throw new NullPointerException();
jaroslav@1890:         if (c == this)
jaroslav@1890:             throw new IllegalArgumentException();
jaroslav@1890:         int n = 0;
jaroslav@1890:         E e;
jaroslav@1890:         while ( (e = poll()) != null) {
jaroslav@1890:             c.add(e);
jaroslav@1890:             ++n;
jaroslav@1890:         }
jaroslav@1890:         return n;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * @throws UnsupportedOperationException {@inheritDoc}
jaroslav@1890:      * @throws ClassCastException            {@inheritDoc}
jaroslav@1890:      * @throws NullPointerException          {@inheritDoc}
jaroslav@1890:      * @throws IllegalArgumentException      {@inheritDoc}
jaroslav@1890:      */
jaroslav@1890:     public int drainTo(Collection<? super E> c, int maxElements) {
jaroslav@1890:         if (c == null)
jaroslav@1890:             throw new NullPointerException();
jaroslav@1890:         if (c == this)
jaroslav@1890:             throw new IllegalArgumentException();
jaroslav@1890:         int n = 0;
jaroslav@1890:         E e;
jaroslav@1890:         while (n < maxElements && (e = poll()) != null) {
jaroslav@1890:             c.add(e);
jaroslav@1890:             ++n;
jaroslav@1890:         }
jaroslav@1890:         return n;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /*
jaroslav@1890:      * To cope with serialization strategy in the 1.5 version of
jaroslav@1890:      * SynchronousQueue, we declare some unused classes and fields
jaroslav@1890:      * that exist solely to enable serializability across versions.
jaroslav@1890:      * These fields are never used, so are initialized only if this
jaroslav@1890:      * object is ever serialized or deserialized.
jaroslav@1890:      */
jaroslav@1890: 
jaroslav@1890:     static class WaitQueue implements java.io.Serializable { }
jaroslav@1890:     static class LifoWaitQueue extends WaitQueue {
jaroslav@1890:         private static final long serialVersionUID = -3633113410248163686L;
jaroslav@1890:     }
jaroslav@1890:     static class FifoWaitQueue extends WaitQueue {
jaroslav@1890:         private static final long serialVersionUID = -3623113410248163686L;
jaroslav@1890:     }
jaroslav@1890:     private ReentrantLock qlock;
jaroslav@1890:     private WaitQueue waitingProducers;
jaroslav@1890:     private WaitQueue waitingConsumers;
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Save the state to a stream (that is, serialize it).
jaroslav@1890:      *
jaroslav@1890:      * @param s the stream
jaroslav@1890:      */
jaroslav@1890:     private void writeObject(java.io.ObjectOutputStream s)
jaroslav@1890:         throws java.io.IOException {
jaroslav@1890:         boolean fair = transferer instanceof TransferQueue;
jaroslav@1890:         if (fair) {
jaroslav@1890:             qlock = new ReentrantLock(true);
jaroslav@1890:             waitingProducers = new FifoWaitQueue();
jaroslav@1890:             waitingConsumers = new FifoWaitQueue();
jaroslav@1890:         }
jaroslav@1890:         else {
jaroslav@1890:             qlock = new ReentrantLock();
jaroslav@1890:             waitingProducers = new LifoWaitQueue();
jaroslav@1890:             waitingConsumers = new LifoWaitQueue();
jaroslav@1890:         }
jaroslav@1890:         s.defaultWriteObject();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     private void readObject(final java.io.ObjectInputStream s)
jaroslav@1890:         throws java.io.IOException, ClassNotFoundException {
jaroslav@1890:         s.defaultReadObject();
jaroslav@1890:         if (waitingProducers instanceof FifoWaitQueue)
jaroslav@1890:             transferer = new TransferQueue();
jaroslav@1890:         else
jaroslav@1890:             transferer = new TransferStack();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890: 
jaroslav@1890: }