1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.  Oracle designates this
   7  * particular file as subject to the "Classpath" exception as provided
   8  * by Oracle in the LICENSE file that accompanied this code.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
  23  */
  24 
  25 /*
  26  * This file is available under and governed by the GNU General Public
  27  * License version 2 only, as published by the Free Software Foundation.
  28  * However, the following notice accompanied the original version of this
  29  * file:
  30  *
  31  * Written by Doug Lea, Bill Scherer, and Michael Scott with
  32  * assistance from members of JCP JSR-166 Expert Group and released to
  33  * the public domain, as explained at
  34  * http://creativecommons.org/licenses/publicdomain
  35  */
  36 
  37 package java.util.concurrent;
  38 import java.util.concurrent.locks.*;
  39 import java.util.concurrent.atomic.*;
  40 import java.util.*;
  41 
  42 /**
  43  * A {@linkplain BlockingQueue blocking queue} in which each insert
  44  * operation must wait for a corresponding remove operation by another
  45  * thread, and vice versa.  A synchronous queue does not have any
  46  * internal capacity, not even a capacity of one.  You cannot
  47  * <tt>peek</tt> at a synchronous queue because an element is only
  48  * present when you try to remove it; you cannot insert an element
  49  * (using any method) unless another thread is trying to remove it;
  50  * you cannot iterate as there is nothing to iterate.  The
  51  * <em>head</em> of the queue is the element that the first queued
  52  * inserting thread is trying to add to the queue; if there is no such
  53  * queued thread then no element is available for removal and
  54  * <tt>poll()</tt> will return <tt>null</tt>.  For purposes of other
  55  * <tt>Collection</tt> methods (for example <tt>contains</tt>), a
  56  * <tt>SynchronousQueue</tt> acts as an empty collection.  This queue
  57  * does not permit <tt>null</tt> elements.
  58  *
  59  * <p>Synchronous queues are similar to rendezvous channels used in
  60  * CSP and Ada. They are well suited for handoff designs, in which an
  61  * object running in one thread must sync up with an object running
  62  * in another thread in order to hand it some information, event, or
  63  * task.
  64  *
  65  * <p> This class supports an optional fairness policy for ordering
  66  * waiting producer and consumer threads.  By default, this ordering
  67  * is not guaranteed. However, a queue constructed with fairness set
  68  * to <tt>true</tt> grants threads access in FIFO order.
  69  *
  70  * <p>This class and its iterator implement all of the
  71  * <em>optional</em> methods of the {@link Collection} and {@link
  72  * Iterator} interfaces.
  73  *
  74  * <p>This class is a member of the
  75  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  76  * Java Collections Framework</a>.
  77  *
  78  * @since 1.5
  79  * @author Doug Lea and Bill Scherer and Michael Scott
  80  * @param <E> the type of elements held in this collection
  81  */
  82 public class SynchronousQueue<E> extends AbstractQueue<E>
  83     implements BlockingQueue<E>, java.io.Serializable {
  84     private static final long serialVersionUID = -3223113410248163686L;
  85 
  86     /*
  87      * This class implements extensions of the dual stack and dual
  88      * queue algorithms described in "Nonblocking Concurrent Objects
  89      * with Condition Synchronization", by W. N. Scherer III and
  90      * M. L. Scott.  18th Annual Conf. on Distributed Computing,
  91      * Oct. 2004 (see also
  92      * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
  93      * The (Lifo) stack is used for non-fair mode, and the (Fifo)
  94      * queue for fair mode. The performance of the two is generally
  95      * similar. Fifo usually supports higher throughput under
  96      * contention but Lifo maintains higher thread locality in common
  97      * applications.
  98      *
  99      * A dual queue (and similarly stack) is one that at any given
 100      * time either holds "data" -- items provided by put operations,
 101      * or "requests" -- slots representing take operations, or is
 102      * empty. A call to "fulfill" (i.e., a call requesting an item
 103      * from a queue holding data or vice versa) dequeues a
 104      * complementary node.  The most interesting feature of these
 105      * queues is that any operation can figure out which mode the
 106      * queue is in, and act accordingly without needing locks.
 107      *
 108      * Both the queue and stack extend abstract class Transferer
 109      * defining the single method transfer that does a put or a
 110      * take. These are unified into a single method because in dual
 111      * data structures, the put and take operations are symmetrical,
 112      * so nearly all code can be combined. The resulting transfer
 113      * methods are on the long side, but are easier to follow than
 114      * they would be if broken up into nearly-duplicated parts.
 115      *
 116      * The queue and stack data structures share many conceptual
 117      * similarities but very few concrete details. For simplicity,
 118      * they are kept distinct so that they can later evolve
 119      * separately.
 120      *
 121      * The algorithms here differ from the versions in the above paper
 122      * in extending them for use in synchronous queues, as well as
 123      * dealing with cancellation. The main differences include:
 124      *
 125      *  1. The original algorithms used bit-marked pointers, but
 126      *     the ones here use mode bits in nodes, leading to a number
 127      *     of further adaptations.
 128      *  2. SynchronousQueues must block threads waiting to become
 129      *     fulfilled.
 130      *  3. Support for cancellation via timeout and interrupts,
 131      *     including cleaning out cancelled nodes/threads
 132      *     from lists to avoid garbage retention and memory depletion.
 133      *
 134      * Blocking is mainly accomplished using LockSupport park/unpark,
 135      * except that nodes that appear to be the next ones to become
 136      * fulfilled first spin a bit (on multiprocessors only). On very
 137      * busy synchronous queues, spinning can dramatically improve
 138      * throughput. And on less busy ones, the amount of spinning is
 139      * small enough not to be noticeable.
 140      *
 141      * Cleaning is done in different ways in queues vs stacks.  For
 142      * queues, we can almost always remove a node immediately in O(1)
 143      * time (modulo retries for consistency checks) when it is
 144      * cancelled. But if it may be pinned as the current tail, it must
 145      * wait until some subsequent cancellation. For stacks, we need a
 146      * potentially O(n) traversal to be sure that we can remove the
 147      * node, but this can run concurrently with other threads
 148      * accessing the stack.
 149      *
 150      * While garbage collection takes care of most node reclamation
 151      * issues that otherwise complicate nonblocking algorithms, care
 152      * is taken to "forget" references to data, other nodes, and
 153      * threads that might be held on to long-term by blocked
 154      * threads. In cases where setting to null would otherwise
 155      * conflict with main algorithms, this is done by changing a
 156      * node's link to now point to the node itself. This doesn't arise
 157      * much for Stack nodes (because blocked threads do not hang on to
 158      * old head pointers), but references in Queue nodes must be
 159      * aggressively forgotten to avoid reachability of everything any
 160      * node has ever referred to since arrival.
 161      */
 162 
 163     /**
 164      * Shared internal API for dual stacks and queues.
 165      */
 166     abstract static class Transferer {
 167         /**
 168          * Performs a put or take.
 169          *
 170          * @param e if non-null, the item to be handed to a consumer;
 171          *          if null, requests that transfer return an item
 172          *          offered by producer.
 173          * @param timed if this operation should timeout
 174          * @param nanos the timeout, in nanoseconds
 175          * @return if non-null, the item provided or received; if null,
 176          *         the operation failed due to timeout or interrupt --
 177          *         the caller can distinguish which of these occurred
 178          *         by checking Thread.interrupted.
 179          */
 180         abstract Object transfer(Object e, boolean timed, long nanos);
 181     }
 182 
 183     /** The number of CPUs, for spin control */
 184     static final int NCPUS = Runtime.getRuntime().availableProcessors();
 185 
 186     /**
 187      * The number of times to spin before blocking in timed waits.
 188      * The value is empirically derived -- it works well across a
 189      * variety of processors and OSes. Empirically, the best value
 190      * seems not to vary with number of CPUs (beyond 2) so is just
 191      * a constant.
 192      */
 193     static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
 194 
 195     /**
 196      * The number of times to spin before blocking in untimed waits.
 197      * This is greater than timed value because untimed waits spin
 198      * faster since they don't need to check times on each spin.
 199      */
 200     static final int maxUntimedSpins = maxTimedSpins * 16;
 201 
 202     /**
 203      * The number of nanoseconds for which it is faster to spin
 204      * rather than to use timed park. A rough estimate suffices.
 205      */
 206     static final long spinForTimeoutThreshold = 1000L;
 207 
 208     /** Dual stack */
 209     static final class TransferStack extends Transferer {
 210         /*
 211          * This extends Scherer-Scott dual stack algorithm, differing,
 212          * among other ways, by using "covering" nodes rather than
 213          * bit-marked pointers: Fulfilling operations push on marker
 214          * nodes (with FULFILLING bit set in mode) to reserve a spot
 215          * to match a waiting node.
 216          */
 217 
 218         /* Modes for SNodes, ORed together in node fields */
 219         /** Node represents an unfulfilled consumer */
 220         static final int REQUEST    = 0;
 221         /** Node represents an unfulfilled producer */
 222         static final int DATA       = 1;
 223         /** Node is fulfilling another unfulfilled DATA or REQUEST */
 224         static final int FULFILLING = 2;
 225 
 226         /** Return true if m has fulfilling bit set */
 227         static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
 228 
 229         /** Node class for TransferStacks. */
 230         static final class SNode {
 231             volatile SNode next;        // next node in stack
 232             volatile SNode match;       // the node matched to this
 233             volatile Thread waiter;     // to control park/unpark
 234             Object item;                // data; or null for REQUESTs
 235             int mode;
 236             // Note: item and mode fields don't need to be volatile
 237             // since they are always written before, and read after,
 238             // other volatile/atomic operations.
 239 
 240             SNode(Object item) {
 241                 this.item = item;
 242             }
 243 
 244             boolean casNext(SNode cmp, SNode val) {
 245                 return cmp == next &&
 246                     UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
 247             }
 248 
 249             /**
 250              * Tries to match node s to this node, if so, waking up thread.
 251              * Fulfillers call tryMatch to identify their waiters.
 252              * Waiters block until they have been matched.
 253              *
 254              * @param s the node to match
 255              * @return true if successfully matched to s
 256              */
 257             boolean tryMatch(SNode s) {
 258                 if (match == null &&
 259                     UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
 260                     Thread w = waiter;
 261                     if (w != null) {    // waiters need at most one unpark
 262                         waiter = null;
 263                         LockSupport.unpark(w);
 264                     }
 265                     return true;
 266                 }
 267                 return match == s;
 268             }
 269 
 270             /**
 271              * Tries to cancel a wait by matching node to itself.
 272              */
 273             void tryCancel() {
 274                 UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
 275             }
 276 
 277             boolean isCancelled() {
 278                 return match == this;
 279             }
 280 
 281             // Unsafe mechanics
 282             private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
 283             private static final long nextOffset =
 284                 objectFieldOffset(UNSAFE, "next", SNode.class);
 285             private static final long matchOffset =
 286                 objectFieldOffset(UNSAFE, "match", SNode.class);
 287 
 288         }
 289 
 290         /** The head (top) of the stack */
 291         volatile SNode head;
 292 
 293         boolean casHead(SNode h, SNode nh) {
 294             return h == head &&
 295                 UNSAFE.compareAndSwapObject(this, headOffset, h, nh);
 296         }
 297 
 298         /**
 299          * Creates or resets fields of a node. Called only from transfer
 300          * where the node to push on stack is lazily created and
 301          * reused when possible to help reduce intervals between reads
 302          * and CASes of head and to avoid surges of garbage when CASes
 303          * to push nodes fail due to contention.
 304          */
 305         static SNode snode(SNode s, Object e, SNode next, int mode) {
 306             if (s == null) s = new SNode(e);
 307             s.mode = mode;
 308             s.next = next;
 309             return s;
 310         }
 311 
 312         /**
 313          * Puts or takes an item.
 314          */
 315         Object transfer(Object e, boolean timed, long nanos) {
 316             /*
 317              * Basic algorithm is to loop trying one of three actions:
 318              *
 319              * 1. If apparently empty or already containing nodes of same
 320              *    mode, try to push node on stack and wait for a match,
 321              *    returning it, or null if cancelled.
 322              *
 323              * 2. If apparently containing node of complementary mode,
 324              *    try to push a fulfilling node on to stack, match
 325              *    with corresponding waiting node, pop both from
 326              *    stack, and return matched item. The matching or
 327              *    unlinking might not actually be necessary because of
 328              *    other threads performing action 3:
 329              *
 330              * 3. If top of stack already holds another fulfilling node,
 331              *    help it out by doing its match and/or pop
 332              *    operations, and then continue. The code for helping
 333              *    is essentially the same as for fulfilling, except
 334              *    that it doesn't return the item.
 335              */
 336 
 337             SNode s = null; // constructed/reused as needed
 338             int mode = (e == null) ? REQUEST : DATA;
 339 
 340             for (;;) {
 341                 SNode h = head;
 342                 if (h == null || h.mode == mode) {  // empty or same-mode
 343                     if (timed && nanos <= 0) {      // can't wait
 344                         if (h != null && h.isCancelled())
 345                             casHead(h, h.next);     // pop cancelled node
 346                         else
 347                             return null;
 348                     } else if (casHead(h, s = snode(s, e, h, mode))) {
 349                         SNode m = awaitFulfill(s, timed, nanos);
 350                         if (m == s) {               // wait was cancelled
 351                             clean(s);
 352                             return null;
 353                         }
 354                         if ((h = head) != null && h.next == s)
 355                             casHead(h, s.next);     // help s's fulfiller
 356                         return (mode == REQUEST) ? m.item : s.item;
 357                     }
 358                 } else if (!isFulfilling(h.mode)) { // try to fulfill
 359                     if (h.isCancelled())            // already cancelled
 360                         casHead(h, h.next);         // pop and retry
 361                     else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
 362                         for (;;) { // loop until matched or waiters disappear
 363                             SNode m = s.next;       // m is s's match
 364                             if (m == null) {        // all waiters are gone
 365                                 casHead(s, null);   // pop fulfill node
 366                                 s = null;           // use new node next time
 367                                 break;              // restart main loop
 368                             }
 369                             SNode mn = m.next;
 370                             if (m.tryMatch(s)) {
 371                                 casHead(s, mn);     // pop both s and m
 372                                 return (mode == REQUEST) ? m.item : s.item;
 373                             } else                  // lost match
 374                                 s.casNext(m, mn);   // help unlink
 375                         }
 376                     }
 377                 } else {                            // help a fulfiller
 378                     SNode m = h.next;               // m is h's match
 379                     if (m == null)                  // waiter is gone
 380                         casHead(h, null);           // pop fulfilling node
 381                     else {
 382                         SNode mn = m.next;
 383                         if (m.tryMatch(h))          // help match
 384                             casHead(h, mn);         // pop both h and m
 385                         else                        // lost match
 386                             h.casNext(m, mn);       // help unlink
 387                     }
 388                 }
 389             }
 390         }
 391 
 392         /**
 393          * Spins/blocks until node s is matched by a fulfill operation.
 394          *
 395          * @param s the waiting node
 396          * @param timed true if timed wait
 397          * @param nanos timeout value
 398          * @return matched node, or s if cancelled
 399          */
 400         SNode awaitFulfill(SNode s, boolean timed, long nanos) {
 401             /*
 402              * When a node/thread is about to block, it sets its waiter
 403              * field and then rechecks state at least one more time
 404              * before actually parking, thus covering race vs
 405              * fulfiller noticing that waiter is non-null so should be
 406              * woken.
 407              *
 408              * When invoked by nodes that appear at the point of call
 409              * to be at the head of the stack, calls to park are
 410              * preceded by spins to avoid blocking when producers and
 411              * consumers are arriving very close in time.  This can
 412              * happen enough to bother only on multiprocessors.
 413              *
 414              * The order of checks for returning out of main loop
 415              * reflects fact that interrupts have precedence over
 416              * normal returns, which have precedence over
 417              * timeouts. (So, on timeout, one last check for match is
 418              * done before giving up.) Except that calls from untimed
 419              * SynchronousQueue.{poll/offer} don't check interrupts
 420              * and don't wait at all, so are trapped in transfer
 421              * method rather than calling awaitFulfill.
 422              */
 423             long lastTime = timed ? System.nanoTime() : 0;
 424             Thread w = Thread.currentThread();
 425             SNode h = head;
 426             int spins = (shouldSpin(s) ?
 427                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);
 428             for (;;) {
 429                 if (w.isInterrupted())
 430                     s.tryCancel();
 431                 SNode m = s.match;
 432                 if (m != null)
 433                     return m;
 434                 if (timed) {
 435                     long now = System.nanoTime();
 436                     nanos -= now - lastTime;
 437                     lastTime = now;
 438                     if (nanos <= 0) {
 439                         s.tryCancel();
 440                         continue;
 441                     }
 442                 }
 443                 if (spins > 0)
 444                     spins = shouldSpin(s) ? (spins-1) : 0;
 445                 else if (s.waiter == null)
 446                     s.waiter = w; // establish waiter so can park next iter
 447                 else if (!timed)
 448                     LockSupport.park(this);
 449                 else if (nanos > spinForTimeoutThreshold)
 450                     LockSupport.parkNanos(this, nanos);
 451             }
 452         }
 453 
 454         /**
 455          * Returns true if node s is at head or there is an active
 456          * fulfiller.
 457          */
 458         boolean shouldSpin(SNode s) {
 459             SNode h = head;
 460             return (h == s || h == null || isFulfilling(h.mode));
 461         }
 462 
 463         /**
 464          * Unlinks s from the stack.
 465          */
 466         void clean(SNode s) {
 467             s.item = null;   // forget item
 468             s.waiter = null; // forget thread
 469 
 470             /*
 471              * At worst we may need to traverse entire stack to unlink
 472              * s. If there are multiple concurrent calls to clean, we
 473              * might not see s if another thread has already removed
 474              * it. But we can stop when we see any node known to
 475              * follow s. We use s.next unless it too is cancelled, in
 476              * which case we try the node one past. We don't check any
 477              * further because we don't want to doubly traverse just to
 478              * find sentinel.
 479              */
 480 
 481             SNode past = s.next;
 482             if (past != null && past.isCancelled())
 483                 past = past.next;
 484 
 485             // Absorb cancelled nodes at head
 486             SNode p;
 487             while ((p = head) != null && p != past && p.isCancelled())
 488                 casHead(p, p.next);
 489 
 490             // Unsplice embedded nodes
 491             while (p != null && p != past) {
 492                 SNode n = p.next;
 493                 if (n != null && n.isCancelled())
 494                     p.casNext(n, n.next);
 495                 else
 496                     p = n;
 497             }
 498         }
 499 
 500         // Unsafe mechanics
 501         private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
 502         private static final long headOffset =
 503             objectFieldOffset(UNSAFE, "head", TransferStack.class);
 504 
 505     }
 506 
 507     /** Dual Queue */
 508     static final class TransferQueue extends Transferer {
 509         /*
 510          * This extends Scherer-Scott dual queue algorithm, differing,
 511          * among other ways, by using modes within nodes rather than
 512          * marked pointers. The algorithm is a little simpler than
 513          * that for stacks because fulfillers do not need explicit
 514          * nodes, and matching is done by CAS'ing QNode.item field
 515          * from non-null to null (for put) or vice versa (for take).
 516          */
 517 
 518         /** Node class for TransferQueue. */
 519         static final class QNode {
 520             volatile QNode next;          // next node in queue
 521             volatile Object item;         // CAS'ed to or from null
 522             volatile Thread waiter;       // to control park/unpark
 523             final boolean isData;
 524 
 525             QNode(Object item, boolean isData) {
 526                 this.item = item;
 527                 this.isData = isData;
 528             }
 529 
 530             boolean casNext(QNode cmp, QNode val) {
 531                 return next == cmp &&
 532                     UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
 533             }
 534 
 535             boolean casItem(Object cmp, Object val) {
 536                 return item == cmp &&
 537                     UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
 538             }
 539 
 540             /**
 541              * Tries to cancel by CAS'ing ref to this as item.
 542              */
 543             void tryCancel(Object cmp) {
 544                 UNSAFE.compareAndSwapObject(this, itemOffset, cmp, this);
 545             }
 546 
 547             boolean isCancelled() {
 548                 return item == this;
 549             }
 550 
 551             /**
 552              * Returns true if this node is known to be off the queue
 553              * because its next pointer has been forgotten due to
 554              * an advanceHead operation.
 555              */
 556             boolean isOffList() {
 557                 return next == this;
 558             }
 559 
 560             // Unsafe mechanics
 561             private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
 562             private static final long nextOffset =
 563                 objectFieldOffset(UNSAFE, "next", QNode.class);
 564             private static final long itemOffset =
 565                 objectFieldOffset(UNSAFE, "item", QNode.class);
 566         }
 567 
 568         /** Head of queue */
 569         transient volatile QNode head;
 570         /** Tail of queue */
 571         transient volatile QNode tail;
 572         /**
 573          * Reference to a cancelled node that might not yet have been
 574          * unlinked from queue because it was the last inserted node
 575          * when it cancelled.
 576          */
 577         transient volatile QNode cleanMe;
 578 
 579         TransferQueue() {
 580             QNode h = new QNode(null, false); // initialize to dummy node.
 581             head = h;
 582             tail = h;
 583         }
 584 
 585         /**
 586          * Tries to cas nh as new head; if successful, unlink
 587          * old head's next node to avoid garbage retention.
 588          */
 589         void advanceHead(QNode h, QNode nh) {
 590             if (h == head &&
 591                 UNSAFE.compareAndSwapObject(this, headOffset, h, nh))
 592                 h.next = h; // forget old next
 593         }
 594 
 595         /**
 596          * Tries to cas nt as new tail.
 597          */
 598         void advanceTail(QNode t, QNode nt) {
 599             if (tail == t)
 600                 UNSAFE.compareAndSwapObject(this, tailOffset, t, nt);
 601         }
 602 
 603         /**
 604          * Tries to CAS cleanMe slot.
 605          */
 606         boolean casCleanMe(QNode cmp, QNode val) {
 607             return cleanMe == cmp &&
 608                 UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
 609         }
 610 
 611         /**
 612          * Puts or takes an item.
 613          */
 614         Object transfer(Object e, boolean timed, long nanos) {
 615             /* Basic algorithm is to loop trying to take either of
 616              * two actions:
 617              *
 618              * 1. If queue apparently empty or holding same-mode nodes,
 619              *    try to add node to queue of waiters, wait to be
 620              *    fulfilled (or cancelled) and return matching item.
 621              *
 622              * 2. If queue apparently contains waiting items, and this
 623              *    call is of complementary mode, try to fulfill by CAS'ing
 624              *    item field of waiting node and dequeuing it, and then
 625              *    returning matching item.
 626              *
 627              * In each case, along the way, check for and try to help
 628              * advance head and tail on behalf of other stalled/slow
 629              * threads.
 630              *
 631              * The loop starts off with a null check guarding against
 632              * seeing uninitialized head or tail values. This never
 633              * happens in current SynchronousQueue, but could if
 634              * callers held non-volatile/final ref to the
 635              * transferer. The check is here anyway because it places
 636              * null checks at top of loop, which is usually faster
 637              * than having them implicitly interspersed.
 638              */
 639 
 640             QNode s = null; // constructed/reused as needed
 641             boolean isData = (e != null);
 642 
 643             for (;;) {
 644                 QNode t = tail;
 645                 QNode h = head;
 646                 if (t == null || h == null)         // saw uninitialized value
 647                     continue;                       // spin
 648 
 649                 if (h == t || t.isData == isData) { // empty or same-mode
 650                     QNode tn = t.next;
 651                     if (t != tail)                  // inconsistent read
 652                         continue;
 653                     if (tn != null) {               // lagging tail
 654                         advanceTail(t, tn);
 655                         continue;
 656                     }
 657                     if (timed && nanos <= 0)        // can't wait
 658                         return null;
 659                     if (s == null)
 660                         s = new QNode(e, isData);
 661                     if (!t.casNext(null, s))        // failed to link in
 662                         continue;
 663 
 664                     advanceTail(t, s);              // swing tail and wait
 665                     Object x = awaitFulfill(s, e, timed, nanos);
 666                     if (x == s) {                   // wait was cancelled
 667                         clean(t, s);
 668                         return null;
 669                     }
 670 
 671                     if (!s.isOffList()) {           // not already unlinked
 672                         advanceHead(t, s);          // unlink if head
 673                         if (x != null)              // and forget fields
 674                             s.item = s;
 675                         s.waiter = null;
 676                     }
 677                     return (x != null) ? x : e;
 678 
 679                 } else {                            // complementary-mode
 680                     QNode m = h.next;               // node to fulfill
 681                     if (t != tail || m == null || h != head)
 682                         continue;                   // inconsistent read
 683 
 684                     Object x = m.item;
 685                     if (isData == (x != null) ||    // m already fulfilled
 686                         x == m ||                   // m cancelled
 687                         !m.casItem(x, e)) {         // lost CAS
 688                         advanceHead(h, m);          // dequeue and retry
 689                         continue;
 690                     }
 691 
 692                     advanceHead(h, m);              // successfully fulfilled
 693                     LockSupport.unpark(m.waiter);
 694                     return (x != null) ? x : e;
 695                 }
 696             }
 697         }
 698 
 699         /**
 700          * Spins/blocks until node s is fulfilled.
 701          *
 702          * @param s the waiting node
 703          * @param e the comparison value for checking match
 704          * @param timed true if timed wait
 705          * @param nanos timeout value
 706          * @return matched item, or s if cancelled
 707          */
 708         Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
 709             /* Same idea as TransferStack.awaitFulfill */
 710             long lastTime = timed ? System.nanoTime() : 0;
 711             Thread w = Thread.currentThread();
 712             int spins = ((head.next == s) ?
 713                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);
 714             for (;;) {
 715                 if (w.isInterrupted())
 716                     s.tryCancel(e);
 717                 Object x = s.item;
 718                 if (x != e)
 719                     return x;
 720                 if (timed) {
 721                     long now = System.nanoTime();
 722                     nanos -= now - lastTime;
 723                     lastTime = now;
 724                     if (nanos <= 0) {
 725                         s.tryCancel(e);
 726                         continue;
 727                     }
 728                 }
 729                 if (spins > 0)
 730                     --spins;
 731                 else if (s.waiter == null)
 732                     s.waiter = w;
 733                 else if (!timed)
 734                     LockSupport.park(this);
 735                 else if (nanos > spinForTimeoutThreshold)
 736                     LockSupport.parkNanos(this, nanos);
 737             }
 738         }
 739 
 740         /**
 741          * Gets rid of cancelled node s with original predecessor pred.
 742          */
 743         void clean(QNode pred, QNode s) {
 744             s.waiter = null; // forget thread
 745             /*
 746              * At any given time, exactly one node on list cannot be
 747              * deleted -- the last inserted node. To accommodate this,
 748              * if we cannot delete s, we save its predecessor as
 749              * "cleanMe", deleting the previously saved version
 750              * first. At least one of node s or the node previously
 751              * saved can always be deleted, so this always terminates.
 752              */
 753             while (pred.next == s) { // Return early if already unlinked
 754                 QNode h = head;
 755                 QNode hn = h.next;   // Absorb cancelled first node as head
 756                 if (hn != null && hn.isCancelled()) {
 757                     advanceHead(h, hn);
 758                     continue;
 759                 }
 760                 QNode t = tail;      // Ensure consistent read for tail
 761                 if (t == h)
 762                     return;
 763                 QNode tn = t.next;
 764                 if (t != tail)
 765                     continue;
 766                 if (tn != null) {
 767                     advanceTail(t, tn);
 768                     continue;
 769                 }
 770                 if (s != t) {        // If not tail, try to unsplice
 771                     QNode sn = s.next;
 772                     if (sn == s || pred.casNext(s, sn))
 773                         return;
 774                 }
 775                 QNode dp = cleanMe;
 776                 if (dp != null) {    // Try unlinking previous cancelled node
 777                     QNode d = dp.next;
 778                     QNode dn;
 779                     if (d == null ||               // d is gone or
 780                         d == dp ||                 // d is off list or
 781                         !d.isCancelled() ||        // d not cancelled or
 782                         (d != t &&                 // d not tail and
 783                          (dn = d.next) != null &&  //   has successor
 784                          dn != d &&                //   that is on list
 785                          dp.casNext(d, dn)))       // d unspliced
 786                         casCleanMe(dp, null);
 787                     if (dp == pred)
 788                         return;      // s is already saved node
 789                 } else if (casCleanMe(null, pred))
 790                     return;          // Postpone cleaning s
 791             }
 792         }
 793 
 794         // unsafe mechanics
 795         private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
 796         private static final long headOffset =
 797             objectFieldOffset(UNSAFE, "head", TransferQueue.class);
 798         private static final long tailOffset =
 799             objectFieldOffset(UNSAFE, "tail", TransferQueue.class);
 800         private static final long cleanMeOffset =
 801             objectFieldOffset(UNSAFE, "cleanMe", TransferQueue.class);
 802 
 803     }
 804 
 805     /**
 806      * The transferer. Set only in constructor, but cannot be declared
 807      * as final without further complicating serialization.  Since
 808      * this is accessed only at most once per public method, there
 809      * isn't a noticeable performance penalty for using volatile
 810      * instead of final here.
 811      */
 812     private transient volatile Transferer transferer;
 813 
 814     /**
 815      * Creates a <tt>SynchronousQueue</tt> with nonfair access policy.
 816      */
 817     public SynchronousQueue() {
 818         this(false);
 819     }
 820 
 821     /**
 822      * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.
 823      *
 824      * @param fair if true, waiting threads contend in FIFO order for
 825      *        access; otherwise the order is unspecified.
 826      */
 827     public SynchronousQueue(boolean fair) {
 828         transferer = fair ? new TransferQueue() : new TransferStack();
 829     }
 830 
 831     /**
 832      * Adds the specified element to this queue, waiting if necessary for
 833      * another thread to receive it.
 834      *
 835      * @throws InterruptedException {@inheritDoc}
 836      * @throws NullPointerException {@inheritDoc}
 837      */
 838     public void put(E o) throws InterruptedException {
 839         if (o == null) throw new NullPointerException();
 840         if (transferer.transfer(o, false, 0) == null) {
 841             Thread.interrupted();
 842             throw new InterruptedException();
 843         }
 844     }
 845 
 846     /**
 847      * Inserts the specified element into this queue, waiting if necessary
 848      * up to the specified wait time for another thread to receive it.
 849      *
 850      * @return <tt>true</tt> if successful, or <tt>false</tt> if the
 851      *         specified waiting time elapses before a consumer appears.
 852      * @throws InterruptedException {@inheritDoc}
 853      * @throws NullPointerException {@inheritDoc}
 854      */
 855     public boolean offer(E o, long timeout, TimeUnit unit)
 856         throws InterruptedException {
 857         if (o == null) throw new NullPointerException();
 858         if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
 859             return true;
 860         if (!Thread.interrupted())
 861             return false;
 862         throw new InterruptedException();
 863     }
 864 
 865     /**
 866      * Inserts the specified element into this queue, if another thread is
 867      * waiting to receive it.
 868      *
 869      * @param e the element to add
 870      * @return <tt>true</tt> if the element was added to this queue, else
 871      *         <tt>false</tt>
 872      * @throws NullPointerException if the specified element is null
 873      */
 874     public boolean offer(E e) {
 875         if (e == null) throw new NullPointerException();
 876         return transferer.transfer(e, true, 0) != null;
 877     }
 878 
 879     /**
 880      * Retrieves and removes the head of this queue, waiting if necessary
 881      * for another thread to insert it.
 882      *
 883      * @return the head of this queue
 884      * @throws InterruptedException {@inheritDoc}
 885      */
 886     public E take() throws InterruptedException {
 887         Object e = transferer.transfer(null, false, 0);
 888         if (e != null)
 889             return (E)e;
 890         Thread.interrupted();
 891         throw new InterruptedException();
 892     }
 893 
 894     /**
 895      * Retrieves and removes the head of this queue, waiting
 896      * if necessary up to the specified wait time, for another thread
 897      * to insert it.
 898      *
 899      * @return the head of this queue, or <tt>null</tt> if the
 900      *         specified waiting time elapses before an element is present.
 901      * @throws InterruptedException {@inheritDoc}
 902      */
 903     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
 904         Object e = transferer.transfer(null, true, unit.toNanos(timeout));
 905         if (e != null || !Thread.interrupted())
 906             return (E)e;
 907         throw new InterruptedException();
 908     }
 909 
 910     /**
 911      * Retrieves and removes the head of this queue, if another thread
 912      * is currently making an element available.
 913      *
 914      * @return the head of this queue, or <tt>null</tt> if no
 915      *         element is available.
 916      */
 917     public E poll() {
 918         return (E)transferer.transfer(null, true, 0);
 919     }
 920 
 921     /**
 922      * Always returns <tt>true</tt>.
 923      * A <tt>SynchronousQueue</tt> has no internal capacity.
 924      *
 925      * @return <tt>true</tt>
 926      */
 927     public boolean isEmpty() {
 928         return true;
 929     }
 930 
 931     /**
 932      * Always returns zero.
 933      * A <tt>SynchronousQueue</tt> has no internal capacity.
 934      *
 935      * @return zero.
 936      */
 937     public int size() {
 938         return 0;
 939     }
 940 
 941     /**
 942      * Always returns zero.
 943      * A <tt>SynchronousQueue</tt> has no internal capacity.
 944      *
 945      * @return zero.
 946      */
 947     public int remainingCapacity() {
 948         return 0;
 949     }
 950 
 951     /**
 952      * Does nothing.
 953      * A <tt>SynchronousQueue</tt> has no internal capacity.
 954      */
 955     public void clear() {
 956     }
 957 
 958     /**
 959      * Always returns <tt>false</tt>.
 960      * A <tt>SynchronousQueue</tt> has no internal capacity.
 961      *
 962      * @param o the element
 963      * @return <tt>false</tt>
 964      */
 965     public boolean contains(Object o) {
 966         return false;
 967     }
 968 
 969     /**
 970      * Always returns <tt>false</tt>.
 971      * A <tt>SynchronousQueue</tt> has no internal capacity.
 972      *
 973      * @param o the element to remove
 974      * @return <tt>false</tt>
 975      */
 976     public boolean remove(Object o) {
 977         return false;
 978     }
 979 
 980     /**
 981      * Returns <tt>false</tt> unless the given collection is empty.
 982      * A <tt>SynchronousQueue</tt> has no internal capacity.
 983      *
 984      * @param c the collection
 985      * @return <tt>false</tt> unless given collection is empty
 986      */
 987     public boolean containsAll(Collection<?> c) {
 988         return c.isEmpty();
 989     }
 990 
 991     /**
 992      * Always returns <tt>false</tt>.
 993      * A <tt>SynchronousQueue</tt> has no internal capacity.
 994      *
 995      * @param c the collection
 996      * @return <tt>false</tt>
 997      */
 998     public boolean removeAll(Collection<?> c) {
 999         return false;
1000     }
1001 
1002     /**
1003      * Always returns <tt>false</tt>.
1004      * A <tt>SynchronousQueue</tt> has no internal capacity.
1005      *
1006      * @param c the collection
1007      * @return <tt>false</tt>
1008      */
1009     public boolean retainAll(Collection<?> c) {
1010         return false;
1011     }
1012 
1013     /**
1014      * Always returns <tt>null</tt>.
1015      * A <tt>SynchronousQueue</tt> does not return elements
1016      * unless actively waited on.
1017      *
1018      * @return <tt>null</tt>
1019      */
1020     public E peek() {
1021         return null;
1022     }
1023 
1024     /**
1025      * Returns an empty iterator in which <tt>hasNext</tt> always returns
1026      * <tt>false</tt>.
1027      *
1028      * @return an empty iterator
1029      */
1030     public Iterator<E> iterator() {
1031         return Collections.emptyIterator();
1032     }
1033 
1034     /**
1035      * Returns a zero-length array.
1036      * @return a zero-length array
1037      */
1038     public Object[] toArray() {
1039         return new Object[0];
1040     }
1041 
1042     /**
1043      * Sets the zeroeth element of the specified array to <tt>null</tt>
1044      * (if the array has non-zero length) and returns it.
1045      *
1046      * @param a the array
1047      * @return the specified array
1048      * @throws NullPointerException if the specified array is null
1049      */
1050     public <T> T[] toArray(T[] a) {
1051         if (a.length > 0)
1052             a[0] = null;
1053         return a;
1054     }
1055 
1056     /**
1057      * @throws UnsupportedOperationException {@inheritDoc}
1058      * @throws ClassCastException            {@inheritDoc}
1059      * @throws NullPointerException          {@inheritDoc}
1060      * @throws IllegalArgumentException      {@inheritDoc}
1061      */
1062     public int drainTo(Collection<? super E> c) {
1063         if (c == null)
1064             throw new NullPointerException();
1065         if (c == this)
1066             throw new IllegalArgumentException();
1067         int n = 0;
1068         E e;
1069         while ( (e = poll()) != null) {
1070             c.add(e);
1071             ++n;
1072         }
1073         return n;
1074     }
1075 
1076     /**
1077      * @throws UnsupportedOperationException {@inheritDoc}
1078      * @throws ClassCastException            {@inheritDoc}
1079      * @throws NullPointerException          {@inheritDoc}
1080      * @throws IllegalArgumentException      {@inheritDoc}
1081      */
1082     public int drainTo(Collection<? super E> c, int maxElements) {
1083         if (c == null)
1084             throw new NullPointerException();
1085         if (c == this)
1086             throw new IllegalArgumentException();
1087         int n = 0;
1088         E e;
1089         while (n < maxElements && (e = poll()) != null) {
1090             c.add(e);
1091             ++n;
1092         }
1093         return n;
1094     }
1095 
1096     /*
1097      * To cope with serialization strategy in the 1.5 version of
1098      * SynchronousQueue, we declare some unused classes and fields
1099      * that exist solely to enable serializability across versions.
1100      * These fields are never used, so are initialized only if this
1101      * object is ever serialized or deserialized.
1102      */
1103 
1104     static class WaitQueue implements java.io.Serializable { }
1105     static class LifoWaitQueue extends WaitQueue {
1106         private static final long serialVersionUID = -3633113410248163686L;
1107     }
1108     static class FifoWaitQueue extends WaitQueue {
1109         private static final long serialVersionUID = -3623113410248163686L;
1110     }
1111     private ReentrantLock qlock;
1112     private WaitQueue waitingProducers;
1113     private WaitQueue waitingConsumers;
1114 
1115     /**
1116      * Save the state to a stream (that is, serialize it).
1117      *
1118      * @param s the stream
1119      */
1120     private void writeObject(java.io.ObjectOutputStream s)
1121         throws java.io.IOException {
1122         boolean fair = transferer instanceof TransferQueue;
1123         if (fair) {
1124             qlock = new ReentrantLock(true);
1125             waitingProducers = new FifoWaitQueue();
1126             waitingConsumers = new FifoWaitQueue();
1127         }
1128         else {
1129             qlock = new ReentrantLock();
1130             waitingProducers = new LifoWaitQueue();
1131             waitingConsumers = new LifoWaitQueue();
1132         }
1133         s.defaultWriteObject();
1134     }
1135 
1136     private void readObject(final java.io.ObjectInputStream s)
1137         throws java.io.IOException, ClassNotFoundException {
1138         s.defaultReadObject();
1139         if (waitingProducers instanceof FifoWaitQueue)
1140             transferer = new TransferQueue();
1141         else
1142             transferer = new TransferStack();
1143     }
1144 
1145     // Unsafe mechanics
1146     static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1147                                   String field, Class<?> klazz) {
1148         try {
1149             return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1150         } catch (NoSuchFieldException e) {
1151             // Convert Exception to corresponding Error
1152             NoSuchFieldError error = new NoSuchFieldError(field);
1153             error.initCause(e);
1154             throw error;
1155         }
1156     }
1157 
1158 }