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/publicdomain/zero/1.0/
  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;
 283             private static final long matchOffset;
 284             private static final long nextOffset;
 285 
 286             static {
 287                 try {
 288                     UNSAFE = sun.misc.Unsafe.getUnsafe();
 289                     Class k = SNode.class;
 290                     matchOffset = UNSAFE.objectFieldOffset
 291                         (k.getDeclaredField("match"));
 292                     nextOffset = UNSAFE.objectFieldOffset
 293                         (k.getDeclaredField("next"));
 294                 } catch (Exception e) {
 295                     throw new Error(e);
 296                 }
 297             }
 298         }
 299 
 300         /** The head (top) of the stack */
 301         volatile SNode head;
 302 
 303         boolean casHead(SNode h, SNode nh) {
 304             return h == head &&
 305                 UNSAFE.compareAndSwapObject(this, headOffset, h, nh);
 306         }
 307 
 308         /**
 309          * Creates or resets fields of a node. Called only from transfer
 310          * where the node to push on stack is lazily created and
 311          * reused when possible to help reduce intervals between reads
 312          * and CASes of head and to avoid surges of garbage when CASes
 313          * to push nodes fail due to contention.
 314          */
 315         static SNode snode(SNode s, Object e, SNode next, int mode) {
 316             if (s == null) s = new SNode(e);
 317             s.mode = mode;
 318             s.next = next;
 319             return s;
 320         }
 321 
 322         /**
 323          * Puts or takes an item.
 324          */
 325         Object transfer(Object e, boolean timed, long nanos) {
 326             /*
 327              * Basic algorithm is to loop trying one of three actions:
 328              *
 329              * 1. If apparently empty or already containing nodes of same
 330              *    mode, try to push node on stack and wait for a match,
 331              *    returning it, or null if cancelled.
 332              *
 333              * 2. If apparently containing node of complementary mode,
 334              *    try to push a fulfilling node on to stack, match
 335              *    with corresponding waiting node, pop both from
 336              *    stack, and return matched item. The matching or
 337              *    unlinking might not actually be necessary because of
 338              *    other threads performing action 3:
 339              *
 340              * 3. If top of stack already holds another fulfilling node,
 341              *    help it out by doing its match and/or pop
 342              *    operations, and then continue. The code for helping
 343              *    is essentially the same as for fulfilling, except
 344              *    that it doesn't return the item.
 345              */
 346 
 347             SNode s = null; // constructed/reused as needed
 348             int mode = (e == null) ? REQUEST : DATA;
 349 
 350             for (;;) {
 351                 SNode h = head;
 352                 if (h == null || h.mode == mode) {  // empty or same-mode
 353                     if (timed && nanos <= 0) {      // can't wait
 354                         if (h != null && h.isCancelled())
 355                             casHead(h, h.next);     // pop cancelled node
 356                         else
 357                             return null;
 358                     } else if (casHead(h, s = snode(s, e, h, mode))) {
 359                         SNode m = awaitFulfill(s, timed, nanos);
 360                         if (m == s) {               // wait was cancelled
 361                             clean(s);
 362                             return null;
 363                         }
 364                         if ((h = head) != null && h.next == s)
 365                             casHead(h, s.next);     // help s's fulfiller
 366                         return (mode == REQUEST) ? m.item : s.item;
 367                     }
 368                 } else if (!isFulfilling(h.mode)) { // try to fulfill
 369                     if (h.isCancelled())            // already cancelled
 370                         casHead(h, h.next);         // pop and retry
 371                     else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
 372                         for (;;) { // loop until matched or waiters disappear
 373                             SNode m = s.next;       // m is s's match
 374                             if (m == null) {        // all waiters are gone
 375                                 casHead(s, null);   // pop fulfill node
 376                                 s = null;           // use new node next time
 377                                 break;              // restart main loop
 378                             }
 379                             SNode mn = m.next;
 380                             if (m.tryMatch(s)) {
 381                                 casHead(s, mn);     // pop both s and m
 382                                 return (mode == REQUEST) ? m.item : s.item;
 383                             } else                  // lost match
 384                                 s.casNext(m, mn);   // help unlink
 385                         }
 386                     }
 387                 } else {                            // help a fulfiller
 388                     SNode m = h.next;               // m is h's match
 389                     if (m == null)                  // waiter is gone
 390                         casHead(h, null);           // pop fulfilling node
 391                     else {
 392                         SNode mn = m.next;
 393                         if (m.tryMatch(h))          // help match
 394                             casHead(h, mn);         // pop both h and m
 395                         else                        // lost match
 396                             h.casNext(m, mn);       // help unlink
 397                     }
 398                 }
 399             }
 400         }
 401 
 402         /**
 403          * Spins/blocks until node s is matched by a fulfill operation.
 404          *
 405          * @param s the waiting node
 406          * @param timed true if timed wait
 407          * @param nanos timeout value
 408          * @return matched node, or s if cancelled
 409          */
 410         SNode awaitFulfill(SNode s, boolean timed, long nanos) {
 411             /*
 412              * When a node/thread is about to block, it sets its waiter
 413              * field and then rechecks state at least one more time
 414              * before actually parking, thus covering race vs
 415              * fulfiller noticing that waiter is non-null so should be
 416              * woken.
 417              *
 418              * When invoked by nodes that appear at the point of call
 419              * to be at the head of the stack, calls to park are
 420              * preceded by spins to avoid blocking when producers and
 421              * consumers are arriving very close in time.  This can
 422              * happen enough to bother only on multiprocessors.
 423              *
 424              * The order of checks for returning out of main loop
 425              * reflects fact that interrupts have precedence over
 426              * normal returns, which have precedence over
 427              * timeouts. (So, on timeout, one last check for match is
 428              * done before giving up.) Except that calls from untimed
 429              * SynchronousQueue.{poll/offer} don't check interrupts
 430              * and don't wait at all, so are trapped in transfer
 431              * method rather than calling awaitFulfill.
 432              */
 433             long lastTime = timed ? System.nanoTime() : 0;
 434             Thread w = Thread.currentThread();
 435             SNode h = head;
 436             int spins = (shouldSpin(s) ?
 437                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);
 438             for (;;) {
 439                 if (w.isInterrupted())
 440                     s.tryCancel();
 441                 SNode m = s.match;
 442                 if (m != null)
 443                     return m;
 444                 if (timed) {
 445                     long now = System.nanoTime();
 446                     nanos -= now - lastTime;
 447                     lastTime = now;
 448                     if (nanos <= 0) {
 449                         s.tryCancel();
 450                         continue;
 451                     }
 452                 }
 453                 if (spins > 0)
 454                     spins = shouldSpin(s) ? (spins-1) : 0;
 455                 else if (s.waiter == null)
 456                     s.waiter = w; // establish waiter so can park next iter
 457                 else if (!timed)
 458                     LockSupport.park(this);
 459                 else if (nanos > spinForTimeoutThreshold)
 460                     LockSupport.parkNanos(this, nanos);
 461             }
 462         }
 463 
 464         /**
 465          * Returns true if node s is at head or there is an active
 466          * fulfiller.
 467          */
 468         boolean shouldSpin(SNode s) {
 469             SNode h = head;
 470             return (h == s || h == null || isFulfilling(h.mode));
 471         }
 472 
 473         /**
 474          * Unlinks s from the stack.
 475          */
 476         void clean(SNode s) {
 477             s.item = null;   // forget item
 478             s.waiter = null; // forget thread
 479 
 480             /*
 481              * At worst we may need to traverse entire stack to unlink
 482              * s. If there are multiple concurrent calls to clean, we
 483              * might not see s if another thread has already removed
 484              * it. But we can stop when we see any node known to
 485              * follow s. We use s.next unless it too is cancelled, in
 486              * which case we try the node one past. We don't check any
 487              * further because we don't want to doubly traverse just to
 488              * find sentinel.
 489              */
 490 
 491             SNode past = s.next;
 492             if (past != null && past.isCancelled())
 493                 past = past.next;
 494 
 495             // Absorb cancelled nodes at head
 496             SNode p;
 497             while ((p = head) != null && p != past && p.isCancelled())
 498                 casHead(p, p.next);
 499 
 500             // Unsplice embedded nodes
 501             while (p != null && p != past) {
 502                 SNode n = p.next;
 503                 if (n != null && n.isCancelled())
 504                     p.casNext(n, n.next);
 505                 else
 506                     p = n;
 507             }
 508         }
 509 
 510         // Unsafe mechanics
 511         private static final sun.misc.Unsafe UNSAFE;
 512         private static final long headOffset;
 513         static {
 514             try {
 515                 UNSAFE = sun.misc.Unsafe.getUnsafe();
 516                 Class k = TransferStack.class;
 517                 headOffset = UNSAFE.objectFieldOffset
 518                     (k.getDeclaredField("head"));
 519             } catch (Exception e) {
 520                 throw new Error(e);
 521             }
 522         }
 523     }
 524 
 525     /** Dual Queue */
 526     static final class TransferQueue extends Transferer {
 527         /*
 528          * This extends Scherer-Scott dual queue algorithm, differing,
 529          * among other ways, by using modes within nodes rather than
 530          * marked pointers. The algorithm is a little simpler than
 531          * that for stacks because fulfillers do not need explicit
 532          * nodes, and matching is done by CAS'ing QNode.item field
 533          * from non-null to null (for put) or vice versa (for take).
 534          */
 535 
 536         /** Node class for TransferQueue. */
 537         static final class QNode {
 538             volatile QNode next;          // next node in queue
 539             volatile Object item;         // CAS'ed to or from null
 540             volatile Thread waiter;       // to control park/unpark
 541             final boolean isData;
 542 
 543             QNode(Object item, boolean isData) {
 544                 this.item = item;
 545                 this.isData = isData;
 546             }
 547 
 548             boolean casNext(QNode cmp, QNode val) {
 549                 return next == cmp &&
 550                     UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
 551             }
 552 
 553             boolean casItem(Object cmp, Object val) {
 554                 return item == cmp &&
 555                     UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
 556             }
 557 
 558             /**
 559              * Tries to cancel by CAS'ing ref to this as item.
 560              */
 561             void tryCancel(Object cmp) {
 562                 UNSAFE.compareAndSwapObject(this, itemOffset, cmp, this);
 563             }
 564 
 565             boolean isCancelled() {
 566                 return item == this;
 567             }
 568 
 569             /**
 570              * Returns true if this node is known to be off the queue
 571              * because its next pointer has been forgotten due to
 572              * an advanceHead operation.
 573              */
 574             boolean isOffList() {
 575                 return next == this;
 576             }
 577 
 578             // Unsafe mechanics
 579             private static final sun.misc.Unsafe UNSAFE;
 580             private static final long itemOffset;
 581             private static final long nextOffset;
 582 
 583             static {
 584                 try {
 585                     UNSAFE = sun.misc.Unsafe.getUnsafe();
 586                     Class k = QNode.class;
 587                     itemOffset = UNSAFE.objectFieldOffset
 588                         (k.getDeclaredField("item"));
 589                     nextOffset = UNSAFE.objectFieldOffset
 590                         (k.getDeclaredField("next"));
 591                 } catch (Exception e) {
 592                     throw new Error(e);
 593                 }
 594             }
 595         }
 596 
 597         /** Head of queue */
 598         transient volatile QNode head;
 599         /** Tail of queue */
 600         transient volatile QNode tail;
 601         /**
 602          * Reference to a cancelled node that might not yet have been
 603          * unlinked from queue because it was the last inserted node
 604          * when it cancelled.
 605          */
 606         transient volatile QNode cleanMe;
 607 
 608         TransferQueue() {
 609             QNode h = new QNode(null, false); // initialize to dummy node.
 610             head = h;
 611             tail = h;
 612         }
 613 
 614         /**
 615          * Tries to cas nh as new head; if successful, unlink
 616          * old head's next node to avoid garbage retention.
 617          */
 618         void advanceHead(QNode h, QNode nh) {
 619             if (h == head &&
 620                 UNSAFE.compareAndSwapObject(this, headOffset, h, nh))
 621                 h.next = h; // forget old next
 622         }
 623 
 624         /**
 625          * Tries to cas nt as new tail.
 626          */
 627         void advanceTail(QNode t, QNode nt) {
 628             if (tail == t)
 629                 UNSAFE.compareAndSwapObject(this, tailOffset, t, nt);
 630         }
 631 
 632         /**
 633          * Tries to CAS cleanMe slot.
 634          */
 635         boolean casCleanMe(QNode cmp, QNode val) {
 636             return cleanMe == cmp &&
 637                 UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
 638         }
 639 
 640         /**
 641          * Puts or takes an item.
 642          */
 643         Object transfer(Object e, boolean timed, long nanos) {
 644             /* Basic algorithm is to loop trying to take either of
 645              * two actions:
 646              *
 647              * 1. If queue apparently empty or holding same-mode nodes,
 648              *    try to add node to queue of waiters, wait to be
 649              *    fulfilled (or cancelled) and return matching item.
 650              *
 651              * 2. If queue apparently contains waiting items, and this
 652              *    call is of complementary mode, try to fulfill by CAS'ing
 653              *    item field of waiting node and dequeuing it, and then
 654              *    returning matching item.
 655              *
 656              * In each case, along the way, check for and try to help
 657              * advance head and tail on behalf of other stalled/slow
 658              * threads.
 659              *
 660              * The loop starts off with a null check guarding against
 661              * seeing uninitialized head or tail values. This never
 662              * happens in current SynchronousQueue, but could if
 663              * callers held non-volatile/final ref to the
 664              * transferer. The check is here anyway because it places
 665              * null checks at top of loop, which is usually faster
 666              * than having them implicitly interspersed.
 667              */
 668 
 669             QNode s = null; // constructed/reused as needed
 670             boolean isData = (e != null);
 671 
 672             for (;;) {
 673                 QNode t = tail;
 674                 QNode h = head;
 675                 if (t == null || h == null)         // saw uninitialized value
 676                     continue;                       // spin
 677 
 678                 if (h == t || t.isData == isData) { // empty or same-mode
 679                     QNode tn = t.next;
 680                     if (t != tail)                  // inconsistent read
 681                         continue;
 682                     if (tn != null) {               // lagging tail
 683                         advanceTail(t, tn);
 684                         continue;
 685                     }
 686                     if (timed && nanos <= 0)        // can't wait
 687                         return null;
 688                     if (s == null)
 689                         s = new QNode(e, isData);
 690                     if (!t.casNext(null, s))        // failed to link in
 691                         continue;
 692 
 693                     advanceTail(t, s);              // swing tail and wait
 694                     Object x = awaitFulfill(s, e, timed, nanos);
 695                     if (x == s) {                   // wait was cancelled
 696                         clean(t, s);
 697                         return null;
 698                     }
 699 
 700                     if (!s.isOffList()) {           // not already unlinked
 701                         advanceHead(t, s);          // unlink if head
 702                         if (x != null)              // and forget fields
 703                             s.item = s;
 704                         s.waiter = null;
 705                     }
 706                     return (x != null) ? x : e;
 707 
 708                 } else {                            // complementary-mode
 709                     QNode m = h.next;               // node to fulfill
 710                     if (t != tail || m == null || h != head)
 711                         continue;                   // inconsistent read
 712 
 713                     Object x = m.item;
 714                     if (isData == (x != null) ||    // m already fulfilled
 715                         x == m ||                   // m cancelled
 716                         !m.casItem(x, e)) {         // lost CAS
 717                         advanceHead(h, m);          // dequeue and retry
 718                         continue;
 719                     }
 720 
 721                     advanceHead(h, m);              // successfully fulfilled
 722                     LockSupport.unpark(m.waiter);
 723                     return (x != null) ? x : e;
 724                 }
 725             }
 726         }
 727 
 728         /**
 729          * Spins/blocks until node s is fulfilled.
 730          *
 731          * @param s the waiting node
 732          * @param e the comparison value for checking match
 733          * @param timed true if timed wait
 734          * @param nanos timeout value
 735          * @return matched item, or s if cancelled
 736          */
 737         Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
 738             /* Same idea as TransferStack.awaitFulfill */
 739             long lastTime = timed ? System.nanoTime() : 0;
 740             Thread w = Thread.currentThread();
 741             int spins = ((head.next == s) ?
 742                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);
 743             for (;;) {
 744                 if (w.isInterrupted())
 745                     s.tryCancel(e);
 746                 Object x = s.item;
 747                 if (x != e)
 748                     return x;
 749                 if (timed) {
 750                     long now = System.nanoTime();
 751                     nanos -= now - lastTime;
 752                     lastTime = now;
 753                     if (nanos <= 0) {
 754                         s.tryCancel(e);
 755                         continue;
 756                     }
 757                 }
 758                 if (spins > 0)
 759                     --spins;
 760                 else if (s.waiter == null)
 761                     s.waiter = w;
 762                 else if (!timed)
 763                     LockSupport.park(this);
 764                 else if (nanos > spinForTimeoutThreshold)
 765                     LockSupport.parkNanos(this, nanos);
 766             }
 767         }
 768 
 769         /**
 770          * Gets rid of cancelled node s with original predecessor pred.
 771          */
 772         void clean(QNode pred, QNode s) {
 773             s.waiter = null; // forget thread
 774             /*
 775              * At any given time, exactly one node on list cannot be
 776              * deleted -- the last inserted node. To accommodate this,
 777              * if we cannot delete s, we save its predecessor as
 778              * "cleanMe", deleting the previously saved version
 779              * first. At least one of node s or the node previously
 780              * saved can always be deleted, so this always terminates.
 781              */
 782             while (pred.next == s) { // Return early if already unlinked
 783                 QNode h = head;
 784                 QNode hn = h.next;   // Absorb cancelled first node as head
 785                 if (hn != null && hn.isCancelled()) {
 786                     advanceHead(h, hn);
 787                     continue;
 788                 }
 789                 QNode t = tail;      // Ensure consistent read for tail
 790                 if (t == h)
 791                     return;
 792                 QNode tn = t.next;
 793                 if (t != tail)
 794                     continue;
 795                 if (tn != null) {
 796                     advanceTail(t, tn);
 797                     continue;
 798                 }
 799                 if (s != t) {        // If not tail, try to unsplice
 800                     QNode sn = s.next;
 801                     if (sn == s || pred.casNext(s, sn))
 802                         return;
 803                 }
 804                 QNode dp = cleanMe;
 805                 if (dp != null) {    // Try unlinking previous cancelled node
 806                     QNode d = dp.next;
 807                     QNode dn;
 808                     if (d == null ||               // d is gone or
 809                         d == dp ||                 // d is off list or
 810                         !d.isCancelled() ||        // d not cancelled or
 811                         (d != t &&                 // d not tail and
 812                          (dn = d.next) != null &&  //   has successor
 813                          dn != d &&                //   that is on list
 814                          dp.casNext(d, dn)))       // d unspliced
 815                         casCleanMe(dp, null);
 816                     if (dp == pred)
 817                         return;      // s is already saved node
 818                 } else if (casCleanMe(null, pred))
 819                     return;          // Postpone cleaning s
 820             }
 821         }
 822 
 823         private static final sun.misc.Unsafe UNSAFE;
 824         private static final long headOffset;
 825         private static final long tailOffset;
 826         private static final long cleanMeOffset;
 827         static {
 828             try {
 829                 UNSAFE = sun.misc.Unsafe.getUnsafe();
 830                 Class k = TransferQueue.class;
 831                 headOffset = UNSAFE.objectFieldOffset
 832                     (k.getDeclaredField("head"));
 833                 tailOffset = UNSAFE.objectFieldOffset
 834                     (k.getDeclaredField("tail"));
 835                 cleanMeOffset = UNSAFE.objectFieldOffset
 836                     (k.getDeclaredField("cleanMe"));
 837             } catch (Exception e) {
 838                 throw new Error(e);
 839             }
 840         }
 841     }
 842 
 843     /**
 844      * The transferer. Set only in constructor, but cannot be declared
 845      * as final without further complicating serialization.  Since
 846      * this is accessed only at most once per public method, there
 847      * isn't a noticeable performance penalty for using volatile
 848      * instead of final here.
 849      */
 850     private transient volatile Transferer transferer;
 851 
 852     /**
 853      * Creates a <tt>SynchronousQueue</tt> with nonfair access policy.
 854      */
 855     public SynchronousQueue() {
 856         this(false);
 857     }
 858 
 859     /**
 860      * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.
 861      *
 862      * @param fair if true, waiting threads contend in FIFO order for
 863      *        access; otherwise the order is unspecified.
 864      */
 865     public SynchronousQueue(boolean fair) {
 866         transferer = fair ? new TransferQueue() : new TransferStack();
 867     }
 868 
 869     /**
 870      * Adds the specified element to this queue, waiting if necessary for
 871      * another thread to receive it.
 872      *
 873      * @throws InterruptedException {@inheritDoc}
 874      * @throws NullPointerException {@inheritDoc}
 875      */
 876     public void put(E o) throws InterruptedException {
 877         if (o == null) throw new NullPointerException();
 878         if (transferer.transfer(o, false, 0) == null) {
 879             Thread.interrupted();
 880             throw new InterruptedException();
 881         }
 882     }
 883 
 884     /**
 885      * Inserts the specified element into this queue, waiting if necessary
 886      * up to the specified wait time for another thread to receive it.
 887      *
 888      * @return <tt>true</tt> if successful, or <tt>false</tt> if the
 889      *         specified waiting time elapses before a consumer appears.
 890      * @throws InterruptedException {@inheritDoc}
 891      * @throws NullPointerException {@inheritDoc}
 892      */
 893     public boolean offer(E o, long timeout, TimeUnit unit)
 894         throws InterruptedException {
 895         if (o == null) throw new NullPointerException();
 896         if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
 897             return true;
 898         if (!Thread.interrupted())
 899             return false;
 900         throw new InterruptedException();
 901     }
 902 
 903     /**
 904      * Inserts the specified element into this queue, if another thread is
 905      * waiting to receive it.
 906      *
 907      * @param e the element to add
 908      * @return <tt>true</tt> if the element was added to this queue, else
 909      *         <tt>false</tt>
 910      * @throws NullPointerException if the specified element is null
 911      */
 912     public boolean offer(E e) {
 913         if (e == null) throw new NullPointerException();
 914         return transferer.transfer(e, true, 0) != null;
 915     }
 916 
 917     /**
 918      * Retrieves and removes the head of this queue, waiting if necessary
 919      * for another thread to insert it.
 920      *
 921      * @return the head of this queue
 922      * @throws InterruptedException {@inheritDoc}
 923      */
 924     public E take() throws InterruptedException {
 925         Object e = transferer.transfer(null, false, 0);
 926         if (e != null)
 927             return (E)e;
 928         Thread.interrupted();
 929         throw new InterruptedException();
 930     }
 931 
 932     /**
 933      * Retrieves and removes the head of this queue, waiting
 934      * if necessary up to the specified wait time, for another thread
 935      * to insert it.
 936      *
 937      * @return the head of this queue, or <tt>null</tt> if the
 938      *         specified waiting time elapses before an element is present.
 939      * @throws InterruptedException {@inheritDoc}
 940      */
 941     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
 942         Object e = transferer.transfer(null, true, unit.toNanos(timeout));
 943         if (e != null || !Thread.interrupted())
 944             return (E)e;
 945         throw new InterruptedException();
 946     }
 947 
 948     /**
 949      * Retrieves and removes the head of this queue, if another thread
 950      * is currently making an element available.
 951      *
 952      * @return the head of this queue, or <tt>null</tt> if no
 953      *         element is available.
 954      */
 955     public E poll() {
 956         return (E)transferer.transfer(null, true, 0);
 957     }
 958 
 959     /**
 960      * Always returns <tt>true</tt>.
 961      * A <tt>SynchronousQueue</tt> has no internal capacity.
 962      *
 963      * @return <tt>true</tt>
 964      */
 965     public boolean isEmpty() {
 966         return true;
 967     }
 968 
 969     /**
 970      * Always returns zero.
 971      * A <tt>SynchronousQueue</tt> has no internal capacity.
 972      *
 973      * @return zero.
 974      */
 975     public int size() {
 976         return 0;
 977     }
 978 
 979     /**
 980      * Always returns zero.
 981      * A <tt>SynchronousQueue</tt> has no internal capacity.
 982      *
 983      * @return zero.
 984      */
 985     public int remainingCapacity() {
 986         return 0;
 987     }
 988 
 989     /**
 990      * Does nothing.
 991      * A <tt>SynchronousQueue</tt> has no internal capacity.
 992      */
 993     public void clear() {
 994     }
 995 
 996     /**
 997      * Always returns <tt>false</tt>.
 998      * A <tt>SynchronousQueue</tt> has no internal capacity.
 999      *
1000      * @param o the element
1001      * @return <tt>false</tt>
1002      */
1003     public boolean contains(Object o) {
1004         return false;
1005     }
1006 
1007     /**
1008      * Always returns <tt>false</tt>.
1009      * A <tt>SynchronousQueue</tt> has no internal capacity.
1010      *
1011      * @param o the element to remove
1012      * @return <tt>false</tt>
1013      */
1014     public boolean remove(Object o) {
1015         return false;
1016     }
1017 
1018     /**
1019      * Returns <tt>false</tt> unless the given collection is empty.
1020      * A <tt>SynchronousQueue</tt> has no internal capacity.
1021      *
1022      * @param c the collection
1023      * @return <tt>false</tt> unless given collection is empty
1024      */
1025     public boolean containsAll(Collection<?> c) {
1026         return c.isEmpty();
1027     }
1028 
1029     /**
1030      * Always returns <tt>false</tt>.
1031      * A <tt>SynchronousQueue</tt> has no internal capacity.
1032      *
1033      * @param c the collection
1034      * @return <tt>false</tt>
1035      */
1036     public boolean removeAll(Collection<?> c) {
1037         return false;
1038     }
1039 
1040     /**
1041      * Always returns <tt>false</tt>.
1042      * A <tt>SynchronousQueue</tt> has no internal capacity.
1043      *
1044      * @param c the collection
1045      * @return <tt>false</tt>
1046      */
1047     public boolean retainAll(Collection<?> c) {
1048         return false;
1049     }
1050 
1051     /**
1052      * Always returns <tt>null</tt>.
1053      * A <tt>SynchronousQueue</tt> does not return elements
1054      * unless actively waited on.
1055      *
1056      * @return <tt>null</tt>
1057      */
1058     public E peek() {
1059         return null;
1060     }
1061 
1062     /**
1063      * Returns an empty iterator in which <tt>hasNext</tt> always returns
1064      * <tt>false</tt>.
1065      *
1066      * @return an empty iterator
1067      */
1068     public Iterator<E> iterator() {
1069         return Collections.emptyIterator();
1070     }
1071 
1072     /**
1073      * Returns a zero-length array.
1074      * @return a zero-length array
1075      */
1076     public Object[] toArray() {
1077         return new Object[0];
1078     }
1079 
1080     /**
1081      * Sets the zeroeth element of the specified array to <tt>null</tt>
1082      * (if the array has non-zero length) and returns it.
1083      *
1084      * @param a the array
1085      * @return the specified array
1086      * @throws NullPointerException if the specified array is null
1087      */
1088     public <T> T[] toArray(T[] a) {
1089         if (a.length > 0)
1090             a[0] = null;
1091         return a;
1092     }
1093 
1094     /**
1095      * @throws UnsupportedOperationException {@inheritDoc}
1096      * @throws ClassCastException            {@inheritDoc}
1097      * @throws NullPointerException          {@inheritDoc}
1098      * @throws IllegalArgumentException      {@inheritDoc}
1099      */
1100     public int drainTo(Collection<? super E> c) {
1101         if (c == null)
1102             throw new NullPointerException();
1103         if (c == this)
1104             throw new IllegalArgumentException();
1105         int n = 0;
1106         E e;
1107         while ( (e = poll()) != null) {
1108             c.add(e);
1109             ++n;
1110         }
1111         return n;
1112     }
1113 
1114     /**
1115      * @throws UnsupportedOperationException {@inheritDoc}
1116      * @throws ClassCastException            {@inheritDoc}
1117      * @throws NullPointerException          {@inheritDoc}
1118      * @throws IllegalArgumentException      {@inheritDoc}
1119      */
1120     public int drainTo(Collection<? super E> c, int maxElements) {
1121         if (c == null)
1122             throw new NullPointerException();
1123         if (c == this)
1124             throw new IllegalArgumentException();
1125         int n = 0;
1126         E e;
1127         while (n < maxElements && (e = poll()) != null) {
1128             c.add(e);
1129             ++n;
1130         }
1131         return n;
1132     }
1133 
1134     /*
1135      * To cope with serialization strategy in the 1.5 version of
1136      * SynchronousQueue, we declare some unused classes and fields
1137      * that exist solely to enable serializability across versions.
1138      * These fields are never used, so are initialized only if this
1139      * object is ever serialized or deserialized.
1140      */
1141 
1142     static class WaitQueue implements java.io.Serializable { }
1143     static class LifoWaitQueue extends WaitQueue {
1144         private static final long serialVersionUID = -3633113410248163686L;
1145     }
1146     static class FifoWaitQueue extends WaitQueue {
1147         private static final long serialVersionUID = -3623113410248163686L;
1148     }
1149     private ReentrantLock qlock;
1150     private WaitQueue waitingProducers;
1151     private WaitQueue waitingConsumers;
1152 
1153     /**
1154      * Save the state to a stream (that is, serialize it).
1155      *
1156      * @param s the stream
1157      */
1158     private void writeObject(java.io.ObjectOutputStream s)
1159         throws java.io.IOException {
1160         boolean fair = transferer instanceof TransferQueue;
1161         if (fair) {
1162             qlock = new ReentrantLock(true);
1163             waitingProducers = new FifoWaitQueue();
1164             waitingConsumers = new FifoWaitQueue();
1165         }
1166         else {
1167             qlock = new ReentrantLock();
1168             waitingProducers = new LifoWaitQueue();
1169             waitingConsumers = new LifoWaitQueue();
1170         }
1171         s.defaultWriteObject();
1172     }
1173 
1174     private void readObject(final java.io.ObjectInputStream s)
1175         throws java.io.IOException, ClassNotFoundException {
1176         s.defaultReadObject();
1177         if (waitingProducers instanceof FifoWaitQueue)
1178             transferer = new TransferQueue();
1179         else
1180             transferer = new TransferStack();
1181     }
1182 
1183     // Unsafe mechanics
1184     static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1185                                   String field, Class<?> klazz) {
1186         try {
1187             return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1188         } catch (NoSuchFieldException e) {
1189             // Convert Exception to corresponding Error
1190             NoSuchFieldError error = new NoSuchFieldError(field);
1191             error.initCause(e);
1192             throw error;
1193         }
1194     }
1195 
1196 }