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 static abstract 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 static final AtomicReferenceFieldUpdater<SNode, SNode> 245 nextUpdater = AtomicReferenceFieldUpdater.newUpdater 246 (SNode.class, SNode.class, "next"); 247 248 boolean casNext(SNode cmp, SNode val) { 249 return (cmp == next && 250 nextUpdater.compareAndSet(this, cmp, val)); 251 } 252 253 static final AtomicReferenceFieldUpdater<SNode, SNode> 254 matchUpdater = AtomicReferenceFieldUpdater.newUpdater 255 (SNode.class, SNode.class, "match"); 256 257 /** 258 * Tries to match node s to this node, if so, waking up thread. 259 * Fulfillers call tryMatch to identify their waiters. 260 * Waiters block until they have been matched. 261 * 262 * @param s the node to match 263 * @return true if successfully matched to s 264 */ 265 boolean tryMatch(SNode s) { 266 if (match == null && 267 matchUpdater.compareAndSet(this, null, s)) { 268 Thread w = waiter; 269 if (w != null) { // waiters need at most one unpark 270 waiter = null; 271 LockSupport.unpark(w); 272 } 273 return true; 274 } 275 return match == s; 276 } 277 278 /** 279 * Tries to cancel a wait by matching node to itself. 280 */ 281 void tryCancel() { 282 matchUpdater.compareAndSet(this, null, this); 283 } 284 285 boolean isCancelled() { 286 return match == this; 287 } 288 } 289 290 /** The head (top) of the stack */ 291 volatile SNode head; 292 293 static final AtomicReferenceFieldUpdater<TransferStack, SNode> 294 headUpdater = AtomicReferenceFieldUpdater.newUpdater 295 (TransferStack.class, SNode.class, "head"); 296 297 boolean casHead(SNode h, SNode nh) { 298 return h == head && headUpdater.compareAndSet(this, h, nh); 299 } 300 301 /** 302 * Creates or resets fields of a node. Called only from transfer 303 * where the node to push on stack is lazily created and 304 * reused when possible to help reduce intervals between reads 305 * and CASes of head and to avoid surges of garbage when CASes 306 * to push nodes fail due to contention. 307 */ 308 static SNode snode(SNode s, Object e, SNode next, int mode) { 309 if (s == null) s = new SNode(e); 310 s.mode = mode; 311 s.next = next; 312 return s; 313 } 314 315 /** 316 * Puts or takes an item. 317 */ 318 Object transfer(Object e, boolean timed, long nanos) { 319 /* 320 * Basic algorithm is to loop trying one of three actions: 321 * 322 * 1. If apparently empty or already containing nodes of same 323 * mode, try to push node on stack and wait for a match, 324 * returning it, or null if cancelled. 325 * 326 * 2. If apparently containing node of complementary mode, 327 * try to push a fulfilling node on to stack, match 328 * with corresponding waiting node, pop both from 329 * stack, and return matched item. The matching or 330 * unlinking might not actually be necessary because of 331 * other threads performing action 3: 332 * 333 * 3. If top of stack already holds another fulfilling node, 334 * help it out by doing its match and/or pop 335 * operations, and then continue. The code for helping 336 * is essentially the same as for fulfilling, except 337 * that it doesn't return the item. 338 */ 339 340 SNode s = null; // constructed/reused as needed 341 int mode = (e == null)? REQUEST : DATA; 342 343 for (;;) { 344 SNode h = head; 345 if (h == null || h.mode == mode) { // empty or same-mode 346 if (timed && nanos <= 0) { // can't wait 347 if (h != null && h.isCancelled()) 348 casHead(h, h.next); // pop cancelled node 349 else 350 return null; 351 } else if (casHead(h, s = snode(s, e, h, mode))) { 352 SNode m = awaitFulfill(s, timed, nanos); 353 if (m == s) { // wait was cancelled 354 clean(s); 355 return null; 356 } 357 if ((h = head) != null && h.next == s) 358 casHead(h, s.next); // help s's fulfiller 359 return mode == REQUEST? m.item : s.item; 360 } 361 } else if (!isFulfilling(h.mode)) { // try to fulfill 362 if (h.isCancelled()) // already cancelled 363 casHead(h, h.next); // pop and retry 364 else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) { 365 for (;;) { // loop until matched or waiters disappear 366 SNode m = s.next; // m is s's match 367 if (m == null) { // all waiters are gone 368 casHead(s, null); // pop fulfill node 369 s = null; // use new node next time 370 break; // restart main loop 371 } 372 SNode mn = m.next; 373 if (m.tryMatch(s)) { 374 casHead(s, mn); // pop both s and m 375 return (mode == REQUEST)? m.item : s.item; 376 } else // lost match 377 s.casNext(m, mn); // help unlink 378 } 379 } 380 } else { // help a fulfiller 381 SNode m = h.next; // m is h's match 382 if (m == null) // waiter is gone 383 casHead(h, null); // pop fulfilling node 384 else { 385 SNode mn = m.next; 386 if (m.tryMatch(h)) // help match 387 casHead(h, mn); // pop both h and m 388 else // lost match 389 h.casNext(m, mn); // help unlink 390 } 391 } 392 } 393 } 394 395 /** 396 * Spins/blocks until node s is matched by a fulfill operation. 397 * 398 * @param s the waiting node 399 * @param timed true if timed wait 400 * @param nanos timeout value 401 * @return matched node, or s if cancelled 402 */ 403 SNode awaitFulfill(SNode s, boolean timed, long nanos) { 404 /* 405 * When a node/thread is about to block, it sets its waiter 406 * field and then rechecks state at least one more time 407 * before actually parking, thus covering race vs 408 * fulfiller noticing that waiter is non-null so should be 409 * woken. 410 * 411 * When invoked by nodes that appear at the point of call 412 * to be at the head of the stack, calls to park are 413 * preceded by spins to avoid blocking when producers and 414 * consumers are arriving very close in time. This can 415 * happen enough to bother only on multiprocessors. 416 * 417 * The order of checks for returning out of main loop 418 * reflects fact that interrupts have precedence over 419 * normal returns, which have precedence over 420 * timeouts. (So, on timeout, one last check for match is 421 * done before giving up.) Except that calls from untimed 422 * SynchronousQueue.{poll/offer} don't check interrupts 423 * and don't wait at all, so are trapped in transfer 424 * method rather than calling awaitFulfill. 425 */ 426 long lastTime = (timed)? System.nanoTime() : 0; 427 Thread w = Thread.currentThread(); 428 SNode h = head; 429 int spins = (shouldSpin(s)? 430 (timed? maxTimedSpins : maxUntimedSpins) : 0); 431 for (;;) { 432 if (w.isInterrupted()) 433 s.tryCancel(); 434 SNode m = s.match; 435 if (m != null) 436 return m; 437 if (timed) { 438 long now = System.nanoTime(); 439 nanos -= now - lastTime; 440 lastTime = now; 441 if (nanos <= 0) { 442 s.tryCancel(); 443 continue; 444 } 445 } 446 if (spins > 0) 447 spins = shouldSpin(s)? (spins-1) : 0; 448 else if (s.waiter == null) 449 s.waiter = w; // establish waiter so can park next iter 450 else if (!timed) 451 LockSupport.park(this); 452 else if (nanos > spinForTimeoutThreshold) 453 LockSupport.parkNanos(this, nanos); 454 } 455 } 456 457 /** 458 * Returns true if node s is at head or there is an active 459 * fulfiller. 460 */ 461 boolean shouldSpin(SNode s) { 462 SNode h = head; 463 return (h == s || h == null || isFulfilling(h.mode)); 464 } 465 466 /** 467 * Unlinks s from the stack. 468 */ 469 void clean(SNode s) { 470 s.item = null; // forget item 471 s.waiter = null; // forget thread 472 473 /* 474 * At worst we may need to traverse entire stack to unlink 475 * s. If there are multiple concurrent calls to clean, we 476 * might not see s if another thread has already removed 477 * it. But we can stop when we see any node known to 478 * follow s. We use s.next unless it too is cancelled, in 479 * which case we try the node one past. We don't check any 480 * further because we don't want to doubly traverse just to 481 * find sentinel. 482 */ 483 484 SNode past = s.next; 485 if (past != null && past.isCancelled()) 486 past = past.next; 487 488 // Absorb cancelled nodes at head 489 SNode p; 490 while ((p = head) != null && p != past && p.isCancelled()) 491 casHead(p, p.next); 492 493 // Unsplice embedded nodes 494 while (p != null && p != past) { 495 SNode n = p.next; 496 if (n != null && n.isCancelled()) 497 p.casNext(n, n.next); 498 else 499 p = n; 500 } 501 } 502 } 503 504 /** Dual Queue */ 505 static final class TransferQueue extends Transferer { 506 /* 507 * This extends Scherer-Scott dual queue algorithm, differing, 508 * among other ways, by using modes within nodes rather than 509 * marked pointers. The algorithm is a little simpler than 510 * that for stacks because fulfillers do not need explicit 511 * nodes, and matching is done by CAS'ing QNode.item field 512 * from non-null to null (for put) or vice versa (for take). 513 */ 514 515 /** Node class for TransferQueue. */ 516 static final class QNode { 517 volatile QNode next; // next node in queue 518 volatile Object item; // CAS'ed to or from null 519 volatile Thread waiter; // to control park/unpark 520 final boolean isData; 521 522 QNode(Object item, boolean isData) { 523 this.item = item; 524 this.isData = isData; 525 } 526 527 static final AtomicReferenceFieldUpdater<QNode, QNode> 528 nextUpdater = AtomicReferenceFieldUpdater.newUpdater 529 (QNode.class, QNode.class, "next"); 530 531 boolean casNext(QNode cmp, QNode val) { 532 return (next == cmp && 533 nextUpdater.compareAndSet(this, cmp, val)); 534 } 535 536 static final AtomicReferenceFieldUpdater<QNode, Object> 537 itemUpdater = AtomicReferenceFieldUpdater.newUpdater 538 (QNode.class, Object.class, "item"); 539 540 boolean casItem(Object cmp, Object val) { 541 return (item == cmp && 542 itemUpdater.compareAndSet(this, cmp, val)); 543 } 544 545 /** 546 * Tries to cancel by CAS'ing ref to this as item. 547 */ 548 void tryCancel(Object cmp) { 549 itemUpdater.compareAndSet(this, cmp, this); 550 } 551 552 boolean isCancelled() { 553 return item == this; 554 } 555 556 /** 557 * Returns true if this node is known to be off the queue 558 * because its next pointer has been forgotten due to 559 * an advanceHead operation. 560 */ 561 boolean isOffList() { 562 return next == this; 563 } 564 } 565 566 /** Head of queue */ 567 transient volatile QNode head; 568 /** Tail of queue */ 569 transient volatile QNode tail; 570 /** 571 * Reference to a cancelled node that might not yet have been 572 * unlinked from queue because it was the last inserted node 573 * when it cancelled. 574 */ 575 transient volatile QNode cleanMe; 576 577 TransferQueue() { 578 QNode h = new QNode(null, false); // initialize to dummy node. 579 head = h; 580 tail = h; 581 } 582 583 static final AtomicReferenceFieldUpdater<TransferQueue, QNode> 584 headUpdater = AtomicReferenceFieldUpdater.newUpdater 585 (TransferQueue.class, QNode.class, "head"); 586 587 /** 588 * Tries to cas nh as new head; if successful, unlink 589 * old head's next node to avoid garbage retention. 590 */ 591 void advanceHead(QNode h, QNode nh) { 592 if (h == head && headUpdater.compareAndSet(this, h, nh)) 593 h.next = h; // forget old next 594 } 595 596 static final AtomicReferenceFieldUpdater<TransferQueue, QNode> 597 tailUpdater = AtomicReferenceFieldUpdater.newUpdater 598 (TransferQueue.class, QNode.class, "tail"); 599 600 /** 601 * Tries to cas nt as new tail. 602 */ 603 void advanceTail(QNode t, QNode nt) { 604 if (tail == t) 605 tailUpdater.compareAndSet(this, t, nt); 606 } 607 608 static final AtomicReferenceFieldUpdater<TransferQueue, QNode> 609 cleanMeUpdater = AtomicReferenceFieldUpdater.newUpdater 610 (TransferQueue.class, QNode.class, "cleanMe"); 611 612 /** 613 * Tries to CAS cleanMe slot. 614 */ 615 boolean casCleanMe(QNode cmp, QNode val) { 616 return (cleanMe == cmp && 617 cleanMeUpdater.compareAndSet(this, cmp, val)); 618 } 619 620 /** 621 * Puts or takes an item. 622 */ 623 Object transfer(Object e, boolean timed, long nanos) { 624 /* Basic algorithm is to loop trying to take either of 625 * two actions: 626 * 627 * 1. If queue apparently empty or holding same-mode nodes, 628 * try to add node to queue of waiters, wait to be 629 * fulfilled (or cancelled) and return matching item. 630 * 631 * 2. If queue apparently contains waiting items, and this 632 * call is of complementary mode, try to fulfill by CAS'ing 633 * item field of waiting node and dequeuing it, and then 634 * returning matching item. 635 * 636 * In each case, along the way, check for and try to help 637 * advance head and tail on behalf of other stalled/slow 638 * threads. 639 * 640 * The loop starts off with a null check guarding against 641 * seeing uninitialized head or tail values. This never 642 * happens in current SynchronousQueue, but could if 643 * callers held non-volatile/final ref to the 644 * transferer. The check is here anyway because it places 645 * null checks at top of loop, which is usually faster 646 * than having them implicitly interspersed. 647 */ 648 649 QNode s = null; // constructed/reused as needed 650 boolean isData = (e != null); 651 652 for (;;) { 653 QNode t = tail; 654 QNode h = head; 655 if (t == null || h == null) // saw uninitialized value 656 continue; // spin 657 658 if (h == t || t.isData == isData) { // empty or same-mode 659 QNode tn = t.next; 660 if (t != tail) // inconsistent read 661 continue; 662 if (tn != null) { // lagging tail 663 advanceTail(t, tn); 664 continue; 665 } 666 if (timed && nanos <= 0) // can't wait 667 return null; 668 if (s == null) 669 s = new QNode(e, isData); 670 if (!t.casNext(null, s)) // failed to link in 671 continue; 672 673 advanceTail(t, s); // swing tail and wait 674 Object x = awaitFulfill(s, e, timed, nanos); 675 if (x == s) { // wait was cancelled 676 clean(t, s); 677 return null; 678 } 679 680 if (!s.isOffList()) { // not already unlinked 681 advanceHead(t, s); // unlink if head 682 if (x != null) // and forget fields 683 s.item = s; 684 s.waiter = null; 685 } 686 return (x != null)? x : e; 687 688 } else { // complementary-mode 689 QNode m = h.next; // node to fulfill 690 if (t != tail || m == null || h != head) 691 continue; // inconsistent read 692 693 Object x = m.item; 694 if (isData == (x != null) || // m already fulfilled 695 x == m || // m cancelled 696 !m.casItem(x, e)) { // lost CAS 697 advanceHead(h, m); // dequeue and retry 698 continue; 699 } 700 701 advanceHead(h, m); // successfully fulfilled 702 LockSupport.unpark(m.waiter); 703 return (x != null)? x : e; 704 } 705 } 706 } 707 708 /** 709 * Spins/blocks until node s is fulfilled. 710 * 711 * @param s the waiting node 712 * @param e the comparison value for checking match 713 * @param timed true if timed wait 714 * @param nanos timeout value 715 * @return matched item, or s if cancelled 716 */ 717 Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) { 718 /* Same idea as TransferStack.awaitFulfill */ 719 long lastTime = (timed)? System.nanoTime() : 0; 720 Thread w = Thread.currentThread(); 721 int spins = ((head.next == s) ? 722 (timed? maxTimedSpins : maxUntimedSpins) : 0); 723 for (;;) { 724 if (w.isInterrupted()) 725 s.tryCancel(e); 726 Object x = s.item; 727 if (x != e) 728 return x; 729 if (timed) { 730 long now = System.nanoTime(); 731 nanos -= now - lastTime; 732 lastTime = now; 733 if (nanos <= 0) { 734 s.tryCancel(e); 735 continue; 736 } 737 } 738 if (spins > 0) 739 --spins; 740 else if (s.waiter == null) 741 s.waiter = w; 742 else if (!timed) 743 LockSupport.park(this); 744 else if (nanos > spinForTimeoutThreshold) 745 LockSupport.parkNanos(this, nanos); 746 } 747 } 748 749 /** 750 * Gets rid of cancelled node s with original predecessor pred. 751 */ 752 void clean(QNode pred, QNode s) { 753 s.waiter = null; // forget thread 754 /* 755 * At any given time, exactly one node on list cannot be 756 * deleted -- the last inserted node. To accommodate this, 757 * if we cannot delete s, we save its predecessor as 758 * "cleanMe", deleting the previously saved version 759 * first. At least one of node s or the node previously 760 * saved can always be deleted, so this always terminates. 761 */ 762 while (pred.next == s) { // Return early if already unlinked 763 QNode h = head; 764 QNode hn = h.next; // Absorb cancelled first node as head 765 if (hn != null && hn.isCancelled()) { 766 advanceHead(h, hn); 767 continue; 768 } 769 QNode t = tail; // Ensure consistent read for tail 770 if (t == h) 771 return; 772 QNode tn = t.next; 773 if (t != tail) 774 continue; 775 if (tn != null) { 776 advanceTail(t, tn); 777 continue; 778 } 779 if (s != t) { // If not tail, try to unsplice 780 QNode sn = s.next; 781 if (sn == s || pred.casNext(s, sn)) 782 return; 783 } 784 QNode dp = cleanMe; 785 if (dp != null) { // Try unlinking previous cancelled node 786 QNode d = dp.next; 787 QNode dn; 788 if (d == null || // d is gone or 789 d == dp || // d is off list or 790 !d.isCancelled() || // d not cancelled or 791 (d != t && // d not tail and 792 (dn = d.next) != null && // has successor 793 dn != d && // that is on list 794 dp.casNext(d, dn))) // d unspliced 795 casCleanMe(dp, null); 796 if (dp == pred) 797 return; // s is already saved node 798 } else if (casCleanMe(null, pred)) 799 return; // Postpone cleaning s 800 } 801 } 802 } 803 804 /** 805 * The transferer. Set only in constructor, but cannot be declared 806 * as final without further complicating serialization. Since 807 * this is accessed only at most once per public method, there 808 * isn't a noticeable performance penalty for using volatile 809 * instead of final here. 810 */ 811 private transient volatile Transferer transferer; 812 813 /** 814 * Creates a <tt>SynchronousQueue</tt> with nonfair access policy. 815 */ 816 public SynchronousQueue() { 817 this(false); 818 } 819 820 /** 821 * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy. 822 * 823 * @param fair if true, waiting threads contend in FIFO order for 824 * access; otherwise the order is unspecified. 825 */ 826 public SynchronousQueue(boolean fair) { 827 transferer = (fair)? new TransferQueue() : new TransferStack(); 828 } 829 830 /** 831 * Adds the specified element to this queue, waiting if necessary for 832 * another thread to receive it. 833 * 834 * @throws InterruptedException {@inheritDoc} 835 * @throws NullPointerException {@inheritDoc} 836 */ 837 public void put(E o) throws InterruptedException { 838 if (o == null) throw new NullPointerException(); 839 if (transferer.transfer(o, false, 0) == null) { 840 Thread.interrupted(); 841 throw new InterruptedException(); 842 } 843 } 844 845 /** 846 * Inserts the specified element into this queue, waiting if necessary 847 * up to the specified wait time for another thread to receive it. 848 * 849 * @return <tt>true</tt> if successful, or <tt>false</tt> if the 850 * specified waiting time elapses before a consumer appears. 851 * @throws InterruptedException {@inheritDoc} 852 * @throws NullPointerException {@inheritDoc} 853 */ 854 public boolean offer(E o, long timeout, TimeUnit unit) 855 throws InterruptedException { 856 if (o == null) throw new NullPointerException(); 857 if (transferer.transfer(o, true, unit.toNanos(timeout)) != null) 858 return true; 859 if (!Thread.interrupted()) 860 return false; 861 throw new InterruptedException(); 862 } 863 864 /** 865 * Inserts the specified element into this queue, if another thread is 866 * waiting to receive it. 867 * 868 * @param e the element to add 869 * @return <tt>true</tt> if the element was added to this queue, else 870 * <tt>false</tt> 871 * @throws NullPointerException if the specified element is null 872 */ 873 public boolean offer(E e) { 874 if (e == null) throw new NullPointerException(); 875 return transferer.transfer(e, true, 0) != null; 876 } 877 878 /** 879 * Retrieves and removes the head of this queue, waiting if necessary 880 * for another thread to insert it. 881 * 882 * @return the head of this queue 883 * @throws InterruptedException {@inheritDoc} 884 */ 885 public E take() throws InterruptedException { 886 Object e = transferer.transfer(null, false, 0); 887 if (e != null) 888 return (E)e; 889 Thread.interrupted(); 890 throw new InterruptedException(); 891 } 892 893 /** 894 * Retrieves and removes the head of this queue, waiting 895 * if necessary up to the specified wait time, for another thread 896 * to insert it. 897 * 898 * @return the head of this queue, or <tt>null</tt> if the 899 * specified waiting time elapses before an element is present. 900 * @throws InterruptedException {@inheritDoc} 901 */ 902 public E poll(long timeout, TimeUnit unit) throws InterruptedException { 903 Object e = transferer.transfer(null, true, unit.toNanos(timeout)); 904 if (e != null || !Thread.interrupted()) 905 return (E)e; 906 throw new InterruptedException(); 907 } 908 909 /** 910 * Retrieves and removes the head of this queue, if another thread 911 * is currently making an element available. 912 * 913 * @return the head of this queue, or <tt>null</tt> if no 914 * element is available. 915 */ 916 public E poll() { 917 return (E)transferer.transfer(null, true, 0); 918 } 919 920 /** 921 * Always returns <tt>true</tt>. 922 * A <tt>SynchronousQueue</tt> has no internal capacity. 923 * 924 * @return <tt>true</tt> 925 */ 926 public boolean isEmpty() { 927 return true; 928 } 929 930 /** 931 * Always returns zero. 932 * A <tt>SynchronousQueue</tt> has no internal capacity. 933 * 934 * @return zero. 935 */ 936 public int size() { 937 return 0; 938 } 939 940 /** 941 * Always returns zero. 942 * A <tt>SynchronousQueue</tt> has no internal capacity. 943 * 944 * @return zero. 945 */ 946 public int remainingCapacity() { 947 return 0; 948 } 949 950 /** 951 * Does nothing. 952 * A <tt>SynchronousQueue</tt> has no internal capacity. 953 */ 954 public void clear() { 955 } 956 957 /** 958 * Always returns <tt>false</tt>. 959 * A <tt>SynchronousQueue</tt> has no internal capacity. 960 * 961 * @param o the element 962 * @return <tt>false</tt> 963 */ 964 public boolean contains(Object o) { 965 return false; 966 } 967 968 /** 969 * Always returns <tt>false</tt>. 970 * A <tt>SynchronousQueue</tt> has no internal capacity. 971 * 972 * @param o the element to remove 973 * @return <tt>false</tt> 974 */ 975 public boolean remove(Object o) { 976 return false; 977 } 978 979 /** 980 * Returns <tt>false</tt> unless the given collection is empty. 981 * A <tt>SynchronousQueue</tt> has no internal capacity. 982 * 983 * @param c the collection 984 * @return <tt>false</tt> unless given collection is empty 985 */ 986 public boolean containsAll(Collection<?> c) { 987 return c.isEmpty(); 988 } 989 990 /** 991 * Always returns <tt>false</tt>. 992 * A <tt>SynchronousQueue</tt> has no internal capacity. 993 * 994 * @param c the collection 995 * @return <tt>false</tt> 996 */ 997 public boolean removeAll(Collection<?> c) { 998 return false; 999 } 1000 1001 /** 1002 * Always returns <tt>false</tt>. 1003 * A <tt>SynchronousQueue</tt> has no internal capacity. 1004 * 1005 * @param c the collection 1006 * @return <tt>false</tt> 1007 */ 1008 public boolean retainAll(Collection<?> c) { 1009 return false; 1010 } 1011 1012 /** 1013 * Always returns <tt>null</tt>. 1014 * A <tt>SynchronousQueue</tt> does not return elements 1015 * unless actively waited on. 1016 * 1017 * @return <tt>null</tt> 1018 */ 1019 public E peek() { 1020 return null; 1021 } 1022 1023 /** 1024 * Returns an empty iterator in which <tt>hasNext</tt> always returns 1025 * <tt>false</tt>. 1026 * 1027 * @return an empty iterator 1028 */ 1029 public Iterator<E> iterator() { 1030 return Collections.emptyIterator(); 1031 } 1032 1033 /** 1034 * Returns a zero-length array. 1035 * @return a zero-length array 1036 */ 1037 public Object[] toArray() { 1038 return new Object[0]; 1039 } 1040 1041 /** 1042 * Sets the zeroeth element of the specified array to <tt>null</tt> 1043 * (if the array has non-zero length) and returns it. 1044 * 1045 * @param a the array 1046 * @return the specified array 1047 * @throws NullPointerException if the specified array is null 1048 */ 1049 public <T> T[] toArray(T[] a) { 1050 if (a.length > 0) 1051 a[0] = null; 1052 return a; 1053 } 1054 1055 /** 1056 * @throws UnsupportedOperationException {@inheritDoc} 1057 * @throws ClassCastException {@inheritDoc} 1058 * @throws NullPointerException {@inheritDoc} 1059 * @throws IllegalArgumentException {@inheritDoc} 1060 */ 1061 public int drainTo(Collection<? super E> c) { 1062 if (c == null) 1063 throw new NullPointerException(); 1064 if (c == this) 1065 throw new IllegalArgumentException(); 1066 int n = 0; 1067 E e; 1068 while ( (e = poll()) != null) { 1069 c.add(e); 1070 ++n; 1071 } 1072 return n; 1073 } 1074 1075 /** 1076 * @throws UnsupportedOperationException {@inheritDoc} 1077 * @throws ClassCastException {@inheritDoc} 1078 * @throws NullPointerException {@inheritDoc} 1079 * @throws IllegalArgumentException {@inheritDoc} 1080 */ 1081 public int drainTo(Collection<? super E> c, int maxElements) { 1082 if (c == null) 1083 throw new NullPointerException(); 1084 if (c == this) 1085 throw new IllegalArgumentException(); 1086 int n = 0; 1087 E e; 1088 while (n < maxElements && (e = poll()) != null) { 1089 c.add(e); 1090 ++n; 1091 } 1092 return n; 1093 } 1094 1095 /* 1096 * To cope with serialization strategy in the 1.5 version of 1097 * SynchronousQueue, we declare some unused classes and fields 1098 * that exist solely to enable serializability across versions. 1099 * These fields are never used, so are initialized only if this 1100 * object is ever serialized or deserialized. 1101 */ 1102 1103 static class WaitQueue implements java.io.Serializable { } 1104 static class LifoWaitQueue extends WaitQueue { 1105 private static final long serialVersionUID = -3633113410248163686L; 1106 } 1107 static class FifoWaitQueue extends WaitQueue { 1108 private static final long serialVersionUID = -3623113410248163686L; 1109 } 1110 private ReentrantLock qlock; 1111 private WaitQueue waitingProducers; 1112 private WaitQueue waitingConsumers; 1113 1114 /** 1115 * Save the state to a stream (that is, serialize it). 1116 * 1117 * @param s the stream 1118 */ 1119 private void writeObject(java.io.ObjectOutputStream s) 1120 throws java.io.IOException { 1121 boolean fair = transferer instanceof TransferQueue; 1122 if (fair) { 1123 qlock = new ReentrantLock(true); 1124 waitingProducers = new FifoWaitQueue(); 1125 waitingConsumers = new FifoWaitQueue(); 1126 } 1127 else { 1128 qlock = new ReentrantLock(); 1129 waitingProducers = new LifoWaitQueue(); 1130 waitingConsumers = new LifoWaitQueue(); 1131 } 1132 s.defaultWriteObject(); 1133 } 1134 1135 private void readObject(final java.io.ObjectInputStream s) 1136 throws java.io.IOException, ClassNotFoundException { 1137 s.defaultReadObject(); 1138 if (waitingProducers instanceof FifoWaitQueue) 1139 transferer = new TransferQueue(); 1140 else 1141 transferer = new TransferStack(); 1142 } 1143 1144 } --- EOF ---