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 39 import java.util.AbstractQueue; 40 import java.util.Collection; 41 import java.util.Collections; 42 import java.util.Iterator; 43 import java.util.Spliterator; 44 import java.util.Spliterators; 45 import java.util.concurrent.locks.LockSupport; 46 import java.util.concurrent.locks.ReentrantLock; 47 48 // BEGIN android-note 49 // removed link to collections framework docs 50 // END android-note 51 52 /** 53 * A {@linkplain BlockingQueue blocking queue} in which each insert 54 * operation must wait for a corresponding remove operation by another 55 * thread, and vice versa. A synchronous queue does not have any 56 * internal capacity, not even a capacity of one. You cannot 57 * {@code peek} at a synchronous queue because an element is only 58 * present when you try to remove it; you cannot insert an element 59 * (using any method) unless another thread is trying to remove it; 60 * you cannot iterate as there is nothing to iterate. The 61 * <em>head</em> of the queue is the element that the first queued 62 * inserting thread is trying to add to the queue; if there is no such 63 * queued thread then no element is available for removal and 64 * {@code poll()} will return {@code null}. For purposes of other 65 * {@code Collection} methods (for example {@code contains}), a 66 * {@code SynchronousQueue} acts as an empty collection. This queue 67 * does not permit {@code null} elements. 68 * 69 * <p>Synchronous queues are similar to rendezvous channels used in 70 * CSP and Ada. They are well suited for handoff designs, in which an 71 * object running in one thread must sync up with an object running 72 * in another thread in order to hand it some information, event, or 73 * task. 74 * 75 * <p>This class supports an optional fairness policy for ordering 76 * waiting producer and consumer threads. By default, this ordering 77 * is not guaranteed. However, a queue constructed with fairness set 78 * to {@code true} grants threads access in FIFO order. 79 * 80 * <p>This class and its iterator implement all of the 81 * <em>optional</em> methods of the {@link Collection} and {@link 82 * Iterator} interfaces. 83 * 84 * @since 1.5 85 * @author Doug Lea and Bill Scherer and Michael Scott 86 * @param <E> the type of elements held in this queue 87 */ 88 public class SynchronousQueue<E> extends AbstractQueue<E> 89 implements BlockingQueue<E>, java.io.Serializable { 90 private static final long serialVersionUID = -3223113410248163686L; 91 92 /* 93 * This class implements extensions of the dual stack and dual 94 * queue algorithms described in "Nonblocking Concurrent Objects 95 * with Condition Synchronization", by W. N. Scherer III and 96 * M. L. Scott. 18th Annual Conf. on Distributed Computing, 97 * Oct. 2004 (see also 98 * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html). 99 * The (Lifo) stack is used for non-fair mode, and the (Fifo) 100 * queue for fair mode. The performance of the two is generally 101 * similar. Fifo usually supports higher throughput under 102 * contention but Lifo maintains higher thread locality in common 103 * applications. 104 * 105 * A dual queue (and similarly stack) is one that at any given 106 * time either holds "data" -- items provided by put operations, 107 * or "requests" -- slots representing take operations, or is 108 * empty. A call to "fulfill" (i.e., a call requesting an item 109 * from a queue holding data or vice versa) dequeues a 110 * complementary node. The most interesting feature of these 111 * queues is that any operation can figure out which mode the 112 * queue is in, and act accordingly without needing locks. 113 * 114 * Both the queue and stack extend abstract class Transferer 115 * defining the single method transfer that does a put or a 116 * take. These are unified into a single method because in dual 117 * data structures, the put and take operations are symmetrical, 118 * so nearly all code can be combined. The resulting transfer 119 * methods are on the long side, but are easier to follow than 120 * they would be if broken up into nearly-duplicated parts. 121 * 122 * The queue and stack data structures share many conceptual 123 * similarities but very few concrete details. For simplicity, 124 * they are kept distinct so that they can later evolve 125 * separately. 126 * 127 * The algorithms here differ from the versions in the above paper 128 * in extending them for use in synchronous queues, as well as 129 * dealing with cancellation. The main differences include: 130 * 131 * 1. The original algorithms used bit-marked pointers, but 132 * the ones here use mode bits in nodes, leading to a number 133 * of further adaptations. 134 * 2. SynchronousQueues must block threads waiting to become 135 * fulfilled. 136 * 3. Support for cancellation via timeout and interrupts, 137 * including cleaning out cancelled nodes/threads 138 * from lists to avoid garbage retention and memory depletion. 139 * 140 * Blocking is mainly accomplished using LockSupport park/unpark, 141 * except that nodes that appear to be the next ones to become 142 * fulfilled first spin a bit (on multiprocessors only). On very 143 * busy synchronous queues, spinning can dramatically improve 144 * throughput. And on less busy ones, the amount of spinning is 145 * small enough not to be noticeable. 146 * 147 * Cleaning is done in different ways in queues vs stacks. For 148 * queues, we can almost always remove a node immediately in O(1) 149 * time (modulo retries for consistency checks) when it is 150 * cancelled. But if it may be pinned as the current tail, it must 151 * wait until some subsequent cancellation. For stacks, we need a 152 * potentially O(n) traversal to be sure that we can remove the 153 * node, but this can run concurrently with other threads 154 * accessing the stack. 155 * 156 * While garbage collection takes care of most node reclamation 157 * issues that otherwise complicate nonblocking algorithms, care 158 * is taken to "forget" references to data, other nodes, and 159 * threads that might be held on to long-term by blocked 160 * threads. In cases where setting to null would otherwise 161 * conflict with main algorithms, this is done by changing a 162 * node's link to now point to the node itself. This doesn't arise 163 * much for Stack nodes (because blocked threads do not hang on to 164 * old head pointers), but references in Queue nodes must be 165 * aggressively forgotten to avoid reachability of everything any 166 * node has ever referred to since arrival. 167 */ 168 169 /** 170 * Shared internal API for dual stacks and queues. 171 */ 172 abstract static class Transferer<E> { 173 /** 174 * Performs a put or take. 175 * 176 * @param e if non-null, the item to be handed to a consumer; 177 * if null, requests that transfer return an item 178 * offered by producer. 179 * @param timed if this operation should timeout 180 * @param nanos the timeout, in nanoseconds 181 * @return if non-null, the item provided or received; if null, 182 * the operation failed due to timeout or interrupt -- 183 * the caller can distinguish which of these occurred 184 * by checking Thread.interrupted. 185 */ 186 abstract E transfer(E e, boolean timed, long nanos); 187 } 188 189 /** 190 * The number of times to spin before blocking in timed waits. 191 * The value is empirically derived -- it works well across a 192 * variety of processors and OSes. Empirically, the best value 193 * seems not to vary with number of CPUs (beyond 2) so is just 194 * a constant. 195 */ 196 static final int MAX_TIMED_SPINS = 197 (Runtime.getRuntime().availableProcessors() < 2) ? 0 : 32; 198 199 /** 200 * The number of times to spin before blocking in untimed waits. 201 * This is greater than timed value because untimed waits spin 202 * faster since they don't need to check times on each spin. 203 */ 204 static final int MAX_UNTIMED_SPINS = MAX_TIMED_SPINS * 16; 205 206 /** 207 * The number of nanoseconds for which it is faster to spin 208 * rather than to use timed park. A rough estimate suffices. 209 */ 210 static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1000L; 211 212 /** Dual stack */ 213 static final class TransferStack<E> extends Transferer<E> { 214 /* 215 * This extends Scherer-Scott dual stack algorithm, differing, 216 * among other ways, by using "covering" nodes rather than 217 * bit-marked pointers: Fulfilling operations push on marker 218 * nodes (with FULFILLING bit set in mode) to reserve a spot 219 * to match a waiting node. 220 */ 221 222 /* Modes for SNodes, ORed together in node fields */ 223 /** Node represents an unfulfilled consumer */ 224 static final int REQUEST = 0; 225 /** Node represents an unfulfilled producer */ 226 static final int DATA = 1; 227 /** Node is fulfilling another unfulfilled DATA or REQUEST */ 228 static final int FULFILLING = 2; 229 230 /** Returns true if m has fulfilling bit set. */ 231 static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; } 232 233 /** Node class for TransferStacks. */ 234 static final class SNode { 235 volatile SNode next; // next node in stack 236 volatile SNode match; // the node matched to this 237 volatile Thread waiter; // to control park/unpark 238 Object item; // data; or null for REQUESTs 239 int mode; 240 // Note: item and mode fields don't need to be volatile 241 // since they are always written before, and read after, 242 // other volatile/atomic operations. 243 244 SNode(Object item) { 245 this.item = item; 246 } 247 248 boolean casNext(SNode cmp, SNode val) { 249 return cmp == next && 250 U.compareAndSwapObject(this, NEXT, cmp, val); 251 } 252 253 /** 254 * Tries to match node s to this node, if so, waking up thread. 255 * Fulfillers call tryMatch to identify their waiters. 256 * Waiters block until they have been matched. 257 * 258 * @param s the node to match 259 * @return true if successfully matched to s 260 */ 261 boolean tryMatch(SNode s) { 262 if (match == null && 263 U.compareAndSwapObject(this, MATCH, null, s)) { 264 Thread w = waiter; 265 if (w != null) { // waiters need at most one unpark 266 waiter = null; 267 LockSupport.unpark(w); 268 } 269 return true; 270 } 271 return match == s; 272 } 273 274 /** 275 * Tries to cancel a wait by matching node to itself. 276 */ 277 void tryCancel() { 278 U.compareAndSwapObject(this, MATCH, null, this); 279 } 280 281 boolean isCancelled() { 282 return match == this; 283 } 284 285 // Unsafe mechanics 286 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); 287 private static final long MATCH; 288 private static final long NEXT; 289 290 static { 291 try { 292 MATCH = U.objectFieldOffset 293 (SNode.class.getDeclaredField("match")); 294 NEXT = U.objectFieldOffset 295 (SNode.class.getDeclaredField("next")); 296 } catch (ReflectiveOperationException e) { 297 throw new Error(e); 298 } 299 } 300 } 301 302 /** The head (top) of the stack */ 303 volatile SNode head; 304 305 boolean casHead(SNode h, SNode nh) { 306 return h == head && 307 U.compareAndSwapObject(this, HEAD, h, nh); 308 } 309 310 /** 311 * Creates or resets fields of a node. Called only from transfer 312 * where the node to push on stack is lazily created and 313 * reused when possible to help reduce intervals between reads 314 * and CASes of head and to avoid surges of garbage when CASes 315 * to push nodes fail due to contention. 316 */ 317 static SNode snode(SNode s, Object e, SNode next, int mode) { 318 if (s == null) s = new SNode(e); 319 s.mode = mode; 320 s.next = next; 321 return s; 322 } 323 324 /** 325 * Puts or takes an item. 326 */ 327 @SuppressWarnings("unchecked") 328 E transfer(E e, boolean timed, long nanos) { 329 /* 330 * Basic algorithm is to loop trying one of three actions: 331 * 332 * 1. If apparently empty or already containing nodes of same 333 * mode, try to push node on stack and wait for a match, 334 * returning it, or null if cancelled. 335 * 336 * 2. If apparently containing node of complementary mode, 337 * try to push a fulfilling node on to stack, match 338 * with corresponding waiting node, pop both from 339 * stack, and return matched item. The matching or 340 * unlinking might not actually be necessary because of 341 * other threads performing action 3: 342 * 343 * 3. If top of stack already holds another fulfilling node, 344 * help it out by doing its match and/or pop 345 * operations, and then continue. The code for helping 346 * is essentially the same as for fulfilling, except 347 * that it doesn't return the item. 348 */ 349 350 SNode s = null; // constructed/reused as needed 351 int mode = (e == null) ? REQUEST : DATA; 352 353 for (;;) { 354 SNode h = head; 355 if (h == null || h.mode == mode) { // empty or same-mode 356 if (timed && nanos <= 0L) { // can't wait 357 if (h != null && h.isCancelled()) 358 casHead(h, h.next); // pop cancelled node 359 else 360 return null; 361 } else if (casHead(h, s = snode(s, e, h, mode))) { 362 SNode m = awaitFulfill(s, timed, nanos); 363 if (m == s) { // wait was cancelled 364 clean(s); 365 return null; 366 } 367 if ((h = head) != null && h.next == s) 368 casHead(h, s.next); // help s's fulfiller 369 return (E) ((mode == REQUEST) ? m.item : s.item); 370 } 371 } else if (!isFulfilling(h.mode)) { // try to fulfill 372 if (h.isCancelled()) // already cancelled 373 casHead(h, h.next); // pop and retry 374 else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) { 375 for (;;) { // loop until matched or waiters disappear 376 SNode m = s.next; // m is s's match 377 if (m == null) { // all waiters are gone 378 casHead(s, null); // pop fulfill node 379 s = null; // use new node next time 380 break; // restart main loop 381 } 382 SNode mn = m.next; 383 if (m.tryMatch(s)) { 384 casHead(s, mn); // pop both s and m 385 return (E) ((mode == REQUEST) ? m.item : s.item); 386 } else // lost match 387 s.casNext(m, mn); // help unlink 388 } 389 } 390 } else { // help a fulfiller 391 SNode m = h.next; // m is h's match 392 if (m == null) // waiter is gone 393 casHead(h, null); // pop fulfilling node 394 else { 395 SNode mn = m.next; 396 if (m.tryMatch(h)) // help match 397 casHead(h, mn); // pop both h and m 398 else // lost match 399 h.casNext(m, mn); // help unlink 400 } 401 } 402 } 403 } 404 405 /** 406 * Spins/blocks until node s is matched by a fulfill operation. 407 * 408 * @param s the waiting node 409 * @param timed true if timed wait 410 * @param nanos timeout value 411 * @return matched node, or s if cancelled 412 */ 413 SNode awaitFulfill(SNode s, boolean timed, long nanos) { 414 /* 415 * When a node/thread is about to block, it sets its waiter 416 * field and then rechecks state at least one more time 417 * before actually parking, thus covering race vs 418 * fulfiller noticing that waiter is non-null so should be 419 * woken. 420 * 421 * When invoked by nodes that appear at the point of call 422 * to be at the head of the stack, calls to park are 423 * preceded by spins to avoid blocking when producers and 424 * consumers are arriving very close in time. This can 425 * happen enough to bother only on multiprocessors. 426 * 427 * The order of checks for returning out of main loop 428 * reflects fact that interrupts have precedence over 429 * normal returns, which have precedence over 430 * timeouts. (So, on timeout, one last check for match is 431 * done before giving up.) Except that calls from untimed 432 * SynchronousQueue.{poll/offer} don't check interrupts 433 * and don't wait at all, so are trapped in transfer 434 * method rather than calling awaitFulfill. 435 */ 436 final long deadline = timed ? System.nanoTime() + nanos : 0L; 437 Thread w = Thread.currentThread(); 438 int spins = shouldSpin(s) 439 ? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS) 440 : 0; 441 for (;;) { 442 if (w.isInterrupted()) 443 s.tryCancel(); 444 SNode m = s.match; 445 if (m != null) 446 return m; 447 if (timed) { 448 nanos = deadline - System.nanoTime(); 449 if (nanos <= 0L) { 450 s.tryCancel(); 451 continue; 452 } 453 } 454 if (spins > 0) 455 spins = shouldSpin(s) ? (spins - 1) : 0; 456 else if (s.waiter == null) 457 s.waiter = w; // establish waiter so can park next iter 458 else if (!timed) 459 LockSupport.park(this); 460 else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) 461 LockSupport.parkNanos(this, nanos); 462 } 463 } 464 465 /** 466 * Returns true if node s is at head or there is an active 467 * fulfiller. 468 */ 469 boolean shouldSpin(SNode s) { 470 SNode h = head; 471 return (h == s || h == null || isFulfilling(h.mode)); 472 } 473 474 /** 475 * Unlinks s from the stack. 476 */ 477 void clean(SNode s) { 478 s.item = null; // forget item 479 s.waiter = null; // forget thread 480 481 /* 482 * At worst we may need to traverse entire stack to unlink 483 * s. If there are multiple concurrent calls to clean, we 484 * might not see s if another thread has already removed 485 * it. But we can stop when we see any node known to 486 * follow s. We use s.next unless it too is cancelled, in 487 * which case we try the node one past. We don't check any 488 * further because we don't want to doubly traverse just to 489 * find sentinel. 490 */ 491 492 SNode past = s.next; 493 if (past != null && past.isCancelled()) 494 past = past.next; 495 496 // Absorb cancelled nodes at head 497 SNode p; 498 while ((p = head) != null && p != past && p.isCancelled()) 499 casHead(p, p.next); 500 501 // Unsplice embedded nodes 502 while (p != null && p != past) { 503 SNode n = p.next; 504 if (n != null && n.isCancelled()) 505 p.casNext(n, n.next); 506 else 507 p = n; 508 } 509 } 510 511 // Unsafe mechanics 512 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); 513 private static final long HEAD; 514 static { 515 try { 516 HEAD = U.objectFieldOffset 517 (TransferStack.class.getDeclaredField("head")); 518 } catch (ReflectiveOperationException e) { 519 throw new Error(e); 520 } 521 } 522 } 523 524 /** Dual Queue */ 525 static final class TransferQueue<E> extends Transferer<E> { 526 /* 527 * This extends Scherer-Scott dual queue algorithm, differing, 528 * among other ways, by using modes within nodes rather than 529 * marked pointers. The algorithm is a little simpler than 530 * that for stacks because fulfillers do not need explicit 531 * nodes, and matching is done by CAS'ing QNode.item field 532 * from non-null to null (for put) or vice versa (for take). 533 */ 534 535 /** Node class for TransferQueue. */ 536 static final class QNode { 537 volatile QNode next; // next node in queue 538 volatile Object item; // CAS'ed to or from null 539 volatile Thread waiter; // to control park/unpark 540 final boolean isData; 541 542 QNode(Object item, boolean isData) { 543 this.item = item; 544 this.isData = isData; 545 } 546 547 boolean casNext(QNode cmp, QNode val) { 548 return next == cmp && 549 U.compareAndSwapObject(this, NEXT, cmp, val); 550 } 551 552 boolean casItem(Object cmp, Object val) { 553 return item == cmp && 554 U.compareAndSwapObject(this, ITEM, cmp, val); 555 } 556 557 /** 558 * Tries to cancel by CAS'ing ref to this as item. 559 */ 560 void tryCancel(Object cmp) { 561 U.compareAndSwapObject(this, ITEM, cmp, this); 562 } 563 564 boolean isCancelled() { 565 return item == this; 566 } 567 568 /** 569 * Returns true if this node is known to be off the queue 570 * because its next pointer has been forgotten due to 571 * an advanceHead operation. 572 */ 573 boolean isOffList() { 574 return next == this; 575 } 576 577 // Unsafe mechanics 578 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); 579 private static final long ITEM; 580 private static final long NEXT; 581 582 static { 583 try { 584 ITEM = U.objectFieldOffset 585 (QNode.class.getDeclaredField("item")); 586 NEXT = U.objectFieldOffset 587 (QNode.class.getDeclaredField("next")); 588 } catch (ReflectiveOperationException e) { 589 throw new Error(e); 590 } 591 } 592 } 593 594 /** Head of queue */ 595 transient volatile QNode head; 596 /** Tail of queue */ 597 transient volatile QNode tail; 598 /** 599 * Reference to a cancelled node that might not yet have been 600 * unlinked from queue because it was the last inserted node 601 * when it was cancelled. 602 */ 603 transient volatile QNode cleanMe; 604 605 TransferQueue() { 606 QNode h = new QNode(null, false); // initialize to dummy node. 607 head = h; 608 tail = h; 609 } 610 611 /** 612 * Tries to cas nh as new head; if successful, unlink 613 * old head's next node to avoid garbage retention. 614 */ 615 void advanceHead(QNode h, QNode nh) { 616 if (h == head && 617 U.compareAndSwapObject(this, HEAD, h, nh)) 618 h.next = h; // forget old next 619 } 620 621 /** 622 * Tries to cas nt as new tail. 623 */ 624 void advanceTail(QNode t, QNode nt) { 625 if (tail == t) 626 U.compareAndSwapObject(this, TAIL, t, nt); 627 } 628 629 /** 630 * Tries to CAS cleanMe slot. 631 */ 632 boolean casCleanMe(QNode cmp, QNode val) { 633 return cleanMe == cmp && 634 U.compareAndSwapObject(this, CLEANME, cmp, val); 635 } 636 637 /** 638 * Puts or takes an item. 639 */ 640 @SuppressWarnings("unchecked") 641 E transfer(E e, boolean timed, long nanos) { 642 /* Basic algorithm is to loop trying to take either of 643 * two actions: 644 * 645 * 1. If queue apparently empty or holding same-mode nodes, 646 * try to add node to queue of waiters, wait to be 647 * fulfilled (or cancelled) and return matching item. 648 * 649 * 2. If queue apparently contains waiting items, and this 650 * call is of complementary mode, try to fulfill by CAS'ing 651 * item field of waiting node and dequeuing it, and then 652 * returning matching item. 653 * 654 * In each case, along the way, check for and try to help 655 * advance head and tail on behalf of other stalled/slow 656 * threads. 657 * 658 * The loop starts off with a null check guarding against 659 * seeing uninitialized head or tail values. This never 660 * happens in current SynchronousQueue, but could if 661 * callers held non-volatile/final ref to the 662 * transferer. The check is here anyway because it places 663 * null checks at top of loop, which is usually faster 664 * than having them implicitly interspersed. 665 */ 666 667 QNode s = null; // constructed/reused as needed 668 boolean isData = (e != null); 669 670 for (;;) { 671 QNode t = tail; 672 QNode h = head; 673 if (t == null || h == null) // saw uninitialized value 674 continue; // spin 675 676 if (h == t || t.isData == isData) { // empty or same-mode 677 QNode tn = t.next; 678 if (t != tail) // inconsistent read 679 continue; 680 if (tn != null) { // lagging tail 681 advanceTail(t, tn); 682 continue; 683 } 684 if (timed && nanos <= 0L) // can't wait 685 return null; 686 if (s == null) 687 s = new QNode(e, isData); 688 if (!t.casNext(null, s)) // failed to link in 689 continue; 690 691 advanceTail(t, s); // swing tail and wait 692 Object x = awaitFulfill(s, e, timed, nanos); 693 if (x == s) { // wait was cancelled 694 clean(t, s); 695 return null; 696 } 697 698 if (!s.isOffList()) { // not already unlinked 699 advanceHead(t, s); // unlink if head 700 if (x != null) // and forget fields 701 s.item = s; 702 s.waiter = null; 703 } 704 return (x != null) ? (E)x : e; 705 706 } else { // complementary-mode 707 QNode m = h.next; // node to fulfill 708 if (t != tail || m == null || h != head) 709 continue; // inconsistent read 710 711 Object x = m.item; 712 if (isData == (x != null) || // m already fulfilled 713 x == m || // m cancelled 714 !m.casItem(x, e)) { // lost CAS 715 advanceHead(h, m); // dequeue and retry 716 continue; 717 } 718 719 advanceHead(h, m); // successfully fulfilled 720 LockSupport.unpark(m.waiter); 721 return (x != null) ? (E)x : e; 722 } 723 } 724 } 725 726 /** 727 * Spins/blocks until node s is fulfilled. 728 * 729 * @param s the waiting node 730 * @param e the comparison value for checking match 731 * @param timed true if timed wait 732 * @param nanos timeout value 733 * @return matched item, or s if cancelled 734 */ 735 Object awaitFulfill(QNode s, E e, boolean timed, long nanos) { 736 /* Same idea as TransferStack.awaitFulfill */ 737 final long deadline = timed ? System.nanoTime() + nanos : 0L; 738 Thread w = Thread.currentThread(); 739 int spins = (head.next == s) 740 ? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS) 741 : 0; 742 for (;;) { 743 if (w.isInterrupted()) 744 s.tryCancel(e); 745 Object x = s.item; 746 if (x != e) 747 return x; 748 if (timed) { 749 nanos = deadline - System.nanoTime(); 750 if (nanos <= 0L) { 751 s.tryCancel(e); 752 continue; 753 } 754 } 755 if (spins > 0) 756 --spins; 757 else if (s.waiter == null) 758 s.waiter = w; 759 else if (!timed) 760 LockSupport.park(this); 761 else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) 762 LockSupport.parkNanos(this, nanos); 763 } 764 } 765 766 /** 767 * Gets rid of cancelled node s with original predecessor pred. 768 */ 769 void clean(QNode pred, QNode s) { 770 s.waiter = null; // forget thread 771 /* 772 * At any given time, exactly one node on list cannot be 773 * deleted -- the last inserted node. To accommodate this, 774 * if we cannot delete s, we save its predecessor as 775 * "cleanMe", deleting the previously saved version 776 * first. At least one of node s or the node previously 777 * saved can always be deleted, so this always terminates. 778 */ 779 while (pred.next == s) { // Return early if already unlinked 780 QNode h = head; 781 QNode hn = h.next; // Absorb cancelled first node as head 782 if (hn != null && hn.isCancelled()) { 783 advanceHead(h, hn); 784 continue; 785 } 786 QNode t = tail; // Ensure consistent read for tail 787 if (t == h) 788 return; 789 QNode tn = t.next; 790 if (t != tail) 791 continue; 792 if (tn != null) { 793 advanceTail(t, tn); 794 continue; 795 } 796 if (s != t) { // If not tail, try to unsplice 797 QNode sn = s.next; 798 if (sn == s || pred.casNext(s, sn)) 799 return; 800 } 801 QNode dp = cleanMe; 802 if (dp != null) { // Try unlinking previous cancelled node 803 QNode d = dp.next; 804 QNode dn; 805 if (d == null || // d is gone or 806 d == dp || // d is off list or 807 !d.isCancelled() || // d not cancelled or 808 (d != t && // d not tail and 809 (dn = d.next) != null && // has successor 810 dn != d && // that is on list 811 dp.casNext(d, dn))) // d unspliced 812 casCleanMe(dp, null); 813 if (dp == pred) 814 return; // s is already saved node 815 } else if (casCleanMe(null, pred)) 816 return; // Postpone cleaning s 817 } 818 } 819 820 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); 821 private static final long HEAD; 822 private static final long TAIL; 823 private static final long CLEANME; 824 static { 825 try { 826 HEAD = U.objectFieldOffset 827 (TransferQueue.class.getDeclaredField("head")); 828 TAIL = U.objectFieldOffset 829 (TransferQueue.class.getDeclaredField("tail")); 830 CLEANME = U.objectFieldOffset 831 (TransferQueue.class.getDeclaredField("cleanMe")); 832 } catch (ReflectiveOperationException e) { 833 throw new Error(e); 834 } 835 } 836 } 837 838 /** 839 * The transferer. Set only in constructor, but cannot be declared 840 * as final without further complicating serialization. Since 841 * this is accessed only at most once per public method, there 842 * isn't a noticeable performance penalty for using volatile 843 * instead of final here. 844 */ 845 private transient volatile Transferer<E> transferer; 846 847 /** 848 * Creates a {@code SynchronousQueue} with nonfair access policy. 849 */ 850 public SynchronousQueue() { 851 this(false); 852 } 853 854 /** 855 * Creates a {@code SynchronousQueue} with the specified fairness policy. 856 * 857 * @param fair if true, waiting threads contend in FIFO order for 858 * access; otherwise the order is unspecified. 859 */ 860 public SynchronousQueue(boolean fair) { 861 transferer = fair ? new TransferQueue<E>() : new TransferStack<E>(); 862 } 863 864 /** 865 * Adds the specified element to this queue, waiting if necessary for 866 * another thread to receive it. 867 * 868 * @throws InterruptedException {@inheritDoc} 869 * @throws NullPointerException {@inheritDoc} 870 */ 871 public void put(E e) throws InterruptedException { 872 if (e == null) throw new NullPointerException(); 873 if (transferer.transfer(e, false, 0) == null) { 874 Thread.interrupted(); 875 throw new InterruptedException(); 876 } 877 } 878 879 /** 880 * Inserts the specified element into this queue, waiting if necessary 881 * up to the specified wait time for another thread to receive it. 882 * 883 * @return {@code true} if successful, or {@code false} if the 884 * specified waiting time elapses before a consumer appears 885 * @throws InterruptedException {@inheritDoc} 886 * @throws NullPointerException {@inheritDoc} 887 */ 888 public boolean offer(E e, long timeout, TimeUnit unit) 889 throws InterruptedException { 890 if (e == null) throw new NullPointerException(); 891 if (transferer.transfer(e, true, unit.toNanos(timeout)) != null) 892 return true; 893 if (!Thread.interrupted()) 894 return false; 895 throw new InterruptedException(); 896 } 897 898 /** 899 * Inserts the specified element into this queue, if another thread is 900 * waiting to receive it. 901 * 902 * @param e the element to add 903 * @return {@code true} if the element was added to this queue, else 904 * {@code false} 905 * @throws NullPointerException if the specified element is null 906 */ 907 public boolean offer(E e) { 908 if (e == null) throw new NullPointerException(); 909 return transferer.transfer(e, true, 0) != null; 910 } 911 912 /** 913 * Retrieves and removes the head of this queue, waiting if necessary 914 * for another thread to insert it. 915 * 916 * @return the head of this queue 917 * @throws InterruptedException {@inheritDoc} 918 */ 919 public E take() throws InterruptedException { 920 E e = transferer.transfer(null, false, 0); 921 if (e != null) 922 return e; 923 Thread.interrupted(); 924 throw new InterruptedException(); 925 } 926 927 /** 928 * Retrieves and removes the head of this queue, waiting 929 * if necessary up to the specified wait time, for another thread 930 * to insert it. 931 * 932 * @return the head of this queue, or {@code null} if the 933 * specified waiting time elapses before an element is present 934 * @throws InterruptedException {@inheritDoc} 935 */ 936 public E poll(long timeout, TimeUnit unit) throws InterruptedException { 937 E e = transferer.transfer(null, true, unit.toNanos(timeout)); 938 if (e != null || !Thread.interrupted()) 939 return e; 940 throw new InterruptedException(); 941 } 942 943 /** 944 * Retrieves and removes the head of this queue, if another thread 945 * is currently making an element available. 946 * 947 * @return the head of this queue, or {@code null} if no 948 * element is available 949 */ 950 public E poll() { 951 return transferer.transfer(null, true, 0); 952 } 953 954 /** 955 * Always returns {@code true}. 956 * A {@code SynchronousQueue} has no internal capacity. 957 * 958 * @return {@code true} 959 */ 960 public boolean isEmpty() { 961 return true; 962 } 963 964 /** 965 * Always returns zero. 966 * A {@code SynchronousQueue} has no internal capacity. 967 * 968 * @return zero 969 */ 970 public int size() { 971 return 0; 972 } 973 974 /** 975 * Always returns zero. 976 * A {@code SynchronousQueue} has no internal capacity. 977 * 978 * @return zero 979 */ 980 public int remainingCapacity() { 981 return 0; 982 } 983 984 /** 985 * Does nothing. 986 * A {@code SynchronousQueue} has no internal capacity. 987 */ 988 public void clear() { 989 } 990 991 /** 992 * Always returns {@code false}. 993 * A {@code SynchronousQueue} has no internal capacity. 994 * 995 * @param o the element 996 * @return {@code false} 997 */ 998 public boolean contains(Object o) { 999 return false; 1000 } 1001 1002 /** 1003 * Always returns {@code false}. 1004 * A {@code SynchronousQueue} has no internal capacity. 1005 * 1006 * @param o the element to remove 1007 * @return {@code false} 1008 */ 1009 public boolean remove(Object o) { 1010 return false; 1011 } 1012 1013 /** 1014 * Returns {@code false} unless the given collection is empty. 1015 * A {@code SynchronousQueue} has no internal capacity. 1016 * 1017 * @param c the collection 1018 * @return {@code false} unless given collection is empty 1019 */ 1020 public boolean containsAll(Collection<?> c) { 1021 return c.isEmpty(); 1022 } 1023 1024 /** 1025 * Always returns {@code false}. 1026 * A {@code SynchronousQueue} has no internal capacity. 1027 * 1028 * @param c the collection 1029 * @return {@code false} 1030 */ 1031 public boolean removeAll(Collection<?> c) { 1032 return false; 1033 } 1034 1035 /** 1036 * Always returns {@code false}. 1037 * A {@code SynchronousQueue} has no internal capacity. 1038 * 1039 * @param c the collection 1040 * @return {@code false} 1041 */ 1042 public boolean retainAll(Collection<?> c) { 1043 return false; 1044 } 1045 1046 /** 1047 * Always returns {@code null}. 1048 * A {@code SynchronousQueue} does not return elements 1049 * unless actively waited on. 1050 * 1051 * @return {@code null} 1052 */ 1053 public E peek() { 1054 return null; 1055 } 1056 1057 /** 1058 * Returns an empty iterator in which {@code hasNext} always returns 1059 * {@code false}. 1060 * 1061 * @return an empty iterator 1062 */ 1063 public Iterator<E> iterator() { 1064 return Collections.emptyIterator(); 1065 } 1066 1067 /** 1068 * Returns an empty spliterator in which calls to 1069 * {@link java.util.Spliterator#trySplit()} always return {@code null}. 1070 * 1071 * @return an empty spliterator 1072 * @since 1.8 1073 */ 1074 public Spliterator<E> spliterator() { 1075 return Spliterators.emptySpliterator(); 1076 } 1077 1078 /** 1079 * Returns a zero-length array. 1080 * @return a zero-length array 1081 */ 1082 public Object[] toArray() { 1083 return new Object[0]; 1084 } 1085 1086 /** 1087 * Sets the zeroth element of the specified array to {@code null} 1088 * (if the array has non-zero length) and returns it. 1089 * 1090 * @param a the array 1091 * @return the specified array 1092 * @throws NullPointerException if the specified array is null 1093 */ 1094 public <T> T[] toArray(T[] a) { 1095 if (a.length > 0) 1096 a[0] = null; 1097 return a; 1098 } 1099 1100 /** 1101 * Always returns {@code "[]"}. 1102 * @return {@code "[]"} 1103 */ 1104 public String toString() { 1105 return "[]"; 1106 } 1107 1108 /** 1109 * @throws UnsupportedOperationException {@inheritDoc} 1110 * @throws ClassCastException {@inheritDoc} 1111 * @throws NullPointerException {@inheritDoc} 1112 * @throws IllegalArgumentException {@inheritDoc} 1113 */ 1114 public int drainTo(Collection<? super E> c) { 1115 if (c == null) 1116 throw new NullPointerException(); 1117 if (c == this) 1118 throw new IllegalArgumentException(); 1119 int n = 0; 1120 for (E e; (e = poll()) != null;) { 1121 c.add(e); 1122 ++n; 1123 } 1124 return n; 1125 } 1126 1127 /** 1128 * @throws UnsupportedOperationException {@inheritDoc} 1129 * @throws ClassCastException {@inheritDoc} 1130 * @throws NullPointerException {@inheritDoc} 1131 * @throws IllegalArgumentException {@inheritDoc} 1132 */ 1133 public int drainTo(Collection<? super E> c, int maxElements) { 1134 if (c == null) 1135 throw new NullPointerException(); 1136 if (c == this) 1137 throw new IllegalArgumentException(); 1138 int n = 0; 1139 for (E e; n < maxElements && (e = poll()) != null;) { 1140 c.add(e); 1141 ++n; 1142 } 1143 return n; 1144 } 1145 1146 /* 1147 * To cope with serialization strategy in the 1.5 version of 1148 * SynchronousQueue, we declare some unused classes and fields 1149 * that exist solely to enable serializability across versions. 1150 * These fields are never used, so are initialized only if this 1151 * object is ever serialized or deserialized. 1152 */ 1153 1154 @SuppressWarnings("serial") 1155 static class WaitQueue implements java.io.Serializable { } 1156 static class LifoWaitQueue extends WaitQueue { 1157 private static final long serialVersionUID = -3633113410248163686L; 1158 } 1159 static class FifoWaitQueue extends WaitQueue { 1160 private static final long serialVersionUID = -3623113410248163686L; 1161 } 1162 private ReentrantLock qlock; 1163 private WaitQueue waitingProducers; 1164 private WaitQueue waitingConsumers; 1165 1166 /** 1167 * Saves this queue to a stream (that is, serializes it). 1168 * @param s the stream 1169 * @throws java.io.IOException if an I/O error occurs 1170 */ 1171 private void writeObject(java.io.ObjectOutputStream s) 1172 throws java.io.IOException { 1173 boolean fair = transferer instanceof TransferQueue; 1174 if (fair) { 1175 qlock = new ReentrantLock(true); 1176 waitingProducers = new FifoWaitQueue(); 1177 waitingConsumers = new FifoWaitQueue(); 1178 } 1179 else { 1180 qlock = new ReentrantLock(); 1181 waitingProducers = new LifoWaitQueue(); 1182 waitingConsumers = new LifoWaitQueue(); 1183 } 1184 s.defaultWriteObject(); 1185 } 1186 1187 /** 1188 * Reconstitutes this queue from a stream (that is, deserializes it). 1189 * @param s the stream 1190 * @throws ClassNotFoundException if the class of a serialized object 1191 * could not be found 1192 * @throws java.io.IOException if an I/O error occurs 1193 */ 1194 private void readObject(java.io.ObjectInputStream s) 1195 throws java.io.IOException, ClassNotFoundException { 1196 s.defaultReadObject(); 1197 if (waitingProducers instanceof FifoWaitQueue) 1198 transferer = new TransferQueue<E>(); 1199 else 1200 transferer = new TransferStack<E>(); 1201 } 1202 1203 static { 1204 // Reduce the risk of rare disastrous classloading in first call to 1205 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 1206 Class<?> ensureLoaded = LockSupport.class; 1207 } 1208 } 1209
