1/* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7package java.util.concurrent; 8 9import java.util.AbstractQueue; 10import java.util.Collection; 11import java.util.Iterator; 12import java.util.NoSuchElementException; 13import java.util.Queue; 14import java.util.concurrent.TimeUnit; 15import java.util.concurrent.locks.LockSupport; 16 17// BEGIN android-note 18// removed link to collections framework docs 19// END android-note 20 21/** 22 * An unbounded {@link TransferQueue} based on linked nodes. 23 * This queue orders elements FIFO (first-in-first-out) with respect 24 * to any given producer. The <em>head</em> of the queue is that 25 * element that has been on the queue the longest time for some 26 * producer. The <em>tail</em> of the queue is that element that has 27 * been on the queue the shortest time for some producer. 28 * 29 * <p>Beware that, unlike in most collections, the {@code size} method 30 * is <em>NOT</em> a constant-time operation. Because of the 31 * asynchronous nature of these queues, determining the current number 32 * of elements requires a traversal of the elements, and so may report 33 * inaccurate results if this collection is modified during traversal. 34 * Additionally, the bulk operations {@code addAll}, 35 * {@code removeAll}, {@code retainAll}, {@code containsAll}, 36 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed 37 * to be performed atomically. For example, an iterator operating 38 * concurrently with an {@code addAll} operation might view only some 39 * of the added elements. 40 * 41 * <p>This class and its iterator implement all of the 42 * <em>optional</em> methods of the {@link Collection} and {@link 43 * Iterator} interfaces. 44 * 45 * <p>Memory consistency effects: As with other concurrent 46 * collections, actions in a thread prior to placing an object into a 47 * {@code LinkedTransferQueue} 48 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 49 * actions subsequent to the access or removal of that element from 50 * the {@code LinkedTransferQueue} in another thread. 51 * 52 * @since 1.7 53 * @author Doug Lea 54 * @param <E> the type of elements held in this collection 55 */ 56public class LinkedTransferQueue<E> extends AbstractQueue<E> 57 implements TransferQueue<E>, java.io.Serializable { 58 private static final long serialVersionUID = -3223113410248163686L; 59 60 /* 61 * *** Overview of Dual Queues with Slack *** 62 * 63 * Dual Queues, introduced by Scherer and Scott 64 * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are 65 * (linked) queues in which nodes may represent either data or 66 * requests. When a thread tries to enqueue a data node, but 67 * encounters a request node, it instead "matches" and removes it; 68 * and vice versa for enqueuing requests. Blocking Dual Queues 69 * arrange that threads enqueuing unmatched requests block until 70 * other threads provide the match. Dual Synchronous Queues (see 71 * Scherer, Lea, & Scott 72 * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) 73 * additionally arrange that threads enqueuing unmatched data also 74 * block. Dual Transfer Queues support all of these modes, as 75 * dictated by callers. 76 * 77 * A FIFO dual queue may be implemented using a variation of the 78 * Michael & Scott (M&S) lock-free queue algorithm 79 * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). 80 * It maintains two pointer fields, "head", pointing to a 81 * (matched) node that in turn points to the first actual 82 * (unmatched) queue node (or null if empty); and "tail" that 83 * points to the last node on the queue (or again null if 84 * empty). For example, here is a possible queue with four data 85 * elements: 86 * 87 * head tail 88 * | | 89 * v v 90 * M -> U -> U -> U -> U 91 * 92 * The M&S queue algorithm is known to be prone to scalability and 93 * overhead limitations when maintaining (via CAS) these head and 94 * tail pointers. This has led to the development of 95 * contention-reducing variants such as elimination arrays (see 96 * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and 97 * optimistic back pointers (see Ladan-Mozes & Shavit 98 * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). 99 * However, the nature of dual queues enables a simpler tactic for 100 * improving M&S-style implementations when dual-ness is needed. 101 * 102 * In a dual queue, each node must atomically maintain its match 103 * status. While there are other possible variants, we implement 104 * this here as: for a data-mode node, matching entails CASing an 105 * "item" field from a non-null data value to null upon match, and 106 * vice-versa for request nodes, CASing from null to a data 107 * value. (Note that the linearization properties of this style of 108 * queue are easy to verify -- elements are made available by 109 * linking, and unavailable by matching.) Compared to plain M&S 110 * queues, this property of dual queues requires one additional 111 * successful atomic operation per enq/deq pair. But it also 112 * enables lower cost variants of queue maintenance mechanics. (A 113 * variation of this idea applies even for non-dual queues that 114 * support deletion of interior elements, such as 115 * j.u.c.ConcurrentLinkedQueue.) 116 * 117 * Once a node is matched, its match status can never again 118 * change. We may thus arrange that the linked list of them 119 * contain a prefix of zero or more matched nodes, followed by a 120 * suffix of zero or more unmatched nodes. (Note that we allow 121 * both the prefix and suffix to be zero length, which in turn 122 * means that we do not use a dummy header.) If we were not 123 * concerned with either time or space efficiency, we could 124 * correctly perform enqueue and dequeue operations by traversing 125 * from a pointer to the initial node; CASing the item of the 126 * first unmatched node on match and CASing the next field of the 127 * trailing node on appends. (Plus some special-casing when 128 * initially empty). While this would be a terrible idea in 129 * itself, it does have the benefit of not requiring ANY atomic 130 * updates on head/tail fields. 131 * 132 * We introduce here an approach that lies between the extremes of 133 * never versus always updating queue (head and tail) pointers. 134 * This offers a tradeoff between sometimes requiring extra 135 * traversal steps to locate the first and/or last unmatched 136 * nodes, versus the reduced overhead and contention of fewer 137 * updates to queue pointers. For example, a possible snapshot of 138 * a queue is: 139 * 140 * head tail 141 * | | 142 * v v 143 * M -> M -> U -> U -> U -> U 144 * 145 * The best value for this "slack" (the targeted maximum distance 146 * between the value of "head" and the first unmatched node, and 147 * similarly for "tail") is an empirical matter. We have found 148 * that using very small constants in the range of 1-3 work best 149 * over a range of platforms. Larger values introduce increasing 150 * costs of cache misses and risks of long traversal chains, while 151 * smaller values increase CAS contention and overhead. 152 * 153 * Dual queues with slack differ from plain M&S dual queues by 154 * virtue of only sometimes updating head or tail pointers when 155 * matching, appending, or even traversing nodes; in order to 156 * maintain a targeted slack. The idea of "sometimes" may be 157 * operationalized in several ways. The simplest is to use a 158 * per-operation counter incremented on each traversal step, and 159 * to try (via CAS) to update the associated queue pointer 160 * whenever the count exceeds a threshold. Another, that requires 161 * more overhead, is to use random number generators to update 162 * with a given probability per traversal step. 163 * 164 * In any strategy along these lines, because CASes updating 165 * fields may fail, the actual slack may exceed targeted 166 * slack. However, they may be retried at any time to maintain 167 * targets. Even when using very small slack values, this 168 * approach works well for dual queues because it allows all 169 * operations up to the point of matching or appending an item 170 * (hence potentially allowing progress by another thread) to be 171 * read-only, thus not introducing any further contention. As 172 * described below, we implement this by performing slack 173 * maintenance retries only after these points. 174 * 175 * As an accompaniment to such techniques, traversal overhead can 176 * be further reduced without increasing contention of head 177 * pointer updates: Threads may sometimes shortcut the "next" link 178 * path from the current "head" node to be closer to the currently 179 * known first unmatched node, and similarly for tail. Again, this 180 * may be triggered with using thresholds or randomization. 181 * 182 * These ideas must be further extended to avoid unbounded amounts 183 * of costly-to-reclaim garbage caused by the sequential "next" 184 * links of nodes starting at old forgotten head nodes: As first 185 * described in detail by Boehm 186 * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC 187 * delays noticing that any arbitrarily old node has become 188 * garbage, all newer dead nodes will also be unreclaimed. 189 * (Similar issues arise in non-GC environments.) To cope with 190 * this in our implementation, upon CASing to advance the head 191 * pointer, we set the "next" link of the previous head to point 192 * only to itself; thus limiting the length of connected dead lists. 193 * (We also take similar care to wipe out possibly garbage 194 * retaining values held in other Node fields.) However, doing so 195 * adds some further complexity to traversal: If any "next" 196 * pointer links to itself, it indicates that the current thread 197 * has lagged behind a head-update, and so the traversal must 198 * continue from the "head". Traversals trying to find the 199 * current tail starting from "tail" may also encounter 200 * self-links, in which case they also continue at "head". 201 * 202 * It is tempting in slack-based scheme to not even use CAS for 203 * updates (similarly to Ladan-Mozes & Shavit). However, this 204 * cannot be done for head updates under the above link-forgetting 205 * mechanics because an update may leave head at a detached node. 206 * And while direct writes are possible for tail updates, they 207 * increase the risk of long retraversals, and hence long garbage 208 * chains, which can be much more costly than is worthwhile 209 * considering that the cost difference of performing a CAS vs 210 * write is smaller when they are not triggered on each operation 211 * (especially considering that writes and CASes equally require 212 * additional GC bookkeeping ("write barriers") that are sometimes 213 * more costly than the writes themselves because of contention). 214 * 215 * *** Overview of implementation *** 216 * 217 * We use a threshold-based approach to updates, with a slack 218 * threshold of two -- that is, we update head/tail when the 219 * current pointer appears to be two or more steps away from the 220 * first/last node. The slack value is hard-wired: a path greater 221 * than one is naturally implemented by checking equality of 222 * traversal pointers except when the list has only one element, 223 * in which case we keep slack threshold at one. Avoiding tracking 224 * explicit counts across method calls slightly simplifies an 225 * already-messy implementation. Using randomization would 226 * probably work better if there were a low-quality dirt-cheap 227 * per-thread one available, but even ThreadLocalRandom is too 228 * heavy for these purposes. 229 * 230 * With such a small slack threshold value, it is not worthwhile 231 * to augment this with path short-circuiting (i.e., unsplicing 232 * interior nodes) except in the case of cancellation/removal (see 233 * below). 234 * 235 * We allow both the head and tail fields to be null before any 236 * nodes are enqueued; initializing upon first append. This 237 * simplifies some other logic, as well as providing more 238 * efficient explicit control paths instead of letting JVMs insert 239 * implicit NullPointerExceptions when they are null. While not 240 * currently fully implemented, we also leave open the possibility 241 * of re-nulling these fields when empty (which is complicated to 242 * arrange, for little benefit.) 243 * 244 * All enqueue/dequeue operations are handled by the single method 245 * "xfer" with parameters indicating whether to act as some form 246 * of offer, put, poll, take, or transfer (each possibly with 247 * timeout). The relative complexity of using one monolithic 248 * method outweighs the code bulk and maintenance problems of 249 * using separate methods for each case. 250 * 251 * Operation consists of up to three phases. The first is 252 * implemented within method xfer, the second in tryAppend, and 253 * the third in method awaitMatch. 254 * 255 * 1. Try to match an existing node 256 * 257 * Starting at head, skip already-matched nodes until finding 258 * an unmatched node of opposite mode, if one exists, in which 259 * case matching it and returning, also if necessary updating 260 * head to one past the matched node (or the node itself if the 261 * list has no other unmatched nodes). If the CAS misses, then 262 * a loop retries advancing head by two steps until either 263 * success or the slack is at most two. By requiring that each 264 * attempt advances head by two (if applicable), we ensure that 265 * the slack does not grow without bound. Traversals also check 266 * if the initial head is now off-list, in which case they 267 * start at the new head. 268 * 269 * If no candidates are found and the call was untimed 270 * poll/offer, (argument "how" is NOW) return. 271 * 272 * 2. Try to append a new node (method tryAppend) 273 * 274 * Starting at current tail pointer, find the actual last node 275 * and try to append a new node (or if head was null, establish 276 * the first node). Nodes can be appended only if their 277 * predecessors are either already matched or are of the same 278 * mode. If we detect otherwise, then a new node with opposite 279 * mode must have been appended during traversal, so we must 280 * restart at phase 1. The traversal and update steps are 281 * otherwise similar to phase 1: Retrying upon CAS misses and 282 * checking for staleness. In particular, if a self-link is 283 * encountered, then we can safely jump to a node on the list 284 * by continuing the traversal at current head. 285 * 286 * On successful append, if the call was ASYNC, return. 287 * 288 * 3. Await match or cancellation (method awaitMatch) 289 * 290 * Wait for another thread to match node; instead cancelling if 291 * the current thread was interrupted or the wait timed out. On 292 * multiprocessors, we use front-of-queue spinning: If a node 293 * appears to be the first unmatched node in the queue, it 294 * spins a bit before blocking. In either case, before blocking 295 * it tries to unsplice any nodes between the current "head" 296 * and the first unmatched node. 297 * 298 * Front-of-queue spinning vastly improves performance of 299 * heavily contended queues. And so long as it is relatively 300 * brief and "quiet", spinning does not much impact performance 301 * of less-contended queues. During spins threads check their 302 * interrupt status and generate a thread-local random number 303 * to decide to occasionally perform a Thread.yield. While 304 * yield has underdefined specs, we assume that it might help, 305 * and will not hurt, in limiting impact of spinning on busy 306 * systems. We also use smaller (1/2) spins for nodes that are 307 * not known to be front but whose predecessors have not 308 * blocked -- these "chained" spins avoid artifacts of 309 * front-of-queue rules which otherwise lead to alternating 310 * nodes spinning vs blocking. Further, front threads that 311 * represent phase changes (from data to request node or vice 312 * versa) compared to their predecessors receive additional 313 * chained spins, reflecting longer paths typically required to 314 * unblock threads during phase changes. 315 * 316 * 317 * ** Unlinking removed interior nodes ** 318 * 319 * In addition to minimizing garbage retention via self-linking 320 * described above, we also unlink removed interior nodes. These 321 * may arise due to timed out or interrupted waits, or calls to 322 * remove(x) or Iterator.remove. Normally, given a node that was 323 * at one time known to be the predecessor of some node s that is 324 * to be removed, we can unsplice s by CASing the next field of 325 * its predecessor if it still points to s (otherwise s must 326 * already have been removed or is now offlist). But there are two 327 * situations in which we cannot guarantee to make node s 328 * unreachable in this way: (1) If s is the trailing node of list 329 * (i.e., with null next), then it is pinned as the target node 330 * for appends, so can only be removed later after other nodes are 331 * appended. (2) We cannot necessarily unlink s given a 332 * predecessor node that is matched (including the case of being 333 * cancelled): the predecessor may already be unspliced, in which 334 * case some previous reachable node may still point to s. 335 * (For further explanation see Herlihy & Shavit "The Art of 336 * Multiprocessor Programming" chapter 9). Although, in both 337 * cases, we can rule out the need for further action if either s 338 * or its predecessor are (or can be made to be) at, or fall off 339 * from, the head of list. 340 * 341 * Without taking these into account, it would be possible for an 342 * unbounded number of supposedly removed nodes to remain 343 * reachable. Situations leading to such buildup are uncommon but 344 * can occur in practice; for example when a series of short timed 345 * calls to poll repeatedly time out but never otherwise fall off 346 * the list because of an untimed call to take at the front of the 347 * queue. 348 * 349 * When these cases arise, rather than always retraversing the 350 * entire list to find an actual predecessor to unlink (which 351 * won't help for case (1) anyway), we record a conservative 352 * estimate of possible unsplice failures (in "sweepVotes"). 353 * We trigger a full sweep when the estimate exceeds a threshold 354 * ("SWEEP_THRESHOLD") indicating the maximum number of estimated 355 * removal failures to tolerate before sweeping through, unlinking 356 * cancelled nodes that were not unlinked upon initial removal. 357 * We perform sweeps by the thread hitting threshold (rather than 358 * background threads or by spreading work to other threads) 359 * because in the main contexts in which removal occurs, the 360 * caller is already timed-out, cancelled, or performing a 361 * potentially O(n) operation (e.g. remove(x)), none of which are 362 * time-critical enough to warrant the overhead that alternatives 363 * would impose on other threads. 364 * 365 * Because the sweepVotes estimate is conservative, and because 366 * nodes become unlinked "naturally" as they fall off the head of 367 * the queue, and because we allow votes to accumulate even while 368 * sweeps are in progress, there are typically significantly fewer 369 * such nodes than estimated. Choice of a threshold value 370 * balances the likelihood of wasted effort and contention, versus 371 * providing a worst-case bound on retention of interior nodes in 372 * quiescent queues. The value defined below was chosen 373 * empirically to balance these under various timeout scenarios. 374 * 375 * Note that we cannot self-link unlinked interior nodes during 376 * sweeps. However, the associated garbage chains terminate when 377 * some successor ultimately falls off the head of the list and is 378 * self-linked. 379 */ 380 381 /** True if on multiprocessor */ 382 private static final boolean MP = 383 Runtime.getRuntime().availableProcessors() > 1; 384 385 /** 386 * The number of times to spin (with randomly interspersed calls 387 * to Thread.yield) on multiprocessor before blocking when a node 388 * is apparently the first waiter in the queue. See above for 389 * explanation. Must be a power of two. The value is empirically 390 * derived -- it works pretty well across a variety of processors, 391 * numbers of CPUs, and OSes. 392 */ 393 private static final int FRONT_SPINS = 1 << 7; 394 395 /** 396 * The number of times to spin before blocking when a node is 397 * preceded by another node that is apparently spinning. Also 398 * serves as an increment to FRONT_SPINS on phase changes, and as 399 * base average frequency for yielding during spins. Must be a 400 * power of two. 401 */ 402 private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; 403 404 /** 405 * The maximum number of estimated removal failures (sweepVotes) 406 * to tolerate before sweeping through the queue unlinking 407 * cancelled nodes that were not unlinked upon initial 408 * removal. See above for explanation. The value must be at least 409 * two to avoid useless sweeps when removing trailing nodes. 410 */ 411 static final int SWEEP_THRESHOLD = 32; 412 413 /** 414 * Queue nodes. Uses Object, not E, for items to allow forgetting 415 * them after use. Relies heavily on Unsafe mechanics to minimize 416 * unnecessary ordering constraints: Writes that are intrinsically 417 * ordered wrt other accesses or CASes use simple relaxed forms. 418 */ 419 static final class Node { 420 final boolean isData; // false if this is a request node 421 volatile Object item; // initially non-null if isData; CASed to match 422 volatile Node next; 423 volatile Thread waiter; // null until waiting 424 425 // CAS methods for fields 426 final boolean casNext(Node cmp, Node val) { 427 return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); 428 } 429 430 final boolean casItem(Object cmp, Object val) { 431 // assert cmp == null || cmp.getClass() != Node.class; 432 return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); 433 } 434 435 /** 436 * Constructs a new node. Uses relaxed write because item can 437 * only be seen after publication via casNext. 438 */ 439 Node(Object item, boolean isData) { 440 UNSAFE.putObject(this, itemOffset, item); // relaxed write 441 this.isData = isData; 442 } 443 444 /** 445 * Links node to itself to avoid garbage retention. Called 446 * only after CASing head field, so uses relaxed write. 447 */ 448 final void forgetNext() { 449 UNSAFE.putObject(this, nextOffset, this); 450 } 451 452 /** 453 * Sets item to self and waiter to null, to avoid garbage 454 * retention after matching or cancelling. Uses relaxed writes 455 * because order is already constrained in the only calling 456 * contexts: item is forgotten only after volatile/atomic 457 * mechanics that extract items. Similarly, clearing waiter 458 * follows either CAS or return from park (if ever parked; 459 * else we don't care). 460 */ 461 final void forgetContents() { 462 UNSAFE.putObject(this, itemOffset, this); 463 UNSAFE.putObject(this, waiterOffset, null); 464 } 465 466 /** 467 * Returns true if this node has been matched, including the 468 * case of artificial matches due to cancellation. 469 */ 470 final boolean isMatched() { 471 Object x = item; 472 return (x == this) || ((x == null) == isData); 473 } 474 475 /** 476 * Returns true if this is an unmatched request node. 477 */ 478 final boolean isUnmatchedRequest() { 479 return !isData && item == null; 480 } 481 482 /** 483 * Returns true if a node with the given mode cannot be 484 * appended to this node because this node is unmatched and 485 * has opposite data mode. 486 */ 487 final boolean cannotPrecede(boolean haveData) { 488 boolean d = isData; 489 Object x; 490 return d != haveData && (x = item) != this && (x != null) == d; 491 } 492 493 /** 494 * Tries to artificially match a data node -- used by remove. 495 */ 496 final boolean tryMatchData() { 497 // assert isData; 498 Object x = item; 499 if (x != null && x != this && casItem(x, null)) { 500 LockSupport.unpark(waiter); 501 return true; 502 } 503 return false; 504 } 505 506 private static final long serialVersionUID = -3375979862319811754L; 507 508 // Unsafe mechanics 509 private static final sun.misc.Unsafe UNSAFE; 510 private static final long itemOffset; 511 private static final long nextOffset; 512 private static final long waiterOffset; 513 static { 514 try { 515 UNSAFE = sun.misc.Unsafe.getUnsafe(); 516 Class<?> k = Node.class; 517 itemOffset = UNSAFE.objectFieldOffset 518 (k.getDeclaredField("item")); 519 nextOffset = UNSAFE.objectFieldOffset 520 (k.getDeclaredField("next")); 521 waiterOffset = UNSAFE.objectFieldOffset 522 (k.getDeclaredField("waiter")); 523 } catch (Exception e) { 524 throw new Error(e); 525 } 526 } 527 } 528 529 /** head of the queue; null until first enqueue */ 530 transient volatile Node head; 531 532 /** tail of the queue; null until first append */ 533 private transient volatile Node tail; 534 535 /** The number of apparent failures to unsplice removed nodes */ 536 private transient volatile int sweepVotes; 537 538 // CAS methods for fields 539 private boolean casTail(Node cmp, Node val) { 540 return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); 541 } 542 543 private boolean casHead(Node cmp, Node val) { 544 return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); 545 } 546 547 private boolean casSweepVotes(int cmp, int val) { 548 return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); 549 } 550 551 /* 552 * Possible values for "how" argument in xfer method. 553 */ 554 private static final int NOW = 0; // for untimed poll, tryTransfer 555 private static final int ASYNC = 1; // for offer, put, add 556 private static final int SYNC = 2; // for transfer, take 557 private static final int TIMED = 3; // for timed poll, tryTransfer 558 559 @SuppressWarnings("unchecked") 560 static <E> E cast(Object item) { 561 // assert item == null || item.getClass() != Node.class; 562 return (E) item; 563 } 564 565 /** 566 * Implements all queuing methods. See above for explanation. 567 * 568 * @param e the item or null for take 569 * @param haveData true if this is a put, else a take 570 * @param how NOW, ASYNC, SYNC, or TIMED 571 * @param nanos timeout in nanosecs, used only if mode is TIMED 572 * @return an item if matched, else e 573 * @throws NullPointerException if haveData mode but e is null 574 */ 575 private E xfer(E e, boolean haveData, int how, long nanos) { 576 if (haveData && (e == null)) 577 throw new NullPointerException(); 578 Node s = null; // the node to append, if needed 579 580 retry: 581 for (;;) { // restart on append race 582 583 for (Node h = head, p = h; p != null;) { // find & match first node 584 boolean isData = p.isData; 585 Object item = p.item; 586 if (item != p && (item != null) == isData) { // unmatched 587 if (isData == haveData) // can't match 588 break; 589 if (p.casItem(item, e)) { // match 590 for (Node q = p; q != h;) { 591 Node n = q.next; // update by 2 unless singleton 592 if (head == h && casHead(h, n == null ? q : n)) { 593 h.forgetNext(); 594 break; 595 } // advance and retry 596 if ((h = head) == null || 597 (q = h.next) == null || !q.isMatched()) 598 break; // unless slack < 2 599 } 600 LockSupport.unpark(p.waiter); 601 return LinkedTransferQueue.<E>cast(item); 602 } 603 } 604 Node n = p.next; 605 p = (p != n) ? n : (h = head); // Use head if p offlist 606 } 607 608 if (how != NOW) { // No matches available 609 if (s == null) 610 s = new Node(e, haveData); 611 Node pred = tryAppend(s, haveData); 612 if (pred == null) 613 continue retry; // lost race vs opposite mode 614 if (how != ASYNC) 615 return awaitMatch(s, pred, e, (how == TIMED), nanos); 616 } 617 return e; // not waiting 618 } 619 } 620 621 /** 622 * Tries to append node s as tail. 623 * 624 * @param s the node to append 625 * @param haveData true if appending in data mode 626 * @return null on failure due to losing race with append in 627 * different mode, else s's predecessor, or s itself if no 628 * predecessor 629 */ 630 private Node tryAppend(Node s, boolean haveData) { 631 for (Node t = tail, p = t;;) { // move p to last node and append 632 Node n, u; // temps for reads of next & tail 633 if (p == null && (p = head) == null) { 634 if (casHead(null, s)) 635 return s; // initialize 636 } 637 else if (p.cannotPrecede(haveData)) 638 return null; // lost race vs opposite mode 639 else if ((n = p.next) != null) // not last; keep traversing 640 p = p != t && t != (u = tail) ? (t = u) : // stale tail 641 (p != n) ? n : null; // restart if off list 642 else if (!p.casNext(null, s)) 643 p = p.next; // re-read on CAS failure 644 else { 645 if (p != t) { // update if slack now >= 2 646 while ((tail != t || !casTail(t, s)) && 647 (t = tail) != null && 648 (s = t.next) != null && // advance and retry 649 (s = s.next) != null && s != t); 650 } 651 return p; 652 } 653 } 654 } 655 656 /** 657 * Spins/yields/blocks until node s is matched or caller gives up. 658 * 659 * @param s the waiting node 660 * @param pred the predecessor of s, or s itself if it has no 661 * predecessor, or null if unknown (the null case does not occur 662 * in any current calls but may in possible future extensions) 663 * @param e the comparison value for checking match 664 * @param timed if true, wait only until timeout elapses 665 * @param nanos timeout in nanosecs, used only if timed is true 666 * @return matched item, or e if unmatched on interrupt or timeout 667 */ 668 private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { 669 final long deadline = timed ? System.nanoTime() + nanos : 0L; 670 Thread w = Thread.currentThread(); 671 int spins = -1; // initialized after first item and cancel checks 672 ThreadLocalRandom randomYields = null; // bound if needed 673 674 for (;;) { 675 Object item = s.item; 676 if (item != e) { // matched 677 // assert item != s; 678 s.forgetContents(); // avoid garbage 679 return LinkedTransferQueue.<E>cast(item); 680 } 681 if ((w.isInterrupted() || (timed && nanos <= 0)) && 682 s.casItem(e, s)) { // cancel 683 unsplice(pred, s); 684 return e; 685 } 686 687 if (spins < 0) { // establish spins at/near front 688 if ((spins = spinsFor(pred, s.isData)) > 0) 689 randomYields = ThreadLocalRandom.current(); 690 } 691 else if (spins > 0) { // spin 692 --spins; 693 if (randomYields.nextInt(CHAINED_SPINS) == 0) 694 Thread.yield(); // occasionally yield 695 } 696 else if (s.waiter == null) { 697 s.waiter = w; // request unpark then recheck 698 } 699 else if (timed) { 700 nanos = deadline - System.nanoTime(); 701 if (nanos > 0L) 702 LockSupport.parkNanos(this, nanos); 703 } 704 else { 705 LockSupport.park(this); 706 } 707 } 708 } 709 710 /** 711 * Returns spin/yield value for a node with given predecessor and 712 * data mode. See above for explanation. 713 */ 714 private static int spinsFor(Node pred, boolean haveData) { 715 if (MP && pred != null) { 716 if (pred.isData != haveData) // phase change 717 return FRONT_SPINS + CHAINED_SPINS; 718 if (pred.isMatched()) // probably at front 719 return FRONT_SPINS; 720 if (pred.waiter == null) // pred apparently spinning 721 return CHAINED_SPINS; 722 } 723 return 0; 724 } 725 726 /* -------------- Traversal methods -------------- */ 727 728 /** 729 * Returns the successor of p, or the head node if p.next has been 730 * linked to self, which will only be true if traversing with a 731 * stale pointer that is now off the list. 732 */ 733 final Node succ(Node p) { 734 Node next = p.next; 735 return (p == next) ? head : next; 736 } 737 738 /** 739 * Returns the first unmatched node of the given mode, or null if 740 * none. Used by methods isEmpty, hasWaitingConsumer. 741 */ 742 private Node firstOfMode(boolean isData) { 743 for (Node p = head; p != null; p = succ(p)) { 744 if (!p.isMatched()) 745 return (p.isData == isData) ? p : null; 746 } 747 return null; 748 } 749 750 /** 751 * Returns the item in the first unmatched node with isData; or 752 * null if none. Used by peek. 753 */ 754 private E firstDataItem() { 755 for (Node p = head; p != null; p = succ(p)) { 756 Object item = p.item; 757 if (p.isData) { 758 if (item != null && item != p) 759 return LinkedTransferQueue.<E>cast(item); 760 } 761 else if (item == null) 762 return null; 763 } 764 return null; 765 } 766 767 /** 768 * Traverses and counts unmatched nodes of the given mode. 769 * Used by methods size and getWaitingConsumerCount. 770 */ 771 private int countOfMode(boolean data) { 772 int count = 0; 773 for (Node p = head; p != null; ) { 774 if (!p.isMatched()) { 775 if (p.isData != data) 776 return 0; 777 if (++count == Integer.MAX_VALUE) // saturated 778 break; 779 } 780 Node n = p.next; 781 if (n != p) 782 p = n; 783 else { 784 count = 0; 785 p = head; 786 } 787 } 788 return count; 789 } 790 791 final class Itr implements Iterator<E> { 792 private Node nextNode; // next node to return item for 793 private E nextItem; // the corresponding item 794 private Node lastRet; // last returned node, to support remove 795 private Node lastPred; // predecessor to unlink lastRet 796 797 /** 798 * Moves to next node after prev, or first node if prev null. 799 */ 800 private void advance(Node prev) { 801 /* 802 * To track and avoid buildup of deleted nodes in the face 803 * of calls to both Queue.remove and Itr.remove, we must 804 * include variants of unsplice and sweep upon each 805 * advance: Upon Itr.remove, we may need to catch up links 806 * from lastPred, and upon other removes, we might need to 807 * skip ahead from stale nodes and unsplice deleted ones 808 * found while advancing. 809 */ 810 811 Node r, b; // reset lastPred upon possible deletion of lastRet 812 if ((r = lastRet) != null && !r.isMatched()) 813 lastPred = r; // next lastPred is old lastRet 814 else if ((b = lastPred) == null || b.isMatched()) 815 lastPred = null; // at start of list 816 else { 817 Node s, n; // help with removal of lastPred.next 818 while ((s = b.next) != null && 819 s != b && s.isMatched() && 820 (n = s.next) != null && n != s) 821 b.casNext(s, n); 822 } 823 824 this.lastRet = prev; 825 826 for (Node p = prev, s, n;;) { 827 s = (p == null) ? head : p.next; 828 if (s == null) 829 break; 830 else if (s == p) { 831 p = null; 832 continue; 833 } 834 Object item = s.item; 835 if (s.isData) { 836 if (item != null && item != s) { 837 nextItem = LinkedTransferQueue.<E>cast(item); 838 nextNode = s; 839 return; 840 } 841 } 842 else if (item == null) 843 break; 844 // assert s.isMatched(); 845 if (p == null) 846 p = s; 847 else if ((n = s.next) == null) 848 break; 849 else if (s == n) 850 p = null; 851 else 852 p.casNext(s, n); 853 } 854 nextNode = null; 855 nextItem = null; 856 } 857 858 Itr() { 859 advance(null); 860 } 861 862 public final boolean hasNext() { 863 return nextNode != null; 864 } 865 866 public final E next() { 867 Node p = nextNode; 868 if (p == null) throw new NoSuchElementException(); 869 E e = nextItem; 870 advance(p); 871 return e; 872 } 873 874 public final void remove() { 875 final Node lastRet = this.lastRet; 876 if (lastRet == null) 877 throw new IllegalStateException(); 878 this.lastRet = null; 879 if (lastRet.tryMatchData()) 880 unsplice(lastPred, lastRet); 881 } 882 } 883 884 /* -------------- Removal methods -------------- */ 885 886 /** 887 * Unsplices (now or later) the given deleted/cancelled node with 888 * the given predecessor. 889 * 890 * @param pred a node that was at one time known to be the 891 * predecessor of s, or null or s itself if s is/was at head 892 * @param s the node to be unspliced 893 */ 894 final void unsplice(Node pred, Node s) { 895 s.forgetContents(); // forget unneeded fields 896 /* 897 * See above for rationale. Briefly: if pred still points to 898 * s, try to unlink s. If s cannot be unlinked, because it is 899 * trailing node or pred might be unlinked, and neither pred 900 * nor s are head or offlist, add to sweepVotes, and if enough 901 * votes have accumulated, sweep. 902 */ 903 if (pred != null && pred != s && pred.next == s) { 904 Node n = s.next; 905 if (n == null || 906 (n != s && pred.casNext(s, n) && pred.isMatched())) { 907 for (;;) { // check if at, or could be, head 908 Node h = head; 909 if (h == pred || h == s || h == null) 910 return; // at head or list empty 911 if (!h.isMatched()) 912 break; 913 Node hn = h.next; 914 if (hn == null) 915 return; // now empty 916 if (hn != h && casHead(h, hn)) 917 h.forgetNext(); // advance head 918 } 919 if (pred.next != pred && s.next != s) { // recheck if offlist 920 for (;;) { // sweep now if enough votes 921 int v = sweepVotes; 922 if (v < SWEEP_THRESHOLD) { 923 if (casSweepVotes(v, v + 1)) 924 break; 925 } 926 else if (casSweepVotes(v, 0)) { 927 sweep(); 928 break; 929 } 930 } 931 } 932 } 933 } 934 } 935 936 /** 937 * Unlinks matched (typically cancelled) nodes encountered in a 938 * traversal from head. 939 */ 940 private void sweep() { 941 for (Node p = head, s, n; p != null && (s = p.next) != null; ) { 942 if (!s.isMatched()) 943 // Unmatched nodes are never self-linked 944 p = s; 945 else if ((n = s.next) == null) // trailing node is pinned 946 break; 947 else if (s == n) // stale 948 // No need to also check for p == s, since that implies s == n 949 p = head; 950 else 951 p.casNext(s, n); 952 } 953 } 954 955 /** 956 * Main implementation of remove(Object) 957 */ 958 private boolean findAndRemove(Object e) { 959 if (e != null) { 960 for (Node pred = null, p = head; p != null; ) { 961 Object item = p.item; 962 if (p.isData) { 963 if (item != null && item != p && e.equals(item) && 964 p.tryMatchData()) { 965 unsplice(pred, p); 966 return true; 967 } 968 } 969 else if (item == null) 970 break; 971 pred = p; 972 if ((p = p.next) == pred) { // stale 973 pred = null; 974 p = head; 975 } 976 } 977 } 978 return false; 979 } 980 981 /** 982 * Creates an initially empty {@code LinkedTransferQueue}. 983 */ 984 public LinkedTransferQueue() { 985 } 986 987 /** 988 * Creates a {@code LinkedTransferQueue} 989 * initially containing the elements of the given collection, 990 * added in traversal order of the collection's iterator. 991 * 992 * @param c the collection of elements to initially contain 993 * @throws NullPointerException if the specified collection or any 994 * of its elements are null 995 */ 996 public LinkedTransferQueue(Collection<? extends E> c) { 997 this(); 998 addAll(c); 999 } 1000 1001 /** 1002 * Inserts the specified element at the tail of this queue. 1003 * As the queue is unbounded, this method will never block. 1004 * 1005 * @throws NullPointerException if the specified element is null 1006 */ 1007 public void put(E e) { 1008 xfer(e, true, ASYNC, 0); 1009 } 1010 1011 /** 1012 * Inserts the specified element at the tail of this queue. 1013 * As the queue is unbounded, this method will never block or 1014 * return {@code false}. 1015 * 1016 * @return {@code true} (as specified by 1017 * {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit) 1018 * BlockingQueue.offer}) 1019 * @throws NullPointerException if the specified element is null 1020 */ 1021 public boolean offer(E e, long timeout, TimeUnit unit) { 1022 xfer(e, true, ASYNC, 0); 1023 return true; 1024 } 1025 1026 /** 1027 * Inserts the specified element at the tail of this queue. 1028 * As the queue is unbounded, this method will never return {@code false}. 1029 * 1030 * @return {@code true} (as specified by {@link Queue#offer}) 1031 * @throws NullPointerException if the specified element is null 1032 */ 1033 public boolean offer(E e) { 1034 xfer(e, true, ASYNC, 0); 1035 return true; 1036 } 1037 1038 /** 1039 * Inserts the specified element at the tail of this queue. 1040 * As the queue is unbounded, this method will never throw 1041 * {@link IllegalStateException} or return {@code false}. 1042 * 1043 * @return {@code true} (as specified by {@link Collection#add}) 1044 * @throws NullPointerException if the specified element is null 1045 */ 1046 public boolean add(E e) { 1047 xfer(e, true, ASYNC, 0); 1048 return true; 1049 } 1050 1051 /** 1052 * Transfers the element to a waiting consumer immediately, if possible. 1053 * 1054 * <p>More precisely, transfers the specified element immediately 1055 * if there exists a consumer already waiting to receive it (in 1056 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 1057 * otherwise returning {@code false} without enqueuing the element. 1058 * 1059 * @throws NullPointerException if the specified element is null 1060 */ 1061 public boolean tryTransfer(E e) { 1062 return xfer(e, true, NOW, 0) == null; 1063 } 1064 1065 /** 1066 * Transfers the element to a consumer, waiting if necessary to do so. 1067 * 1068 * <p>More precisely, transfers the specified element immediately 1069 * if there exists a consumer already waiting to receive it (in 1070 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 1071 * else inserts the specified element at the tail of this queue 1072 * and waits until the element is received by a consumer. 1073 * 1074 * @throws NullPointerException if the specified element is null 1075 */ 1076 public void transfer(E e) throws InterruptedException { 1077 if (xfer(e, true, SYNC, 0) != null) { 1078 Thread.interrupted(); // failure possible only due to interrupt 1079 throw new InterruptedException(); 1080 } 1081 } 1082 1083 /** 1084 * Transfers the element to a consumer if it is possible to do so 1085 * before the timeout elapses. 1086 * 1087 * <p>More precisely, transfers the specified element immediately 1088 * if there exists a consumer already waiting to receive it (in 1089 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 1090 * else inserts the specified element at the tail of this queue 1091 * and waits until the element is received by a consumer, 1092 * returning {@code false} if the specified wait time elapses 1093 * before the element can be transferred. 1094 * 1095 * @throws NullPointerException if the specified element is null 1096 */ 1097 public boolean tryTransfer(E e, long timeout, TimeUnit unit) 1098 throws InterruptedException { 1099 if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) 1100 return true; 1101 if (!Thread.interrupted()) 1102 return false; 1103 throw new InterruptedException(); 1104 } 1105 1106 public E take() throws InterruptedException { 1107 E e = xfer(null, false, SYNC, 0); 1108 if (e != null) 1109 return e; 1110 Thread.interrupted(); 1111 throw new InterruptedException(); 1112 } 1113 1114 public E poll(long timeout, TimeUnit unit) throws InterruptedException { 1115 E e = xfer(null, false, TIMED, unit.toNanos(timeout)); 1116 if (e != null || !Thread.interrupted()) 1117 return e; 1118 throw new InterruptedException(); 1119 } 1120 1121 public E poll() { 1122 return xfer(null, false, NOW, 0); 1123 } 1124 1125 /** 1126 * @throws NullPointerException {@inheritDoc} 1127 * @throws IllegalArgumentException {@inheritDoc} 1128 */ 1129 public int drainTo(Collection<? super E> c) { 1130 if (c == null) 1131 throw new NullPointerException(); 1132 if (c == this) 1133 throw new IllegalArgumentException(); 1134 int n = 0; 1135 for (E e; (e = poll()) != null;) { 1136 c.add(e); 1137 ++n; 1138 } 1139 return n; 1140 } 1141 1142 /** 1143 * @throws NullPointerException {@inheritDoc} 1144 * @throws IllegalArgumentException {@inheritDoc} 1145 */ 1146 public int drainTo(Collection<? super E> c, int maxElements) { 1147 if (c == null) 1148 throw new NullPointerException(); 1149 if (c == this) 1150 throw new IllegalArgumentException(); 1151 int n = 0; 1152 for (E e; n < maxElements && (e = poll()) != null;) { 1153 c.add(e); 1154 ++n; 1155 } 1156 return n; 1157 } 1158 1159 /** 1160 * Returns an iterator over the elements in this queue in proper sequence. 1161 * The elements will be returned in order from first (head) to last (tail). 1162 * 1163 * <p>The returned iterator is a "weakly consistent" iterator that 1164 * will never throw {@link java.util.ConcurrentModificationException 1165 * ConcurrentModificationException}, and guarantees to traverse 1166 * elements as they existed upon construction of the iterator, and 1167 * may (but is not guaranteed to) reflect any modifications 1168 * subsequent to construction. 1169 * 1170 * @return an iterator over the elements in this queue in proper sequence 1171 */ 1172 public Iterator<E> iterator() { 1173 return new Itr(); 1174 } 1175 1176 public E peek() { 1177 return firstDataItem(); 1178 } 1179 1180 /** 1181 * Returns {@code true} if this queue contains no elements. 1182 * 1183 * @return {@code true} if this queue contains no elements 1184 */ 1185 public boolean isEmpty() { 1186 for (Node p = head; p != null; p = succ(p)) { 1187 if (!p.isMatched()) 1188 return !p.isData; 1189 } 1190 return true; 1191 } 1192 1193 public boolean hasWaitingConsumer() { 1194 return firstOfMode(false) != null; 1195 } 1196 1197 /** 1198 * Returns the number of elements in this queue. If this queue 1199 * contains more than {@code Integer.MAX_VALUE} elements, returns 1200 * {@code Integer.MAX_VALUE}. 1201 * 1202 * <p>Beware that, unlike in most collections, this method is 1203 * <em>NOT</em> a constant-time operation. Because of the 1204 * asynchronous nature of these queues, determining the current 1205 * number of elements requires an O(n) traversal. 1206 * 1207 * @return the number of elements in this queue 1208 */ 1209 public int size() { 1210 return countOfMode(true); 1211 } 1212 1213 public int getWaitingConsumerCount() { 1214 return countOfMode(false); 1215 } 1216 1217 /** 1218 * Removes a single instance of the specified element from this queue, 1219 * if it is present. More formally, removes an element {@code e} such 1220 * that {@code o.equals(e)}, if this queue contains one or more such 1221 * elements. 1222 * Returns {@code true} if this queue contained the specified element 1223 * (or equivalently, if this queue changed as a result of the call). 1224 * 1225 * @param o element to be removed from this queue, if present 1226 * @return {@code true} if this queue changed as a result of the call 1227 */ 1228 public boolean remove(Object o) { 1229 return findAndRemove(o); 1230 } 1231 1232 /** 1233 * Returns {@code true} if this queue contains the specified element. 1234 * More formally, returns {@code true} if and only if this queue contains 1235 * at least one element {@code e} such that {@code o.equals(e)}. 1236 * 1237 * @param o object to be checked for containment in this queue 1238 * @return {@code true} if this queue contains the specified element 1239 */ 1240 public boolean contains(Object o) { 1241 if (o == null) return false; 1242 for (Node p = head; p != null; p = succ(p)) { 1243 Object item = p.item; 1244 if (p.isData) { 1245 if (item != null && item != p && o.equals(item)) 1246 return true; 1247 } 1248 else if (item == null) 1249 break; 1250 } 1251 return false; 1252 } 1253 1254 /** 1255 * Always returns {@code Integer.MAX_VALUE} because a 1256 * {@code LinkedTransferQueue} is not capacity constrained. 1257 * 1258 * @return {@code Integer.MAX_VALUE} (as specified by 1259 * {@link java.util.concurrent.BlockingQueue#remainingCapacity() 1260 * BlockingQueue.remainingCapacity}) 1261 */ 1262 public int remainingCapacity() { 1263 return Integer.MAX_VALUE; 1264 } 1265 1266 /** 1267 * Saves this queue to a stream (that is, serializes it). 1268 * 1269 * @serialData All of the elements (each an {@code E}) in 1270 * the proper order, followed by a null 1271 */ 1272 private void writeObject(java.io.ObjectOutputStream s) 1273 throws java.io.IOException { 1274 s.defaultWriteObject(); 1275 for (E e : this) 1276 s.writeObject(e); 1277 // Use trailing null as sentinel 1278 s.writeObject(null); 1279 } 1280 1281 /** 1282 * Reconstitutes this queue from a stream (that is, deserializes it). 1283 */ 1284 private void readObject(java.io.ObjectInputStream s) 1285 throws java.io.IOException, ClassNotFoundException { 1286 s.defaultReadObject(); 1287 for (;;) { 1288 @SuppressWarnings("unchecked") 1289 E item = (E) s.readObject(); 1290 if (item == null) 1291 break; 1292 else 1293 offer(item); 1294 } 1295 } 1296 1297 // Unsafe mechanics 1298 1299 private static final sun.misc.Unsafe UNSAFE; 1300 private static final long headOffset; 1301 private static final long tailOffset; 1302 private static final long sweepVotesOffset; 1303 static { 1304 try { 1305 UNSAFE = sun.misc.Unsafe.getUnsafe(); 1306 Class<?> k = LinkedTransferQueue.class; 1307 headOffset = UNSAFE.objectFieldOffset 1308 (k.getDeclaredField("head")); 1309 tailOffset = UNSAFE.objectFieldOffset 1310 (k.getDeclaredField("tail")); 1311 sweepVotesOffset = UNSAFE.objectFieldOffset 1312 (k.getDeclaredField("sweepVotes")); 1313 } catch (Exception e) { 1314 throw new Error(e); 1315 } 1316 } 1317} 1318