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 with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37
38 import java.lang.invoke.MethodHandles;
39 import java.lang.invoke.VarHandle;
40 import java.util.AbstractQueue;
41 import java.util.Arrays;
42 import java.util.Collection;
43 import java.util.Iterator;
44 import java.util.NoSuchElementException;
45 import java.util.Objects;
46 import java.util.Queue;
47 import java.util.Spliterator;
48 import java.util.Spliterators;
49 import java.util.concurrent.locks.LockSupport;
50 import java.util.function.Consumer;
51 import java.util.function.Predicate;
52
53 /**
54 * An unbounded {@link TransferQueue} based on linked nodes.
55 * This queue orders elements FIFO (first-in-first-out) with respect
56 * to any given producer. The <em>head</em> of the queue is that
57 * element that has been on the queue the longest time for some
58 * producer. The <em>tail</em> of the queue is that element that has
59 * been on the queue the shortest time for some producer.
60 *
61 * <p>Beware that, unlike in most collections, the {@code size} method
62 * is <em>NOT</em> a constant-time operation. Because of the
63 * asynchronous nature of these queues, determining the current number
64 * of elements requires a traversal of the elements, and so may report
65 * inaccurate results if this collection is modified during traversal.
66 *
67 * <p>Bulk operations that add, remove, or examine multiple elements,
68 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
69 * are <em>not</em> guaranteed to be performed atomically.
70 * For example, a {@code forEach} traversal concurrent with an {@code
71 * addAll} operation might observe only some of the added elements.
72 *
73 * <p>This class and its iterator implement all of the <em>optional</em>
74 * methods of the {@link Collection} and {@link Iterator} interfaces.
75 *
76 * <p>Memory consistency effects: As with other concurrent
77 * collections, actions in a thread prior to placing an object into a
78 * {@code LinkedTransferQueue}
79 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
80 * actions subsequent to the access or removal of that element from
81 * the {@code LinkedTransferQueue} in another thread.
82 *
83 * <p>This class is a member of the
84 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
85 * Java Collections Framework</a>.
86 *
87 * @since 1.7
88 * @author Doug Lea
89 * @param <E> the type of elements held in this queue
90 */
91 public class LinkedTransferQueue<E> extends AbstractQueue<E>
92 implements TransferQueue<E>, java.io.Serializable {
93 private static final long serialVersionUID = -3223113410248163686L;
94
95 /*
96 * *** Overview of Dual Queues with Slack ***
97 *
98 * Dual Queues, introduced by Scherer and Scott
99 * (http://www.cs.rochester.edu/~scott/papers/2004_DISC_dual_DS.pdf)
100 * are (linked) queues in which nodes may represent either data or
101 * requests. When a thread tries to enqueue a data node, but
102 * encounters a request node, it instead "matches" and removes it;
103 * and vice versa for enqueuing requests. Blocking Dual Queues
104 * arrange that threads enqueuing unmatched requests block until
105 * other threads provide the match. Dual Synchronous Queues (see
106 * Scherer, Lea, & Scott
107 * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
108 * additionally arrange that threads enqueuing unmatched data also
109 * block. Dual Transfer Queues support all of these modes, as
110 * dictated by callers.
111 *
112 * A FIFO dual queue may be implemented using a variation of the
113 * Michael & Scott (M&S) lock-free queue algorithm
114 * (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf).
115 * It maintains two pointer fields, "head", pointing to a
116 * (matched) node that in turn points to the first actual
117 * (unmatched) queue node (or null if empty); and "tail" that
118 * points to the last node on the queue (or again null if
119 * empty). For example, here is a possible queue with four data
120 * elements:
121 *
122 * head tail
123 * | |
124 * v v
125 * M -> U -> U -> U -> U
126 *
127 * The M&S queue algorithm is known to be prone to scalability and
128 * overhead limitations when maintaining (via CAS) these head and
129 * tail pointers. This has led to the development of
130 * contention-reducing variants such as elimination arrays (see
131 * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
132 * optimistic back pointers (see Ladan-Mozes & Shavit
133 * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
134 * However, the nature of dual queues enables a simpler tactic for
135 * improving M&S-style implementations when dual-ness is needed.
136 *
137 * In a dual queue, each node must atomically maintain its match
138 * status. While there are other possible variants, we implement
139 * this here as: for a data-mode node, matching entails CASing an
140 * "item" field from a non-null data value to null upon match, and
141 * vice-versa for request nodes, CASing from null to a data
142 * value. (Note that the linearization properties of this style of
143 * queue are easy to verify -- elements are made available by
144 * linking, and unavailable by matching.) Compared to plain M&S
145 * queues, this property of dual queues requires one additional
146 * successful atomic operation per enq/deq pair. But it also
147 * enables lower cost variants of queue maintenance mechanics. (A
148 * variation of this idea applies even for non-dual queues that
149 * support deletion of interior elements, such as
150 * j.u.c.ConcurrentLinkedQueue.)
151 *
152 * Once a node is matched, its match status can never again
153 * change. We may thus arrange that the linked list of them
154 * contain a prefix of zero or more matched nodes, followed by a
155 * suffix of zero or more unmatched nodes. (Note that we allow
156 * both the prefix and suffix to be zero length, which in turn
157 * means that we do not use a dummy header.) If we were not
158 * concerned with either time or space efficiency, we could
159 * correctly perform enqueue and dequeue operations by traversing
160 * from a pointer to the initial node; CASing the item of the
161 * first unmatched node on match and CASing the next field of the
162 * trailing node on appends. While this would be a terrible idea
163 * in itself, it does have the benefit of not requiring ANY atomic
164 * updates on head/tail fields.
165 *
166 * We introduce here an approach that lies between the extremes of
167 * never versus always updating queue (head and tail) pointers.
168 * This offers a tradeoff between sometimes requiring extra
169 * traversal steps to locate the first and/or last unmatched
170 * nodes, versus the reduced overhead and contention of fewer
171 * updates to queue pointers. For example, a possible snapshot of
172 * a queue is:
173 *
174 * head tail
175 * | |
176 * v v
177 * M -> M -> U -> U -> U -> U
178 *
179 * The best value for this "slack" (the targeted maximum distance
180 * between the value of "head" and the first unmatched node, and
181 * similarly for "tail") is an empirical matter. We have found
182 * that using very small constants in the range of 1-3 work best
183 * over a range of platforms. Larger values introduce increasing
184 * costs of cache misses and risks of long traversal chains, while
185 * smaller values increase CAS contention and overhead.
186 *
187 * Dual queues with slack differ from plain M&S dual queues by
188 * virtue of only sometimes updating head or tail pointers when
189 * matching, appending, or even traversing nodes; in order to
190 * maintain a targeted slack. The idea of "sometimes" may be
191 * operationalized in several ways. The simplest is to use a
192 * per-operation counter incremented on each traversal step, and
193 * to try (via CAS) to update the associated queue pointer
194 * whenever the count exceeds a threshold. Another, that requires
195 * more overhead, is to use random number generators to update
196 * with a given probability per traversal step.
197 *
198 * In any strategy along these lines, because CASes updating
199 * fields may fail, the actual slack may exceed targeted slack.
200 * However, they may be retried at any time to maintain targets.
201 * Even when using very small slack values, this approach works
202 * well for dual queues because it allows all operations up to the
203 * point of matching or appending an item (hence potentially
204 * allowing progress by another thread) to be read-only, thus not
205 * introducing any further contention. As described below, we
206 * implement this by performing slack maintenance retries only
207 * after these points.
208 *
209 * As an accompaniment to such techniques, traversal overhead can
210 * be further reduced without increasing contention of head
211 * pointer updates: Threads may sometimes shortcut the "next" link
212 * path from the current "head" node to be closer to the currently
213 * known first unmatched node, and similarly for tail. Again, this
214 * may be triggered with using thresholds or randomization.
215 *
216 * These ideas must be further extended to avoid unbounded amounts
217 * of costly-to-reclaim garbage caused by the sequential "next"
218 * links of nodes starting at old forgotten head nodes: As first
219 * described in detail by Boehm
220 * (http://portal.acm.org/citation.cfm?doid=503272.503282), if a GC
221 * delays noticing that any arbitrarily old node has become
222 * garbage, all newer dead nodes will also be unreclaimed.
223 * (Similar issues arise in non-GC environments.) To cope with
224 * this in our implementation, upon CASing to advance the head
225 * pointer, we set the "next" link of the previous head to point
226 * only to itself; thus limiting the length of chains of dead nodes.
227 * (We also take similar care to wipe out possibly garbage
228 * retaining values held in other Node fields.) However, doing so
229 * adds some further complexity to traversal: If any "next"
230 * pointer links to itself, it indicates that the current thread
231 * has lagged behind a head-update, and so the traversal must
232 * continue from the "head". Traversals trying to find the
233 * current tail starting from "tail" may also encounter
234 * self-links, in which case they also continue at "head".
235 *
236 * It is tempting in slack-based scheme to not even use CAS for
237 * updates (similarly to Ladan-Mozes & Shavit). However, this
238 * cannot be done for head updates under the above link-forgetting
239 * mechanics because an update may leave head at a detached node.
240 * And while direct writes are possible for tail updates, they
241 * increase the risk of long retraversals, and hence long garbage
242 * chains, which can be much more costly than is worthwhile
243 * considering that the cost difference of performing a CAS vs
244 * write is smaller when they are not triggered on each operation
245 * (especially considering that writes and CASes equally require
246 * additional GC bookkeeping ("write barriers") that are sometimes
247 * more costly than the writes themselves because of contention).
248 *
249 * *** Overview of implementation ***
250 *
251 * We use a threshold-based approach to updates, with a slack
252 * threshold of two -- that is, we update head/tail when the
253 * current pointer appears to be two or more steps away from the
254 * first/last node. The slack value is hard-wired: a path greater
255 * than one is naturally implemented by checking equality of
256 * traversal pointers except when the list has only one element,
257 * in which case we keep slack threshold at one. Avoiding tracking
258 * explicit counts across method calls slightly simplifies an
259 * already-messy implementation. Using randomization would
260 * probably work better if there were a low-quality dirt-cheap
261 * per-thread one available, but even ThreadLocalRandom is too
262 * heavy for these purposes.
263 *
264 * With such a small slack threshold value, it is not worthwhile
265 * to augment this with path short-circuiting (i.e., unsplicing
266 * interior nodes) except in the case of cancellation/removal (see
267 * below).
268 *
269 * All enqueue/dequeue operations are handled by the single method
270 * "xfer" with parameters indicating whether to act as some form
271 * of offer, put, poll, take, or transfer (each possibly with
272 * timeout). The relative complexity of using one monolithic
273 * method outweighs the code bulk and maintenance problems of
274 * using separate methods for each case.
275 *
276 * Operation consists of up to two phases. The first is implemented
277 * in method xfer, the second in method awaitMatch.
278 *
279 * 1. Traverse until matching or appending (method xfer)
280 *
281 * Conceptually, we simply traverse all nodes starting from head.
282 * If we encounter an unmatched node of opposite mode, we match
283 * it and return, also updating head (by at least 2 hops) to
284 * one past the matched node (or the node itself if it's the
285 * pinned trailing node). Traversals also check for the
286 * possibility of falling off-list, in which case they restart.
287 *
288 * If the trailing node of the list is reached, a match is not
289 * possible. If this call was untimed poll or tryTransfer
290 * (argument "how" is NOW), return empty-handed immediately.
291 * Else a new node is CAS-appended. On successful append, if
292 * this call was ASYNC (e.g. offer), an element was
293 * successfully added to the end of the queue and we return.
294 *
295 * Of course, this naive traversal is O(n) when no match is
296 * possible. We optimize the traversal by maintaining a tail
297 * pointer, which is expected to be "near" the end of the list.
298 * It is only safe to fast-forward to tail (in the presence of
299 * arbitrary concurrent changes) if it is pointing to a node of
300 * the same mode, even if it is dead (in this case no preceding
301 * node could still be matchable by this traversal). If we
302 * need to restart due to falling off-list, we can again
303 * fast-forward to tail, but only if it has changed since the
304 * last traversal (else we might loop forever). If tail cannot
305 * be used, traversal starts at head (but in this case we
306 * expect to be able to match near head). As with head, we
307 * CAS-advance the tail pointer by at least two hops.
308 *
309 * 2. Await match or cancellation (method awaitMatch)
310 *
311 * Wait for another thread to match node; instead cancelling if
312 * the current thread was interrupted or the wait timed out. On
313 * multiprocessors, we use front-of-queue spinning: If a node
314 * appears to be the first unmatched node in the queue, it
315 * spins a bit before blocking. In either case, before blocking
316 * it tries to unsplice any nodes between the current "head"
317 * and the first unmatched node.
318 *
319 * Front-of-queue spinning vastly improves performance of
320 * heavily contended queues. And so long as it is relatively
321 * brief and "quiet", spinning does not much impact performance
322 * of less-contended queues. During spins threads check their
323 * interrupt status and generate a thread-local random number
324 * to decide to occasionally perform a Thread.yield. While
325 * yield has underdefined specs, we assume that it might help,
326 * and will not hurt, in limiting impact of spinning on busy
327 * systems. We also use smaller (1/2) spins for nodes that are
328 * not known to be front but whose predecessors have not
329 * blocked -- these "chained" spins avoid artifacts of
330 * front-of-queue rules which otherwise lead to alternating
331 * nodes spinning vs blocking. Further, front threads that
332 * represent phase changes (from data to request node or vice
333 * versa) compared to their predecessors receive additional
334 * chained spins, reflecting longer paths typically required to
335 * unblock threads during phase changes.
336 *
337 *
338 * ** Unlinking removed interior nodes **
339 *
340 * In addition to minimizing garbage retention via self-linking
341 * described above, we also unlink removed interior nodes. These
342 * may arise due to timed out or interrupted waits, or calls to
343 * remove(x) or Iterator.remove. Normally, given a node that was
344 * at one time known to be the predecessor of some node s that is
345 * to be removed, we can unsplice s by CASing the next field of
346 * its predecessor if it still points to s (otherwise s must
347 * already have been removed or is now offlist). But there are two
348 * situations in which we cannot guarantee to make node s
349 * unreachable in this way: (1) If s is the trailing node of list
350 * (i.e., with null next), then it is pinned as the target node
351 * for appends, so can only be removed later after other nodes are
352 * appended. (2) We cannot necessarily unlink s given a
353 * predecessor node that is matched (including the case of being
354 * cancelled): the predecessor may already be unspliced, in which
355 * case some previous reachable node may still point to s.
356 * (For further explanation see Herlihy & Shavit "The Art of
357 * Multiprocessor Programming" chapter 9). Although, in both
358 * cases, we can rule out the need for further action if either s
359 * or its predecessor are (or can be made to be) at, or fall off
360 * from, the head of list.
361 *
362 * Without taking these into account, it would be possible for an
363 * unbounded number of supposedly removed nodes to remain reachable.
364 * Situations leading to such buildup are uncommon but can occur
365 * in practice; for example when a series of short timed calls to
366 * poll repeatedly time out at the trailing node but otherwise
367 * never fall off the list because of an untimed call to take() at
368 * the front of the queue.
369 *
370 * When these cases arise, rather than always retraversing the
371 * entire list to find an actual predecessor to unlink (which
372 * won't help for case (1) anyway), we record a conservative
373 * estimate of possible unsplice failures (in "sweepVotes").
374 * We trigger a full sweep when the estimate exceeds a threshold
375 * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
376 * removal failures to tolerate before sweeping through, unlinking
377 * cancelled nodes that were not unlinked upon initial removal.
378 * We perform sweeps by the thread hitting threshold (rather than
379 * background threads or by spreading work to other threads)
380 * because in the main contexts in which removal occurs, the
381 * caller is timed-out or cancelled, which are not time-critical
382 * enough to warrant the overhead that alternatives would impose
383 * on other threads.
384 *
385 * Because the sweepVotes estimate is conservative, and because
386 * nodes become unlinked "naturally" as they fall off the head of
387 * the queue, and because we allow votes to accumulate even while
388 * sweeps are in progress, there are typically significantly fewer
389 * such nodes than estimated. Choice of a threshold value
390 * balances the likelihood of wasted effort and contention, versus
391 * providing a worst-case bound on retention of interior nodes in
392 * quiescent queues. The value defined below was chosen
393 * empirically to balance these under various timeout scenarios.
394 *
395 * Because traversal operations on the linked list of nodes are a
396 * natural opportunity to sweep dead nodes, we generally do so,
397 * including all the operations that might remove elements as they
398 * traverse, such as removeIf and Iterator.remove. This largely
399 * eliminates long chains of dead interior nodes, except from
400 * cancelled or timed out blocking operations.
401 *
402 * Note that we cannot self-link unlinked interior nodes during
403 * sweeps. However, the associated garbage chains terminate when
404 * some successor ultimately falls off the head of the list and is
405 * self-linked.
406 */
407
408 /** True if on multiprocessor */
409 private static final boolean MP =
410 Runtime.getRuntime().availableProcessors() > 1;
411
412 /**
413 * The number of times to spin (with randomly interspersed calls
414 * to Thread.yield) on multiprocessor before blocking when a node
415 * is apparently the first waiter in the queue. See above for
416 * explanation. Must be a power of two. The value is empirically
417 * derived -- it works pretty well across a variety of processors,
418 * numbers of CPUs, and OSes.
419 */
420 private static final int FRONT_SPINS = 1 << 7;
421
422 /**
423 * The number of times to spin before blocking when a node is
424 * preceded by another node that is apparently spinning. Also
425 * serves as an increment to FRONT_SPINS on phase changes, and as
426 * base average frequency for yielding during spins. Must be a
427 * power of two.
428 */
429 private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
430
431 /**
432 * The maximum number of estimated removal failures (sweepVotes)
433 * to tolerate before sweeping through the queue unlinking
434 * cancelled nodes that were not unlinked upon initial
435 * removal. See above for explanation. The value must be at least
436 * two to avoid useless sweeps when removing trailing nodes.
437 */
438 static final int SWEEP_THRESHOLD = 32;
439
440 /**
441 * Queue nodes. Uses Object, not E, for items to allow forgetting
442 * them after use. Writes that are intrinsically ordered wrt
443 * other accesses or CASes use simple relaxed forms.
444 */
445 static final class Node {
446 final boolean isData; // false if this is a request node
447 volatile Object item; // initially non-null if isData; CASed to match
448 volatile Node next;
449 volatile Thread waiter; // null when not waiting for a match
450
451 /**
452 * Constructs a data node holding item if item is non-null,
453 * else a request node. Uses relaxed write because item can
454 * only be seen after piggy-backing publication via CAS.
455 */
456 Node(Object item) {
457 ITEM.set(this, item);
458 isData = (item != null);
459 }
460
461 /** Constructs a (matched data) dummy node. */
462 Node() {
463 isData = true;
464 }
465
466 final boolean casNext(Node cmp, Node val) {
467 // assert val != null;
468 return NEXT.compareAndSet(this, cmp, val);
469 }
470
471 final boolean casItem(Object cmp, Object val) {
472 // assert isData == (cmp != null);
473 // assert isData == (val == null);
474 // assert !(cmp instanceof Node);
475 return ITEM.compareAndSet(this, cmp, val);
476 }
477
478 /**
479 * Links node to itself to avoid garbage retention. Called
480 * only after CASing head field, so uses relaxed write.
481 */
482 final void selfLink() {
483 // assert isMatched();
484 NEXT.setRelease(this, this);
485 }
486
487 final void appendRelaxed(Node next) {
488 // assert next != null;
489 // assert this.next == null;
490 NEXT.set(this, next);
491 }
492
493 /**
494 * Sets item (of a request node) to self and waiter to null,
495 * to avoid garbage retention after matching or cancelling.
496 * Uses relaxed writes because order is already constrained in
497 * the only calling contexts: item is forgotten only after
498 * volatile/atomic mechanics that extract items, and visitors
499 * of request nodes only ever check whether item is null.
500 * Similarly, clearing waiter follows either CAS or return
501 * from park (if ever parked; else we don't care).
502 */
503 final void forgetContents() {
504 // assert isMatched();
505 if (!isData)
506 ITEM.set(this, this);
507 WAITER.set(this, null);
508 }
509
510 /**
511 * Returns true if this node has been matched, including the
512 * case of artificial matches due to cancellation.
513 */
514 final boolean isMatched() {
515 return isData == (item == null);
516 }
517
518 /** Tries to CAS-match this node; if successful, wakes waiter. */
519 final boolean tryMatch(Object cmp, Object val) {
520 if (casItem(cmp, val)) {
521 LockSupport.unpark(waiter);
522 return true;
523 }
524 return false;
525 }
526
527 /**
528 * Returns true if a node with the given mode cannot be
529 * appended to this node because this node is unmatched and
530 * has opposite data mode.
531 */
532 final boolean cannotPrecede(boolean haveData) {
533 boolean d = isData;
534 return d != haveData && d != (item == null);
535 }
536
537 private static final long serialVersionUID = -3375979862319811754L;
538 }
539
540 /**
541 * A node from which the first live (non-matched) node (if any)
542 * can be reached in O(1) time.
543 * Invariants:
544 * - all live nodes are reachable from head via .next
545 * - head != null
546 * - (tmp = head).next != tmp || tmp != head
547 * Non-invariants:
548 * - head may or may not be live
549 * - it is permitted for tail to lag behind head, that is, for tail
550 * to not be reachable from head!
551 */
552 transient volatile Node head;
553
554 /**
555 * A node from which the last node on list (that is, the unique
556 * node with node.next == null) can be reached in O(1) time.
557 * Invariants:
558 * - the last node is always reachable from tail via .next
559 * - tail != null
560 * Non-invariants:
561 * - tail may or may not be live
562 * - it is permitted for tail to lag behind head, that is, for tail
563 * to not be reachable from head!
564 * - tail.next may or may not be self-linked.
565 */
566 private transient volatile Node tail;
567
568 /** The number of apparent failures to unsplice cancelled nodes */
569 private transient volatile int sweepVotes;
570
571 private boolean casTail(Node cmp, Node val) {
572 // assert cmp != null;
573 // assert val != null;
574 return TAIL.compareAndSet(this, cmp, val);
575 }
576
577 private boolean casHead(Node cmp, Node val) {
578 return HEAD.compareAndSet(this, cmp, val);
579 }
580
581 /** Atomic version of ++sweepVotes. */
582 private int incSweepVotes() {
583 return (int) SWEEPVOTES.getAndAdd(this, 1) + 1;
584 }
585
586 /**
587 * Tries to CAS pred.next (or head, if pred is null) from c to p.
588 * Caller must ensure that we're not unlinking the trailing node.
589 */
590 private boolean tryCasSuccessor(Node pred, Node c, Node p) {
591 // assert p != null;
592 // assert c.isData != (c.item != null);
593 // assert c != p;
594 if (pred != null)
595 return pred.casNext(c, p);
596 if (casHead(c, p)) {
597 c.selfLink();
598 return true;
599 }
600 return false;
601 }
602
603 /**
604 * Collapses dead (matched) nodes between pred and q.
605 * @param pred the last known live node, or null if none
606 * @param c the first dead node
607 * @param p the last dead node
608 * @param q p.next: the next live node, or null if at end
609 * @return pred if pred still alive and CAS succeeded; else p
610 */
611 private Node skipDeadNodes(Node pred, Node c, Node p, Node q) {
612 // assert pred != c;
613 // assert p != q;
614 // assert c.isMatched();
615 // assert p.isMatched();
616 if (q == null) {
617 // Never unlink trailing node.
618 if (c == p) return pred;
619 q = p;
620 }
621 return (tryCasSuccessor(pred, c, q)
622 && (pred == null || !pred.isMatched()))
623 ? pred : p;
624 }
625
626 /**
627 * Collapses dead (matched) nodes from h (which was once head) to p.
628 * Caller ensures all nodes from h up to and including p are dead.
629 */
630 private void skipDeadNodesNearHead(Node h, Node p) {
631 // assert h != null;
632 // assert h != p;
633 // assert p.isMatched();
634 for (;;) {
635 final Node q;
636 if ((q = p.next) == null) break;
637 else if (!q.isMatched()) { p = q; break; }
638 else if (p == (p = q)) return;
639 }
640 if (casHead(h, p))
641 h.selfLink();
642 }
643
644 /* Possible values for "how" argument in xfer method. */
645
646 private static final int NOW = 0; // for untimed poll, tryTransfer
647 private static final int ASYNC = 1; // for offer, put, add
648 private static final int SYNC = 2; // for transfer, take
649 private static final int TIMED = 3; // for timed poll, tryTransfer
650
651 /**
652 * Implements all queuing methods. See above for explanation.
653 *
654 * @param e the item or null for take
655 * @param haveData true if this is a put, else a take
656 * @param how NOW, ASYNC, SYNC, or TIMED
657 * @param nanos timeout in nanosecs, used only if mode is TIMED
658 * @return an item if matched, else e
659 * @throws NullPointerException if haveData mode but e is null
660 */
661 @SuppressWarnings("unchecked")
662 private E xfer(E e, boolean haveData, int how, long nanos) {
663 if (haveData && (e == null))
664 throw new NullPointerException();
665
666 restart: for (Node s = null, t = null, h = null;;) {
667 for (Node p = (t != (t = tail) && t.isData == haveData) ? t
668 : (h = head);; ) {
669 final Node q; final Object item;
670 if (p.isData != haveData
671 && haveData == ((item = p.item) == null)) {
672 if (h == null) h = head;
673 if (p.tryMatch(item, e)) {
674 if (h != p) skipDeadNodesNearHead(h, p);
675 return (E) item;
676 }
677 }
678 if ((q = p.next) == null) {
679 if (how == NOW) return e;
680 if (s == null) s = new Node(e);
681 if (!p.casNext(null, s)) continue;
682 if (p != t) casTail(t, s);
683 if (how == ASYNC) return e;
684 return awaitMatch(s, p, e, (how == TIMED), nanos);
685 }
686 if (p == (p = q)) continue restart;
687 }
688 }
689 }
690
691 /**
692 * Spins/yields/blocks until node s is matched or caller gives up.
693 *
694 * @param s the waiting node
695 * @param pred the predecessor of s, or null if unknown (the null
696 * case does not occur in any current calls but may in possible
697 * future extensions)
698 * @param e the comparison value for checking match
699 * @param timed if true, wait only until timeout elapses
700 * @param nanos timeout in nanosecs, used only if timed is true
701 * @return matched item, or e if unmatched on interrupt or timeout
702 */
703 private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
704 final long deadline = timed ? System.nanoTime() + nanos : 0L;
705 Thread w = Thread.currentThread();
706 int spins = -1; // initialized after first item and cancel checks
707 ThreadLocalRandom randomYields = null; // bound if needed
708
709 for (;;) {
710 final Object item;
711 if ((item = s.item) != e) { // matched
712 // assert item != s;
713 s.forgetContents(); // avoid garbage
714 @SuppressWarnings("unchecked") E itemE = (E) item;
715 return itemE;
716 }
717 else if (w.isInterrupted() || (timed && nanos <= 0L)) {
718 // try to cancel and unlink
719 if (s.casItem(e, s.isData ? null : s)) {
720 unsplice(pred, s);
721 return e;
722 }
723 // return normally if lost CAS
724 }
725 else if (spins < 0) { // establish spins at/near front
726 if ((spins = spinsFor(pred, s.isData)) > 0)
727 randomYields = ThreadLocalRandom.current();
728 }
729 else if (spins > 0) { // spin
730 --spins;
731 if (randomYields.nextInt(CHAINED_SPINS) == 0)
732 Thread.yield(); // occasionally yield
733 }
734 else if (s.waiter == null) {
735 s.waiter = w; // request unpark then recheck
736 }
737 else if (timed) {
738 nanos = deadline - System.nanoTime();
739 if (nanos > 0L)
740 LockSupport.parkNanos(this, nanos);
741 }
742 else {
743 LockSupport.park(this);
744 }
745 }
746 }
747
748 /**
749 * Returns spin/yield value for a node with given predecessor and
750 * data mode. See above for explanation.
751 */
752 private static int spinsFor(Node pred, boolean haveData) {
753 if (MP && pred != null) {
754 if (pred.isData != haveData) // phase change
755 return FRONT_SPINS + CHAINED_SPINS;
756 if (pred.isMatched()) // probably at front
757 return FRONT_SPINS;
758 if (pred.waiter == null) // pred apparently spinning
759 return CHAINED_SPINS;
760 }
761 return 0;
762 }
763
764 /* -------------- Traversal methods -------------- */
765
766 /**
767 * Returns the first unmatched data node, or null if none.
768 * Callers must recheck if the returned node is unmatched
769 * before using.
770 */
771 final Node firstDataNode() {
772 Node first = null;
773 restartFromHead: for (;;) {
774 Node h = head, p = h;
775 for (; p != null;) {
776 final Object item;
777 if ((item = p.item) != null) {
778 if (p.isData) {
779 first = p;
780 break;
781 }
782 }
783 else if (!p.isData)
784 break;
785 final Node q;
786 if ((q = p.next) == null)
787 break;
788 if (p == (p = q))
789 continue restartFromHead;
790 }
791 if (p != h && casHead(h, p))
792 h.selfLink();
793 return first;
794 }
795 }
796
797 /**
798 * Traverses and counts unmatched nodes of the given mode.
799 * Used by methods size and getWaitingConsumerCount.
800 */
801 private int countOfMode(boolean data) {
802 restartFromHead: for (;;) {
803 int count = 0;
804 for (Node p = head; p != null;) {
805 if (!p.isMatched()) {
806 if (p.isData != data)
807 return 0;
808 if (++count == Integer.MAX_VALUE)
809 break; // @see Collection.size()
810 }
811 if (p == (p = p.next))
812 continue restartFromHead;
813 }
814 return count;
815 }
816 }
817
818 public String toString() {
819 String[] a = null;
820 restartFromHead: for (;;) {
821 int charLength = 0;
822 int size = 0;
823 for (Node p = head; p != null;) {
824 Object item = p.item;
825 if (p.isData) {
826 if (item != null) {
827 if (a == null)
828 a = new String[4];
829 else if (size == a.length)
830 a = Arrays.copyOf(a, 2 * size);
831 String s = item.toString();
832 a[size++] = s;
833 charLength += s.length();
834 }
835 } else if (item == null)
836 break;
837 if (p == (p = p.next))
838 continue restartFromHead;
839 }
840
841 if (size == 0)
842 return "[]";
843
844 return Helpers.toString(a, size, charLength);
845 }
846 }
847
848 private Object[] toArrayInternal(Object[] a) {
849 Object[] x = a;
850 restartFromHead: for (;;) {
851 int size = 0;
852 for (Node p = head; p != null;) {
853 Object item = p.item;
854 if (p.isData) {
855 if (item != null) {
856 if (x == null)
857 x = new Object[4];
858 else if (size == x.length)
859 x = Arrays.copyOf(x, 2 * (size + 4));
860 x[size++] = item;
861 }
862 } else if (item == null)
863 break;
864 if (p == (p = p.next))
865 continue restartFromHead;
866 }
867 if (x == null)
868 return new Object[0];
869 else if (a != null && size <= a.length) {
870 if (a != x)
871 System.arraycopy(x, 0, a, 0, size);
872 if (size < a.length)
873 a[size] = null;
874 return a;
875 }
876 return (size == x.length) ? x : Arrays.copyOf(x, size);
877 }
878 }
879
880 /**
881 * Returns an array containing all of the elements in this queue, in
882 * proper sequence.
883 *
884 * <p>The returned array will be "safe" in that no references to it are
885 * maintained by this queue. (In other words, this method must allocate
886 * a new array). The caller is thus free to modify the returned array.
887 *
888 * <p>This method acts as bridge between array-based and collection-based
889 * APIs.
890 *
891 * @return an array containing all of the elements in this queue
892 */
893 public Object[] toArray() {
894 return toArrayInternal(null);
895 }
896
897 /**
898 * Returns an array containing all of the elements in this queue, in
899 * proper sequence; the runtime type of the returned array is that of
900 * the specified array. If the queue fits in the specified array, it
901 * is returned therein. Otherwise, a new array is allocated with the
902 * runtime type of the specified array and the size of this queue.
903 *
904 * <p>If this queue fits in the specified array with room to spare
905 * (i.e., the array has more elements than this queue), the element in
906 * the array immediately following the end of the queue is set to
907 * {@code null}.
908 *
909 * <p>Like the {@link #toArray()} method, this method acts as bridge between
910 * array-based and collection-based APIs. Further, this method allows
911 * precise control over the runtime type of the output array, and may,
912 * under certain circumstances, be used to save allocation costs.
913 *
914 * <p>Suppose {@code x} is a queue known to contain only strings.
915 * The following code can be used to dump the queue into a newly
916 * allocated array of {@code String}:
917 *
918 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
919 *
920 * Note that {@code toArray(new Object[0])} is identical in function to
921 * {@code toArray()}.
922 *
923 * @param a the array into which the elements of the queue are to
924 * be stored, if it is big enough; otherwise, a new array of the
925 * same runtime type is allocated for this purpose
926 * @return an array containing all of the elements in this queue
927 * @throws ArrayStoreException if the runtime type of the specified array
928 * is not a supertype of the runtime type of every element in
929 * this queue
930 * @throws NullPointerException if the specified array is null
931 */
932 @SuppressWarnings("unchecked")
933 public <T> T[] toArray(T[] a) {
934 Objects.requireNonNull(a);
935 return (T[]) toArrayInternal(a);
936 }
937
938 /**
939 * Weakly-consistent iterator.
940 *
941 * Lazily updated ancestor is expected to be amortized O(1) remove(),
942 * but O(n) in the worst case, when lastRet is concurrently deleted.
943 */
944 final class Itr implements Iterator<E> {
945 private Node nextNode; // next node to return item for
946 private E nextItem; // the corresponding item
947 private Node lastRet; // last returned node, to support remove
948 private Node ancestor; // Helps unlink lastRet on remove()
949
950 /**
951 * Moves to next node after pred, or first node if pred null.
952 */
953 @SuppressWarnings("unchecked")
954 private void advance(Node pred) {
955 for (Node p = (pred == null) ? head : pred.next, c = p;
956 p != null; ) {
957 final Object item;
958 if ((item = p.item) != null && p.isData) {
959 nextNode = p;
960 nextItem = (E) item;
961 if (c != p)
962 tryCasSuccessor(pred, c, p);
963 return;
964 }
965 else if (!p.isData && item == null)
966 break;
967 if (c != p && !tryCasSuccessor(pred, c, c = p)) {
968 pred = p;
969 c = p = p.next;
970 }
971 else if (p == (p = p.next)) {
972 pred = null;
973 c = p = head;
974 }
975 }
976 nextItem = null;
977 nextNode = null;
978 }
979
980 Itr() {
981 advance(null);
982 }
983
984 public final boolean hasNext() {
985 return nextNode != null;
986 }
987
988 public final E next() {
989 final Node p;
990 if ((p = nextNode) == null) throw new NoSuchElementException();
991 E e = nextItem;
992 advance(lastRet = p);
993 return e;
994 }
995
996 public void forEachRemaining(Consumer<? super E> action) {
997 Objects.requireNonNull(action);
998 Node q = null;
999 for (Node p; (p = nextNode) != null; advance(q = p))
1000 action.accept(nextItem);
1001 if (q != null)
1002 lastRet = q;
1003 }
1004
1005 public final void remove() {
1006 final Node lastRet = this.lastRet;
1007 if (lastRet == null)
1008 throw new IllegalStateException();
1009 this.lastRet = null;
1010 if (lastRet.item == null) // already deleted?
1011 return;
1012 // Advance ancestor, collapsing intervening dead nodes
1013 Node pred = ancestor;
1014 for (Node p = (pred == null) ? head : pred.next, c = p, q;
1015 p != null; ) {
1016 if (p == lastRet) {
1017 final Object item;
1018 if ((item = p.item) != null)
1019 p.tryMatch(item, null);
1020 if ((q = p.next) == null) q = p;
1021 if (c != q) tryCasSuccessor(pred, c, q);
1022 ancestor = pred;
1023 return;
1024 }
1025 final Object item; final boolean pAlive;
1026 if (pAlive = ((item = p.item) != null && p.isData)) {
1027 // exceptionally, nothing to do
1028 }
1029 else if (!p.isData && item == null)
1030 break;
1031 if ((c != p && !tryCasSuccessor(pred, c, c = p)) || pAlive) {
1032 pred = p;
1033 c = p = p.next;
1034 }
1035 else if (p == (p = p.next)) {
1036 pred = null;
1037 c = p = head;
1038 }
1039 }
1040 // traversal failed to find lastRet; must have been deleted;
1041 // leave ancestor at original location to avoid overshoot;
1042 // better luck next time!
1043
1044 // assert lastRet.isMatched();
1045 }
1046 }
1047
1048 /** A customized variant of Spliterators.IteratorSpliterator */
1049 final class LTQSpliterator implements Spliterator<E> {
1050 static final int MAX_BATCH = 1 << 25; // max batch array size;
1051 Node current; // current node; null until initialized
1052 int batch; // batch size for splits
1053 boolean exhausted; // true when no more nodes
1054 LTQSpliterator() {}
1055
1056 public Spliterator<E> trySplit() {
1057 Node p, q;
1058 if ((p = current()) == null || (q = p.next) == null)
1059 return null;
1060 int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH);
1061 Object[] a = null;
1062 do {
1063 final Object item = p.item;
1064 if (p.isData) {
1065 if (item != null) {
1066 if (a == null)
1067 a = new Object[n];
1068 a[i++] = item;
1069 }
1070 } else if (item == null) {
1071 p = null;
1072 break;
1073 }
1074 if (p == (p = q))
1075 p = firstDataNode();
1076 } while (p != null && (q = p.next) != null && i < n);
1077 setCurrent(p);
1078 return (i == 0) ? null :
1079 Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED |
1080 Spliterator.NONNULL |
1081 Spliterator.CONCURRENT));
1082 }
1083
1084 public void forEachRemaining(Consumer<? super E> action) {
1085 Objects.requireNonNull(action);
1086 final Node p;
1087 if ((p = current()) != null) {
1088 current = null;
1089 exhausted = true;
1090 forEachFrom(action, p);
1091 }
1092 }
1093
1094 @SuppressWarnings("unchecked")
1095 public boolean tryAdvance(Consumer<? super E> action) {
1096 Objects.requireNonNull(action);
1097 Node p;
1098 if ((p = current()) != null) {
1099 E e = null;
1100 do {
1101 final Object item = p.item;
1102 final boolean isData = p.isData;
1103 if (p == (p = p.next))
1104 p = head;
1105 if (isData) {
1106 if (item != null) {
1107 e = (E) item;
1108 break;
1109 }
1110 }
1111 else if (item == null)
1112 p = null;
1113 } while (p != null);
1114 setCurrent(p);
1115 if (e != null) {
1116 action.accept(e);
1117 return true;
1118 }
1119 }
1120 return false;
1121 }
1122
1123 private void setCurrent(Node p) {
1124 if ((current = p) == null)
1125 exhausted = true;
1126 }
1127
1128 private Node current() {
1129 Node p;
1130 if ((p = current) == null && !exhausted)
1131 setCurrent(p = firstDataNode());
1132 return p;
1133 }
1134
1135 public long estimateSize() { return Long.MAX_VALUE; }
1136
1137 public int characteristics() {
1138 return (Spliterator.ORDERED |
1139 Spliterator.NONNULL |
1140 Spliterator.CONCURRENT);
1141 }
1142 }
1143
1144 /**
1145 * Returns a {@link Spliterator} over the elements in this queue.
1146 *
1147 * <p>The returned spliterator is
1148 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1149 *
1150 * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
1151 * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
1152 *
1153 * @implNote
1154 * The {@code Spliterator} implements {@code trySplit} to permit limited
1155 * parallelism.
1156 *
1157 * @return a {@code Spliterator} over the elements in this queue
1158 * @since 1.8
1159 */
1160 public Spliterator<E> spliterator() {
1161 return new LTQSpliterator();
1162 }
1163
1164 /* -------------- Removal methods -------------- */
1165
1166 /**
1167 * Unsplices (now or later) the given deleted/cancelled node with
1168 * the given predecessor.
1169 *
1170 * @param pred a node that was at one time known to be the
1171 * predecessor of s
1172 * @param s the node to be unspliced
1173 */
1174 final void unsplice(Node pred, Node s) {
1175 // assert pred != null;
1176 // assert pred != s;
1177 // assert s != null;
1178 // assert s.isMatched();
1179 // assert (SWEEP_THRESHOLD & (SWEEP_THRESHOLD - 1)) == 0;
1180 s.waiter = null; // disable signals
1181 /*
1182 * See above for rationale. Briefly: if pred still points to
1183 * s, try to unlink s. If s cannot be unlinked, because it is
1184 * trailing node or pred might be unlinked, and neither pred
1185 * nor s are head or offlist, add to sweepVotes, and if enough
1186 * votes have accumulated, sweep.
1187 */
1188 if (pred != null && pred.next == s) {
1189 Node n = s.next;
1190 if (n == null ||
1191 (n != s && pred.casNext(s, n) && pred.isMatched())) {
1192 for (;;) { // check if at, or could be, head
1193 Node h = head;
1194 if (h == pred || h == s)
1195 return; // at head or list empty
1196 if (!h.isMatched())
1197 break;
1198 Node hn = h.next;
1199 if (hn == null)
1200 return; // now empty
1201 if (hn != h && casHead(h, hn))
1202 h.selfLink(); // advance head
1203 }
1204 // sweep every SWEEP_THRESHOLD votes
1205 if (pred.next != pred && s.next != s // recheck if offlist
1206 && (incSweepVotes() & (SWEEP_THRESHOLD - 1)) == 0)
1207 sweep();
1208 }
1209 }
1210 }
1211
1212 /**
1213 * Unlinks matched (typically cancelled) nodes encountered in a
1214 * traversal from head.
1215 */
1216 private void sweep() {
1217 for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
1218 if (!s.isMatched())
1219 // Unmatched nodes are never self-linked
1220 p = s;
1221 else if ((n = s.next) == null) // trailing node is pinned
1222 break;
1223 else if (s == n) // stale
1224 // No need to also check for p == s, since that implies s == n
1225 p = head;
1226 else
1227 p.casNext(s, n);
1228 }
1229 }
1230
1231 /**
1232 * Creates an initially empty {@code LinkedTransferQueue}.
1233 */
1234 public LinkedTransferQueue() {
1235 head = tail = new Node();
1236 }
1237
1238 /**
1239 * Creates a {@code LinkedTransferQueue}
1240 * initially containing the elements of the given collection,
1241 * added in traversal order of the collection's iterator.
1242 *
1243 * @param c the collection of elements to initially contain
1244 * @throws NullPointerException if the specified collection or any
1245 * of its elements are null
1246 */
1247 public LinkedTransferQueue(Collection<? extends E> c) {
1248 Node h = null, t = null;
1249 for (E e : c) {
1250 Node newNode = new Node(Objects.requireNonNull(e));
1251 if (h == null)
1252 h = t = newNode;
1253 else
1254 t.appendRelaxed(t = newNode);
1255 }
1256 if (h == null)
1257 h = t = new Node();
1258 head = h;
1259 tail = t;
1260 }
1261
1262 /**
1263 * Inserts the specified element at the tail of this queue.
1264 * As the queue is unbounded, this method will never block.
1265 *
1266 * @throws NullPointerException if the specified element is null
1267 */
1268 public void put(E e) {
1269 xfer(e, true, ASYNC, 0);
1270 }
1271
1272 /**
1273 * Inserts the specified element at the tail of this queue.
1274 * As the queue is unbounded, this method will never block or
1275 * return {@code false}.
1276 *
1277 * @return {@code true} (as specified by
1278 * {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit)
1279 * BlockingQueue.offer})
1280 * @throws NullPointerException if the specified element is null
1281 */
1282 public boolean offer(E e, long timeout, TimeUnit unit) {
1283 xfer(e, true, ASYNC, 0);
1284 return true;
1285 }
1286
1287 /**
1288 * Inserts the specified element at the tail of this queue.
1289 * As the queue is unbounded, this method will never return {@code false}.
1290 *
1291 * @return {@code true} (as specified by {@link Queue#offer})
1292 * @throws NullPointerException if the specified element is null
1293 */
1294 public boolean offer(E e) {
1295 xfer(e, true, ASYNC, 0);
1296 return true;
1297 }
1298
1299 /**
1300 * Inserts the specified element at the tail of this queue.
1301 * As the queue is unbounded, this method will never throw
1302 * {@link IllegalStateException} or return {@code false}.
1303 *
1304 * @return {@code true} (as specified by {@link Collection#add})
1305 * @throws NullPointerException if the specified element is null
1306 */
1307 public boolean add(E e) {
1308 xfer(e, true, ASYNC, 0);
1309 return true;
1310 }
1311
1312 /**
1313 * Transfers the element to a waiting consumer immediately, if possible.
1314 *
1315 * <p>More precisely, transfers the specified element immediately
1316 * if there exists a consumer already waiting to receive it (in
1317 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1318 * otherwise returning {@code false} without enqueuing the element.
1319 *
1320 * @throws NullPointerException if the specified element is null
1321 */
1322 public boolean tryTransfer(E e) {
1323 return xfer(e, true, NOW, 0) == null;
1324 }
1325
1326 /**
1327 * Transfers the element to a consumer, waiting if necessary to do so.
1328 *
1329 * <p>More precisely, transfers the specified element immediately
1330 * if there exists a consumer already waiting to receive it (in
1331 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1332 * else inserts the specified element at the tail of this queue
1333 * and waits until the element is received by a consumer.
1334 *
1335 * @throws NullPointerException if the specified element is null
1336 */
1337 public void transfer(E e) throws InterruptedException {
1338 if (xfer(e, true, SYNC, 0) != null) {
1339 Thread.interrupted(); // failure possible only due to interrupt
1340 throw new InterruptedException();
1341 }
1342 }
1343
1344 /**
1345 * Transfers the element to a consumer if it is possible to do so
1346 * before the timeout elapses.
1347 *
1348 * <p>More precisely, transfers the specified element immediately
1349 * if there exists a consumer already waiting to receive it (in
1350 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1351 * else inserts the specified element at the tail of this queue
1352 * and waits until the element is received by a consumer,
1353 * returning {@code false} if the specified wait time elapses
1354 * before the element can be transferred.
1355 *
1356 * @throws NullPointerException if the specified element is null
1357 */
1358 public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1359 throws InterruptedException {
1360 if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
1361 return true;
1362 if (!Thread.interrupted())
1363 return false;
1364 throw new InterruptedException();
1365 }
1366
1367 public E take() throws InterruptedException {
1368 E e = xfer(null, false, SYNC, 0);
1369 if (e != null)
1370 return e;
1371 Thread.interrupted();
1372 throw new InterruptedException();
1373 }
1374
1375 public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1376 E e = xfer(null, false, TIMED, unit.toNanos(timeout));
1377 if (e != null || !Thread.interrupted())
1378 return e;
1379 throw new InterruptedException();
1380 }
1381
1382 public E poll() {
1383 return xfer(null, false, NOW, 0);
1384 }
1385
1386 /**
1387 * @throws NullPointerException {@inheritDoc}
1388 * @throws IllegalArgumentException {@inheritDoc}
1389 */
1390 public int drainTo(Collection<? super E> c) {
1391 Objects.requireNonNull(c);
1392 if (c == this)
1393 throw new IllegalArgumentException();
1394 int n = 0;
1395 for (E e; (e = poll()) != null; n++)
1396 c.add(e);
1397 return n;
1398 }
1399
1400 /**
1401 * @throws NullPointerException {@inheritDoc}
1402 * @throws IllegalArgumentException {@inheritDoc}
1403 */
1404 public int drainTo(Collection<? super E> c, int maxElements) {
1405 Objects.requireNonNull(c);
1406 if (c == this)
1407 throw new IllegalArgumentException();
1408 int n = 0;
1409 for (E e; n < maxElements && (e = poll()) != null; n++)
1410 c.add(e);
1411 return n;
1412 }
1413
1414 /**
1415 * Returns an iterator over the elements in this queue in proper sequence.
1416 * The elements will be returned in order from first (head) to last (tail).
1417 *
1418 * <p>The returned iterator is
1419 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1420 *
1421 * @return an iterator over the elements in this queue in proper sequence
1422 */
1423 public Iterator<E> iterator() {
1424 return new Itr();
1425 }
1426
1427 public E peek() {
1428 restartFromHead: for (;;) {
1429 for (Node p = head; p != null;) {
1430 Object item = p.item;
1431 if (p.isData) {
1432 if (item != null) {
1433 @SuppressWarnings("unchecked") E e = (E) item;
1434 return e;
1435 }
1436 }
1437 else if (item == null)
1438 break;
1439 if (p == (p = p.next))
1440 continue restartFromHead;
1441 }
1442 return null;
1443 }
1444 }
1445
1446 /**
1447 * Returns {@code true} if this queue contains no elements.
1448 *
1449 * @return {@code true} if this queue contains no elements
1450 */
1451 public boolean isEmpty() {
1452 return firstDataNode() == null;
1453 }
1454
1455 public boolean hasWaitingConsumer() {
1456 restartFromHead: for (;;) {
1457 for (Node p = head; p != null;) {
1458 Object item = p.item;
1459 if (p.isData) {
1460 if (item != null)
1461 break;
1462 }
1463 else if (item == null)
1464 return true;
1465 if (p == (p = p.next))
1466 continue restartFromHead;
1467 }
1468 return false;
1469 }
1470 }
1471
1472 /**
1473 * Returns the number of elements in this queue. If this queue
1474 * contains more than {@code Integer.MAX_VALUE} elements, returns
1475 * {@code Integer.MAX_VALUE}.
1476 *
1477 * <p>Beware that, unlike in most collections, this method is
1478 * <em>NOT</em> a constant-time operation. Because of the
1479 * asynchronous nature of these queues, determining the current
1480 * number of elements requires an O(n) traversal.
1481 *
1482 * @return the number of elements in this queue
1483 */
1484 public int size() {
1485 return countOfMode(true);
1486 }
1487
1488 public int getWaitingConsumerCount() {
1489 return countOfMode(false);
1490 }
1491
1492 /**
1493 * Removes a single instance of the specified element from this queue,
1494 * if it is present. More formally, removes an element {@code e} such
1495 * that {@code o.equals(e)}, if this queue contains one or more such
1496 * elements.
1497 * Returns {@code true} if this queue contained the specified element
1498 * (or equivalently, if this queue changed as a result of the call).
1499 *
1500 * @param o element to be removed from this queue, if present
1501 * @return {@code true} if this queue changed as a result of the call
1502 */
1503 public boolean remove(Object o) {
1504 if (o == null) return false;
1505 restartFromHead: for (;;) {
1506 for (Node p = head, pred = null; p != null; ) {
1507 Node q = p.next;
1508 final Object item;
1509 if ((item = p.item) != null) {
1510 if (p.isData) {
1511 if (o.equals(item) && p.tryMatch(item, null)) {
1512 skipDeadNodes(pred, p, p, q);
1513 return true;
1514 }
1515 pred = p; p = q; continue;
1516 }
1517 }
1518 else if (!p.isData)
1519 break;
1520 for (Node c = p;; q = p.next) {
1521 if (q == null || !q.isMatched()) {
1522 pred = skipDeadNodes(pred, c, p, q); p = q; break;
1523 }
1524 if (p == (p = q)) continue restartFromHead;
1525 }
1526 }
1527 return false;
1528 }
1529 }
1530
1531 /**
1532 * Returns {@code true} if this queue contains the specified element.
1533 * More formally, returns {@code true} if and only if this queue contains
1534 * at least one element {@code e} such that {@code o.equals(e)}.
1535 *
1536 * @param o object to be checked for containment in this queue
1537 * @return {@code true} if this queue contains the specified element
1538 */
1539 public boolean contains(Object o) {
1540 if (o == null) return false;
1541 restartFromHead: for (;;) {
1542 for (Node p = head, pred = null; p != null; ) {
1543 Node q = p.next;
1544 final Object item;
1545 if ((item = p.item) != null) {
1546 if (p.isData) {
1547 if (o.equals(item))
1548 return true;
1549 pred = p; p = q; continue;
1550 }
1551 }
1552 else if (!p.isData)
1553 break;
1554 for (Node c = p;; q = p.next) {
1555 if (q == null || !q.isMatched()) {
1556 pred = skipDeadNodes(pred, c, p, q); p = q; break;
1557 }
1558 if (p == (p = q)) continue restartFromHead;
1559 }
1560 }
1561 return false;
1562 }
1563 }
1564
1565 /**
1566 * Always returns {@code Integer.MAX_VALUE} because a
1567 * {@code LinkedTransferQueue} is not capacity constrained.
1568 *
1569 * @return {@code Integer.MAX_VALUE} (as specified by
1570 * {@link java.util.concurrent.BlockingQueue#remainingCapacity()
1571 * BlockingQueue.remainingCapacity})
1572 */
1573 public int remainingCapacity() {
1574 return Integer.MAX_VALUE;
1575 }
1576
1577 /**
1578 * Saves this queue to a stream (that is, serializes it).
1579 *
1580 * @param s the stream
1581 * @throws java.io.IOException if an I/O error occurs
1582 * @serialData All of the elements (each an {@code E}) in
1583 * the proper order, followed by a null
1584 */
1585 private void writeObject(java.io.ObjectOutputStream s)
1586 throws java.io.IOException {
1587 s.defaultWriteObject();
1588 for (E e : this)
1589 s.writeObject(e);
1590 // Use trailing null as sentinel
1591 s.writeObject(null);
1592 }
1593
1594 /**
1595 * Reconstitutes this queue from a stream (that is, deserializes it).
1596 * @param s the stream
1597 * @throws ClassNotFoundException if the class of a serialized object
1598 * could not be found
1599 * @throws java.io.IOException if an I/O error occurs
1600 */
1601 private void readObject(java.io.ObjectInputStream s)
1602 throws java.io.IOException, ClassNotFoundException {
1603
1604 // Read in elements until trailing null sentinel found
1605 Node h = null, t = null;
1606 for (Object item; (item = s.readObject()) != null; ) {
1607 @SuppressWarnings("unchecked")
1608 Node newNode = new Node((E) item);
1609 if (h == null)
1610 h = t = newNode;
1611 else
1612 t.appendRelaxed(t = newNode);
1613 }
1614 if (h == null)
1615 h = t = new Node();
1616 head = h;
1617 tail = t;
1618 }
1619
1620 /**
1621 * @throws NullPointerException {@inheritDoc}
1622 */
1623 public boolean removeIf(Predicate<? super E> filter) {
1624 Objects.requireNonNull(filter);
1625 return bulkRemove(filter);
1626 }
1627
1628 /**
1629 * @throws NullPointerException {@inheritDoc}
1630 */
1631 public boolean removeAll(Collection<?> c) {
1632 Objects.requireNonNull(c);
1633 return bulkRemove(e -> c.contains(e));
1634 }
1635
1636 /**
1637 * @throws NullPointerException {@inheritDoc}
1638 */
1639 public boolean retainAll(Collection<?> c) {
1640 Objects.requireNonNull(c);
1641 return bulkRemove(e -> !c.contains(e));
1642 }
1643
1644 public void clear() {
1645 bulkRemove(e -> true);
1646 }
1647
1648 /**
1649 * Tolerate this many consecutive dead nodes before CAS-collapsing.
1650 * Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element.
1651 */
1652 private static final int MAX_HOPS = 8;
1653
1654 /** Implementation of bulk remove methods. */
1655 @SuppressWarnings("unchecked")
1656 private boolean bulkRemove(Predicate<? super E> filter) {
1657 boolean removed = false;
1658 restartFromHead: for (;;) {
1659 int hops = MAX_HOPS;
1660 // c will be CASed to collapse intervening dead nodes between
1661 // pred (or head if null) and p.
1662 for (Node p = head, c = p, pred = null, q; p != null; p = q) {
1663 q = p.next;
1664 final Object item; boolean pAlive;
1665 if (pAlive = ((item = p.item) != null && p.isData)) {
1666 if (filter.test((E) item)) {
1667 if (p.tryMatch(item, null))
1668 removed = true;
1669 pAlive = false;
1670 }
1671 }
1672 else if (!p.isData && item == null)
1673 break;
1674 if (pAlive || q == null || --hops == 0) {
1675 // p might already be self-linked here, but if so:
1676 // - CASing head will surely fail
1677 // - CASing pred's next will be useless but harmless.
1678 if ((c != p && !tryCasSuccessor(pred, c, c = p))
1679 || pAlive) {
1680 // if CAS failed or alive, abandon old pred
1681 hops = MAX_HOPS;
1682 pred = p;
1683 c = q;
1684 }
1685 } else if (p == q)
1686 continue restartFromHead;
1687 }
1688 return removed;
1689 }
1690 }
1691
1692 /**
1693 * Runs action on each element found during a traversal starting at p.
1694 * If p is null, the action is not run.
1695 */
1696 @SuppressWarnings("unchecked")
1697 void forEachFrom(Consumer<? super E> action, Node p) {
1698 for (Node pred = null; p != null; ) {
1699 Node q = p.next;
1700 final Object item;
1701 if ((item = p.item) != null) {
1702 if (p.isData) {
1703 action.accept((E) item);
1704 pred = p; p = q; continue;
1705 }
1706 }
1707 else if (!p.isData)
1708 break;
1709 for (Node c = p;; q = p.next) {
1710 if (q == null || !q.isMatched()) {
1711 pred = skipDeadNodes(pred, c, p, q); p = q; break;
1712 }
1713 if (p == (p = q)) { pred = null; p = head; break; }
1714 }
1715 }
1716 }
1717
1718 /**
1719 * @throws NullPointerException {@inheritDoc}
1720 */
1721 public void forEach(Consumer<? super E> action) {
1722 Objects.requireNonNull(action);
1723 forEachFrom(action, head);
1724 }
1725
1726 // VarHandle mechanics
1727 private static final VarHandle HEAD;
1728 private static final VarHandle TAIL;
1729 private static final VarHandle SWEEPVOTES;
1730 static final VarHandle ITEM;
1731 static final VarHandle NEXT;
1732 static final VarHandle WAITER;
1733 static {
1734 try {
1735 MethodHandles.Lookup l = MethodHandles.lookup();
1736 HEAD = l.findVarHandle(LinkedTransferQueue.class, "head",
1737 Node.class);
1738 TAIL = l.findVarHandle(LinkedTransferQueue.class, "tail",
1739 Node.class);
1740 SWEEPVOTES = l.findVarHandle(LinkedTransferQueue.class, "sweepVotes",
1741 int.class);
1742 ITEM = l.findVarHandle(Node.class, "item", Object.class);
1743 NEXT = l.findVarHandle(Node.class, "next", Node.class);
1744 WAITER = l.findVarHandle(Node.class, "waiter", Thread.class);
1745 } catch (ReflectiveOperationException e) {
1746 throw new Error(e);
1747 }
1748
1749 // Reduce the risk of rare disastrous classloading in first call to
1750 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
1751 Class<?> ensureLoaded = LockSupport.class;
1752 }
1753 }