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.util.AbstractQueue;
39 import java.util.Arrays;
40 import java.util.Collection;
41 import java.util.Iterator;
42 import java.util.NoSuchElementException;
43 import java.util.Queue;
44 import java.util.Spliterator;
45 import java.util.Spliterators;
46 import java.util.concurrent.locks.LockSupport;
47 import java.util.function.Consumer;
48
49 /**
50 * An unbounded {@link TransferQueue} based on linked nodes.
51 * This queue orders elements FIFO (first-in-first-out) with respect
52 * to any given producer. The <em>head</em> of the queue is that
53 * element that has been on the queue the longest time for some
54 * producer. The <em>tail</em> of the queue is that element that has
55 * been on the queue the shortest time for some producer.
56 *
57 * <p>Beware that, unlike in most collections, the {@code size} method
58 * is <em>NOT</em> a constant-time operation. Because of the
59 * asynchronous nature of these queues, determining the current number
60 * of elements requires a traversal of the elements, and so may report
61 * inaccurate results if this collection is modified during traversal.
62 * Additionally, the bulk operations {@code addAll},
63 * {@code removeAll}, {@code retainAll}, {@code containsAll},
64 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
65 * to be performed atomically. For example, an iterator operating
66 * concurrently with an {@code addAll} operation might view only some
67 * of the added elements.
68 *
69 * <p>This class and its iterator implement all of the
70 * <em>optional</em> methods of the {@link Collection} and {@link
71 * Iterator} interfaces.
72 *
73 * <p>Memory consistency effects: As with other concurrent
74 * collections, actions in a thread prior to placing an object into a
75 * {@code LinkedTransferQueue}
76 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
77 * actions subsequent to the access or removal of that element from
78 * the {@code LinkedTransferQueue} in another thread.
79 *
80 * <p>This class is a member of the
81 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
82 * Java Collections Framework</a>.
83 *
84 * @since 1.7
85 * @author Doug Lea
86 * @param <E> the type of elements held in this queue
87 */
88 public class LinkedTransferQueue<E> extends AbstractQueue<E>
89 implements TransferQueue<E>, java.io.Serializable {
90 private static final long serialVersionUID = -3223113410248163686L;
91
92 /*
93 * *** Overview of Dual Queues with Slack ***
94 *
95 * Dual Queues, introduced by Scherer and Scott
96 * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
97 * (linked) queues in which nodes may represent either data or
98 * requests. When a thread tries to enqueue a data node, but
99 * encounters a request node, it instead "matches" and removes it;
100 * and vice versa for enqueuing requests. Blocking Dual Queues
101 * arrange that threads enqueuing unmatched requests block until
102 * other threads provide the match. Dual Synchronous Queues (see
103 * Scherer, Lea, & Scott
104 * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
105 * additionally arrange that threads enqueuing unmatched data also
106 * block. Dual Transfer Queues support all of these modes, as
107 * dictated by callers.
108 *
109 * A FIFO dual queue may be implemented using a variation of the
110 * Michael & Scott (M&S) lock-free queue algorithm
111 * (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf).
112 * It maintains two pointer fields, "head", pointing to a
113 * (matched) node that in turn points to the first actual
114 * (unmatched) queue node (or null if empty); and "tail" that
115 * points to the last node on the queue (or again null if
116 * empty). For example, here is a possible queue with four data
117 * elements:
118 *
119 * head tail
120 * | |
121 * v v
122 * M -> U -> U -> U -> U
123 *
124 * The M&S queue algorithm is known to be prone to scalability and
125 * overhead limitations when maintaining (via CAS) these head and
126 * tail pointers. This has led to the development of
127 * contention-reducing variants such as elimination arrays (see
128 * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
129 * optimistic back pointers (see Ladan-Mozes & Shavit
130 * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
131 * However, the nature of dual queues enables a simpler tactic for
132 * improving M&S-style implementations when dual-ness is needed.
133 *
134 * In a dual queue, each node must atomically maintain its match
135 * status. While there are other possible variants, we implement
136 * this here as: for a data-mode node, matching entails CASing an
137 * "item" field from a non-null data value to null upon match, and
138 * vice-versa for request nodes, CASing from null to a data
139 * value. (Note that the linearization properties of this style of
140 * queue are easy to verify -- elements are made available by
141 * linking, and unavailable by matching.) Compared to plain M&S
142 * queues, this property of dual queues requires one additional
143 * successful atomic operation per enq/deq pair. But it also
144 * enables lower cost variants of queue maintenance mechanics. (A
145 * variation of this idea applies even for non-dual queues that
146 * support deletion of interior elements, such as
147 * j.u.c.ConcurrentLinkedQueue.)
148 *
149 * Once a node is matched, its match status can never again
150 * change. We may thus arrange that the linked list of them
151 * contain a prefix of zero or more matched nodes, followed by a
152 * suffix of zero or more unmatched nodes. (Note that we allow
153 * both the prefix and suffix to be zero length, which in turn
154 * means that we do not use a dummy header.) If we were not
155 * concerned with either time or space efficiency, we could
156 * correctly perform enqueue and dequeue operations by traversing
157 * from a pointer to the initial node; CASing the item of the
158 * first unmatched node on match and CASing the next field of the
159 * trailing node on appends. (Plus some special-casing when
160 * initially empty). While this would be a terrible idea in
161 * itself, it does have the benefit of not requiring ANY atomic
162 * updates on head/tail fields.
163 *
164 * We introduce here an approach that lies between the extremes of
165 * never versus always updating queue (head and tail) pointers.
166 * This offers a tradeoff between sometimes requiring extra
167 * traversal steps to locate the first and/or last unmatched
168 * nodes, versus the reduced overhead and contention of fewer
169 * updates to queue pointers. For example, a possible snapshot of
170 * a queue is:
171 *
172 * head tail
173 * | |
174 * v v
175 * M -> M -> U -> U -> U -> U
176 *
177 * The best value for this "slack" (the targeted maximum distance
178 * between the value of "head" and the first unmatched node, and
179 * similarly for "tail") is an empirical matter. We have found
180 * that using very small constants in the range of 1-3 work best
181 * over a range of platforms. Larger values introduce increasing
182 * costs of cache misses and risks of long traversal chains, while
183 * smaller values increase CAS contention and overhead.
184 *
185 * Dual queues with slack differ from plain M&S dual queues by
186 * virtue of only sometimes updating head or tail pointers when
187 * matching, appending, or even traversing nodes; in order to
188 * maintain a targeted slack. The idea of "sometimes" may be
189 * operationalized in several ways. The simplest is to use a
190 * per-operation counter incremented on each traversal step, and
191 * to try (via CAS) to update the associated queue pointer
192 * whenever the count exceeds a threshold. Another, that requires
193 * more overhead, is to use random number generators to update
194 * with a given probability per traversal step.
195 *
196 * In any strategy along these lines, because CASes updating
197 * fields may fail, the actual slack may exceed targeted
198 * slack. However, they may be retried at any time to maintain
199 * targets. Even when using very small slack values, this
200 * approach works well for dual queues because it allows all
201 * operations up to the point of matching or appending an item
202 * (hence potentially allowing progress by another thread) to be
203 * read-only, thus not introducing any further contention. As
204 * described below, we implement this by performing slack
205 * maintenance retries only after these points.
206 *
207 * As an accompaniment to such techniques, traversal overhead can
208 * be further reduced without increasing contention of head
209 * pointer updates: Threads may sometimes shortcut the "next" link
210 * path from the current "head" node to be closer to the currently
211 * known first unmatched node, and similarly for tail. Again, this
212 * may be triggered with using thresholds or randomization.
213 *
214 * These ideas must be further extended to avoid unbounded amounts
215 * of costly-to-reclaim garbage caused by the sequential "next"
216 * links of nodes starting at old forgotten head nodes: As first
217 * described in detail by Boehm
218 * (http://portal.acm.org/citation.cfm?doid=503272.503282), if a GC
219 * delays noticing that any arbitrarily old node has become
220 * garbage, all newer dead nodes will also be unreclaimed.
221 * (Similar issues arise in non-GC environments.) To cope with
222 * this in our implementation, upon CASing to advance the head
223 * pointer, we set the "next" link of the previous head to point
224 * only to itself; thus limiting the length of connected dead lists.
225 * (We also take similar care to wipe out possibly garbage
226 * retaining values held in other Node fields.) However, doing so
227 * adds some further complexity to traversal: If any "next"
228 * pointer links to itself, it indicates that the current thread
229 * has lagged behind a head-update, and so the traversal must
230 * continue from the "head". Traversals trying to find the
231 * current tail starting from "tail" may also encounter
232 * self-links, in which case they also continue at "head".
233 *
234 * It is tempting in slack-based scheme to not even use CAS for
235 * updates (similarly to Ladan-Mozes & Shavit). However, this
236 * cannot be done for head updates under the above link-forgetting
237 * mechanics because an update may leave head at a detached node.
238 * And while direct writes are possible for tail updates, they
239 * increase the risk of long retraversals, and hence long garbage
240 * chains, which can be much more costly than is worthwhile
241 * considering that the cost difference of performing a CAS vs
242 * write is smaller when they are not triggered on each operation
243 * (especially considering that writes and CASes equally require
244 * additional GC bookkeeping ("write barriers") that are sometimes
245 * more costly than the writes themselves because of contention).
246 *
247 * *** Overview of implementation ***
248 *
249 * We use a threshold-based approach to updates, with a slack
250 * threshold of two -- that is, we update head/tail when the
251 * current pointer appears to be two or more steps away from the
252 * first/last node. The slack value is hard-wired: a path greater
253 * than one is naturally implemented by checking equality of
254 * traversal pointers except when the list has only one element,
255 * in which case we keep slack threshold at one. Avoiding tracking
256 * explicit counts across method calls slightly simplifies an
257 * already-messy implementation. Using randomization would
258 * probably work better if there were a low-quality dirt-cheap
259 * per-thread one available, but even ThreadLocalRandom is too
260 * heavy for these purposes.
261 *
262 * With such a small slack threshold value, it is not worthwhile
263 * to augment this with path short-circuiting (i.e., unsplicing
264 * interior nodes) except in the case of cancellation/removal (see
265 * below).
266 *
267 * We allow both the head and tail fields to be null before any
268 * nodes are enqueued; initializing upon first append. This
269 * simplifies some other logic, as well as providing more
270 * efficient explicit control paths instead of letting JVMs insert
271 * implicit NullPointerExceptions when they are null. While not
272 * currently fully implemented, we also leave open the possibility
273 * of re-nulling these fields when empty (which is complicated to
274 * arrange, for little benefit.)
275 *
276 * All enqueue/dequeue operations are handled by the single method
277 * "xfer" with parameters indicating whether to act as some form
278 * of offer, put, poll, take, or transfer (each possibly with
279 * timeout). The relative complexity of using one monolithic
280 * method outweighs the code bulk and maintenance problems of
281 * using separate methods for each case.
282 *
283 * Operation consists of up to three phases. The first is
284 * implemented within method xfer, the second in tryAppend, and
285 * the third in method awaitMatch.
286 *
287 * 1. Try to match an existing node
288 *
289 * Starting at head, skip already-matched nodes until finding
290 * an unmatched node of opposite mode, if one exists, in which
291 * case matching it and returning, also if necessary updating
292 * head to one past the matched node (or the node itself if the
293 * list has no other unmatched nodes). If the CAS misses, then
294 * a loop retries advancing head by two steps until either
295 * success or the slack is at most two. By requiring that each
296 * attempt advances head by two (if applicable), we ensure that
297 * the slack does not grow without bound. Traversals also check
298 * if the initial head is now off-list, in which case they
299 * start at the new head.
300 *
301 * If no candidates are found and the call was untimed
302 * poll/offer, (argument "how" is NOW) return.
303 *
304 * 2. Try to append a new node (method tryAppend)
305 *
306 * Starting at current tail pointer, find the actual last node
307 * and try to append a new node (or if head was null, establish
308 * the first node). Nodes can be appended only if their
309 * predecessors are either already matched or are of the same
310 * mode. If we detect otherwise, then a new node with opposite
311 * mode must have been appended during traversal, so we must
312 * restart at phase 1. The traversal and update steps are
313 * otherwise similar to phase 1: Retrying upon CAS misses and
314 * checking for staleness. In particular, if a self-link is
315 * encountered, then we can safely jump to a node on the list
316 * by continuing the traversal at current head.
317 *
318 * On successful append, if the call was ASYNC, return.
319 *
320 * 3. Await match or cancellation (method awaitMatch)
321 *
322 * Wait for another thread to match node; instead cancelling if
323 * the current thread was interrupted or the wait timed out. On
324 * multiprocessors, we use front-of-queue spinning: If a node
325 * appears to be the first unmatched node in the queue, it
326 * spins a bit before blocking. In either case, before blocking
327 * it tries to unsplice any nodes between the current "head"
328 * and the first unmatched node.
329 *
330 * Front-of-queue spinning vastly improves performance of
331 * heavily contended queues. And so long as it is relatively
332 * brief and "quiet", spinning does not much impact performance
333 * of less-contended queues. During spins threads check their
334 * interrupt status and generate a thread-local random number
335 * to decide to occasionally perform a Thread.yield. While
336 * yield has underdefined specs, we assume that it might help,
337 * and will not hurt, in limiting impact of spinning on busy
338 * systems. We also use smaller (1/2) spins for nodes that are
339 * not known to be front but whose predecessors have not
340 * blocked -- these "chained" spins avoid artifacts of
341 * front-of-queue rules which otherwise lead to alternating
342 * nodes spinning vs blocking. Further, front threads that
343 * represent phase changes (from data to request node or vice
344 * versa) compared to their predecessors receive additional
345 * chained spins, reflecting longer paths typically required to
346 * unblock threads during phase changes.
347 *
348 *
349 * ** Unlinking removed interior nodes **
350 *
351 * In addition to minimizing garbage retention via self-linking
352 * described above, we also unlink removed interior nodes. These
353 * may arise due to timed out or interrupted waits, or calls to
354 * remove(x) or Iterator.remove. Normally, given a node that was
355 * at one time known to be the predecessor of some node s that is
356 * to be removed, we can unsplice s by CASing the next field of
357 * its predecessor if it still points to s (otherwise s must
358 * already have been removed or is now offlist). But there are two
359 * situations in which we cannot guarantee to make node s
360 * unreachable in this way: (1) If s is the trailing node of list
361 * (i.e., with null next), then it is pinned as the target node
362 * for appends, so can only be removed later after other nodes are
363 * appended. (2) We cannot necessarily unlink s given a
364 * predecessor node that is matched (including the case of being
365 * cancelled): the predecessor may already be unspliced, in which
366 * case some previous reachable node may still point to s.
367 * (For further explanation see Herlihy & Shavit "The Art of
368 * Multiprocessor Programming" chapter 9). Although, in both
369 * cases, we can rule out the need for further action if either s
370 * or its predecessor are (or can be made to be) at, or fall off
371 * from, the head of list.
372 *
373 * Without taking these into account, it would be possible for an
374 * unbounded number of supposedly removed nodes to remain
375 * reachable. Situations leading to such buildup are uncommon but
376 * can occur in practice; for example when a series of short timed
377 * calls to poll repeatedly time out but never otherwise fall off
378 * the list because of an untimed call to take at the front of the
379 * queue.
380 *
381 * When these cases arise, rather than always retraversing the
382 * entire list to find an actual predecessor to unlink (which
383 * won't help for case (1) anyway), we record a conservative
384 * estimate of possible unsplice failures (in "sweepVotes").
385 * We trigger a full sweep when the estimate exceeds a threshold
386 * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
387 * removal failures to tolerate before sweeping through, unlinking
388 * cancelled nodes that were not unlinked upon initial removal.
389 * We perform sweeps by the thread hitting threshold (rather than
390 * background threads or by spreading work to other threads)
391 * because in the main contexts in which removal occurs, the
392 * caller is already timed-out, cancelled, or performing a
393 * potentially O(n) operation (e.g. remove(x)), none of which are
394 * time-critical enough to warrant the overhead that alternatives
395 * would impose on other threads.
396 *
397 * Because the sweepVotes estimate is conservative, and because
398 * nodes become unlinked "naturally" as they fall off the head of
399 * the queue, and because we allow votes to accumulate even while
400 * sweeps are in progress, there are typically significantly fewer
401 * such nodes than estimated. Choice of a threshold value
402 * balances the likelihood of wasted effort and contention, versus
403 * providing a worst-case bound on retention of interior nodes in
404 * quiescent queues. The value defined below was chosen
405 * empirically to balance these under various timeout scenarios.
406 *
407 * Note that we cannot self-link unlinked interior nodes during
408 * sweeps. However, the associated garbage chains terminate when
409 * some successor ultimately falls off the head of the list and is
410 * self-linked.
411 */
412
413 /** True if on multiprocessor */
414 private static final boolean MP =
415 Runtime.getRuntime().availableProcessors() > 1;
416
417 /**
418 * The number of times to spin (with randomly interspersed calls
419 * to Thread.yield) on multiprocessor before blocking when a node
420 * is apparently the first waiter in the queue. See above for
421 * explanation. Must be a power of two. The value is empirically
422 * derived -- it works pretty well across a variety of processors,
423 * numbers of CPUs, and OSes.
424 */
425 private static final int FRONT_SPINS = 1 << 7;
426
427 /**
428 * The number of times to spin before blocking when a node is
429 * preceded by another node that is apparently spinning. Also
430 * serves as an increment to FRONT_SPINS on phase changes, and as
431 * base average frequency for yielding during spins. Must be a
432 * power of two.
433 */
434 private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
435
436 /**
437 * The maximum number of estimated removal failures (sweepVotes)
438 * to tolerate before sweeping through the queue unlinking
439 * cancelled nodes that were not unlinked upon initial
440 * removal. See above for explanation. The value must be at least
441 * two to avoid useless sweeps when removing trailing nodes.
442 */
443 static final int SWEEP_THRESHOLD = 32;
444
445 /**
446 * Queue nodes. Uses Object, not E, for items to allow forgetting
447 * them after use. Relies heavily on Unsafe mechanics to minimize
448 * unnecessary ordering constraints: Writes that are intrinsically
449 * ordered wrt other accesses or CASes use simple relaxed forms.
450 */
451 static final class Node {
452 final boolean isData; // false if this is a request node
453 volatile Object item; // initially non-null if isData; CASed to match
454 volatile Node next;
455 volatile Thread waiter; // null until waiting
456
457 // CAS methods for fields
458 final boolean casNext(Node cmp, Node val) {
459 return U.compareAndSwapObject(this, NEXT, cmp, val);
460 }
461
462 final boolean casItem(Object cmp, Object val) {
463 // assert cmp == null || cmp.getClass() != Node.class;
464 return U.compareAndSwapObject(this, ITEM, cmp, val);
465 }
466
467 /**
468 * Constructs a new node. Uses relaxed write because item can
469 * only be seen after publication via casNext.
470 */
471 Node(Object item, boolean isData) {
472 U.putObject(this, ITEM, item); // relaxed write
473 this.isData = isData;
474 }
475
476 /**
477 * Links node to itself to avoid garbage retention. Called
478 * only after CASing head field, so uses relaxed write.
479 */
480 final void forgetNext() {
481 U.putObject(this, NEXT, this);
482 }
483
484 /**
485 * Sets item to self and waiter to null, to avoid garbage
486 * retention after matching or cancelling. Uses relaxed writes
487 * because order is already constrained in the only calling
488 * contexts: item is forgotten only after volatile/atomic
489 * mechanics that extract items. Similarly, clearing waiter
490 * follows either CAS or return from park (if ever parked;
491 * else we don't care).
492 */
493 final void forgetContents() {
494 U.putObject(this, ITEM, this);
495 U.putObject(this, WAITER, null);
496 }
497
498 /**
499 * Returns true if this node has been matched, including the
500 * case of artificial matches due to cancellation.
501 */
502 final boolean isMatched() {
503 Object x = item;
504 return (x == this) || ((x == null) == isData);
505 }
506
507 /**
508 * Returns true if this is an unmatched request node.
509 */
510 final boolean isUnmatchedRequest() {
511 return !isData && item == null;
512 }
513
514 /**
515 * Returns true if a node with the given mode cannot be
516 * appended to this node because this node is unmatched and
517 * has opposite data mode.
518 */
519 final boolean cannotPrecede(boolean haveData) {
520 boolean d = isData;
521 Object x;
522 return d != haveData && (x = item) != this && (x != null) == d;
523 }
524
525 /**
526 * Tries to artificially match a data node -- used by remove.
527 */
528 final boolean tryMatchData() {
529 // assert isData;
530 Object x = item;
531 if (x != null && x != this && casItem(x, null)) {
532 LockSupport.unpark(waiter);
533 return true;
534 }
535 return false;
536 }
537
538 private static final long serialVersionUID = -3375979862319811754L;
539
540 // Unsafe mechanics
541 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
542 private static final long ITEM;
543 private static final long NEXT;
544 private static final long WAITER;
545 static {
546 try {
547 ITEM = U.objectFieldOffset
548 (Node.class.getDeclaredField("item"));
549 NEXT = U.objectFieldOffset
550 (Node.class.getDeclaredField("next"));
551 WAITER = U.objectFieldOffset
552 (Node.class.getDeclaredField("waiter"));
553 } catch (ReflectiveOperationException e) {
554 throw new Error(e);
555 }
556 }
557 }
558
559 /** head of the queue; null until first enqueue */
560 transient volatile Node head;
561
562 /** tail of the queue; null until first append */
563 private transient volatile Node tail;
564
565 /** The number of apparent failures to unsplice removed nodes */
566 private transient volatile int sweepVotes;
567
568 // CAS methods for fields
569 private boolean casTail(Node cmp, Node val) {
570 return U.compareAndSwapObject(this, TAIL, cmp, val);
571 }
572
573 private boolean casHead(Node cmp, Node val) {
574 return U.compareAndSwapObject(this, HEAD, cmp, val);
575 }
576
577 private boolean casSweepVotes(int cmp, int val) {
578 return U.compareAndSwapInt(this, SWEEPVOTES, cmp, val);
579 }
580
581 /*
582 * Possible values for "how" argument in xfer method.
583 */
584 private static final int NOW = 0; // for untimed poll, tryTransfer
585 private static final int ASYNC = 1; // for offer, put, add
586 private static final int SYNC = 2; // for transfer, take
587 private static final int TIMED = 3; // for timed poll, tryTransfer
588
589 /**
590 * Implements all queuing methods. See above for explanation.
591 *
592 * @param e the item or null for take
593 * @param haveData true if this is a put, else a take
594 * @param how NOW, ASYNC, SYNC, or TIMED
595 * @param nanos timeout in nanosecs, used only if mode is TIMED
596 * @return an item if matched, else e
597 * @throws NullPointerException if haveData mode but e is null
598 */
599 private E xfer(E e, boolean haveData, int how, long nanos) {
600 if (haveData && (e == null))
601 throw new NullPointerException();
602 Node s = null; // the node to append, if needed
603
604 retry:
605 for (;;) { // restart on append race
606
607 for (Node h = head, p = h; p != null;) { // find & match first node
608 boolean isData = p.isData;
609 Object item = p.item;
610 if (item != p && (item != null) == isData) { // unmatched
611 if (isData == haveData) // can't match
612 break;
613 if (p.casItem(item, e)) { // match
614 for (Node q = p; q != h;) {
615 Node n = q.next; // update by 2 unless singleton
616 if (head == h && casHead(h, n == null ? q : n)) {
617 h.forgetNext();
618 break;
619 } // advance and retry
620 if ((h = head) == null ||
621 (q = h.next) == null || !q.isMatched())
622 break; // unless slack < 2
623 }
624 LockSupport.unpark(p.waiter);
625 @SuppressWarnings("unchecked") E itemE = (E) item;
626 return itemE;
627 }
628 }
629 Node n = p.next;
630 p = (p != n) ? n : (h = head); // Use head if p offlist
631 }
632
633 if (how != NOW) { // No matches available
634 if (s == null)
635 s = new Node(e, haveData);
636 Node pred = tryAppend(s, haveData);
637 if (pred == null)
638 continue retry; // lost race vs opposite mode
639 if (how != ASYNC)
640 return awaitMatch(s, pred, e, (how == TIMED), nanos);
641 }
642 return e; // not waiting
643 }
644 }
645
646 /**
647 * Tries to append node s as tail.
648 *
649 * @param s the node to append
650 * @param haveData true if appending in data mode
651 * @return null on failure due to losing race with append in
652 * different mode, else s's predecessor, or s itself if no
653 * predecessor
654 */
655 private Node tryAppend(Node s, boolean haveData) {
656 for (Node t = tail, p = t;;) { // move p to last node and append
657 Node n, u; // temps for reads of next & tail
658 if (p == null && (p = head) == null) {
659 if (casHead(null, s))
660 return s; // initialize
661 }
662 else if (p.cannotPrecede(haveData))
663 return null; // lost race vs opposite mode
664 else if ((n = p.next) != null) // not last; keep traversing
665 p = p != t && t != (u = tail) ? (t = u) : // stale tail
666 (p != n) ? n : null; // restart if off list
667 else if (!p.casNext(null, s))
668 p = p.next; // re-read on CAS failure
669 else {
670 if (p != t) { // update if slack now >= 2
671 while ((tail != t || !casTail(t, s)) &&
672 (t = tail) != null &&
673 (s = t.next) != null && // advance and retry
674 (s = s.next) != null && s != t);
675 }
676 return p;
677 }
678 }
679 }
680
681 /**
682 * Spins/yields/blocks until node s is matched or caller gives up.
683 *
684 * @param s the waiting node
685 * @param pred the predecessor of s, or s itself if it has no
686 * predecessor, or null if unknown (the null case does not occur
687 * in any current calls but may in possible future extensions)
688 * @param e the comparison value for checking match
689 * @param timed if true, wait only until timeout elapses
690 * @param nanos timeout in nanosecs, used only if timed is true
691 * @return matched item, or e if unmatched on interrupt or timeout
692 */
693 private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
694 final long deadline = timed ? System.nanoTime() + nanos : 0L;
695 Thread w = Thread.currentThread();
696 int spins = -1; // initialized after first item and cancel checks
697 ThreadLocalRandom randomYields = null; // bound if needed
698
699 for (;;) {
700 Object item = s.item;
701 if (item != e) { // matched
702 // assert item != s;
703 s.forgetContents(); // avoid garbage
704 @SuppressWarnings("unchecked") E itemE = (E) item;
705 return itemE;
706 }
707 else if (w.isInterrupted() || (timed && nanos <= 0L)) {
708 unsplice(pred, s); // try to unlink and cancel
709 if (s.casItem(e, s)) // return normally if lost CAS
710 return e;
711 }
712 else if (spins < 0) { // establish spins at/near front
713 if ((spins = spinsFor(pred, s.isData)) > 0)
714 randomYields = ThreadLocalRandom.current();
715 }
716 else if (spins > 0) { // spin
717 --spins;
718 if (randomYields.nextInt(CHAINED_SPINS) == 0)
719 Thread.yield(); // occasionally yield
720 }
721 else if (s.waiter == null) {
722 s.waiter = w; // request unpark then recheck
723 }
724 else if (timed) {
725 nanos = deadline - System.nanoTime();
726 if (nanos > 0L)
727 LockSupport.parkNanos(this, nanos);
728 }
729 else {
730 LockSupport.park(this);
731 }
732 }
733 }
734
735 /**
736 * Returns spin/yield value for a node with given predecessor and
737 * data mode. See above for explanation.
738 */
739 private static int spinsFor(Node pred, boolean haveData) {
740 if (MP && pred != null) {
741 if (pred.isData != haveData) // phase change
742 return FRONT_SPINS + CHAINED_SPINS;
743 if (pred.isMatched()) // probably at front
744 return FRONT_SPINS;
745 if (pred.waiter == null) // pred apparently spinning
746 return CHAINED_SPINS;
747 }
748 return 0;
749 }
750
751 /* -------------- Traversal methods -------------- */
752
753 /**
754 * Returns the successor of p, or the head node if p.next has been
755 * linked to self, which will only be true if traversing with a
756 * stale pointer that is now off the list.
757 */
758 final Node succ(Node p) {
759 Node next = p.next;
760 return (p == next) ? head : next;
761 }
762
763 /**
764 * Returns the first unmatched data node, or null if none.
765 * Callers must recheck if the returned node's item field is null
766 * or self-linked before using.
767 */
768 final Node firstDataNode() {
769 restartFromHead: for (;;) {
770 for (Node p = head; p != null;) {
771 Object item = p.item;
772 if (p.isData) {
773 if (item != null && item != p)
774 return p;
775 }
776 else if (item == null)
777 break;
778 if (p == (p = p.next))
779 continue restartFromHead;
780 }
781 return null;
782 }
783 }
784
785 /**
786 * Traverses and counts unmatched nodes of the given mode.
787 * Used by methods size and getWaitingConsumerCount.
788 */
789 private int countOfMode(boolean data) {
790 restartFromHead: for (;;) {
791 int count = 0;
792 for (Node p = head; p != null;) {
793 if (!p.isMatched()) {
794 if (p.isData != data)
795 return 0;
796 if (++count == Integer.MAX_VALUE)
797 break; // @see Collection.size()
798 }
799 if (p == (p = p.next))
800 continue restartFromHead;
801 }
802 return count;
803 }
804 }
805
806 public String toString() {
807 String[] a = null;
808 restartFromHead: for (;;) {
809 int charLength = 0;
810 int size = 0;
811 for (Node p = head; p != null;) {
812 Object item = p.item;
813 if (p.isData) {
814 if (item != null && item != p) {
815 if (a == null)
816 a = new String[4];
817 else if (size == a.length)
818 a = Arrays.copyOf(a, 2 * size);
819 String s = item.toString();
820 a[size++] = s;
821 charLength += s.length();
822 }
823 } else if (item == null)
824 break;
825 if (p == (p = p.next))
826 continue restartFromHead;
827 }
828
829 if (size == 0)
830 return "[]";
831
832 return Helpers.toString(a, size, charLength);
833 }
834 }
835
836 private Object[] toArrayInternal(Object[] a) {
837 Object[] x = a;
838 restartFromHead: for (;;) {
839 int size = 0;
840 for (Node p = head; p != null;) {
841 Object item = p.item;
842 if (p.isData) {
843 if (item != null && item != p) {
844 if (x == null)
845 x = new Object[4];
846 else if (size == x.length)
847 x = Arrays.copyOf(x, 2 * (size + 4));
848 x[size++] = item;
849 }
850 } else if (item == null)
851 break;
852 if (p == (p = p.next))
853 continue restartFromHead;
854 }
855 if (x == null)
856 return new Object[0];
857 else if (a != null && size <= a.length) {
858 if (a != x)
859 System.arraycopy(x, 0, a, 0, size);
860 if (size < a.length)
861 a[size] = null;
862 return a;
863 }
864 return (size == x.length) ? x : Arrays.copyOf(x, size);
865 }
866 }
867
868 /**
869 * Returns an array containing all of the elements in this queue, in
870 * proper sequence.
871 *
872 * <p>The returned array will be "safe" in that no references to it are
873 * maintained by this queue. (In other words, this method must allocate
874 * a new array). The caller is thus free to modify the returned array.
875 *
876 * <p>This method acts as bridge between array-based and collection-based
877 * APIs.
878 *
879 * @return an array containing all of the elements in this queue
880 */
881 public Object[] toArray() {
882 return toArrayInternal(null);
883 }
884
885 /**
886 * Returns an array containing all of the elements in this queue, in
887 * proper sequence; the runtime type of the returned array is that of
888 * the specified array. If the queue fits in the specified array, it
889 * is returned therein. Otherwise, a new array is allocated with the
890 * runtime type of the specified array and the size of this queue.
891 *
892 * <p>If this queue fits in the specified array with room to spare
893 * (i.e., the array has more elements than this queue), the element in
894 * the array immediately following the end of the queue is set to
895 * {@code null}.
896 *
897 * <p>Like the {@link #toArray()} method, this method acts as bridge between
898 * array-based and collection-based APIs. Further, this method allows
899 * precise control over the runtime type of the output array, and may,
900 * under certain circumstances, be used to save allocation costs.
901 *
902 * <p>Suppose {@code x} is a queue known to contain only strings.
903 * The following code can be used to dump the queue into a newly
904 * allocated array of {@code String}:
905 *
906 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
907 *
908 * Note that {@code toArray(new Object[0])} is identical in function to
909 * {@code toArray()}.
910 *
911 * @param a the array into which the elements of the queue are to
912 * be stored, if it is big enough; otherwise, a new array of the
913 * same runtime type is allocated for this purpose
914 * @return an array containing all of the elements in this queue
915 * @throws ArrayStoreException if the runtime type of the specified array
916 * is not a supertype of the runtime type of every element in
917 * this queue
918 * @throws NullPointerException if the specified array is null
919 */
920 @SuppressWarnings("unchecked")
921 public <T> T[] toArray(T[] a) {
922 if (a == null) throw new NullPointerException();
923 return (T[]) toArrayInternal(a);
924 }
925
926 final class Itr implements Iterator<E> {
927 private Node nextNode; // next node to return item for
928 private E nextItem; // the corresponding item
929 private Node lastRet; // last returned node, to support remove
930 private Node lastPred; // predecessor to unlink lastRet
931
932 /**
933 * Moves to next node after prev, or first node if prev null.
934 */
935 private void advance(Node prev) {
936 /*
937 * To track and avoid buildup of deleted nodes in the face
938 * of calls to both Queue.remove and Itr.remove, we must
939 * include variants of unsplice and sweep upon each
940 * advance: Upon Itr.remove, we may need to catch up links
941 * from lastPred, and upon other removes, we might need to
942 * skip ahead from stale nodes and unsplice deleted ones
943 * found while advancing.
944 */
945
946 Node r, b; // reset lastPred upon possible deletion of lastRet
947 if ((r = lastRet) != null && !r.isMatched())
948 lastPred = r; // next lastPred is old lastRet
949 else if ((b = lastPred) == null || b.isMatched())
950 lastPred = null; // at start of list
951 else {
952 Node s, n; // help with removal of lastPred.next
953 while ((s = b.next) != null &&
954 s != b && s.isMatched() &&
955 (n = s.next) != null && n != s)
956 b.casNext(s, n);
957 }
958
959 this.lastRet = prev;
960
961 for (Node p = prev, s, n;;) {
962 s = (p == null) ? head : p.next;
963 if (s == null)
964 break;
965 else if (s == p) {
966 p = null;
967 continue;
968 }
969 Object item = s.item;
970 if (s.isData) {
971 if (item != null && item != s) {
972 @SuppressWarnings("unchecked") E itemE = (E) item;
973 nextItem = itemE;
974 nextNode = s;
975 return;
976 }
977 }
978 else if (item == null)
979 break;
980 // assert s.isMatched();
981 if (p == null)
982 p = s;
983 else if ((n = s.next) == null)
984 break;
985 else if (s == n)
986 p = null;
987 else
988 p.casNext(s, n);
989 }
990 nextNode = null;
991 nextItem = null;
992 }
993
994 Itr() {
995 advance(null);
996 }
997
998 public final boolean hasNext() {
999 return nextNode != null;
1000 }
1001
1002 public final E next() {
1003 Node p = nextNode;
1004 if (p == null) throw new NoSuchElementException();
1005 E e = nextItem;
1006 advance(p);
1007 return e;
1008 }
1009
1010 public final void remove() {
1011 final Node lastRet = this.lastRet;
1012 if (lastRet == null)
1013 throw new IllegalStateException();
1014 this.lastRet = null;
1015 if (lastRet.tryMatchData())
1016 unsplice(lastPred, lastRet);
1017 }
1018 }
1019
1020 /** A customized variant of Spliterators.IteratorSpliterator */
1021 final class LTQSpliterator<E> implements Spliterator<E> {
1022 static final int MAX_BATCH = 1 << 25; // max batch array size;
1023 Node current; // current node; null until initialized
1024 int batch; // batch size for splits
1025 boolean exhausted; // true when no more nodes
1026 LTQSpliterator() {}
1027
1028 public Spliterator<E> trySplit() {
1029 Node p;
1030 int b = batch;
1031 int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
1032 if (!exhausted &&
1033 ((p = current) != null || (p = firstDataNode()) != null) &&
1034 p.next != null) {
1035 Object[] a = new Object[n];
1036 int i = 0;
1037 do {
1038 Object e = p.item;
1039 if (e != p && (a[i] = e) != null)
1040 ++i;
1041 if (p == (p = p.next))
1042 p = firstDataNode();
1043 } while (p != null && i < n && p.isData);
1044 if ((current = p) == null)
1045 exhausted = true;
1046 if (i > 0) {
1047 batch = i;
1048 return Spliterators.spliterator
1049 (a, 0, i, (Spliterator.ORDERED |
1050 Spliterator.NONNULL |
1051 Spliterator.CONCURRENT));
1052 }
1053 }
1054 return null;
1055 }
1056
1057 @SuppressWarnings("unchecked")
1058 public void forEachRemaining(Consumer<? super E> action) {
1059 Node p;
1060 if (action == null) throw new NullPointerException();
1061 if (!exhausted &&
1062 ((p = current) != null || (p = firstDataNode()) != null)) {
1063 exhausted = true;
1064 do {
1065 Object e = p.item;
1066 if (e != null && e != p)
1067 action.accept((E)e);
1068 if (p == (p = p.next))
1069 p = firstDataNode();
1070 } while (p != null && p.isData);
1071 }
1072 }
1073
1074 @SuppressWarnings("unchecked")
1075 public boolean tryAdvance(Consumer<? super E> action) {
1076 Node p;
1077 if (action == null) throw new NullPointerException();
1078 if (!exhausted &&
1079 ((p = current) != null || (p = firstDataNode()) != null)) {
1080 Object e;
1081 do {
1082 if ((e = p.item) == p)
1083 e = null;
1084 if (p == (p = p.next))
1085 p = firstDataNode();
1086 } while (e == null && p != null && p.isData);
1087 if ((current = p) == null)
1088 exhausted = true;
1089 if (e != null) {
1090 action.accept((E)e);
1091 return true;
1092 }
1093 }
1094 return false;
1095 }
1096
1097 public long estimateSize() { return Long.MAX_VALUE; }
1098
1099 public int characteristics() {
1100 return Spliterator.ORDERED | Spliterator.NONNULL |
1101 Spliterator.CONCURRENT;
1102 }
1103 }
1104
1105 /**
1106 * Returns a {@link Spliterator} over the elements in this queue.
1107 *
1108 * <p>The returned spliterator is
1109 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1110 *
1111 * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
1112 * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
1113 *
1114 * @implNote
1115 * The {@code Spliterator} implements {@code trySplit} to permit limited
1116 * parallelism.
1117 *
1118 * @return a {@code Spliterator} over the elements in this queue
1119 * @since 1.8
1120 */
1121 public Spliterator<E> spliterator() {
1122 return new LTQSpliterator<E>();
1123 }
1124
1125 /* -------------- Removal methods -------------- */
1126
1127 /**
1128 * Unsplices (now or later) the given deleted/cancelled node with
1129 * the given predecessor.
1130 *
1131 * @param pred a node that was at one time known to be the
1132 * predecessor of s, or null or s itself if s is/was at head
1133 * @param s the node to be unspliced
1134 */
1135 final void unsplice(Node pred, Node s) {
1136 s.waiter = null; // disable signals
1137 /*
1138 * See above for rationale. Briefly: if pred still points to
1139 * s, try to unlink s. If s cannot be unlinked, because it is
1140 * trailing node or pred might be unlinked, and neither pred
1141 * nor s are head or offlist, add to sweepVotes, and if enough
1142 * votes have accumulated, sweep.
1143 */
1144 if (pred != null && pred != s && pred.next == s) {
1145 Node n = s.next;
1146 if (n == null ||
1147 (n != s && pred.casNext(s, n) && pred.isMatched())) {
1148 for (;;) { // check if at, or could be, head
1149 Node h = head;
1150 if (h == pred || h == s || h == null)
1151 return; // at head or list empty
1152 if (!h.isMatched())
1153 break;
1154 Node hn = h.next;
1155 if (hn == null)
1156 return; // now empty
1157 if (hn != h && casHead(h, hn))
1158 h.forgetNext(); // advance head
1159 }
1160 if (pred.next != pred && s.next != s) { // recheck if offlist
1161 for (;;) { // sweep now if enough votes
1162 int v = sweepVotes;
1163 if (v < SWEEP_THRESHOLD) {
1164 if (casSweepVotes(v, v + 1))
1165 break;
1166 }
1167 else if (casSweepVotes(v, 0)) {
1168 sweep();
1169 break;
1170 }
1171 }
1172 }
1173 }
1174 }
1175 }
1176
1177 /**
1178 * Unlinks matched (typically cancelled) nodes encountered in a
1179 * traversal from head.
1180 */
1181 private void sweep() {
1182 for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
1183 if (!s.isMatched())
1184 // Unmatched nodes are never self-linked
1185 p = s;
1186 else if ((n = s.next) == null) // trailing node is pinned
1187 break;
1188 else if (s == n) // stale
1189 // No need to also check for p == s, since that implies s == n
1190 p = head;
1191 else
1192 p.casNext(s, n);
1193 }
1194 }
1195
1196 /**
1197 * Main implementation of remove(Object)
1198 */
1199 private boolean findAndRemove(Object e) {
1200 if (e != null) {
1201 for (Node pred = null, p = head; p != null; ) {
1202 Object item = p.item;
1203 if (p.isData) {
1204 if (item != null && item != p && e.equals(item) &&
1205 p.tryMatchData()) {
1206 unsplice(pred, p);
1207 return true;
1208 }
1209 }
1210 else if (item == null)
1211 break;
1212 pred = p;
1213 if ((p = p.next) == pred) { // stale
1214 pred = null;
1215 p = head;
1216 }
1217 }
1218 }
1219 return false;
1220 }
1221
1222 /**
1223 * Creates an initially empty {@code LinkedTransferQueue}.
1224 */
1225 public LinkedTransferQueue() {
1226 }
1227
1228 /**
1229 * Creates a {@code LinkedTransferQueue}
1230 * initially containing the elements of the given collection,
1231 * added in traversal order of the collection's iterator.
1232 *
1233 * @param c the collection of elements to initially contain
1234 * @throws NullPointerException if the specified collection or any
1235 * of its elements are null
1236 */
1237 public LinkedTransferQueue(Collection<? extends E> c) {
1238 this();
1239 addAll(c);
1240 }
1241
1242 /**
1243 * Inserts the specified element at the tail of this queue.
1244 * As the queue is unbounded, this method will never block.
1245 *
1246 * @throws NullPointerException if the specified element is null
1247 */
1248 public void put(E e) {
1249 xfer(e, true, ASYNC, 0);
1250 }
1251
1252 /**
1253 * Inserts the specified element at the tail of this queue.
1254 * As the queue is unbounded, this method will never block or
1255 * return {@code false}.
1256 *
1257 * @return {@code true} (as specified by
1258 * {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit)
1259 * BlockingQueue.offer})
1260 * @throws NullPointerException if the specified element is null
1261 */
1262 public boolean offer(E e, long timeout, TimeUnit unit) {
1263 xfer(e, true, ASYNC, 0);
1264 return true;
1265 }
1266
1267 /**
1268 * Inserts the specified element at the tail of this queue.
1269 * As the queue is unbounded, this method will never return {@code false}.
1270 *
1271 * @return {@code true} (as specified by {@link Queue#offer})
1272 * @throws NullPointerException if the specified element is null
1273 */
1274 public boolean offer(E e) {
1275 xfer(e, true, ASYNC, 0);
1276 return true;
1277 }
1278
1279 /**
1280 * Inserts the specified element at the tail of this queue.
1281 * As the queue is unbounded, this method will never throw
1282 * {@link IllegalStateException} or return {@code false}.
1283 *
1284 * @return {@code true} (as specified by {@link Collection#add})
1285 * @throws NullPointerException if the specified element is null
1286 */
1287 public boolean add(E e) {
1288 xfer(e, true, ASYNC, 0);
1289 return true;
1290 }
1291
1292 /**
1293 * Transfers the element to a waiting consumer immediately, if possible.
1294 *
1295 * <p>More precisely, transfers the specified element immediately
1296 * if there exists a consumer already waiting to receive it (in
1297 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1298 * otherwise returning {@code false} without enqueuing the element.
1299 *
1300 * @throws NullPointerException if the specified element is null
1301 */
1302 public boolean tryTransfer(E e) {
1303 return xfer(e, true, NOW, 0) == null;
1304 }
1305
1306 /**
1307 * Transfers the element to a consumer, waiting if necessary to do so.
1308 *
1309 * <p>More precisely, transfers the specified element immediately
1310 * if there exists a consumer already waiting to receive it (in
1311 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1312 * else inserts the specified element at the tail of this queue
1313 * and waits until the element is received by a consumer.
1314 *
1315 * @throws NullPointerException if the specified element is null
1316 */
1317 public void transfer(E e) throws InterruptedException {
1318 if (xfer(e, true, SYNC, 0) != null) {
1319 Thread.interrupted(); // failure possible only due to interrupt
1320 throw new InterruptedException();
1321 }
1322 }
1323
1324 /**
1325 * Transfers the element to a consumer if it is possible to do so
1326 * before the timeout elapses.
1327 *
1328 * <p>More precisely, transfers the specified element immediately
1329 * if there exists a consumer already waiting to receive it (in
1330 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1331 * else inserts the specified element at the tail of this queue
1332 * and waits until the element is received by a consumer,
1333 * returning {@code false} if the specified wait time elapses
1334 * before the element can be transferred.
1335 *
1336 * @throws NullPointerException if the specified element is null
1337 */
1338 public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1339 throws InterruptedException {
1340 if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
1341 return true;
1342 if (!Thread.interrupted())
1343 return false;
1344 throw new InterruptedException();
1345 }
1346
1347 public E take() throws InterruptedException {
1348 E e = xfer(null, false, SYNC, 0);
1349 if (e != null)
1350 return e;
1351 Thread.interrupted();
1352 throw new InterruptedException();
1353 }
1354
1355 public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1356 E e = xfer(null, false, TIMED, unit.toNanos(timeout));
1357 if (e != null || !Thread.interrupted())
1358 return e;
1359 throw new InterruptedException();
1360 }
1361
1362 public E poll() {
1363 return xfer(null, false, NOW, 0);
1364 }
1365
1366 /**
1367 * @throws NullPointerException {@inheritDoc}
1368 * @throws IllegalArgumentException {@inheritDoc}
1369 */
1370 public int drainTo(Collection<? super E> c) {
1371 if (c == null)
1372 throw new NullPointerException();
1373 if (c == this)
1374 throw new IllegalArgumentException();
1375 int n = 0;
1376 for (E e; (e = poll()) != null;) {
1377 c.add(e);
1378 ++n;
1379 }
1380 return n;
1381 }
1382
1383 /**
1384 * @throws NullPointerException {@inheritDoc}
1385 * @throws IllegalArgumentException {@inheritDoc}
1386 */
1387 public int drainTo(Collection<? super E> c, int maxElements) {
1388 if (c == null)
1389 throw new NullPointerException();
1390 if (c == this)
1391 throw new IllegalArgumentException();
1392 int n = 0;
1393 for (E e; n < maxElements && (e = poll()) != null;) {
1394 c.add(e);
1395 ++n;
1396 }
1397 return n;
1398 }
1399
1400 /**
1401 * Returns an iterator over the elements in this queue in proper sequence.
1402 * The elements will be returned in order from first (head) to last (tail).
1403 *
1404 * <p>The returned iterator is
1405 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1406 *
1407 * @return an iterator over the elements in this queue in proper sequence
1408 */
1409 public Iterator<E> iterator() {
1410 return new Itr();
1411 }
1412
1413 public E peek() {
1414 restartFromHead: for (;;) {
1415 for (Node p = head; p != null;) {
1416 Object item = p.item;
1417 if (p.isData) {
1418 if (item != null && item != p) {
1419 @SuppressWarnings("unchecked") E e = (E) item;
1420 return e;
1421 }
1422 }
1423 else if (item == null)
1424 break;
1425 if (p == (p = p.next))
1426 continue restartFromHead;
1427 }
1428 return null;
1429 }
1430 }
1431
1432 /**
1433 * Returns {@code true} if this queue contains no elements.
1434 *
1435 * @return {@code true} if this queue contains no elements
1436 */
1437 public boolean isEmpty() {
1438 return firstDataNode() == null;
1439 }
1440
1441 public boolean hasWaitingConsumer() {
1442 restartFromHead: for (;;) {
1443 for (Node p = head; p != null;) {
1444 Object item = p.item;
1445 if (p.isData) {
1446 if (item != null && item != p)
1447 break;
1448 }
1449 else if (item == null)
1450 return true;
1451 if (p == (p = p.next))
1452 continue restartFromHead;
1453 }
1454 return false;
1455 }
1456 }
1457
1458 /**
1459 * Returns the number of elements in this queue. If this queue
1460 * contains more than {@code Integer.MAX_VALUE} elements, returns
1461 * {@code Integer.MAX_VALUE}.
1462 *
1463 * <p>Beware that, unlike in most collections, this method is
1464 * <em>NOT</em> a constant-time operation. Because of the
1465 * asynchronous nature of these queues, determining the current
1466 * number of elements requires an O(n) traversal.
1467 *
1468 * @return the number of elements in this queue
1469 */
1470 public int size() {
1471 return countOfMode(true);
1472 }
1473
1474 public int getWaitingConsumerCount() {
1475 return countOfMode(false);
1476 }
1477
1478 /**
1479 * Removes a single instance of the specified element from this queue,
1480 * if it is present. More formally, removes an element {@code e} such
1481 * that {@code o.equals(e)}, if this queue contains one or more such
1482 * elements.
1483 * Returns {@code true} if this queue contained the specified element
1484 * (or equivalently, if this queue changed as a result of the call).
1485 *
1486 * @param o element to be removed from this queue, if present
1487 * @return {@code true} if this queue changed as a result of the call
1488 */
1489 public boolean remove(Object o) {
1490 return findAndRemove(o);
1491 }
1492
1493 /**
1494 * Returns {@code true} if this queue contains the specified element.
1495 * More formally, returns {@code true} if and only if this queue contains
1496 * at least one element {@code e} such that {@code o.equals(e)}.
1497 *
1498 * @param o object to be checked for containment in this queue
1499 * @return {@code true} if this queue contains the specified element
1500 */
1501 public boolean contains(Object o) {
1502 if (o != null) {
1503 for (Node p = head; p != null; p = succ(p)) {
1504 Object item = p.item;
1505 if (p.isData) {
1506 if (item != null && item != p && o.equals(item))
1507 return true;
1508 }
1509 else if (item == null)
1510 break;
1511 }
1512 }
1513 return false;
1514 }
1515
1516 /**
1517 * Always returns {@code Integer.MAX_VALUE} because a
1518 * {@code LinkedTransferQueue} is not capacity constrained.
1519 *
1520 * @return {@code Integer.MAX_VALUE} (as specified by
1521 * {@link java.util.concurrent.BlockingQueue#remainingCapacity()
1522 * BlockingQueue.remainingCapacity})
1523 */
1524 public int remainingCapacity() {
1525 return Integer.MAX_VALUE;
1526 }
1527
1528 /**
1529 * Saves this queue to a stream (that is, serializes it).
1530 *
1531 * @param s the stream
1532 * @throws java.io.IOException if an I/O error occurs
1533 * @serialData All of the elements (each an {@code E}) in
1534 * the proper order, followed by a null
1535 */
1536 private void writeObject(java.io.ObjectOutputStream s)
1537 throws java.io.IOException {
1538 s.defaultWriteObject();
1539 for (E e : this)
1540 s.writeObject(e);
1541 // Use trailing null as sentinel
1542 s.writeObject(null);
1543 }
1544
1545 /**
1546 * Reconstitutes this queue from a stream (that is, deserializes it).
1547 * @param s the stream
1548 * @throws ClassNotFoundException if the class of a serialized object
1549 * could not be found
1550 * @throws java.io.IOException if an I/O error occurs
1551 */
1552 private void readObject(java.io.ObjectInputStream s)
1553 throws java.io.IOException, ClassNotFoundException {
1554 s.defaultReadObject();
1555 for (;;) {
1556 @SuppressWarnings("unchecked")
1557 E item = (E) s.readObject();
1558 if (item == null)
1559 break;
1560 else
1561 offer(item);
1562 }
1563 }
1564
1565 // Unsafe mechanics
1566
1567 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
1568 private static final long HEAD;
1569 private static final long TAIL;
1570 private static final long SWEEPVOTES;
1571 static {
1572 try {
1573 HEAD = U.objectFieldOffset
1574 (LinkedTransferQueue.class.getDeclaredField("head"));
1575 TAIL = U.objectFieldOffset
1576 (LinkedTransferQueue.class.getDeclaredField("tail"));
1577 SWEEPVOTES = U.objectFieldOffset
1578 (LinkedTransferQueue.class.getDeclaredField("sweepVotes"));
1579 } catch (ReflectiveOperationException e) {
1580 throw new Error(e);
1581 }
1582
1583 // Reduce the risk of rare disastrous classloading in first call to
1584 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
1585 Class<?> ensureLoaded = LockSupport.class;
1586 }
1587 }