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1 : jsr166 1.1 /*
2 :     * Written by Doug Lea with assistance from members of JCP JSR-166
3 :     * Expert Group and released to the public domain, as explained at
4 : jsr166 1.39 * http://creativecommons.org/publicdomain/zero/1.0/
5 : jsr166 1.1 */
6 :    
7 :     package java.util.concurrent;
8 :    
9 : dl 1.97 import java.lang.invoke.MethodHandles;
10 :     import java.lang.invoke.VarHandle;
11 : jsr166 1.1 import java.util.AbstractQueue;
12 : jsr166 1.82 import java.util.Arrays;
13 : jsr166 1.1 import java.util.Collection;
14 :     import java.util.Iterator;
15 :     import java.util.NoSuchElementException;
16 : jsr166 1.111 import java.util.Objects;
17 : jsr166 1.5 import java.util.Queue;
18 : dl 1.52 import java.util.Spliterator;
19 : dl 1.54 import java.util.Spliterators;
20 : jsr166 1.76 import java.util.concurrent.locks.LockSupport;
21 :     import java.util.function.Consumer;
22 : jsr166 1.116 import java.util.function.Predicate;
23 : dl 1.22
24 : jsr166 1.1 /**
25 : jsr166 1.6 * An unbounded {@link TransferQueue} based on linked nodes.
26 : jsr166 1.1 * This queue orders elements FIFO (first-in-first-out) with respect
27 :     * to any given producer. The <em>head</em> of the queue is that
28 :     * element that has been on the queue the longest time for some
29 :     * producer. The <em>tail</em> of the queue is that element that has
30 :     * been on the queue the shortest time for some producer.
31 :     *
32 : dl 1.40 * <p>Beware that, unlike in most collections, the {@code size} method
33 :     * is <em>NOT</em> a constant-time operation. Because of the
34 : jsr166 1.1 * asynchronous nature of these queues, determining the current number
35 : dl 1.40 * of elements requires a traversal of the elements, and so may report
36 :     * inaccurate results if this collection is modified during traversal.
37 : jsr166 1.1 *
38 : jsr166 1.131 * <p>Bulk operations that add, remove, or examine multiple elements,
39 :     * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
40 :     * are <em>not</em> guaranteed to be performed atomically.
41 :     * For example, a {@code forEach} traversal concurrent with an {@code
42 :     * addAll} operation might observe only some of the added elements.
43 :     *
44 :     * <p>This class and its iterator implement all of the <em>optional</em>
45 :     * methods of the {@link Collection} and {@link Iterator} interfaces.
46 : jsr166 1.1 *
47 :     * <p>Memory consistency effects: As with other concurrent
48 :     * collections, actions in a thread prior to placing an object into a
49 :     * {@code LinkedTransferQueue}
50 :     * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
51 :     * actions subsequent to the access or removal of that element from
52 :     * the {@code LinkedTransferQueue} in another thread.
53 :     *
54 :     * <p>This class is a member of the
55 : jsr166 1.157 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
56 : jsr166 1.1 * Java Collections Framework</a>.
57 :     *
58 :     * @since 1.7
59 :     * @author Doug Lea
60 : jsr166 1.75 * @param <E> the type of elements held in this queue
61 : jsr166 1.1 */
62 :     public class LinkedTransferQueue<E> extends AbstractQueue<E>
63 :     implements TransferQueue<E>, java.io.Serializable {
64 :     private static final long serialVersionUID = -3223113410248163686L;
65 :    
66 :     /*
67 : jsr166 1.8 * *** Overview of Dual Queues with Slack ***
68 : jsr166 1.1 *
69 : jsr166 1.8 * Dual Queues, introduced by Scherer and Scott
70 : jsr166 1.99 * (http://www.cs.rochester.edu/~scott/papers/2004_DISC_dual_DS.pdf)
71 :     * are (linked) queues in which nodes may represent either data or
72 : jsr166 1.8 * requests. When a thread tries to enqueue a data node, but
73 :     * encounters a request node, it instead "matches" and removes it;
74 :     * and vice versa for enqueuing requests. Blocking Dual Queues
75 :     * arrange that threads enqueuing unmatched requests block until
76 :     * other threads provide the match. Dual Synchronous Queues (see
77 :     * Scherer, Lea, & Scott
78 :     * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
79 :     * additionally arrange that threads enqueuing unmatched data also
80 :     * block. Dual Transfer Queues support all of these modes, as
81 :     * dictated by callers.
82 :     *
83 :     * A FIFO dual queue may be implemented using a variation of the
84 :     * Michael & Scott (M&S) lock-free queue algorithm
85 : jsr166 1.72 * (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf).
86 : jsr166 1.8 * It maintains two pointer fields, "head", pointing to a
87 :     * (matched) node that in turn points to the first actual
88 :     * (unmatched) queue node (or null if empty); and "tail" that
89 :     * points to the last node on the queue (or again null if
90 :     * empty). For example, here is a possible queue with four data
91 :     * elements:
92 :     *
93 :     * head tail
94 :     * | |
95 :     * v v
96 :     * M -> U -> U -> U -> U
97 :     *
98 :     * The M&S queue algorithm is known to be prone to scalability and
99 :     * overhead limitations when maintaining (via CAS) these head and
100 :     * tail pointers. This has led to the development of
101 :     * contention-reducing variants such as elimination arrays (see
102 :     * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
103 :     * optimistic back pointers (see Ladan-Mozes & Shavit
104 :     * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
105 :     * However, the nature of dual queues enables a simpler tactic for
106 :     * improving M&S-style implementations when dual-ness is needed.
107 :     *
108 :     * In a dual queue, each node must atomically maintain its match
109 :     * status. While there are other possible variants, we implement
110 :     * this here as: for a data-mode node, matching entails CASing an
111 :     * "item" field from a non-null data value to null upon match, and
112 :     * vice-versa for request nodes, CASing from null to a data
113 :     * value. (Note that the linearization properties of this style of
114 :     * queue are easy to verify -- elements are made available by
115 :     * linking, and unavailable by matching.) Compared to plain M&S
116 :     * queues, this property of dual queues requires one additional
117 :     * successful atomic operation per enq/deq pair. But it also
118 :     * enables lower cost variants of queue maintenance mechanics. (A
119 :     * variation of this idea applies even for non-dual queues that
120 :     * support deletion of interior elements, such as
121 :     * j.u.c.ConcurrentLinkedQueue.)
122 :     *
123 :     * Once a node is matched, its match status can never again
124 :     * change. We may thus arrange that the linked list of them
125 :     * contain a prefix of zero or more matched nodes, followed by a
126 :     * suffix of zero or more unmatched nodes. (Note that we allow
127 :     * both the prefix and suffix to be zero length, which in turn
128 :     * means that we do not use a dummy header.) If we were not
129 :     * concerned with either time or space efficiency, we could
130 :     * correctly perform enqueue and dequeue operations by traversing
131 :     * from a pointer to the initial node; CASing the item of the
132 :     * first unmatched node on match and CASing the next field of the
133 : jsr166 1.140 * trailing node on appends. While this would be a terrible idea
134 :     * in itself, it does have the benefit of not requiring ANY atomic
135 : jsr166 1.8 * updates on head/tail fields.
136 :     *
137 :     * We introduce here an approach that lies between the extremes of
138 :     * never versus always updating queue (head and tail) pointers.
139 :     * This offers a tradeoff between sometimes requiring extra
140 :     * traversal steps to locate the first and/or last unmatched
141 :     * nodes, versus the reduced overhead and contention of fewer
142 :     * updates to queue pointers. For example, a possible snapshot of
143 :     * a queue is:
144 :     *
145 :     * head tail
146 :     * | |
147 :     * v v
148 :     * M -> M -> U -> U -> U -> U
149 :     *
150 :     * The best value for this "slack" (the targeted maximum distance
151 :     * between the value of "head" and the first unmatched node, and
152 :     * similarly for "tail") is an empirical matter. We have found
153 :     * that using very small constants in the range of 1-3 work best
154 :     * over a range of platforms. Larger values introduce increasing
155 :     * costs of cache misses and risks of long traversal chains, while
156 :     * smaller values increase CAS contention and overhead.
157 :     *
158 :     * Dual queues with slack differ from plain M&S dual queues by
159 :     * virtue of only sometimes updating head or tail pointers when
160 :     * matching, appending, or even traversing nodes; in order to
161 :     * maintain a targeted slack. The idea of "sometimes" may be
162 :     * operationalized in several ways. The simplest is to use a
163 :     * per-operation counter incremented on each traversal step, and
164 :     * to try (via CAS) to update the associated queue pointer
165 :     * whenever the count exceeds a threshold. Another, that requires
166 :     * more overhead, is to use random number generators to update
167 :     * with a given probability per traversal step.
168 :     *
169 :     * In any strategy along these lines, because CASes updating
170 : jsr166 1.126 * fields may fail, the actual slack may exceed targeted slack.
171 :     * However, they may be retried at any time to maintain targets.
172 :     * Even when using very small slack values, this approach works
173 :     * well for dual queues because it allows all operations up to the
174 :     * point of matching or appending an item (hence potentially
175 :     * allowing progress by another thread) to be read-only, thus not
176 :     * introducing any further contention. As described below, we
177 :     * implement this by performing slack maintenance retries only
178 :     * after these points.
179 : jsr166 1.8 *
180 :     * As an accompaniment to such techniques, traversal overhead can
181 :     * be further reduced without increasing contention of head
182 :     * pointer updates: Threads may sometimes shortcut the "next" link
183 :     * path from the current "head" node to be closer to the currently
184 :     * known first unmatched node, and similarly for tail. Again, this
185 :     * may be triggered with using thresholds or randomization.
186 :     *
187 :     * These ideas must be further extended to avoid unbounded amounts
188 :     * of costly-to-reclaim garbage caused by the sequential "next"
189 :     * links of nodes starting at old forgotten head nodes: As first
190 :     * described in detail by Boehm
191 : jsr166 1.69 * (http://portal.acm.org/citation.cfm?doid=503272.503282), if a GC
192 : jsr166 1.8 * delays noticing that any arbitrarily old node has become
193 :     * garbage, all newer dead nodes will also be unreclaimed.
194 :     * (Similar issues arise in non-GC environments.) To cope with
195 :     * this in our implementation, upon CASing to advance the head
196 :     * pointer, we set the "next" link of the previous head to point
197 : jsr166 1.127 * only to itself; thus limiting the length of chains of dead nodes.
198 : jsr166 1.8 * (We also take similar care to wipe out possibly garbage
199 :     * retaining values held in other Node fields.) However, doing so
200 :     * adds some further complexity to traversal: If any "next"
201 :     * pointer links to itself, it indicates that the current thread
202 :     * has lagged behind a head-update, and so the traversal must
203 :     * continue from the "head". Traversals trying to find the
204 :     * current tail starting from "tail" may also encounter
205 :     * self-links, in which case they also continue at "head".
206 :     *
207 :     * It is tempting in slack-based scheme to not even use CAS for
208 :     * updates (similarly to Ladan-Mozes & Shavit). However, this
209 :     * cannot be done for head updates under the above link-forgetting
210 :     * mechanics because an update may leave head at a detached node.
211 :     * And while direct writes are possible for tail updates, they
212 :     * increase the risk of long retraversals, and hence long garbage
213 :     * chains, which can be much more costly than is worthwhile
214 :     * considering that the cost difference of performing a CAS vs
215 :     * write is smaller when they are not triggered on each operation
216 :     * (especially considering that writes and CASes equally require
217 :     * additional GC bookkeeping ("write barriers") that are sometimes
218 :     * more costly than the writes themselves because of contention).
219 :     *
220 :     * *** Overview of implementation ***
221 :     *
222 :     * We use a threshold-based approach to updates, with a slack
223 :     * threshold of two -- that is, we update head/tail when the
224 :     * current pointer appears to be two or more steps away from the
225 :     * first/last node. The slack value is hard-wired: a path greater
226 :     * than one is naturally implemented by checking equality of
227 :     * traversal pointers except when the list has only one element,
228 :     * in which case we keep slack threshold at one. Avoiding tracking
229 :     * explicit counts across method calls slightly simplifies an
230 :     * already-messy implementation. Using randomization would
231 :     * probably work better if there were a low-quality dirt-cheap
232 :     * per-thread one available, but even ThreadLocalRandom is too
233 :     * heavy for these purposes.
234 :     *
235 : dl 1.16 * With such a small slack threshold value, it is not worthwhile
236 :     * to augment this with path short-circuiting (i.e., unsplicing
237 :     * interior nodes) except in the case of cancellation/removal (see
238 :     * below).
239 : jsr166 1.8 *
240 :     * All enqueue/dequeue operations are handled by the single method
241 :     * "xfer" with parameters indicating whether to act as some form
242 :     * of offer, put, poll, take, or transfer (each possibly with
243 :     * timeout). The relative complexity of using one monolithic
244 :     * method outweighs the code bulk and maintenance problems of
245 :     * using separate methods for each case.
246 :     *
247 : jsr166 1.153 * Operation consists of up to two phases. The first is implemented
248 :     * in method xfer, the second in method awaitMatch.
249 : jsr166 1.8 *
250 : jsr166 1.153 * 1. Traverse until matching or appending (method xfer)
251 : jsr166 1.8 *
252 : jsr166 1.153 * Conceptually, we simply traverse all nodes starting from head.
253 :     * If we encounter an unmatched node of opposite mode, we match
254 :     * it and return, also updating head (by at least 2 hops) to
255 :     * one past the matched node (or the node itself if it's the
256 :     * pinned trailing node). Traversals also check for the
257 :     * possibility of falling off-list, in which case they restart.
258 :     *
259 :     * If the trailing node of the list is reached, a match is not
260 :     * possible. If this call was untimed poll or tryTransfer
261 :     * (argument "how" is NOW), return empty-handed immediately.
262 :     * Else a new node is CAS-appended. On successful append, if
263 :     * this call was ASYNC (e.g. offer), an element was
264 :     * successfully added to the end of the queue and we return.
265 :     *
266 :     * Of course, this naive traversal is O(n) when no match is
267 :     * possible. We optimize the traversal by maintaining a tail
268 :     * pointer, which is expected to be "near" the end of the list.
269 :     * It is only safe to fast-forward to tail (in the presence of
270 :     * arbitrary concurrent changes) if it is pointing to a node of
271 :     * the same mode, even if it is dead (in this case no preceding
272 :     * node could still be matchable by this traversal). If we
273 :     * need to restart due to falling off-list, we can again
274 :     * fast-forward to tail, but only if it has changed since the
275 :     * last traversal (else we might loop forever). If tail cannot
276 :     * be used, traversal starts at head (but in this case we
277 :     * expect to be able to match near head). As with head, we
278 :     * CAS-advance the tail pointer by at least two hops.
279 :     *
280 :     * 2. Await match or cancellation (method awaitMatch)
281 : jsr166 1.8 *
282 :     * Wait for another thread to match node; instead cancelling if
283 :     * the current thread was interrupted or the wait timed out. On
284 :     * multiprocessors, we use front-of-queue spinning: If a node
285 :     * appears to be the first unmatched node in the queue, it
286 :     * spins a bit before blocking. In either case, before blocking
287 :     * it tries to unsplice any nodes between the current "head"
288 :     * and the first unmatched node.
289 :     *
290 :     * Front-of-queue spinning vastly improves performance of
291 :     * heavily contended queues. And so long as it is relatively
292 :     * brief and "quiet", spinning does not much impact performance
293 :     * of less-contended queues. During spins threads check their
294 :     * interrupt status and generate a thread-local random number
295 :     * to decide to occasionally perform a Thread.yield. While
296 : jsr166 1.44 * yield has underdefined specs, we assume that it might help,
297 : jsr166 1.45 * and will not hurt, in limiting impact of spinning on busy
298 : jsr166 1.8 * systems. We also use smaller (1/2) spins for nodes that are
299 :     * not known to be front but whose predecessors have not
300 :     * blocked -- these "chained" spins avoid artifacts of
301 :     * front-of-queue rules which otherwise lead to alternating
302 :     * nodes spinning vs blocking. Further, front threads that
303 :     * represent phase changes (from data to request node or vice
304 :     * versa) compared to their predecessors receive additional
305 :     * chained spins, reflecting longer paths typically required to
306 :     * unblock threads during phase changes.
307 : dl 1.16 *
308 :     *
309 :     * ** Unlinking removed interior nodes **
310 :     *
311 :     * In addition to minimizing garbage retention via self-linking
312 :     * described above, we also unlink removed interior nodes. These
313 :     * may arise due to timed out or interrupted waits, or calls to
314 :     * remove(x) or Iterator.remove. Normally, given a node that was
315 :     * at one time known to be the predecessor of some node s that is
316 :     * to be removed, we can unsplice s by CASing the next field of
317 :     * its predecessor if it still points to s (otherwise s must
318 :     * already have been removed or is now offlist). But there are two
319 :     * situations in which we cannot guarantee to make node s
320 :     * unreachable in this way: (1) If s is the trailing node of list
321 :     * (i.e., with null next), then it is pinned as the target node
322 : jsr166 1.23 * for appends, so can only be removed later after other nodes are
323 : dl 1.16 * appended. (2) We cannot necessarily unlink s given a
324 :     * predecessor node that is matched (including the case of being
325 : jsr166 1.17 * cancelled): the predecessor may already be unspliced, in which
326 :     * case some previous reachable node may still point to s.
327 :     * (For further explanation see Herlihy & Shavit "The Art of
328 : dl 1.16 * Multiprocessor Programming" chapter 9). Although, in both
329 :     * cases, we can rule out the need for further action if either s
330 :     * or its predecessor are (or can be made to be) at, or fall off
331 :     * from, the head of list.
332 :     *
333 :     * Without taking these into account, it would be possible for an
334 : jsr166 1.152 * unbounded number of supposedly removed nodes to remain reachable.
335 : jsr166 1.155 * Situations leading to such buildup are uncommon but can occur
336 :     * in practice; for example when a series of short timed calls to
337 :     * poll repeatedly time out at the trailing node but otherwise
338 :     * never fall off the list because of an untimed call to take() at
339 :     * the front of the queue.
340 : dl 1.16 *
341 :     * When these cases arise, rather than always retraversing the
342 :     * entire list to find an actual predecessor to unlink (which
343 :     * won't help for case (1) anyway), we record a conservative
344 : jsr166 1.24 * estimate of possible unsplice failures (in "sweepVotes").
345 :     * We trigger a full sweep when the estimate exceeds a threshold
346 :     * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
347 :     * removal failures to tolerate before sweeping through, unlinking
348 :     * cancelled nodes that were not unlinked upon initial removal.
349 :     * We perform sweeps by the thread hitting threshold (rather than
350 :     * background threads or by spreading work to other threads)
351 :     * because in the main contexts in which removal occurs, the
352 : jsr166 1.152 * caller is timed-out or cancelled, which are not time-critical
353 :     * enough to warrant the overhead that alternatives would impose
354 :     * on other threads.
355 : dl 1.16 *
356 :     * Because the sweepVotes estimate is conservative, and because
357 :     * nodes become unlinked "naturally" as they fall off the head of
358 :     * the queue, and because we allow votes to accumulate even while
359 : jsr166 1.17 * sweeps are in progress, there are typically significantly fewer
360 : dl 1.16 * such nodes than estimated. Choice of a threshold value
361 :     * balances the likelihood of wasted effort and contention, versus
362 :     * providing a worst-case bound on retention of interior nodes in
363 :     * quiescent queues. The value defined below was chosen
364 :     * empirically to balance these under various timeout scenarios.
365 :     *
366 : jsr166 1.152 * Because traversal operations on the linked list of nodes are a
367 :     * natural opportunity to sweep dead nodes, we generally do so,
368 :     * including all the operations that might remove elements as they
369 :     * traverse, such as removeIf and Iterator.remove. This largely
370 :     * eliminates long chains of dead interior nodes, except from
371 :     * cancelled or timed out blocking operations.
372 :     *
373 : dl 1.16 * Note that we cannot self-link unlinked interior nodes during
374 :     * sweeps. However, the associated garbage chains terminate when
375 :     * some successor ultimately falls off the head of the list and is
376 :     * self-linked.
377 : jsr166 1.8 */
378 :    
379 :     /** True if on multiprocessor */
380 :     private static final boolean MP =
381 :     Runtime.getRuntime().availableProcessors() > 1;
382 :    
383 :     /**
384 :     * The number of times to spin (with randomly interspersed calls
385 :     * to Thread.yield) on multiprocessor before blocking when a node
386 :     * is apparently the first waiter in the queue. See above for
387 :     * explanation. Must be a power of two. The value is empirically
388 :     * derived -- it works pretty well across a variety of processors,
389 :     * numbers of CPUs, and OSes.
390 :     */
391 :     private static final int FRONT_SPINS = 1 << 7;
392 :    
393 :     /**
394 :     * The number of times to spin before blocking when a node is
395 :     * preceded by another node that is apparently spinning. Also
396 :     * serves as an increment to FRONT_SPINS on phase changes, and as
397 :     * base average frequency for yielding during spins. Must be a
398 :     * power of two.
399 :     */
400 :     private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
401 :    
402 :     /**
403 : dl 1.16 * The maximum number of estimated removal failures (sweepVotes)
404 :     * to tolerate before sweeping through the queue unlinking
405 :     * cancelled nodes that were not unlinked upon initial
406 :     * removal. See above for explanation. The value must be at least
407 :     * two to avoid useless sweeps when removing trailing nodes.
408 :     */
409 :     static final int SWEEP_THRESHOLD = 32;
410 :    
411 :     /**
412 : jsr166 1.8 * Queue nodes. Uses Object, not E, for items to allow forgetting
413 : jsr166 1.142 * them after use. Writes that are intrinsically ordered wrt
414 :     * other accesses or CASes use simple relaxed forms.
415 : jsr166 1.8 */
416 : jsr166 1.14 static final class Node {
417 : jsr166 1.8 final boolean isData; // false if this is a request node
418 :     volatile Object item; // initially non-null if isData; CASed to match
419 : jsr166 1.14 volatile Node next;
420 : jsr166 1.142 volatile Thread waiter; // null when not waiting for a match
421 : jsr166 1.1
422 : jsr166 1.8 /**
423 : jsr166 1.140 * Constructs a data node holding item if item is non-null,
424 :     * else a request node. Uses relaxed write because item can
425 :     * only be seen after piggy-backing publication via CAS.
426 : jsr166 1.8 */
427 : jsr166 1.101 Node(Object item) {
428 : jsr166 1.104 ITEM.set(this, item);
429 : jsr166 1.101 isData = (item != null);
430 : jsr166 1.8 }
431 : jsr166 1.1
432 : jsr166 1.142 /** Constructs a (matched data) dummy node. */
433 : jsr166 1.140 Node() {
434 :     isData = true;
435 :     }
436 :    
437 : jsr166 1.142 final boolean casNext(Node cmp, Node val) {
438 :     // assert val != null;
439 :     return NEXT.compareAndSet(this, cmp, val);
440 :     }
441 :    
442 :     final boolean casItem(Object cmp, Object val) {
443 :     // assert isData == (cmp != null);
444 :     // assert isData == (val == null);
445 :     // assert !(cmp instanceof Node);
446 :     return ITEM.compareAndSet(this, cmp, val);
447 :     }
448 :    
449 : jsr166 1.8 /**
450 :     * Links node to itself to avoid garbage retention. Called
451 :     * only after CASing head field, so uses relaxed write.
452 :     */
453 : jsr166 1.142 final void selfLink() {
454 :     // assert isMatched();
455 : jsr166 1.121 NEXT.setRelease(this, this);
456 : jsr166 1.8 }
457 : jsr166 1.1
458 : jsr166 1.140 final void appendRelaxed(Node next) {
459 :     // assert next != null;
460 :     // assert this.next == null;
461 :     NEXT.set(this, next);
462 :     }
463 :    
464 : jsr166 1.8 /**
465 : jsr166 1.105 * Sets item (of a request node) to self and waiter to null,
466 :     * to avoid garbage retention after matching or cancelling.
467 :     * Uses relaxed writes because order is already constrained in
468 :     * the only calling contexts: item is forgotten only after
469 : jsr166 1.121 * volatile/atomic mechanics that extract items, and visitors
470 :     * of request nodes only ever check whether item is null.
471 :     * Similarly, clearing waiter follows either CAS or return
472 :     * from park (if ever parked; else we don't care).
473 : jsr166 1.8 */
474 :     final void forgetContents() {
475 : jsr166 1.105 // assert isMatched();
476 :     if (!isData)
477 :     ITEM.set(this, this);
478 : dl 1.97 WAITER.set(this, null);
479 : jsr166 1.8 }
480 : jsr166 1.1
481 : jsr166 1.8 /**
482 :     * Returns true if this node has been matched, including the
483 :     * case of artificial matches due to cancellation.
484 :     */
485 :     final boolean isMatched() {
486 : jsr166 1.105 return isData == (item == null);
487 : jsr166 1.11 }
488 :    
489 : jsr166 1.142 /** Tries to CAS-match this node; if successful, wakes waiter. */
490 :     final boolean tryMatch(Object cmp, Object val) {
491 :     if (casItem(cmp, val)) {
492 :     LockSupport.unpark(waiter);
493 :     return true;
494 :     }
495 :     return false;
496 :     }
497 :    
498 : jsr166 1.11 /**
499 : jsr166 1.8 * Returns true if a node with the given mode cannot be
500 :     * appended to this node because this node is unmatched and
501 :     * has opposite data mode.
502 :     */
503 :     final boolean cannotPrecede(boolean haveData) {
504 :     boolean d = isData;
505 : jsr166 1.105 return d != haveData && d != (item == null);
506 : jsr166 1.8 }
507 : jsr166 1.1
508 : dl 1.38 private static final long serialVersionUID = -3375979862319811754L;
509 : jsr166 1.1 }
510 :    
511 : jsr166 1.140 /**
512 :     * A node from which the first live (non-matched) node (if any)
513 :     * can be reached in O(1) time.
514 :     * Invariants:
515 :     * - all live nodes are reachable from head via .next
516 :     * - head != null
517 :     * - (tmp = head).next != tmp || tmp != head
518 :     * Non-invariants:
519 :     * - head may or may not be live
520 :     * - it is permitted for tail to lag behind head, that is, for tail
521 :     * to not be reachable from head!
522 :     */
523 : jsr166 1.14 transient volatile Node head;
524 : jsr166 1.8
525 : jsr166 1.140 /**
526 :     * A node from which the last node on list (that is, the unique
527 :     * node with node.next == null) can be reached in O(1) time.
528 :     * Invariants:
529 :     * - the last node is always reachable from tail via .next
530 :     * - tail != null
531 :     * Non-invariants:
532 :     * - tail may or may not be live
533 :     * - it is permitted for tail to lag behind head, that is, for tail
534 :     * to not be reachable from head!
535 :     * - tail.next may or may not be self-linked.
536 :     */
537 : jsr166 1.14 private transient volatile Node tail;
538 : jsr166 1.1
539 : jsr166 1.152 /** The number of apparent failures to unsplice cancelled nodes */
540 : dl 1.16 private transient volatile int sweepVotes;
541 :    
542 : jsr166 1.14 private boolean casTail(Node cmp, Node val) {
543 : jsr166 1.140 // assert cmp != null;
544 :     // assert val != null;
545 : dl 1.97 return TAIL.compareAndSet(this, cmp, val);
546 : jsr166 1.8 }
547 : jsr166 1.1
548 : jsr166 1.14 private boolean casHead(Node cmp, Node val) {
549 : dl 1.97 return HEAD.compareAndSet(this, cmp, val);
550 : jsr166 1.8 }
551 : jsr166 1.1
552 : jsr166 1.152 /** Atomic version of ++sweepVotes. */
553 :     private int incSweepVotes() {
554 :     return (int) SWEEPVOTES.getAndAdd(this, 1) + 1;
555 : jsr166 1.8 }
556 : jsr166 1.1
557 : jsr166 1.122 /**
558 :     * Tries to CAS pred.next (or head, if pred is null) from c to p.
559 : jsr166 1.133 * Caller must ensure that we're not unlinking the trailing node.
560 : jsr166 1.122 */
561 :     private boolean tryCasSuccessor(Node pred, Node c, Node p) {
562 : jsr166 1.133 // assert p != null;
563 : jsr166 1.136 // assert c.isData != (c.item != null);
564 : jsr166 1.122 // assert c != p;
565 :     if (pred != null)
566 :     return pred.casNext(c, p);
567 :     if (casHead(c, p)) {
568 : jsr166 1.142 c.selfLink();
569 : jsr166 1.122 return true;
570 :     }
571 :     return false;
572 :     }
573 :    
574 : jsr166 1.137 /**
575 : jsr166 1.144 * Collapses dead (matched) nodes between pred and q.
576 : jsr166 1.137 * @param pred the last known live node, or null if none
577 :     * @param c the first dead node
578 :     * @param p the last dead node
579 :     * @param q p.next: the next live node, or null if at end
580 : jsr166 1.153 * @return pred if pred still alive and CAS succeeded; else p
581 : jsr166 1.137 */
582 :     private Node skipDeadNodes(Node pred, Node c, Node p, Node q) {
583 :     // assert pred != c;
584 :     // assert p != q;
585 :     // assert c.isMatched();
586 :     // assert p.isMatched();
587 :     if (q == null) {
588 :     // Never unlink trailing node.
589 :     if (c == p) return pred;
590 :     q = p;
591 :     }
592 :     return (tryCasSuccessor(pred, c, q)
593 :     && (pred == null || !pred.isMatched()))
594 :     ? pred : p;
595 :     }
596 :    
597 : jsr166 1.144 /**
598 : jsr166 1.153 * Collapses dead (matched) nodes from h (which was once head) to p.
599 :     * Caller ensures all nodes from h up to and including p are dead.
600 : jsr166 1.144 */
601 :     private void skipDeadNodesNearHead(Node h, Node p) {
602 : jsr166 1.153 // assert h != null;
603 : jsr166 1.144 // assert h != p;
604 :     // assert p.isMatched();
605 : jsr166 1.153 for (;;) {
606 :     final Node q;
607 :     if ((q = p.next) == null) break;
608 :     else if (!q.isMatched()) { p = q; break; }
609 :     else if (p == (p = q)) return;
610 : jsr166 1.144 }
611 : jsr166 1.153 if (casHead(h, p))
612 : jsr166 1.144 h.selfLink();
613 :     }
614 :    
615 : jsr166 1.137 /* Possible values for "how" argument in xfer method. */
616 :    
617 : jsr166 1.14 private static final int NOW = 0; // for untimed poll, tryTransfer
618 :     private static final int ASYNC = 1; // for offer, put, add
619 :     private static final int SYNC = 2; // for transfer, take
620 :     private static final int TIMED = 3; // for timed poll, tryTransfer
621 : jsr166 1.1
622 :     /**
623 : jsr166 1.8 * Implements all queuing methods. See above for explanation.
624 : jsr166 1.1 *
625 : jsr166 1.8 * @param e the item or null for take
626 :     * @param haveData true if this is a put, else a take
627 : jsr166 1.14 * @param how NOW, ASYNC, SYNC, or TIMED
628 :     * @param nanos timeout in nanosecs, used only if mode is TIMED
629 : jsr166 1.8 * @return an item if matched, else e
630 :     * @throws NullPointerException if haveData mode but e is null
631 : jsr166 1.1 */
632 : jsr166 1.153 @SuppressWarnings("unchecked")
633 : jsr166 1.8 private E xfer(E e, boolean haveData, int how, long nanos) {
634 :     if (haveData && (e == null))
635 :     throw new NullPointerException();
636 : jsr166 1.1
637 : jsr166 1.153 restart: for (Node s = null, t = null, h = null;;) {
638 :     for (Node p = (t != (t = tail) && t.isData == haveData) ? t
639 :     : (h = head);; ) {
640 :     final Node q; final Object item;
641 :     if (p.isData != haveData
642 :     && haveData == ((item = p.item) == null)) {
643 :     if (h == null) h = head;
644 : jsr166 1.142 if (p.tryMatch(item, e)) {
645 : jsr166 1.144 if (h != p) skipDeadNodesNearHead(h, p);
646 : jsr166 1.153 return (E) item;
647 : jsr166 1.1 }
648 :     }
649 : jsr166 1.153 if ((q = p.next) == null) {
650 :     if (how == NOW) return e;
651 :     if (s == null) s = new Node(e);
652 :     if (!p.casNext(null, s)) continue;
653 :     if (p != t) casTail(t, s);
654 :     if (how == ASYNC) return e;
655 :     return awaitMatch(s, p, e, (how == TIMED), nanos);
656 : jsr166 1.1 }
657 : jsr166 1.153 if (p == (p = q)) continue restart;
658 : jsr166 1.1 }
659 :     }
660 :     }
661 :    
662 :     /**
663 : jsr166 1.8 * Spins/yields/blocks until node s is matched or caller gives up.
664 : jsr166 1.1 *
665 :     * @param s the waiting node
666 : jsr166 1.148 * @param pred the predecessor of s, or null if unknown (the null
667 :     * case does not occur in any current calls but may in possible
668 :     * future extensions)
669 : jsr166 1.1 * @param e the comparison value for checking match
670 : jsr166 1.14 * @param timed if true, wait only until timeout elapses
671 :     * @param nanos timeout in nanosecs, used only if timed is true
672 : jsr166 1.8 * @return matched item, or e if unmatched on interrupt or timeout
673 : jsr166 1.1 */
674 : jsr166 1.14 private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
675 : jsr166 1.51 final long deadline = timed ? System.nanoTime() + nanos : 0L;
676 : jsr166 1.8 Thread w = Thread.currentThread();
677 :     int spins = -1; // initialized after first item and cancel checks
678 :     ThreadLocalRandom randomYields = null; // bound if needed
679 : jsr166 1.1
680 :     for (;;) {
681 : jsr166 1.141 final Object item;
682 :     if ((item = s.item) != e) { // matched
683 : dl 1.33 // assert item != s;
684 : jsr166 1.8 s.forgetContents(); // avoid garbage
685 : jsr166 1.70 @SuppressWarnings("unchecked") E itemE = (E) item;
686 :     return itemE;
687 : jsr166 1.8 }
688 : jsr166 1.95 else if (w.isInterrupted() || (timed && nanos <= 0L)) {
689 : jsr166 1.102 // try to cancel and unlink
690 : jsr166 1.105 if (s.casItem(e, s.isData ? null : s)) {
691 : jsr166 1.102 unsplice(pred, s);
692 : jsr166 1.77 return e;
693 : jsr166 1.102 }
694 :     // return normally if lost CAS
695 : jsr166 1.8 }
696 : dl 1.84 else if (spins < 0) { // establish spins at/near front
697 : jsr166 1.8 if ((spins = spinsFor(pred, s.isData)) > 0)
698 :     randomYields = ThreadLocalRandom.current();
699 :     }
700 :     else if (spins > 0) { // spin
701 : dl 1.16 --spins;
702 :     if (randomYields.nextInt(CHAINED_SPINS) == 0)
703 : jsr166 1.8 Thread.yield(); // occasionally yield
704 :     }
705 :     else if (s.waiter == null) {
706 :     s.waiter = w; // request unpark then recheck
707 : jsr166 1.1 }
708 : jsr166 1.14 else if (timed) {
709 : jsr166 1.51 nanos = deadline - System.nanoTime();
710 :     if (nanos > 0L)
711 : jsr166 1.8 LockSupport.parkNanos(this, nanos);
712 : jsr166 1.1 }
713 : jsr166 1.8 else {
714 : jsr166 1.1 LockSupport.park(this);
715 :     }
716 : jsr166 1.8 }
717 :     }
718 :    
719 :     /**
720 :     * Returns spin/yield value for a node with given predecessor and
721 :     * data mode. See above for explanation.
722 :     */
723 : jsr166 1.14 private static int spinsFor(Node pred, boolean haveData) {
724 : jsr166 1.8 if (MP && pred != null) {
725 :     if (pred.isData != haveData) // phase change
726 :     return FRONT_SPINS + CHAINED_SPINS;
727 :     if (pred.isMatched()) // probably at front
728 :     return FRONT_SPINS;
729 :     if (pred.waiter == null) // pred apparently spinning
730 :     return CHAINED_SPINS;
731 :     }
732 :     return 0;
733 :     }
734 :    
735 :     /* -------------- Traversal methods -------------- */
736 :    
737 :     /**
738 : jsr166 1.93 * Returns the first unmatched data node, or null if none.
739 : jsr166 1.105 * Callers must recheck if the returned node is unmatched
740 :     * before using.
741 : dl 1.52 */
742 :     final Node firstDataNode() {
743 : jsr166 1.139 Node first = null;
744 : jsr166 1.91 restartFromHead: for (;;) {
745 : jsr166 1.139 Node h = head, p = h;
746 : jsr166 1.159 while (p != null) {
747 :     if (p.item != null) {
748 : jsr166 1.139 if (p.isData) {
749 :     first = p;
750 :     break;
751 :     }
752 : jsr166 1.91 }
753 : jsr166 1.139 else if (!p.isData)
754 :     break;
755 :     final Node q;
756 :     if ((q = p.next) == null)
757 : jsr166 1.91 break;
758 : jsr166 1.139 if (p == (p = q))
759 : jsr166 1.91 continue restartFromHead;
760 : dl 1.52 }
761 : jsr166 1.139 if (p != h && casHead(h, p))
762 : jsr166 1.142 h.selfLink();
763 : jsr166 1.139 return first;
764 : dl 1.52 }
765 :     }
766 :    
767 :     /**
768 : jsr166 1.8 * Traverses and counts unmatched nodes of the given mode.
769 :     * Used by methods size and getWaitingConsumerCount.
770 : jsr166 1.1 */
771 : jsr166 1.8 private int countOfMode(boolean data) {
772 : jsr166 1.73 restartFromHead: for (;;) {
773 :     int count = 0;
774 :     for (Node p = head; p != null;) {
775 :     if (!p.isMatched()) {
776 :     if (p.isData != data)
777 :     return 0;
778 :     if (++count == Integer.MAX_VALUE)
779 :     break; // @see Collection.size()
780 :     }
781 : jsr166 1.81 if (p == (p = p.next))
782 : jsr166 1.73 continue restartFromHead;
783 : jsr166 1.1 }
784 : jsr166 1.73 return count;
785 : jsr166 1.8 }
786 :     }
787 :    
788 : jsr166 1.82 public String toString() {
789 :     String[] a = null;
790 :     restartFromHead: for (;;) {
791 :     int charLength = 0;
792 :     int size = 0;
793 :     for (Node p = head; p != null;) {
794 :     Object item = p.item;
795 :     if (p.isData) {
796 : jsr166 1.105 if (item != null) {
797 : jsr166 1.82 if (a == null)
798 :     a = new String[4];
799 :     else if (size == a.length)
800 :     a = Arrays.copyOf(a, 2 * size);
801 :     String s = item.toString();
802 :     a[size++] = s;
803 :     charLength += s.length();
804 :     }
805 :     } else if (item == null)
806 :     break;
807 :     if (p == (p = p.next))
808 :     continue restartFromHead;
809 :     }
810 :    
811 :     if (size == 0)
812 :     return "[]";
813 :    
814 : jsr166 1.83 return Helpers.toString(a, size, charLength);
815 : jsr166 1.82 }
816 :     }
817 :    
818 :     private Object[] toArrayInternal(Object[] a) {
819 :     Object[] x = a;
820 :     restartFromHead: for (;;) {
821 :     int size = 0;
822 :     for (Node p = head; p != null;) {
823 :     Object item = p.item;
824 :     if (p.isData) {
825 : jsr166 1.105 if (item != null) {
826 : jsr166 1.82 if (x == null)
827 :     x = new Object[4];
828 :     else if (size == x.length)
829 :     x = Arrays.copyOf(x, 2 * (size + 4));
830 :     x[size++] = item;
831 :     }
832 :     } else if (item == null)
833 :     break;
834 :     if (p == (p = p.next))
835 :     continue restartFromHead;
836 :     }
837 :     if (x == null)
838 :     return new Object[0];
839 :     else if (a != null && size <= a.length) {
840 :     if (a != x)
841 :     System.arraycopy(x, 0, a, 0, size);
842 :     if (size < a.length)
843 :     a[size] = null;
844 :     return a;
845 :     }
846 :     return (size == x.length) ? x : Arrays.copyOf(x, size);
847 :     }
848 :     }
849 :    
850 :     /**
851 :     * Returns an array containing all of the elements in this queue, in
852 :     * proper sequence.
853 :     *
854 :     * <p>The returned array will be "safe" in that no references to it are
855 :     * maintained by this queue. (In other words, this method must allocate
856 :     * a new array). The caller is thus free to modify the returned array.
857 :     *
858 :     * <p>This method acts as bridge between array-based and collection-based
859 :     * APIs.
860 :     *
861 :     * @return an array containing all of the elements in this queue
862 :     */
863 :     public Object[] toArray() {
864 :     return toArrayInternal(null);
865 :     }
866 :    
867 :     /**
868 :     * Returns an array containing all of the elements in this queue, in
869 :     * proper sequence; the runtime type of the returned array is that of
870 :     * the specified array. If the queue fits in the specified array, it
871 :     * is returned therein. Otherwise, a new array is allocated with the
872 :     * runtime type of the specified array and the size of this queue.
873 :     *
874 :     * <p>If this queue fits in the specified array with room to spare
875 :     * (i.e., the array has more elements than this queue), the element in
876 :     * the array immediately following the end of the queue is set to
877 :     * {@code null}.
878 :     *
879 :     * <p>Like the {@link #toArray()} method, this method acts as bridge between
880 :     * array-based and collection-based APIs. Further, this method allows
881 :     * precise control over the runtime type of the output array, and may,
882 :     * under certain circumstances, be used to save allocation costs.
883 :     *
884 :     * <p>Suppose {@code x} is a queue known to contain only strings.
885 :     * The following code can be used to dump the queue into a newly
886 :     * allocated array of {@code String}:
887 :     *
888 :     * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
889 :     *
890 :     * Note that {@code toArray(new Object[0])} is identical in function to
891 :     * {@code toArray()}.
892 :     *
893 :     * @param a the array into which the elements of the queue are to
894 :     * be stored, if it is big enough; otherwise, a new array of the
895 :     * same runtime type is allocated for this purpose
896 :     * @return an array containing all of the elements in this queue
897 :     * @throws ArrayStoreException if the runtime type of the specified array
898 :     * is not a supertype of the runtime type of every element in
899 :     * this queue
900 :     * @throws NullPointerException if the specified array is null
901 :     */
902 :     @SuppressWarnings("unchecked")
903 :     public <T> T[] toArray(T[] a) {
904 : jsr166 1.111 Objects.requireNonNull(a);
905 : jsr166 1.82 return (T[]) toArrayInternal(a);
906 :     }
907 :    
908 : jsr166 1.134 /**
909 :     * Weakly-consistent iterator.
910 :     *
911 :     * Lazily updated ancestor is expected to be amortized O(1) remove(),
912 :     * but O(n) in the worst case, when lastRet is concurrently deleted.
913 :     */
914 : jsr166 1.8 final class Itr implements Iterator<E> {
915 : jsr166 1.14 private Node nextNode; // next node to return item for
916 :     private E nextItem; // the corresponding item
917 :     private Node lastRet; // last returned node, to support remove
918 : jsr166 1.134 private Node ancestor; // Helps unlink lastRet on remove()
919 : jsr166 1.8
920 :     /**
921 : jsr166 1.134 * Moves to next node after pred, or first node if pred null.
922 : jsr166 1.8 */
923 : jsr166 1.134 @SuppressWarnings("unchecked")
924 :     private void advance(Node pred) {
925 :     for (Node p = (pred == null) ? head : pred.next, c = p;
926 :     p != null; ) {
927 :     final Object item;
928 :     if ((item = p.item) != null && p.isData) {
929 :     nextNode = p;
930 :     nextItem = (E) item;
931 :     if (c != p)
932 :     tryCasSuccessor(pred, c, p);
933 :     return;
934 :     }
935 :     else if (!p.isData && item == null)
936 : dl 1.33 break;
937 : jsr166 1.134 if (c != p && !tryCasSuccessor(pred, c, c = p)) {
938 :     pred = p;
939 :     c = p = p.next;
940 : dl 1.33 }
941 : jsr166 1.134 else if (p == (p = p.next)) {
942 :     pred = null;
943 :     c = p = head;
944 : jsr166 1.34 }
945 : jsr166 1.1 }
946 : jsr166 1.134 nextItem = null;
947 : jsr166 1.8 nextNode = null;
948 :     }
949 :    
950 :     Itr() {
951 :     advance(null);
952 :     }
953 :    
954 :     public final boolean hasNext() {
955 :     return nextNode != null;
956 :     }
957 :    
958 :     public final E next() {
959 : jsr166 1.125 final Node p;
960 :     if ((p = nextNode) == null) throw new NoSuchElementException();
961 : jsr166 1.8 E e = nextItem;
962 : jsr166 1.134 advance(lastRet = p);
963 : jsr166 1.8 return e;
964 :     }
965 :    
966 : jsr166 1.134 public void forEachRemaining(Consumer<? super E> action) {
967 :     Objects.requireNonNull(action);
968 :     Node q = null;
969 :     for (Node p; (p = nextNode) != null; advance(q = p))
970 :     action.accept(nextItem);
971 :     if (q != null)
972 :     lastRet = q;
973 :     }
974 : jsr166 1.116
975 : jsr166 1.8 public final void remove() {
976 : dl 1.33 final Node lastRet = this.lastRet;
977 :     if (lastRet == null)
978 :     throw new IllegalStateException();
979 :     this.lastRet = null;
980 : jsr166 1.134 if (lastRet.item == null) // already deleted?
981 :     return;
982 :     // Advance ancestor, collapsing intervening dead nodes
983 :     Node pred = ancestor;
984 :     for (Node p = (pred == null) ? head : pred.next, c = p, q;
985 :     p != null; ) {
986 :     if (p == lastRet) {
987 : jsr166 1.142 final Object item;
988 :     if ((item = p.item) != null)
989 :     p.tryMatch(item, null);
990 : jsr166 1.134 if ((q = p.next) == null) q = p;
991 :     if (c != q) tryCasSuccessor(pred, c, q);
992 :     ancestor = pred;
993 :     return;
994 :     }
995 :     final Object item; final boolean pAlive;
996 :     if (pAlive = ((item = p.item) != null && p.isData)) {
997 :     // exceptionally, nothing to do
998 :     }
999 :     else if (!p.isData && item == null)
1000 :     break;
1001 :     if ((c != p && !tryCasSuccessor(pred, c, c = p)) || pAlive) {
1002 :     pred = p;
1003 :     c = p = p.next;
1004 :     }
1005 :     else if (p == (p = p.next)) {
1006 :     pred = null;
1007 :     c = p = head;
1008 :     }
1009 :     }
1010 :     // traversal failed to find lastRet; must have been deleted;
1011 :     // leave ancestor at original location to avoid overshoot;
1012 :     // better luck next time!
1013 :    
1014 :     // assert lastRet.isMatched();
1015 : jsr166 1.1 }
1016 :     }
1017 : jsr166 1.53
1018 : dl 1.57 /** A customized variant of Spliterators.IteratorSpliterator */
1019 : jsr166 1.109 final class LTQSpliterator implements Spliterator<E> {
1020 : dl 1.60 static final int MAX_BATCH = 1 << 25; // max batch array size;
1021 : jsr166 1.87 Node current; // current node; null until initialized
1022 : dl 1.52 int batch; // batch size for splits
1023 :     boolean exhausted; // true when no more nodes
1024 : jsr166 1.94 LTQSpliterator() {}
1025 : dl 1.52
1026 :     public Spliterator<E> trySplit() {
1027 : jsr166 1.115 Node p, q;
1028 :     if ((p = current()) == null || (q = p.next) == null)
1029 :     return null;
1030 :     int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH);
1031 :     Object[] a = null;
1032 :     do {
1033 :     final Object item = p.item;
1034 :     if (p.isData) {
1035 : jsr166 1.154 if (item != null) {
1036 :     if (a == null)
1037 :     a = new Object[n];
1038 :     a[i++] = item;
1039 :     }
1040 : jsr166 1.115 } else if (item == null) {
1041 :     p = null;
1042 :     break;
1043 : dl 1.60 }
1044 : jsr166 1.117 if (p == (p = q))
1045 :     p = firstDataNode();
1046 : jsr166 1.115 } while (p != null && (q = p.next) != null && i < n);
1047 :     setCurrent(p);
1048 :     return (i == 0) ? null :
1049 :     Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED |
1050 :     Spliterator.NONNULL |
1051 :     Spliterator.CONCURRENT));
1052 : dl 1.52 }
1053 :    
1054 : dl 1.61 public void forEachRemaining(Consumer<? super E> action) {
1055 : jsr166 1.111 Objects.requireNonNull(action);
1056 : jsr166 1.116 final Node p;
1057 : jsr166 1.115 if ((p = current()) != null) {
1058 : jsr166 1.107 current = null;
1059 : dl 1.52 exhausted = true;
1060 : jsr166 1.116 forEachFrom(action, p);
1061 : dl 1.52 }
1062 :     }
1063 :    
1064 :     @SuppressWarnings("unchecked")
1065 :     public boolean tryAdvance(Consumer<? super E> action) {
1066 : jsr166 1.111 Objects.requireNonNull(action);
1067 : dl 1.52 Node p;
1068 : jsr166 1.115 if ((p = current()) != null) {
1069 :     E e = null;
1070 : dl 1.52 do {
1071 : jsr166 1.115 final Object item = p.item;
1072 :     final boolean isData = p.isData;
1073 :     if (p == (p = p.next))
1074 :     p = head;
1075 :     if (isData) {
1076 :     if (item != null) {
1077 :     e = (E) item;
1078 : jsr166 1.107 break;
1079 :     }
1080 :     }
1081 : jsr166 1.115 else if (item == null)
1082 :     p = null;
1083 :     } while (p != null);
1084 :     setCurrent(p);
1085 :     if (e != null) {
1086 :     action.accept(e);
1087 : dl 1.52 return true;
1088 :     }
1089 :     }
1090 :     return false;
1091 :     }
1092 :    
1093 : jsr166 1.115 private void setCurrent(Node p) {
1094 :     if ((current = p) == null)
1095 :     exhausted = true;
1096 :     }
1097 :    
1098 :     private Node current() {
1099 :     Node p;
1100 :     if ((p = current) == null && !exhausted)
1101 :     setCurrent(p = firstDataNode());
1102 :     return p;
1103 :     }
1104 :    
1105 : dl 1.54 public long estimateSize() { return Long.MAX_VALUE; }
1106 :    
1107 : dl 1.52 public int characteristics() {
1108 : jsr166 1.100 return (Spliterator.ORDERED |
1109 :     Spliterator.NONNULL |
1110 :     Spliterator.CONCURRENT);
1111 : dl 1.52 }
1112 :     }
1113 :    
1114 : jsr166 1.67 /**
1115 :     * Returns a {@link Spliterator} over the elements in this queue.
1116 :     *
1117 : jsr166 1.68 * <p>The returned spliterator is
1118 :     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1119 :     *
1120 : jsr166 1.67 * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
1121 :     * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
1122 :     *
1123 :     * @implNote
1124 :     * The {@code Spliterator} implements {@code trySplit} to permit limited
1125 :     * parallelism.
1126 :     *
1127 :     * @return a {@code Spliterator} over the elements in this queue
1128 :     * @since 1.8
1129 :     */
1130 : dl 1.56 public Spliterator<E> spliterator() {
1131 : jsr166 1.109 return new LTQSpliterator();
1132 : dl 1.52 }
1133 :    
1134 : jsr166 1.8 /* -------------- Removal methods -------------- */
1135 :    
1136 : jsr166 1.1 /**
1137 : jsr166 1.8 * Unsplices (now or later) the given deleted/cancelled node with
1138 :     * the given predecessor.
1139 : jsr166 1.1 *
1140 : dl 1.16 * @param pred a node that was at one time known to be the
1141 : jsr166 1.149 * predecessor of s
1142 : jsr166 1.8 * @param s the node to be unspliced
1143 : jsr166 1.1 */
1144 : dl 1.16 final void unsplice(Node pred, Node s) {
1145 : jsr166 1.149 // assert pred != null;
1146 : jsr166 1.150 // assert pred != s;
1147 : jsr166 1.149 // assert s != null;
1148 :     // assert s.isMatched();
1149 : jsr166 1.152 // assert (SWEEP_THRESHOLD & (SWEEP_THRESHOLD - 1)) == 0;
1150 : dl 1.71 s.waiter = null; // disable signals
1151 : jsr166 1.1 /*
1152 : dl 1.16 * See above for rationale. Briefly: if pred still points to
1153 :     * s, try to unlink s. If s cannot be unlinked, because it is
1154 :     * trailing node or pred might be unlinked, and neither pred
1155 :     * nor s are head or offlist, add to sweepVotes, and if enough
1156 :     * votes have accumulated, sweep.
1157 : jsr166 1.1 */
1158 : jsr166 1.150 if (pred != null && pred.next == s) {
1159 : dl 1.16 Node n = s.next;
1160 :     if (n == null ||
1161 :     (n != s && pred.casNext(s, n) && pred.isMatched())) {
1162 :     for (;;) { // check if at, or could be, head
1163 :     Node h = head;
1164 : jsr166 1.151 if (h == pred || h == s)
1165 : dl 1.16 return; // at head or list empty
1166 :     if (!h.isMatched())
1167 :     break;
1168 :     Node hn = h.next;
1169 :     if (hn == null)
1170 :     return; // now empty
1171 :     if (hn != h && casHead(h, hn))
1172 : jsr166 1.142 h.selfLink(); // advance head
1173 : jsr166 1.8 }
1174 : jsr166 1.152 // sweep every SWEEP_THRESHOLD votes
1175 :     if (pred.next != pred && s.next != s // recheck if offlist
1176 :     && (incSweepVotes() & (SWEEP_THRESHOLD - 1)) == 0)
1177 :     sweep();
1178 : jsr166 1.1 }
1179 :     }
1180 :     }
1181 :    
1182 :     /**
1183 : jsr166 1.26 * Unlinks matched (typically cancelled) nodes encountered in a
1184 :     * traversal from head.
1185 : jsr166 1.1 */
1186 : dl 1.16 private void sweep() {
1187 : jsr166 1.20 for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
1188 : jsr166 1.28 if (!s.isMatched())
1189 :     // Unmatched nodes are never self-linked
1190 : jsr166 1.20 p = s;
1191 : jsr166 1.28 else if ((n = s.next) == null) // trailing node is pinned
1192 : jsr166 1.20 break;
1193 : jsr166 1.28 else if (s == n) // stale
1194 :     // No need to also check for p == s, since that implies s == n
1195 :     p = head;
1196 : jsr166 1.20 else
1197 : dl 1.16 p.casNext(s, n);
1198 : jsr166 1.8 }
1199 :     }
1200 :    
1201 :     /**
1202 : jsr166 1.1 * Creates an initially empty {@code LinkedTransferQueue}.
1203 :     */
1204 :     public LinkedTransferQueue() {
1205 : jsr166 1.140 head = tail = new Node();
1206 : jsr166 1.1 }
1207 :    
1208 :     /**
1209 :     * Creates a {@code LinkedTransferQueue}
1210 :     * initially containing the elements of the given collection,
1211 :     * added in traversal order of the collection's iterator.
1212 :     *
1213 :     * @param c the collection of elements to initially contain
1214 :     * @throws NullPointerException if the specified collection or any
1215 :     * of its elements are null
1216 :     */
1217 :     public LinkedTransferQueue(Collection<? extends E> c) {
1218 : jsr166 1.140 Node h = null, t = null;
1219 :     for (E e : c) {
1220 :     Node newNode = new Node(Objects.requireNonNull(e));
1221 :     if (h == null)
1222 :     h = t = newNode;
1223 :     else
1224 :     t.appendRelaxed(t = newNode);
1225 :     }
1226 :     if (h == null)
1227 :     h = t = new Node();
1228 :     head = h;
1229 :     tail = t;
1230 : jsr166 1.1 }
1231 :    
1232 : jsr166 1.4 /**
1233 : jsr166 1.5 * Inserts the specified element at the tail of this queue.
1234 :     * As the queue is unbounded, this method will never block.
1235 :     *
1236 :     * @throws NullPointerException if the specified element is null
1237 : jsr166 1.4 */
1238 : jsr166 1.5 public void put(E e) {
1239 : jsr166 1.8 xfer(e, true, ASYNC, 0);
1240 : jsr166 1.1 }
1241 :    
1242 : jsr166 1.4 /**
1243 : jsr166 1.5 * Inserts the specified element at the tail of this queue.
1244 :     * As the queue is unbounded, this method will never block or
1245 :     * return {@code false}.
1246 :     *
1247 :     * @return {@code true} (as specified by
1248 : jsr166 1.156 * {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
1249 : jsr166 1.5 * @throws NullPointerException if the specified element is null
1250 : jsr166 1.4 */
1251 : jsr166 1.5 public boolean offer(E e, long timeout, TimeUnit unit) {
1252 : jsr166 1.8 xfer(e, true, ASYNC, 0);
1253 :     return true;
1254 : jsr166 1.1 }
1255 :    
1256 : jsr166 1.4 /**
1257 : jsr166 1.5 * Inserts the specified element at the tail of this queue.
1258 :     * As the queue is unbounded, this method will never return {@code false}.
1259 :     *
1260 : jsr166 1.32 * @return {@code true} (as specified by {@link Queue#offer})
1261 : jsr166 1.5 * @throws NullPointerException if the specified element is null
1262 : jsr166 1.4 */
1263 : jsr166 1.1 public boolean offer(E e) {
1264 : jsr166 1.8 xfer(e, true, ASYNC, 0);
1265 : jsr166 1.1 return true;
1266 :     }
1267 :    
1268 : jsr166 1.4 /**
1269 : jsr166 1.5 * Inserts the specified element at the tail of this queue.
1270 :     * As the queue is unbounded, this method will never throw
1271 :     * {@link IllegalStateException} or return {@code false}.
1272 :     *
1273 :     * @return {@code true} (as specified by {@link Collection#add})
1274 :     * @throws NullPointerException if the specified element is null
1275 : jsr166 1.4 */
1276 : jsr166 1.1 public boolean add(E e) {
1277 : jsr166 1.8 xfer(e, true, ASYNC, 0);
1278 :     return true;
1279 : jsr166 1.5 }
1280 :    
1281 :     /**
1282 : jsr166 1.6 * Transfers the element to a waiting consumer immediately, if possible.
1283 :     *
1284 :     * <p>More precisely, transfers the specified element immediately
1285 :     * if there exists a consumer already waiting to receive it (in
1286 :     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1287 :     * otherwise returning {@code false} without enqueuing the element.
1288 : jsr166 1.5 *
1289 :     * @throws NullPointerException if the specified element is null
1290 :     */
1291 :     public boolean tryTransfer(E e) {
1292 : jsr166 1.8 return xfer(e, true, NOW, 0) == null;
1293 : jsr166 1.1 }
1294 :    
1295 : jsr166 1.4 /**
1296 : jsr166 1.6 * Transfers the element to a consumer, waiting if necessary to do so.
1297 :     *
1298 :     * <p>More precisely, transfers the specified element immediately
1299 :     * if there exists a consumer already waiting to receive it (in
1300 :     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1301 :     * else inserts the specified element at the tail of this queue
1302 :     * and waits until the element is received by a consumer.
1303 : jsr166 1.5 *
1304 :     * @throws NullPointerException if the specified element is null
1305 : jsr166 1.4 */
1306 : jsr166 1.1 public void transfer(E e) throws InterruptedException {
1307 : jsr166 1.8 if (xfer(e, true, SYNC, 0) != null) {
1308 :     Thread.interrupted(); // failure possible only due to interrupt
1309 : jsr166 1.1 throw new InterruptedException();
1310 :     }
1311 :     }
1312 :    
1313 : jsr166 1.4 /**
1314 : jsr166 1.6 * Transfers the element to a consumer if it is possible to do so
1315 :     * before the timeout elapses.
1316 :     *
1317 :     * <p>More precisely, transfers the specified element immediately
1318 :     * if there exists a consumer already waiting to receive it (in
1319 :     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1320 :     * else inserts the specified element at the tail of this queue
1321 :     * and waits until the element is received by a consumer,
1322 :     * returning {@code false} if the specified wait time elapses
1323 :     * before the element can be transferred.
1324 : jsr166 1.5 *
1325 :     * @throws NullPointerException if the specified element is null
1326 : jsr166 1.4 */
1327 : jsr166 1.1 public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1328 :     throws InterruptedException {
1329 : jsr166 1.14 if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
1330 : jsr166 1.1 return true;
1331 :     if (!Thread.interrupted())
1332 :     return false;
1333 :     throw new InterruptedException();
1334 :     }
1335 :    
1336 :     public E take() throws InterruptedException {
1337 : jsr166 1.8 E e = xfer(null, false, SYNC, 0);
1338 : jsr166 1.1 if (e != null)
1339 : jsr166 1.5 return e;
1340 : jsr166 1.1 Thread.interrupted();
1341 :     throw new InterruptedException();
1342 :     }
1343 :    
1344 :     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1345 : jsr166 1.14 E e = xfer(null, false, TIMED, unit.toNanos(timeout));
1346 : jsr166 1.1 if (e != null || !Thread.interrupted())
1347 : jsr166 1.5 return e;
1348 : jsr166 1.1 throw new InterruptedException();
1349 :     }
1350 :    
1351 :     public E poll() {
1352 : jsr166 1.8 return xfer(null, false, NOW, 0);
1353 : jsr166 1.1 }
1354 :    
1355 : jsr166 1.4 /**
1356 :     * @throws NullPointerException {@inheritDoc}
1357 :     * @throws IllegalArgumentException {@inheritDoc}
1358 :     */
1359 : jsr166 1.1 public int drainTo(Collection<? super E> c) {
1360 : jsr166 1.111 Objects.requireNonNull(c);
1361 : jsr166 1.1 if (c == this)
1362 :     throw new IllegalArgumentException();
1363 :     int n = 0;
1364 : jsr166 1.112 for (E e; (e = poll()) != null; n++)
1365 : jsr166 1.1 c.add(e);
1366 :     return n;
1367 :     }
1368 :    
1369 : jsr166 1.4 /**
1370 :     * @throws NullPointerException {@inheritDoc}
1371 :     * @throws IllegalArgumentException {@inheritDoc}
1372 :     */
1373 : jsr166 1.1 public int drainTo(Collection<? super E> c, int maxElements) {
1374 : jsr166 1.111 Objects.requireNonNull(c);
1375 : jsr166 1.1 if (c == this)
1376 :     throw new IllegalArgumentException();
1377 :     int n = 0;
1378 : jsr166 1.112 for (E e; n < maxElements && (e = poll()) != null; n++)
1379 : jsr166 1.1 c.add(e);
1380 :     return n;
1381 :     }
1382 :    
1383 : jsr166 1.5 /**
1384 : jsr166 1.36 * Returns an iterator over the elements in this queue in proper sequence.
1385 :     * The elements will be returned in order from first (head) to last (tail).
1386 : jsr166 1.5 *
1387 : jsr166 1.68 * <p>The returned iterator is
1388 :     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1389 : jsr166 1.5 *
1390 :     * @return an iterator over the elements in this queue in proper sequence
1391 :     */
1392 : jsr166 1.1 public Iterator<E> iterator() {
1393 :     return new Itr();
1394 :     }
1395 :    
1396 :     public E peek() {
1397 : jsr166 1.92 restartFromHead: for (;;) {
1398 :     for (Node p = head; p != null;) {
1399 :     Object item = p.item;
1400 :     if (p.isData) {
1401 : jsr166 1.105 if (item != null) {
1402 : jsr166 1.92 @SuppressWarnings("unchecked") E e = (E) item;
1403 :     return e;
1404 :     }
1405 :     }
1406 :     else if (item == null)
1407 :     break;
1408 :     if (p == (p = p.next))
1409 :     continue restartFromHead;
1410 :     }
1411 :     return null;
1412 :     }
1413 : jsr166 1.1 }
1414 :    
1415 : jsr166 1.6 /**
1416 :     * Returns {@code true} if this queue contains no elements.
1417 :     *
1418 :     * @return {@code true} if this queue contains no elements
1419 :     */
1420 : jsr166 1.1 public boolean isEmpty() {
1421 : jsr166 1.90 return firstDataNode() == null;
1422 : jsr166 1.1 }
1423 :    
1424 :     public boolean hasWaitingConsumer() {
1425 : jsr166 1.93 restartFromHead: for (;;) {
1426 :     for (Node p = head; p != null;) {
1427 :     Object item = p.item;
1428 :     if (p.isData) {
1429 : jsr166 1.105 if (item != null)
1430 : jsr166 1.93 break;
1431 :     }
1432 :     else if (item == null)
1433 :     return true;
1434 :     if (p == (p = p.next))
1435 :     continue restartFromHead;
1436 :     }
1437 :     return false;
1438 :     }
1439 : jsr166 1.1 }
1440 :    
1441 :     /**
1442 :     * Returns the number of elements in this queue. If this queue
1443 :     * contains more than {@code Integer.MAX_VALUE} elements, returns
1444 :     * {@code Integer.MAX_VALUE}.
1445 :     *
1446 :     * <p>Beware that, unlike in most collections, this method is
1447 :     * <em>NOT</em> a constant-time operation. Because of the
1448 :     * asynchronous nature of these queues, determining the current
1449 :     * number of elements requires an O(n) traversal.
1450 :     *
1451 :     * @return the number of elements in this queue
1452 :     */
1453 :     public int size() {
1454 : jsr166 1.8 return countOfMode(true);
1455 : jsr166 1.1 }
1456 :    
1457 :     public int getWaitingConsumerCount() {
1458 : jsr166 1.8 return countOfMode(false);
1459 : jsr166 1.1 }
1460 :    
1461 : jsr166 1.6 /**
1462 :     * Removes a single instance of the specified element from this queue,
1463 :     * if it is present. More formally, removes an element {@code e} such
1464 :     * that {@code o.equals(e)}, if this queue contains one or more such
1465 :     * elements.
1466 :     * Returns {@code true} if this queue contained the specified element
1467 :     * (or equivalently, if this queue changed as a result of the call).
1468 :     *
1469 :     * @param o element to be removed from this queue, if present
1470 :     * @return {@code true} if this queue changed as a result of the call
1471 :     */
1472 : jsr166 1.1 public boolean remove(Object o) {
1473 : jsr166 1.137 if (o == null) return false;
1474 : jsr166 1.108 restartFromHead: for (;;) {
1475 : jsr166 1.137 for (Node p = head, pred = null; p != null; ) {
1476 :     Node q = p.next;
1477 :     final Object item;
1478 :     if ((item = p.item) != null) {
1479 :     if (p.isData) {
1480 : jsr166 1.142 if (o.equals(item) && p.tryMatch(item, null)) {
1481 : jsr166 1.137 skipDeadNodes(pred, p, p, q);
1482 :     return true;
1483 :     }
1484 :     pred = p; p = q; continue;
1485 : jsr166 1.108 }
1486 :     }
1487 : jsr166 1.137 else if (!p.isData)
1488 : jsr166 1.108 break;
1489 : jsr166 1.138 for (Node c = p;; q = p.next) {
1490 :     if (q == null || !q.isMatched()) {
1491 : jsr166 1.137 pred = skipDeadNodes(pred, c, p, q); p = q; break;
1492 :     }
1493 :     if (p == (p = q)) continue restartFromHead;
1494 : jsr166 1.122 }
1495 : jsr166 1.108 }
1496 :     return false;
1497 :     }
1498 : jsr166 1.1 }
1499 :    
1500 :     /**
1501 : jsr166 1.30 * Returns {@code true} if this queue contains the specified element.
1502 :     * More formally, returns {@code true} if and only if this queue contains
1503 :     * at least one element {@code e} such that {@code o.equals(e)}.
1504 :     *
1505 :     * @param o object to be checked for containment in this queue
1506 :     * @return {@code true} if this queue contains the specified element
1507 :     */
1508 :     public boolean contains(Object o) {
1509 : jsr166 1.137 if (o == null) return false;
1510 : jsr166 1.122 restartFromHead: for (;;) {
1511 : jsr166 1.137 for (Node p = head, pred = null; p != null; ) {
1512 :     Node q = p.next;
1513 :     final Object item;
1514 :     if ((item = p.item) != null) {
1515 :     if (p.isData) {
1516 :     if (o.equals(item))
1517 :     return true;
1518 :     pred = p; p = q; continue;
1519 :     }
1520 : jsr166 1.74 }
1521 : jsr166 1.137 else if (!p.isData)
1522 : jsr166 1.74 break;
1523 : jsr166 1.138 for (Node c = p;; q = p.next) {
1524 :     if (q == null || !q.isMatched()) {
1525 : jsr166 1.137 pred = skipDeadNodes(pred, c, p, q); p = q; break;
1526 :     }
1527 :     if (p == (p = q)) continue restartFromHead;
1528 : jsr166 1.122 }
1529 : jsr166 1.30 }
1530 : jsr166 1.122 return false;
1531 : jsr166 1.30 }
1532 :     }
1533 :    
1534 :     /**
1535 : jsr166 1.5 * Always returns {@code Integer.MAX_VALUE} because a
1536 :     * {@code LinkedTransferQueue} is not capacity constrained.
1537 :     *
1538 :     * @return {@code Integer.MAX_VALUE} (as specified by
1539 : jsr166 1.156 * {@link BlockingQueue#remainingCapacity()})
1540 : jsr166 1.5 */
1541 :     public int remainingCapacity() {
1542 :     return Integer.MAX_VALUE;
1543 :     }
1544 :    
1545 :     /**
1546 : jsr166 1.50 * Saves this queue to a stream (that is, serializes it).
1547 : jsr166 1.1 *
1548 : jsr166 1.65 * @param s the stream
1549 : jsr166 1.66 * @throws java.io.IOException if an I/O error occurs
1550 : jsr166 1.1 * @serialData All of the elements (each an {@code E}) in
1551 :     * the proper order, followed by a null
1552 :     */
1553 :     private void writeObject(java.io.ObjectOutputStream s)
1554 :     throws java.io.IOException {
1555 :     s.defaultWriteObject();
1556 :     for (E e : this)
1557 :     s.writeObject(e);
1558 :     // Use trailing null as sentinel
1559 :     s.writeObject(null);
1560 :     }
1561 :    
1562 :     /**
1563 : jsr166 1.50 * Reconstitutes this queue from a stream (that is, deserializes it).
1564 : jsr166 1.65 * @param s the stream
1565 : jsr166 1.66 * @throws ClassNotFoundException if the class of a serialized object
1566 :     * could not be found
1567 :     * @throws java.io.IOException if an I/O error occurs
1568 : jsr166 1.1 */
1569 :     private void readObject(java.io.ObjectInputStream s)
1570 :     throws java.io.IOException, ClassNotFoundException {
1571 : jsr166 1.140
1572 :     // Read in elements until trailing null sentinel found
1573 :     Node h = null, t = null;
1574 :     for (Object item; (item = s.readObject()) != null; ) {
1575 : jsr166 1.158 Node newNode = new Node(item);
1576 : jsr166 1.140 if (h == null)
1577 :     h = t = newNode;
1578 : jsr166 1.1 else
1579 : jsr166 1.140 t.appendRelaxed(t = newNode);
1580 : jsr166 1.1 }
1581 : jsr166 1.140 if (h == null)
1582 :     h = t = new Node();
1583 :     head = h;
1584 :     tail = t;
1585 : jsr166 1.1 }
1586 :    
1587 : jsr166 1.116 /**
1588 :     * @throws NullPointerException {@inheritDoc}
1589 :     */
1590 :     public boolean removeIf(Predicate<? super E> filter) {
1591 :     Objects.requireNonNull(filter);
1592 :     return bulkRemove(filter);
1593 :     }
1594 :    
1595 :     /**
1596 :     * @throws NullPointerException {@inheritDoc}
1597 :     */
1598 :     public boolean removeAll(Collection<?> c) {
1599 :     Objects.requireNonNull(c);
1600 :     return bulkRemove(e -> c.contains(e));
1601 :     }
1602 :    
1603 :     /**
1604 :     * @throws NullPointerException {@inheritDoc}
1605 :     */
1606 :     public boolean retainAll(Collection<?> c) {
1607 :     Objects.requireNonNull(c);
1608 :     return bulkRemove(e -> !c.contains(e));
1609 :     }
1610 :    
1611 : jsr166 1.124 public void clear() {
1612 :     bulkRemove(e -> true);
1613 :     }
1614 :    
1615 :     /**
1616 :     * Tolerate this many consecutive dead nodes before CAS-collapsing.
1617 :     * Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element.
1618 :     */
1619 :     private static final int MAX_HOPS = 8;
1620 :    
1621 : jsr166 1.116 /** Implementation of bulk remove methods. */
1622 :     @SuppressWarnings("unchecked")
1623 :     private boolean bulkRemove(Predicate<? super E> filter) {
1624 :     boolean removed = false;
1625 :     restartFromHead: for (;;) {
1626 : jsr166 1.124 int hops = MAX_HOPS;
1627 :     // c will be CASed to collapse intervening dead nodes between
1628 :     // pred (or head if null) and p.
1629 :     for (Node p = head, c = p, pred = null, q; p != null; p = q) {
1630 : jsr166 1.138 q = p.next;
1631 : jsr166 1.124 final Object item; boolean pAlive;
1632 : jsr166 1.132 if (pAlive = ((item = p.item) != null && p.isData)) {
1633 : jsr166 1.124 if (filter.test((E) item)) {
1634 : jsr166 1.142 if (p.tryMatch(item, null))
1635 : jsr166 1.124 removed = true;
1636 :     pAlive = false;
1637 : jsr166 1.116 }
1638 :     }
1639 : jsr166 1.124 else if (!p.isData && item == null)
1640 : jsr166 1.116 break;
1641 : jsr166 1.138 if (pAlive || q == null || --hops == 0) {
1642 : jsr166 1.124 // p might already be self-linked here, but if so:
1643 :     // - CASing head will surely fail
1644 :     // - CASing pred's next will be useless but harmless.
1645 : jsr166 1.134 if ((c != p && !tryCasSuccessor(pred, c, c = p))
1646 :     || pAlive) {
1647 :     // if CAS failed or alive, abandon old pred
1648 : jsr166 1.124 hops = MAX_HOPS;
1649 :     pred = p;
1650 :     c = q;
1651 :     }
1652 :     } else if (p == q)
1653 : jsr166 1.116 continue restartFromHead;
1654 :     }
1655 :     return removed;
1656 :     }
1657 :     }
1658 :    
1659 :     /**
1660 :     * Runs action on each element found during a traversal starting at p.
1661 : jsr166 1.118 * If p is null, the action is not run.
1662 : jsr166 1.116 */
1663 :     @SuppressWarnings("unchecked")
1664 :     void forEachFrom(Consumer<? super E> action, Node p) {
1665 : jsr166 1.137 for (Node pred = null; p != null; ) {
1666 :     Node q = p.next;
1667 :     final Object item;
1668 :     if ((item = p.item) != null) {
1669 :     if (p.isData) {
1670 :     action.accept((E) item);
1671 :     pred = p; p = q; continue;
1672 :     }
1673 :     }
1674 :     else if (!p.isData)
1675 : jsr166 1.122 break;
1676 : jsr166 1.138 for (Node c = p;; q = p.next) {
1677 :     if (q == null || !q.isMatched()) {
1678 : jsr166 1.137 pred = skipDeadNodes(pred, c, p, q); p = q; break;
1679 :     }
1680 :     if (p == (p = q)) { pred = null; p = head; break; }
1681 : jsr166 1.116 }
1682 :     }
1683 :     }
1684 :    
1685 :     /**
1686 :     * @throws NullPointerException {@inheritDoc}
1687 :     */
1688 :     public void forEach(Consumer<? super E> action) {
1689 :     Objects.requireNonNull(action);
1690 :     forEachFrom(action, head);
1691 :     }
1692 :    
1693 : dl 1.97 // VarHandle mechanics
1694 :     private static final VarHandle HEAD;
1695 :     private static final VarHandle TAIL;
1696 :     private static final VarHandle SWEEPVOTES;
1697 : jsr166 1.140 static final VarHandle ITEM;
1698 :     static final VarHandle NEXT;
1699 :     static final VarHandle WAITER;
1700 : dl 1.38 static {
1701 : jsr166 1.1 try {
1702 : dl 1.97 MethodHandles.Lookup l = MethodHandles.lookup();
1703 :     HEAD = l.findVarHandle(LinkedTransferQueue.class, "head",
1704 :     Node.class);
1705 :     TAIL = l.findVarHandle(LinkedTransferQueue.class, "tail",
1706 :     Node.class);
1707 :     SWEEPVOTES = l.findVarHandle(LinkedTransferQueue.class, "sweepVotes",
1708 :     int.class);
1709 : jsr166 1.140 ITEM = l.findVarHandle(Node.class, "item", Object.class);
1710 :     NEXT = l.findVarHandle(Node.class, "next", Node.class);
1711 :     WAITER = l.findVarHandle(Node.class, "waiter", Thread.class);
1712 : jsr166 1.79 } catch (ReflectiveOperationException e) {
1713 : jsr166 1.160 throw new ExceptionInInitializerError(e);
1714 : jsr166 1.1 }
1715 : jsr166 1.85
1716 :     // Reduce the risk of rare disastrous classloading in first call to
1717 :     // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
1718 :     Class<?> ensureLoaded = LockSupport.class;
1719 : jsr166 1.1 }
1720 :     }

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