1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/rt/emul/compact/src/main/java/java/util/concurrent/LinkedTransferQueue.java Sat Mar 19 10:46:31 2016 +0100
1.3 @@ -0,0 +1,1351 @@
1.4 +/*
1.5 + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
1.6 + *
1.7 + * This code is free software; you can redistribute it and/or modify it
1.8 + * under the terms of the GNU General Public License version 2 only, as
1.9 + * published by the Free Software Foundation. Oracle designates this
1.10 + * particular file as subject to the "Classpath" exception as provided
1.11 + * by Oracle in the LICENSE file that accompanied this code.
1.12 + *
1.13 + * This code is distributed in the hope that it will be useful, but WITHOUT
1.14 + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
1.15 + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
1.16 + * version 2 for more details (a copy is included in the LICENSE file that
1.17 + * accompanied this code).
1.18 + *
1.19 + * You should have received a copy of the GNU General Public License version
1.20 + * 2 along with this work; if not, write to the Free Software Foundation,
1.21 + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
1.22 + *
1.23 + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
1.24 + * or visit www.oracle.com if you need additional information or have any
1.25 + * questions.
1.26 + */
1.27 +
1.28 +/*
1.29 + * This file is available under and governed by the GNU General Public
1.30 + * License version 2 only, as published by the Free Software Foundation.
1.31 + * However, the following notice accompanied the original version of this
1.32 + * file:
1.33 + *
1.34 + * Written by Doug Lea with assistance from members of JCP JSR-166
1.35 + * Expert Group and released to the public domain, as explained at
1.36 + * http://creativecommons.org/publicdomain/zero/1.0/
1.37 + */
1.38 +
1.39 +package java.util.concurrent;
1.40 +
1.41 +import java.util.AbstractQueue;
1.42 +import java.util.Collection;
1.43 +import java.util.Iterator;
1.44 +import java.util.NoSuchElementException;
1.45 +import java.util.Queue;
1.46 +import java.util.concurrent.TimeUnit;
1.47 +import java.util.concurrent.locks.LockSupport;
1.48 +
1.49 +/**
1.50 + * An unbounded {@link TransferQueue} based on linked nodes.
1.51 + * This queue orders elements FIFO (first-in-first-out) with respect
1.52 + * to any given producer. The <em>head</em> of the queue is that
1.53 + * element that has been on the queue the longest time for some
1.54 + * producer. The <em>tail</em> of the queue is that element that has
1.55 + * been on the queue the shortest time for some producer.
1.56 + *
1.57 + * <p>Beware that, unlike in most collections, the {@code size} method
1.58 + * is <em>NOT</em> a constant-time operation. Because of the
1.59 + * asynchronous nature of these queues, determining the current number
1.60 + * of elements requires a traversal of the elements, and so may report
1.61 + * inaccurate results if this collection is modified during traversal.
1.62 + * Additionally, the bulk operations {@code addAll},
1.63 + * {@code removeAll}, {@code retainAll}, {@code containsAll},
1.64 + * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
1.65 + * to be performed atomically. For example, an iterator operating
1.66 + * concurrently with an {@code addAll} operation might view only some
1.67 + * of the added elements.
1.68 + *
1.69 + * <p>This class and its iterator implement all of the
1.70 + * <em>optional</em> methods of the {@link Collection} and {@link
1.71 + * Iterator} interfaces.
1.72 + *
1.73 + * <p>Memory consistency effects: As with other concurrent
1.74 + * collections, actions in a thread prior to placing an object into a
1.75 + * {@code LinkedTransferQueue}
1.76 + * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
1.77 + * actions subsequent to the access or removal of that element from
1.78 + * the {@code LinkedTransferQueue} in another thread.
1.79 + *
1.80 + * <p>This class is a member of the
1.81 + * <a href="{@docRoot}/../technotes/guides/collections/index.html">
1.82 + * Java Collections Framework</a>.
1.83 + *
1.84 + * @since 1.7
1.85 + * @author Doug Lea
1.86 + * @param <E> the type of elements held in this collection
1.87 + */
1.88 +public class LinkedTransferQueue<E> extends AbstractQueue<E>
1.89 + implements TransferQueue<E>, java.io.Serializable {
1.90 + private static final long serialVersionUID = -3223113410248163686L;
1.91 +
1.92 + /*
1.93 + * *** Overview of Dual Queues with Slack ***
1.94 + *
1.95 + * Dual Queues, introduced by Scherer and Scott
1.96 + * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
1.97 + * (linked) queues in which nodes may represent either data or
1.98 + * requests. When a thread tries to enqueue a data node, but
1.99 + * encounters a request node, it instead "matches" and removes it;
1.100 + * and vice versa for enqueuing requests. Blocking Dual Queues
1.101 + * arrange that threads enqueuing unmatched requests block until
1.102 + * other threads provide the match. Dual Synchronous Queues (see
1.103 + * Scherer, Lea, & Scott
1.104 + * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
1.105 + * additionally arrange that threads enqueuing unmatched data also
1.106 + * block. Dual Transfer Queues support all of these modes, as
1.107 + * dictated by callers.
1.108 + *
1.109 + * A FIFO dual queue may be implemented using a variation of the
1.110 + * Michael & Scott (M&S) lock-free queue algorithm
1.111 + * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
1.112 + * It maintains two pointer fields, "head", pointing to a
1.113 + * (matched) node that in turn points to the first actual
1.114 + * (unmatched) queue node (or null if empty); and "tail" that
1.115 + * points to the last node on the queue (or again null if
1.116 + * empty). For example, here is a possible queue with four data
1.117 + * elements:
1.118 + *
1.119 + * head tail
1.120 + * | |
1.121 + * v v
1.122 + * M -> U -> U -> U -> U
1.123 + *
1.124 + * The M&S queue algorithm is known to be prone to scalability and
1.125 + * overhead limitations when maintaining (via CAS) these head and
1.126 + * tail pointers. This has led to the development of
1.127 + * contention-reducing variants such as elimination arrays (see
1.128 + * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
1.129 + * optimistic back pointers (see Ladan-Mozes & Shavit
1.130 + * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
1.131 + * However, the nature of dual queues enables a simpler tactic for
1.132 + * improving M&S-style implementations when dual-ness is needed.
1.133 + *
1.134 + * In a dual queue, each node must atomically maintain its match
1.135 + * status. While there are other possible variants, we implement
1.136 + * this here as: for a data-mode node, matching entails CASing an
1.137 + * "item" field from a non-null data value to null upon match, and
1.138 + * vice-versa for request nodes, CASing from null to a data
1.139 + * value. (Note that the linearization properties of this style of
1.140 + * queue are easy to verify -- elements are made available by
1.141 + * linking, and unavailable by matching.) Compared to plain M&S
1.142 + * queues, this property of dual queues requires one additional
1.143 + * successful atomic operation per enq/deq pair. But it also
1.144 + * enables lower cost variants of queue maintenance mechanics. (A
1.145 + * variation of this idea applies even for non-dual queues that
1.146 + * support deletion of interior elements, such as
1.147 + * j.u.c.ConcurrentLinkedQueue.)
1.148 + *
1.149 + * Once a node is matched, its match status can never again
1.150 + * change. We may thus arrange that the linked list of them
1.151 + * contain a prefix of zero or more matched nodes, followed by a
1.152 + * suffix of zero or more unmatched nodes. (Note that we allow
1.153 + * both the prefix and suffix to be zero length, which in turn
1.154 + * means that we do not use a dummy header.) If we were not
1.155 + * concerned with either time or space efficiency, we could
1.156 + * correctly perform enqueue and dequeue operations by traversing
1.157 + * from a pointer to the initial node; CASing the item of the
1.158 + * first unmatched node on match and CASing the next field of the
1.159 + * trailing node on appends. (Plus some special-casing when
1.160 + * initially empty). While this would be a terrible idea in
1.161 + * itself, it does have the benefit of not requiring ANY atomic
1.162 + * updates on head/tail fields.
1.163 + *
1.164 + * We introduce here an approach that lies between the extremes of
1.165 + * never versus always updating queue (head and tail) pointers.
1.166 + * This offers a tradeoff between sometimes requiring extra
1.167 + * traversal steps to locate the first and/or last unmatched
1.168 + * nodes, versus the reduced overhead and contention of fewer
1.169 + * updates to queue pointers. For example, a possible snapshot of
1.170 + * a queue is:
1.171 + *
1.172 + * head tail
1.173 + * | |
1.174 + * v v
1.175 + * M -> M -> U -> U -> U -> U
1.176 + *
1.177 + * The best value for this "slack" (the targeted maximum distance
1.178 + * between the value of "head" and the first unmatched node, and
1.179 + * similarly for "tail") is an empirical matter. We have found
1.180 + * that using very small constants in the range of 1-3 work best
1.181 + * over a range of platforms. Larger values introduce increasing
1.182 + * costs of cache misses and risks of long traversal chains, while
1.183 + * smaller values increase CAS contention and overhead.
1.184 + *
1.185 + * Dual queues with slack differ from plain M&S dual queues by
1.186 + * virtue of only sometimes updating head or tail pointers when
1.187 + * matching, appending, or even traversing nodes; in order to
1.188 + * maintain a targeted slack. The idea of "sometimes" may be
1.189 + * operationalized in several ways. The simplest is to use a
1.190 + * per-operation counter incremented on each traversal step, and
1.191 + * to try (via CAS) to update the associated queue pointer
1.192 + * whenever the count exceeds a threshold. Another, that requires
1.193 + * more overhead, is to use random number generators to update
1.194 + * with a given probability per traversal step.
1.195 + *
1.196 + * In any strategy along these lines, because CASes updating
1.197 + * fields may fail, the actual slack may exceed targeted
1.198 + * slack. However, they may be retried at any time to maintain
1.199 + * targets. Even when using very small slack values, this
1.200 + * approach works well for dual queues because it allows all
1.201 + * operations up to the point of matching or appending an item
1.202 + * (hence potentially allowing progress by another thread) to be
1.203 + * read-only, thus not introducing any further contention. As
1.204 + * described below, we implement this by performing slack
1.205 + * maintenance retries only after these points.
1.206 + *
1.207 + * As an accompaniment to such techniques, traversal overhead can
1.208 + * be further reduced without increasing contention of head
1.209 + * pointer updates: Threads may sometimes shortcut the "next" link
1.210 + * path from the current "head" node to be closer to the currently
1.211 + * known first unmatched node, and similarly for tail. Again, this
1.212 + * may be triggered with using thresholds or randomization.
1.213 + *
1.214 + * These ideas must be further extended to avoid unbounded amounts
1.215 + * of costly-to-reclaim garbage caused by the sequential "next"
1.216 + * links of nodes starting at old forgotten head nodes: As first
1.217 + * described in detail by Boehm
1.218 + * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
1.219 + * delays noticing that any arbitrarily old node has become
1.220 + * garbage, all newer dead nodes will also be unreclaimed.
1.221 + * (Similar issues arise in non-GC environments.) To cope with
1.222 + * this in our implementation, upon CASing to advance the head
1.223 + * pointer, we set the "next" link of the previous head to point
1.224 + * only to itself; thus limiting the length of connected dead lists.
1.225 + * (We also take similar care to wipe out possibly garbage
1.226 + * retaining values held in other Node fields.) However, doing so
1.227 + * adds some further complexity to traversal: If any "next"
1.228 + * pointer links to itself, it indicates that the current thread
1.229 + * has lagged behind a head-update, and so the traversal must
1.230 + * continue from the "head". Traversals trying to find the
1.231 + * current tail starting from "tail" may also encounter
1.232 + * self-links, in which case they also continue at "head".
1.233 + *
1.234 + * It is tempting in slack-based scheme to not even use CAS for
1.235 + * updates (similarly to Ladan-Mozes & Shavit). However, this
1.236 + * cannot be done for head updates under the above link-forgetting
1.237 + * mechanics because an update may leave head at a detached node.
1.238 + * And while direct writes are possible for tail updates, they
1.239 + * increase the risk of long retraversals, and hence long garbage
1.240 + * chains, which can be much more costly than is worthwhile
1.241 + * considering that the cost difference of performing a CAS vs
1.242 + * write is smaller when they are not triggered on each operation
1.243 + * (especially considering that writes and CASes equally require
1.244 + * additional GC bookkeeping ("write barriers") that are sometimes
1.245 + * more costly than the writes themselves because of contention).
1.246 + *
1.247 + * *** Overview of implementation ***
1.248 + *
1.249 + * We use a threshold-based approach to updates, with a slack
1.250 + * threshold of two -- that is, we update head/tail when the
1.251 + * current pointer appears to be two or more steps away from the
1.252 + * first/last node. The slack value is hard-wired: a path greater
1.253 + * than one is naturally implemented by checking equality of
1.254 + * traversal pointers except when the list has only one element,
1.255 + * in which case we keep slack threshold at one. Avoiding tracking
1.256 + * explicit counts across method calls slightly simplifies an
1.257 + * already-messy implementation. Using randomization would
1.258 + * probably work better if there were a low-quality dirt-cheap
1.259 + * per-thread one available, but even ThreadLocalRandom is too
1.260 + * heavy for these purposes.
1.261 + *
1.262 + * With such a small slack threshold value, it is not worthwhile
1.263 + * to augment this with path short-circuiting (i.e., unsplicing
1.264 + * interior nodes) except in the case of cancellation/removal (see
1.265 + * below).
1.266 + *
1.267 + * We allow both the head and tail fields to be null before any
1.268 + * nodes are enqueued; initializing upon first append. This
1.269 + * simplifies some other logic, as well as providing more
1.270 + * efficient explicit control paths instead of letting JVMs insert
1.271 + * implicit NullPointerExceptions when they are null. While not
1.272 + * currently fully implemented, we also leave open the possibility
1.273 + * of re-nulling these fields when empty (which is complicated to
1.274 + * arrange, for little benefit.)
1.275 + *
1.276 + * All enqueue/dequeue operations are handled by the single method
1.277 + * "xfer" with parameters indicating whether to act as some form
1.278 + * of offer, put, poll, take, or transfer (each possibly with
1.279 + * timeout). The relative complexity of using one monolithic
1.280 + * method outweighs the code bulk and maintenance problems of
1.281 + * using separate methods for each case.
1.282 + *
1.283 + * Operation consists of up to three phases. The first is
1.284 + * implemented within method xfer, the second in tryAppend, and
1.285 + * the third in method awaitMatch.
1.286 + *
1.287 + * 1. Try to match an existing node
1.288 + *
1.289 + * Starting at head, skip already-matched nodes until finding
1.290 + * an unmatched node of opposite mode, if one exists, in which
1.291 + * case matching it and returning, also if necessary updating
1.292 + * head to one past the matched node (or the node itself if the
1.293 + * list has no other unmatched nodes). If the CAS misses, then
1.294 + * a loop retries advancing head by two steps until either
1.295 + * success or the slack is at most two. By requiring that each
1.296 + * attempt advances head by two (if applicable), we ensure that
1.297 + * the slack does not grow without bound. Traversals also check
1.298 + * if the initial head is now off-list, in which case they
1.299 + * start at the new head.
1.300 + *
1.301 + * If no candidates are found and the call was untimed
1.302 + * poll/offer, (argument "how" is NOW) return.
1.303 + *
1.304 + * 2. Try to append a new node (method tryAppend)
1.305 + *
1.306 + * Starting at current tail pointer, find the actual last node
1.307 + * and try to append a new node (or if head was null, establish
1.308 + * the first node). Nodes can be appended only if their
1.309 + * predecessors are either already matched or are of the same
1.310 + * mode. If we detect otherwise, then a new node with opposite
1.311 + * mode must have been appended during traversal, so we must
1.312 + * restart at phase 1. The traversal and update steps are
1.313 + * otherwise similar to phase 1: Retrying upon CAS misses and
1.314 + * checking for staleness. In particular, if a self-link is
1.315 + * encountered, then we can safely jump to a node on the list
1.316 + * by continuing the traversal at current head.
1.317 + *
1.318 + * On successful append, if the call was ASYNC, return.
1.319 + *
1.320 + * 3. Await match or cancellation (method awaitMatch)
1.321 + *
1.322 + * Wait for another thread to match node; instead cancelling if
1.323 + * the current thread was interrupted or the wait timed out. On
1.324 + * multiprocessors, we use front-of-queue spinning: If a node
1.325 + * appears to be the first unmatched node in the queue, it
1.326 + * spins a bit before blocking. In either case, before blocking
1.327 + * it tries to unsplice any nodes between the current "head"
1.328 + * and the first unmatched node.
1.329 + *
1.330 + * Front-of-queue spinning vastly improves performance of
1.331 + * heavily contended queues. And so long as it is relatively
1.332 + * brief and "quiet", spinning does not much impact performance
1.333 + * of less-contended queues. During spins threads check their
1.334 + * interrupt status and generate a thread-local random number
1.335 + * to decide to occasionally perform a Thread.yield. While
1.336 + * yield has underdefined specs, we assume that might it help,
1.337 + * and will not hurt in limiting impact of spinning on busy
1.338 + * systems. We also use smaller (1/2) spins for nodes that are
1.339 + * not known to be front but whose predecessors have not
1.340 + * blocked -- these "chained" spins avoid artifacts of
1.341 + * front-of-queue rules which otherwise lead to alternating
1.342 + * nodes spinning vs blocking. Further, front threads that
1.343 + * represent phase changes (from data to request node or vice
1.344 + * versa) compared to their predecessors receive additional
1.345 + * chained spins, reflecting longer paths typically required to
1.346 + * unblock threads during phase changes.
1.347 + *
1.348 + *
1.349 + * ** Unlinking removed interior nodes **
1.350 + *
1.351 + * In addition to minimizing garbage retention via self-linking
1.352 + * described above, we also unlink removed interior nodes. These
1.353 + * may arise due to timed out or interrupted waits, or calls to
1.354 + * remove(x) or Iterator.remove. Normally, given a node that was
1.355 + * at one time known to be the predecessor of some node s that is
1.356 + * to be removed, we can unsplice s by CASing the next field of
1.357 + * its predecessor if it still points to s (otherwise s must
1.358 + * already have been removed or is now offlist). But there are two
1.359 + * situations in which we cannot guarantee to make node s
1.360 + * unreachable in this way: (1) If s is the trailing node of list
1.361 + * (i.e., with null next), then it is pinned as the target node
1.362 + * for appends, so can only be removed later after other nodes are
1.363 + * appended. (2) We cannot necessarily unlink s given a
1.364 + * predecessor node that is matched (including the case of being
1.365 + * cancelled): the predecessor may already be unspliced, in which
1.366 + * case some previous reachable node may still point to s.
1.367 + * (For further explanation see Herlihy & Shavit "The Art of
1.368 + * Multiprocessor Programming" chapter 9). Although, in both
1.369 + * cases, we can rule out the need for further action if either s
1.370 + * or its predecessor are (or can be made to be) at, or fall off
1.371 + * from, the head of list.
1.372 + *
1.373 + * Without taking these into account, it would be possible for an
1.374 + * unbounded number of supposedly removed nodes to remain
1.375 + * reachable. Situations leading to such buildup are uncommon but
1.376 + * can occur in practice; for example when a series of short timed
1.377 + * calls to poll repeatedly time out but never otherwise fall off
1.378 + * the list because of an untimed call to take at the front of the
1.379 + * queue.
1.380 + *
1.381 + * When these cases arise, rather than always retraversing the
1.382 + * entire list to find an actual predecessor to unlink (which
1.383 + * won't help for case (1) anyway), we record a conservative
1.384 + * estimate of possible unsplice failures (in "sweepVotes").
1.385 + * We trigger a full sweep when the estimate exceeds a threshold
1.386 + * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
1.387 + * removal failures to tolerate before sweeping through, unlinking
1.388 + * cancelled nodes that were not unlinked upon initial removal.
1.389 + * We perform sweeps by the thread hitting threshold (rather than
1.390 + * background threads or by spreading work to other threads)
1.391 + * because in the main contexts in which removal occurs, the
1.392 + * caller is already timed-out, cancelled, or performing a
1.393 + * potentially O(n) operation (e.g. remove(x)), none of which are
1.394 + * time-critical enough to warrant the overhead that alternatives
1.395 + * would impose on other threads.
1.396 + *
1.397 + * Because the sweepVotes estimate is conservative, and because
1.398 + * nodes become unlinked "naturally" as they fall off the head of
1.399 + * the queue, and because we allow votes to accumulate even while
1.400 + * sweeps are in progress, there are typically significantly fewer
1.401 + * such nodes than estimated. Choice of a threshold value
1.402 + * balances the likelihood of wasted effort and contention, versus
1.403 + * providing a worst-case bound on retention of interior nodes in
1.404 + * quiescent queues. The value defined below was chosen
1.405 + * empirically to balance these under various timeout scenarios.
1.406 + *
1.407 + * Note that we cannot self-link unlinked interior nodes during
1.408 + * sweeps. However, the associated garbage chains terminate when
1.409 + * some successor ultimately falls off the head of the list and is
1.410 + * self-linked.
1.411 + */
1.412 +
1.413 + /** True if on multiprocessor */
1.414 + private static final boolean MP =
1.415 + Runtime.getRuntime().availableProcessors() > 1;
1.416 +
1.417 + /**
1.418 + * The number of times to spin (with randomly interspersed calls
1.419 + * to Thread.yield) on multiprocessor before blocking when a node
1.420 + * is apparently the first waiter in the queue. See above for
1.421 + * explanation. Must be a power of two. The value is empirically
1.422 + * derived -- it works pretty well across a variety of processors,
1.423 + * numbers of CPUs, and OSes.
1.424 + */
1.425 + private static final int FRONT_SPINS = 1 << 7;
1.426 +
1.427 + /**
1.428 + * The number of times to spin before blocking when a node is
1.429 + * preceded by another node that is apparently spinning. Also
1.430 + * serves as an increment to FRONT_SPINS on phase changes, and as
1.431 + * base average frequency for yielding during spins. Must be a
1.432 + * power of two.
1.433 + */
1.434 + private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
1.435 +
1.436 + /**
1.437 + * The maximum number of estimated removal failures (sweepVotes)
1.438 + * to tolerate before sweeping through the queue unlinking
1.439 + * cancelled nodes that were not unlinked upon initial
1.440 + * removal. See above for explanation. The value must be at least
1.441 + * two to avoid useless sweeps when removing trailing nodes.
1.442 + */
1.443 + static final int SWEEP_THRESHOLD = 32;
1.444 +
1.445 + /**
1.446 + * Queue nodes. Uses Object, not E, for items to allow forgetting
1.447 + * them after use. Relies heavily on Unsafe mechanics to minimize
1.448 + * unnecessary ordering constraints: Writes that are intrinsically
1.449 + * ordered wrt other accesses or CASes use simple relaxed forms.
1.450 + */
1.451 + static final class Node {
1.452 + final boolean isData; // false if this is a request node
1.453 + volatile Object item; // initially non-null if isData; CASed to match
1.454 + volatile Node next;
1.455 + volatile Thread waiter; // null until waiting
1.456 +
1.457 + // CAS methods for fields
1.458 + final boolean casNext(Node cmp, Node val) {
1.459 + return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
1.460 + }
1.461 +
1.462 + final boolean casItem(Object cmp, Object val) {
1.463 + // assert cmp == null || cmp.getClass() != Node.class;
1.464 + return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
1.465 + }
1.466 +
1.467 + /**
1.468 + * Constructs a new node. Uses relaxed write because item can
1.469 + * only be seen after publication via casNext.
1.470 + */
1.471 + Node(Object item, boolean isData) {
1.472 + UNSAFE.putObject(this, itemOffset, item); // relaxed write
1.473 + this.isData = isData;
1.474 + }
1.475 +
1.476 + /**
1.477 + * Links node to itself to avoid garbage retention. Called
1.478 + * only after CASing head field, so uses relaxed write.
1.479 + */
1.480 + final void forgetNext() {
1.481 + UNSAFE.putObject(this, nextOffset, this);
1.482 + }
1.483 +
1.484 + /**
1.485 + * Sets item to self and waiter to null, to avoid garbage
1.486 + * retention after matching or cancelling. Uses relaxed writes
1.487 + * because order is already constrained in the only calling
1.488 + * contexts: item is forgotten only after volatile/atomic
1.489 + * mechanics that extract items. Similarly, clearing waiter
1.490 + * follows either CAS or return from park (if ever parked;
1.491 + * else we don't care).
1.492 + */
1.493 + final void forgetContents() {
1.494 + UNSAFE.putObject(this, itemOffset, this);
1.495 + UNSAFE.putObject(this, waiterOffset, null);
1.496 + }
1.497 +
1.498 + /**
1.499 + * Returns true if this node has been matched, including the
1.500 + * case of artificial matches due to cancellation.
1.501 + */
1.502 + final boolean isMatched() {
1.503 + Object x = item;
1.504 + return (x == this) || ((x == null) == isData);
1.505 + }
1.506 +
1.507 + /**
1.508 + * Returns true if this is an unmatched request node.
1.509 + */
1.510 + final boolean isUnmatchedRequest() {
1.511 + return !isData && item == null;
1.512 + }
1.513 +
1.514 + /**
1.515 + * Returns true if a node with the given mode cannot be
1.516 + * appended to this node because this node is unmatched and
1.517 + * has opposite data mode.
1.518 + */
1.519 + final boolean cannotPrecede(boolean haveData) {
1.520 + boolean d = isData;
1.521 + Object x;
1.522 + return d != haveData && (x = item) != this && (x != null) == d;
1.523 + }
1.524 +
1.525 + /**
1.526 + * Tries to artificially match a data node -- used by remove.
1.527 + */
1.528 + final boolean tryMatchData() {
1.529 + // assert isData;
1.530 + Object x = item;
1.531 + if (x != null && x != this && casItem(x, null)) {
1.532 + LockSupport.unpark(waiter);
1.533 + return true;
1.534 + }
1.535 + return false;
1.536 + }
1.537 +
1.538 + private static final long serialVersionUID = -3375979862319811754L;
1.539 +
1.540 + // Unsafe mechanics
1.541 + private static final sun.misc.Unsafe UNSAFE;
1.542 + private static final long itemOffset;
1.543 + private static final long nextOffset;
1.544 + private static final long waiterOffset;
1.545 + static {
1.546 + try {
1.547 + UNSAFE = sun.misc.Unsafe.getUnsafe();
1.548 + Class k = Node.class;
1.549 + itemOffset = UNSAFE.objectFieldOffset
1.550 + (k.getDeclaredField("item"));
1.551 + nextOffset = UNSAFE.objectFieldOffset
1.552 + (k.getDeclaredField("next"));
1.553 + waiterOffset = UNSAFE.objectFieldOffset
1.554 + (k.getDeclaredField("waiter"));
1.555 + } catch (Exception e) {
1.556 + throw new Error(e);
1.557 + }
1.558 + }
1.559 + }
1.560 +
1.561 + /** head of the queue; null until first enqueue */
1.562 + transient volatile Node head;
1.563 +
1.564 + /** tail of the queue; null until first append */
1.565 + private transient volatile Node tail;
1.566 +
1.567 + /** The number of apparent failures to unsplice removed nodes */
1.568 + private transient volatile int sweepVotes;
1.569 +
1.570 + // CAS methods for fields
1.571 + private boolean casTail(Node cmp, Node val) {
1.572 + return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
1.573 + }
1.574 +
1.575 + private boolean casHead(Node cmp, Node val) {
1.576 + return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
1.577 + }
1.578 +
1.579 + private boolean casSweepVotes(int cmp, int val) {
1.580 + return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
1.581 + }
1.582 +
1.583 + /*
1.584 + * Possible values for "how" argument in xfer method.
1.585 + */
1.586 + private static final int NOW = 0; // for untimed poll, tryTransfer
1.587 + private static final int ASYNC = 1; // for offer, put, add
1.588 + private static final int SYNC = 2; // for transfer, take
1.589 + private static final int TIMED = 3; // for timed poll, tryTransfer
1.590 +
1.591 + @SuppressWarnings("unchecked")
1.592 + static <E> E cast(Object item) {
1.593 + // assert item == null || item.getClass() != Node.class;
1.594 + return (E) item;
1.595 + }
1.596 +
1.597 + /**
1.598 + * Implements all queuing methods. See above for explanation.
1.599 + *
1.600 + * @param e the item or null for take
1.601 + * @param haveData true if this is a put, else a take
1.602 + * @param how NOW, ASYNC, SYNC, or TIMED
1.603 + * @param nanos timeout in nanosecs, used only if mode is TIMED
1.604 + * @return an item if matched, else e
1.605 + * @throws NullPointerException if haveData mode but e is null
1.606 + */
1.607 + private E xfer(E e, boolean haveData, int how, long nanos) {
1.608 + if (haveData && (e == null))
1.609 + throw new NullPointerException();
1.610 + Node s = null; // the node to append, if needed
1.611 +
1.612 + retry:
1.613 + for (;;) { // restart on append race
1.614 +
1.615 + for (Node h = head, p = h; p != null;) { // find & match first node
1.616 + boolean isData = p.isData;
1.617 + Object item = p.item;
1.618 + if (item != p && (item != null) == isData) { // unmatched
1.619 + if (isData == haveData) // can't match
1.620 + break;
1.621 + if (p.casItem(item, e)) { // match
1.622 + for (Node q = p; q != h;) {
1.623 + Node n = q.next; // update by 2 unless singleton
1.624 + if (head == h && casHead(h, n == null ? q : n)) {
1.625 + h.forgetNext();
1.626 + break;
1.627 + } // advance and retry
1.628 + if ((h = head) == null ||
1.629 + (q = h.next) == null || !q.isMatched())
1.630 + break; // unless slack < 2
1.631 + }
1.632 + LockSupport.unpark(p.waiter);
1.633 + return this.<E>cast(item);
1.634 + }
1.635 + }
1.636 + Node n = p.next;
1.637 + p = (p != n) ? n : (h = head); // Use head if p offlist
1.638 + }
1.639 +
1.640 + if (how != NOW) { // No matches available
1.641 + if (s == null)
1.642 + s = new Node(e, haveData);
1.643 + Node pred = tryAppend(s, haveData);
1.644 + if (pred == null)
1.645 + continue retry; // lost race vs opposite mode
1.646 + if (how != ASYNC)
1.647 + return awaitMatch(s, pred, e, (how == TIMED), nanos);
1.648 + }
1.649 + return e; // not waiting
1.650 + }
1.651 + }
1.652 +
1.653 + /**
1.654 + * Tries to append node s as tail.
1.655 + *
1.656 + * @param s the node to append
1.657 + * @param haveData true if appending in data mode
1.658 + * @return null on failure due to losing race with append in
1.659 + * different mode, else s's predecessor, or s itself if no
1.660 + * predecessor
1.661 + */
1.662 + private Node tryAppend(Node s, boolean haveData) {
1.663 + for (Node t = tail, p = t;;) { // move p to last node and append
1.664 + Node n, u; // temps for reads of next & tail
1.665 + if (p == null && (p = head) == null) {
1.666 + if (casHead(null, s))
1.667 + return s; // initialize
1.668 + }
1.669 + else if (p.cannotPrecede(haveData))
1.670 + return null; // lost race vs opposite mode
1.671 + else if ((n = p.next) != null) // not last; keep traversing
1.672 + p = p != t && t != (u = tail) ? (t = u) : // stale tail
1.673 + (p != n) ? n : null; // restart if off list
1.674 + else if (!p.casNext(null, s))
1.675 + p = p.next; // re-read on CAS failure
1.676 + else {
1.677 + if (p != t) { // update if slack now >= 2
1.678 + while ((tail != t || !casTail(t, s)) &&
1.679 + (t = tail) != null &&
1.680 + (s = t.next) != null && // advance and retry
1.681 + (s = s.next) != null && s != t);
1.682 + }
1.683 + return p;
1.684 + }
1.685 + }
1.686 + }
1.687 +
1.688 + /**
1.689 + * Spins/yields/blocks until node s is matched or caller gives up.
1.690 + *
1.691 + * @param s the waiting node
1.692 + * @param pred the predecessor of s, or s itself if it has no
1.693 + * predecessor, or null if unknown (the null case does not occur
1.694 + * in any current calls but may in possible future extensions)
1.695 + * @param e the comparison value for checking match
1.696 + * @param timed if true, wait only until timeout elapses
1.697 + * @param nanos timeout in nanosecs, used only if timed is true
1.698 + * @return matched item, or e if unmatched on interrupt or timeout
1.699 + */
1.700 + private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
1.701 + long lastTime = timed ? System.nanoTime() : 0L;
1.702 + Thread w = Thread.currentThread();
1.703 + int spins = -1; // initialized after first item and cancel checks
1.704 + ThreadLocalRandom randomYields = null; // bound if needed
1.705 +
1.706 + for (;;) {
1.707 + Object item = s.item;
1.708 + if (item != e) { // matched
1.709 + // assert item != s;
1.710 + s.forgetContents(); // avoid garbage
1.711 + return this.<E>cast(item);
1.712 + }
1.713 + if ((w.isInterrupted() || (timed && nanos <= 0)) &&
1.714 + s.casItem(e, s)) { // cancel
1.715 + unsplice(pred, s);
1.716 + return e;
1.717 + }
1.718 +
1.719 + if (spins < 0) { // establish spins at/near front
1.720 + if ((spins = spinsFor(pred, s.isData)) > 0)
1.721 + randomYields = ThreadLocalRandom.current();
1.722 + }
1.723 + else if (spins > 0) { // spin
1.724 + --spins;
1.725 + if (randomYields.nextInt(CHAINED_SPINS) == 0)
1.726 + Thread.yield(); // occasionally yield
1.727 + }
1.728 + else if (s.waiter == null) {
1.729 + s.waiter = w; // request unpark then recheck
1.730 + }
1.731 + else if (timed) {
1.732 + long now = System.nanoTime();
1.733 + if ((nanos -= now - lastTime) > 0)
1.734 + LockSupport.parkNanos(this, nanos);
1.735 + lastTime = now;
1.736 + }
1.737 + else {
1.738 + LockSupport.park(this);
1.739 + }
1.740 + }
1.741 + }
1.742 +
1.743 + /**
1.744 + * Returns spin/yield value for a node with given predecessor and
1.745 + * data mode. See above for explanation.
1.746 + */
1.747 + private static int spinsFor(Node pred, boolean haveData) {
1.748 + if (MP && pred != null) {
1.749 + if (pred.isData != haveData) // phase change
1.750 + return FRONT_SPINS + CHAINED_SPINS;
1.751 + if (pred.isMatched()) // probably at front
1.752 + return FRONT_SPINS;
1.753 + if (pred.waiter == null) // pred apparently spinning
1.754 + return CHAINED_SPINS;
1.755 + }
1.756 + return 0;
1.757 + }
1.758 +
1.759 + /* -------------- Traversal methods -------------- */
1.760 +
1.761 + /**
1.762 + * Returns the successor of p, or the head node if p.next has been
1.763 + * linked to self, which will only be true if traversing with a
1.764 + * stale pointer that is now off the list.
1.765 + */
1.766 + final Node succ(Node p) {
1.767 + Node next = p.next;
1.768 + return (p == next) ? head : next;
1.769 + }
1.770 +
1.771 + /**
1.772 + * Returns the first unmatched node of the given mode, or null if
1.773 + * none. Used by methods isEmpty, hasWaitingConsumer.
1.774 + */
1.775 + private Node firstOfMode(boolean isData) {
1.776 + for (Node p = head; p != null; p = succ(p)) {
1.777 + if (!p.isMatched())
1.778 + return (p.isData == isData) ? p : null;
1.779 + }
1.780 + return null;
1.781 + }
1.782 +
1.783 + /**
1.784 + * Returns the item in the first unmatched node with isData; or
1.785 + * null if none. Used by peek.
1.786 + */
1.787 + private E firstDataItem() {
1.788 + for (Node p = head; p != null; p = succ(p)) {
1.789 + Object item = p.item;
1.790 + if (p.isData) {
1.791 + if (item != null && item != p)
1.792 + return this.<E>cast(item);
1.793 + }
1.794 + else if (item == null)
1.795 + return null;
1.796 + }
1.797 + return null;
1.798 + }
1.799 +
1.800 + /**
1.801 + * Traverses and counts unmatched nodes of the given mode.
1.802 + * Used by methods size and getWaitingConsumerCount.
1.803 + */
1.804 + private int countOfMode(boolean data) {
1.805 + int count = 0;
1.806 + for (Node p = head; p != null; ) {
1.807 + if (!p.isMatched()) {
1.808 + if (p.isData != data)
1.809 + return 0;
1.810 + if (++count == Integer.MAX_VALUE) // saturated
1.811 + break;
1.812 + }
1.813 + Node n = p.next;
1.814 + if (n != p)
1.815 + p = n;
1.816 + else {
1.817 + count = 0;
1.818 + p = head;
1.819 + }
1.820 + }
1.821 + return count;
1.822 + }
1.823 +
1.824 + final class Itr implements Iterator<E> {
1.825 + private Node nextNode; // next node to return item for
1.826 + private E nextItem; // the corresponding item
1.827 + private Node lastRet; // last returned node, to support remove
1.828 + private Node lastPred; // predecessor to unlink lastRet
1.829 +
1.830 + /**
1.831 + * Moves to next node after prev, or first node if prev null.
1.832 + */
1.833 + private void advance(Node prev) {
1.834 + /*
1.835 + * To track and avoid buildup of deleted nodes in the face
1.836 + * of calls to both Queue.remove and Itr.remove, we must
1.837 + * include variants of unsplice and sweep upon each
1.838 + * advance: Upon Itr.remove, we may need to catch up links
1.839 + * from lastPred, and upon other removes, we might need to
1.840 + * skip ahead from stale nodes and unsplice deleted ones
1.841 + * found while advancing.
1.842 + */
1.843 +
1.844 + Node r, b; // reset lastPred upon possible deletion of lastRet
1.845 + if ((r = lastRet) != null && !r.isMatched())
1.846 + lastPred = r; // next lastPred is old lastRet
1.847 + else if ((b = lastPred) == null || b.isMatched())
1.848 + lastPred = null; // at start of list
1.849 + else {
1.850 + Node s, n; // help with removal of lastPred.next
1.851 + while ((s = b.next) != null &&
1.852 + s != b && s.isMatched() &&
1.853 + (n = s.next) != null && n != s)
1.854 + b.casNext(s, n);
1.855 + }
1.856 +
1.857 + this.lastRet = prev;
1.858 +
1.859 + for (Node p = prev, s, n;;) {
1.860 + s = (p == null) ? head : p.next;
1.861 + if (s == null)
1.862 + break;
1.863 + else if (s == p) {
1.864 + p = null;
1.865 + continue;
1.866 + }
1.867 + Object item = s.item;
1.868 + if (s.isData) {
1.869 + if (item != null && item != s) {
1.870 + nextItem = LinkedTransferQueue.<E>cast(item);
1.871 + nextNode = s;
1.872 + return;
1.873 + }
1.874 + }
1.875 + else if (item == null)
1.876 + break;
1.877 + // assert s.isMatched();
1.878 + if (p == null)
1.879 + p = s;
1.880 + else if ((n = s.next) == null)
1.881 + break;
1.882 + else if (s == n)
1.883 + p = null;
1.884 + else
1.885 + p.casNext(s, n);
1.886 + }
1.887 + nextNode = null;
1.888 + nextItem = null;
1.889 + }
1.890 +
1.891 + Itr() {
1.892 + advance(null);
1.893 + }
1.894 +
1.895 + public final boolean hasNext() {
1.896 + return nextNode != null;
1.897 + }
1.898 +
1.899 + public final E next() {
1.900 + Node p = nextNode;
1.901 + if (p == null) throw new NoSuchElementException();
1.902 + E e = nextItem;
1.903 + advance(p);
1.904 + return e;
1.905 + }
1.906 +
1.907 + public final void remove() {
1.908 + final Node lastRet = this.lastRet;
1.909 + if (lastRet == null)
1.910 + throw new IllegalStateException();
1.911 + this.lastRet = null;
1.912 + if (lastRet.tryMatchData())
1.913 + unsplice(lastPred, lastRet);
1.914 + }
1.915 + }
1.916 +
1.917 + /* -------------- Removal methods -------------- */
1.918 +
1.919 + /**
1.920 + * Unsplices (now or later) the given deleted/cancelled node with
1.921 + * the given predecessor.
1.922 + *
1.923 + * @param pred a node that was at one time known to be the
1.924 + * predecessor of s, or null or s itself if s is/was at head
1.925 + * @param s the node to be unspliced
1.926 + */
1.927 + final void unsplice(Node pred, Node s) {
1.928 + s.forgetContents(); // forget unneeded fields
1.929 + /*
1.930 + * See above for rationale. Briefly: if pred still points to
1.931 + * s, try to unlink s. If s cannot be unlinked, because it is
1.932 + * trailing node or pred might be unlinked, and neither pred
1.933 + * nor s are head or offlist, add to sweepVotes, and if enough
1.934 + * votes have accumulated, sweep.
1.935 + */
1.936 + if (pred != null && pred != s && pred.next == s) {
1.937 + Node n = s.next;
1.938 + if (n == null ||
1.939 + (n != s && pred.casNext(s, n) && pred.isMatched())) {
1.940 + for (;;) { // check if at, or could be, head
1.941 + Node h = head;
1.942 + if (h == pred || h == s || h == null)
1.943 + return; // at head or list empty
1.944 + if (!h.isMatched())
1.945 + break;
1.946 + Node hn = h.next;
1.947 + if (hn == null)
1.948 + return; // now empty
1.949 + if (hn != h && casHead(h, hn))
1.950 + h.forgetNext(); // advance head
1.951 + }
1.952 + if (pred.next != pred && s.next != s) { // recheck if offlist
1.953 + for (;;) { // sweep now if enough votes
1.954 + int v = sweepVotes;
1.955 + if (v < SWEEP_THRESHOLD) {
1.956 + if (casSweepVotes(v, v + 1))
1.957 + break;
1.958 + }
1.959 + else if (casSweepVotes(v, 0)) {
1.960 + sweep();
1.961 + break;
1.962 + }
1.963 + }
1.964 + }
1.965 + }
1.966 + }
1.967 + }
1.968 +
1.969 + /**
1.970 + * Unlinks matched (typically cancelled) nodes encountered in a
1.971 + * traversal from head.
1.972 + */
1.973 + private void sweep() {
1.974 + for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
1.975 + if (!s.isMatched())
1.976 + // Unmatched nodes are never self-linked
1.977 + p = s;
1.978 + else if ((n = s.next) == null) // trailing node is pinned
1.979 + break;
1.980 + else if (s == n) // stale
1.981 + // No need to also check for p == s, since that implies s == n
1.982 + p = head;
1.983 + else
1.984 + p.casNext(s, n);
1.985 + }
1.986 + }
1.987 +
1.988 + /**
1.989 + * Main implementation of remove(Object)
1.990 + */
1.991 + private boolean findAndRemove(Object e) {
1.992 + if (e != null) {
1.993 + for (Node pred = null, p = head; p != null; ) {
1.994 + Object item = p.item;
1.995 + if (p.isData) {
1.996 + if (item != null && item != p && e.equals(item) &&
1.997 + p.tryMatchData()) {
1.998 + unsplice(pred, p);
1.999 + return true;
1.1000 + }
1.1001 + }
1.1002 + else if (item == null)
1.1003 + break;
1.1004 + pred = p;
1.1005 + if ((p = p.next) == pred) { // stale
1.1006 + pred = null;
1.1007 + p = head;
1.1008 + }
1.1009 + }
1.1010 + }
1.1011 + return false;
1.1012 + }
1.1013 +
1.1014 +
1.1015 + /**
1.1016 + * Creates an initially empty {@code LinkedTransferQueue}.
1.1017 + */
1.1018 + public LinkedTransferQueue() {
1.1019 + }
1.1020 +
1.1021 + /**
1.1022 + * Creates a {@code LinkedTransferQueue}
1.1023 + * initially containing the elements of the given collection,
1.1024 + * added in traversal order of the collection's iterator.
1.1025 + *
1.1026 + * @param c the collection of elements to initially contain
1.1027 + * @throws NullPointerException if the specified collection or any
1.1028 + * of its elements are null
1.1029 + */
1.1030 + public LinkedTransferQueue(Collection<? extends E> c) {
1.1031 + this();
1.1032 + addAll(c);
1.1033 + }
1.1034 +
1.1035 + /**
1.1036 + * Inserts the specified element at the tail of this queue.
1.1037 + * As the queue is unbounded, this method will never block.
1.1038 + *
1.1039 + * @throws NullPointerException if the specified element is null
1.1040 + */
1.1041 + public void put(E e) {
1.1042 + xfer(e, true, ASYNC, 0);
1.1043 + }
1.1044 +
1.1045 + /**
1.1046 + * Inserts the specified element at the tail of this queue.
1.1047 + * As the queue is unbounded, this method will never block or
1.1048 + * return {@code false}.
1.1049 + *
1.1050 + * @return {@code true} (as specified by
1.1051 + * {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
1.1052 + * @throws NullPointerException if the specified element is null
1.1053 + */
1.1054 + public boolean offer(E e, long timeout, TimeUnit unit) {
1.1055 + xfer(e, true, ASYNC, 0);
1.1056 + return true;
1.1057 + }
1.1058 +
1.1059 + /**
1.1060 + * Inserts the specified element at the tail of this queue.
1.1061 + * As the queue is unbounded, this method will never return {@code false}.
1.1062 + *
1.1063 + * @return {@code true} (as specified by {@link Queue#offer})
1.1064 + * @throws NullPointerException if the specified element is null
1.1065 + */
1.1066 + public boolean offer(E e) {
1.1067 + xfer(e, true, ASYNC, 0);
1.1068 + return true;
1.1069 + }
1.1070 +
1.1071 + /**
1.1072 + * Inserts the specified element at the tail of this queue.
1.1073 + * As the queue is unbounded, this method will never throw
1.1074 + * {@link IllegalStateException} or return {@code false}.
1.1075 + *
1.1076 + * @return {@code true} (as specified by {@link Collection#add})
1.1077 + * @throws NullPointerException if the specified element is null
1.1078 + */
1.1079 + public boolean add(E e) {
1.1080 + xfer(e, true, ASYNC, 0);
1.1081 + return true;
1.1082 + }
1.1083 +
1.1084 + /**
1.1085 + * Transfers the element to a waiting consumer immediately, if possible.
1.1086 + *
1.1087 + * <p>More precisely, transfers the specified element immediately
1.1088 + * if there exists a consumer already waiting to receive it (in
1.1089 + * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1.1090 + * otherwise returning {@code false} without enqueuing the element.
1.1091 + *
1.1092 + * @throws NullPointerException if the specified element is null
1.1093 + */
1.1094 + public boolean tryTransfer(E e) {
1.1095 + return xfer(e, true, NOW, 0) == null;
1.1096 + }
1.1097 +
1.1098 + /**
1.1099 + * Transfers the element to a consumer, waiting if necessary to do so.
1.1100 + *
1.1101 + * <p>More precisely, transfers the specified element immediately
1.1102 + * if there exists a consumer already waiting to receive it (in
1.1103 + * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1.1104 + * else inserts the specified element at the tail of this queue
1.1105 + * and waits until the element is received by a consumer.
1.1106 + *
1.1107 + * @throws NullPointerException if the specified element is null
1.1108 + */
1.1109 + public void transfer(E e) throws InterruptedException {
1.1110 + if (xfer(e, true, SYNC, 0) != null) {
1.1111 + Thread.interrupted(); // failure possible only due to interrupt
1.1112 + throw new InterruptedException();
1.1113 + }
1.1114 + }
1.1115 +
1.1116 + /**
1.1117 + * Transfers the element to a consumer if it is possible to do so
1.1118 + * before the timeout elapses.
1.1119 + *
1.1120 + * <p>More precisely, transfers the specified element immediately
1.1121 + * if there exists a consumer already waiting to receive it (in
1.1122 + * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1.1123 + * else inserts the specified element at the tail of this queue
1.1124 + * and waits until the element is received by a consumer,
1.1125 + * returning {@code false} if the specified wait time elapses
1.1126 + * before the element can be transferred.
1.1127 + *
1.1128 + * @throws NullPointerException if the specified element is null
1.1129 + */
1.1130 + public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1.1131 + throws InterruptedException {
1.1132 + if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
1.1133 + return true;
1.1134 + if (!Thread.interrupted())
1.1135 + return false;
1.1136 + throw new InterruptedException();
1.1137 + }
1.1138 +
1.1139 + public E take() throws InterruptedException {
1.1140 + E e = xfer(null, false, SYNC, 0);
1.1141 + if (e != null)
1.1142 + return e;
1.1143 + Thread.interrupted();
1.1144 + throw new InterruptedException();
1.1145 + }
1.1146 +
1.1147 + public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1.1148 + E e = xfer(null, false, TIMED, unit.toNanos(timeout));
1.1149 + if (e != null || !Thread.interrupted())
1.1150 + return e;
1.1151 + throw new InterruptedException();
1.1152 + }
1.1153 +
1.1154 + public E poll() {
1.1155 + return xfer(null, false, NOW, 0);
1.1156 + }
1.1157 +
1.1158 + /**
1.1159 + * @throws NullPointerException {@inheritDoc}
1.1160 + * @throws IllegalArgumentException {@inheritDoc}
1.1161 + */
1.1162 + public int drainTo(Collection<? super E> c) {
1.1163 + if (c == null)
1.1164 + throw new NullPointerException();
1.1165 + if (c == this)
1.1166 + throw new IllegalArgumentException();
1.1167 + int n = 0;
1.1168 + E e;
1.1169 + while ( (e = poll()) != null) {
1.1170 + c.add(e);
1.1171 + ++n;
1.1172 + }
1.1173 + return n;
1.1174 + }
1.1175 +
1.1176 + /**
1.1177 + * @throws NullPointerException {@inheritDoc}
1.1178 + * @throws IllegalArgumentException {@inheritDoc}
1.1179 + */
1.1180 + public int drainTo(Collection<? super E> c, int maxElements) {
1.1181 + if (c == null)
1.1182 + throw new NullPointerException();
1.1183 + if (c == this)
1.1184 + throw new IllegalArgumentException();
1.1185 + int n = 0;
1.1186 + E e;
1.1187 + while (n < maxElements && (e = poll()) != null) {
1.1188 + c.add(e);
1.1189 + ++n;
1.1190 + }
1.1191 + return n;
1.1192 + }
1.1193 +
1.1194 + /**
1.1195 + * Returns an iterator over the elements in this queue in proper sequence.
1.1196 + * The elements will be returned in order from first (head) to last (tail).
1.1197 + *
1.1198 + * <p>The returned iterator is a "weakly consistent" iterator that
1.1199 + * will never throw {@link java.util.ConcurrentModificationException
1.1200 + * ConcurrentModificationException}, and guarantees to traverse
1.1201 + * elements as they existed upon construction of the iterator, and
1.1202 + * may (but is not guaranteed to) reflect any modifications
1.1203 + * subsequent to construction.
1.1204 + *
1.1205 + * @return an iterator over the elements in this queue in proper sequence
1.1206 + */
1.1207 + public Iterator<E> iterator() {
1.1208 + return new Itr();
1.1209 + }
1.1210 +
1.1211 + public E peek() {
1.1212 + return firstDataItem();
1.1213 + }
1.1214 +
1.1215 + /**
1.1216 + * Returns {@code true} if this queue contains no elements.
1.1217 + *
1.1218 + * @return {@code true} if this queue contains no elements
1.1219 + */
1.1220 + public boolean isEmpty() {
1.1221 + for (Node p = head; p != null; p = succ(p)) {
1.1222 + if (!p.isMatched())
1.1223 + return !p.isData;
1.1224 + }
1.1225 + return true;
1.1226 + }
1.1227 +
1.1228 + public boolean hasWaitingConsumer() {
1.1229 + return firstOfMode(false) != null;
1.1230 + }
1.1231 +
1.1232 + /**
1.1233 + * Returns the number of elements in this queue. If this queue
1.1234 + * contains more than {@code Integer.MAX_VALUE} elements, returns
1.1235 + * {@code Integer.MAX_VALUE}.
1.1236 + *
1.1237 + * <p>Beware that, unlike in most collections, this method is
1.1238 + * <em>NOT</em> a constant-time operation. Because of the
1.1239 + * asynchronous nature of these queues, determining the current
1.1240 + * number of elements requires an O(n) traversal.
1.1241 + *
1.1242 + * @return the number of elements in this queue
1.1243 + */
1.1244 + public int size() {
1.1245 + return countOfMode(true);
1.1246 + }
1.1247 +
1.1248 + public int getWaitingConsumerCount() {
1.1249 + return countOfMode(false);
1.1250 + }
1.1251 +
1.1252 + /**
1.1253 + * Removes a single instance of the specified element from this queue,
1.1254 + * if it is present. More formally, removes an element {@code e} such
1.1255 + * that {@code o.equals(e)}, if this queue contains one or more such
1.1256 + * elements.
1.1257 + * Returns {@code true} if this queue contained the specified element
1.1258 + * (or equivalently, if this queue changed as a result of the call).
1.1259 + *
1.1260 + * @param o element to be removed from this queue, if present
1.1261 + * @return {@code true} if this queue changed as a result of the call
1.1262 + */
1.1263 + public boolean remove(Object o) {
1.1264 + return findAndRemove(o);
1.1265 + }
1.1266 +
1.1267 + /**
1.1268 + * Returns {@code true} if this queue contains the specified element.
1.1269 + * More formally, returns {@code true} if and only if this queue contains
1.1270 + * at least one element {@code e} such that {@code o.equals(e)}.
1.1271 + *
1.1272 + * @param o object to be checked for containment in this queue
1.1273 + * @return {@code true} if this queue contains the specified element
1.1274 + */
1.1275 + public boolean contains(Object o) {
1.1276 + if (o == null) return false;
1.1277 + for (Node p = head; p != null; p = succ(p)) {
1.1278 + Object item = p.item;
1.1279 + if (p.isData) {
1.1280 + if (item != null && item != p && o.equals(item))
1.1281 + return true;
1.1282 + }
1.1283 + else if (item == null)
1.1284 + break;
1.1285 + }
1.1286 + return false;
1.1287 + }
1.1288 +
1.1289 + /**
1.1290 + * Always returns {@code Integer.MAX_VALUE} because a
1.1291 + * {@code LinkedTransferQueue} is not capacity constrained.
1.1292 + *
1.1293 + * @return {@code Integer.MAX_VALUE} (as specified by
1.1294 + * {@link BlockingQueue#remainingCapacity()})
1.1295 + */
1.1296 + public int remainingCapacity() {
1.1297 + return Integer.MAX_VALUE;
1.1298 + }
1.1299 +
1.1300 + /**
1.1301 + * Saves the state to a stream (that is, serializes it).
1.1302 + *
1.1303 + * @serialData All of the elements (each an {@code E}) in
1.1304 + * the proper order, followed by a null
1.1305 + * @param s the stream
1.1306 + */
1.1307 + private void writeObject(java.io.ObjectOutputStream s)
1.1308 + throws java.io.IOException {
1.1309 + s.defaultWriteObject();
1.1310 + for (E e : this)
1.1311 + s.writeObject(e);
1.1312 + // Use trailing null as sentinel
1.1313 + s.writeObject(null);
1.1314 + }
1.1315 +
1.1316 + /**
1.1317 + * Reconstitutes the Queue instance from a stream (that is,
1.1318 + * deserializes it).
1.1319 + *
1.1320 + * @param s the stream
1.1321 + */
1.1322 + private void readObject(java.io.ObjectInputStream s)
1.1323 + throws java.io.IOException, ClassNotFoundException {
1.1324 + s.defaultReadObject();
1.1325 + for (;;) {
1.1326 + @SuppressWarnings("unchecked") E item = (E) s.readObject();
1.1327 + if (item == null)
1.1328 + break;
1.1329 + else
1.1330 + offer(item);
1.1331 + }
1.1332 + }
1.1333 +
1.1334 + // Unsafe mechanics
1.1335 +
1.1336 + private static final sun.misc.Unsafe UNSAFE;
1.1337 + private static final long headOffset;
1.1338 + private static final long tailOffset;
1.1339 + private static final long sweepVotesOffset;
1.1340 + static {
1.1341 + try {
1.1342 + UNSAFE = sun.misc.Unsafe.getUnsafe();
1.1343 + Class k = LinkedTransferQueue.class;
1.1344 + headOffset = UNSAFE.objectFieldOffset
1.1345 + (k.getDeclaredField("head"));
1.1346 + tailOffset = UNSAFE.objectFieldOffset
1.1347 + (k.getDeclaredField("tail"));
1.1348 + sweepVotesOffset = UNSAFE.objectFieldOffset
1.1349 + (k.getDeclaredField("sweepVotes"));
1.1350 + } catch (Exception e) {
1.1351 + throw new Error(e);
1.1352 + }
1.1353 + }
1.1354 +}