| /* |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| |
| /* |
| * This file is available under and governed by the GNU General Public |
| * License version 2 only, as published by the Free Software Foundation. |
| * However, the following notice accompanied the original version of this |
| * file: |
| * |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/publicdomain/zero/1.0/ |
| */ |
| |
| package java.util.concurrent; |
| |
| import java.lang.invoke.MethodHandles; |
| import java.lang.invoke.VarHandle; |
| import java.util.AbstractQueue; |
| import java.util.Arrays; |
| import java.util.Collection; |
| import java.util.Iterator; |
| import java.util.NoSuchElementException; |
| import java.util.Objects; |
| import java.util.Queue; |
| import java.util.Spliterator; |
| import java.util.Spliterators; |
| import java.util.concurrent.locks.LockSupport; |
| import java.util.function.Consumer; |
| import java.util.function.Predicate; |
| |
| /** |
| * An unbounded {@link TransferQueue} based on linked nodes. |
| * This queue orders elements FIFO (first-in-first-out) with respect |
| * to any given producer. The <em>head</em> of the queue is that |
| * element that has been on the queue the longest time for some |
| * producer. The <em>tail</em> of the queue is that element that has |
| * been on the queue the shortest time for some producer. |
| * |
| * <p>Beware that, unlike in most collections, the {@code size} method |
| * is <em>NOT</em> a constant-time operation. Because of the |
| * asynchronous nature of these queues, determining the current number |
| * of elements requires a traversal of the elements, and so may report |
| * inaccurate results if this collection is modified during traversal. |
| * |
| * <p>Bulk operations that add, remove, or examine multiple elements, |
| * such as {@link #addAll}, {@link #removeIf} or {@link #forEach}, |
| * are <em>not</em> guaranteed to be performed atomically. |
| * For example, a {@code forEach} traversal concurrent with an {@code |
| * addAll} operation might observe only some of the added elements. |
| * |
| * <p>This class and its iterator implement all of the <em>optional</em> |
| * methods of the {@link Collection} and {@link Iterator} interfaces. |
| * |
| * <p>Memory consistency effects: As with other concurrent |
| * collections, actions in a thread prior to placing an object into a |
| * {@code LinkedTransferQueue} |
| * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
| * actions subsequent to the access or removal of that element from |
| * the {@code LinkedTransferQueue} in another thread. |
| * |
| * <p>This class is a member of the |
| * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework"> |
| * Java Collections Framework</a>. |
| * |
| * @since 1.7 |
| * @author Doug Lea |
| * @param <E> the type of elements held in this queue |
| */ |
| public class LinkedTransferQueue<E> extends AbstractQueue<E> |
| implements TransferQueue<E>, java.io.Serializable { |
| private static final long serialVersionUID = -3223113410248163686L; |
| |
| /* |
| * *** Overview of Dual Queues with Slack *** |
| * |
| * Dual Queues, introduced by Scherer and Scott |
| * (http://www.cs.rochester.edu/~scott/papers/2004_DISC_dual_DS.pdf) |
| * are (linked) queues in which nodes may represent either data or |
| * requests. When a thread tries to enqueue a data node, but |
| * encounters a request node, it instead "matches" and removes it; |
| * and vice versa for enqueuing requests. Blocking Dual Queues |
| * arrange that threads enqueuing unmatched requests block until |
| * other threads provide the match. Dual Synchronous Queues (see |
| * Scherer, Lea, & Scott |
| * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) |
| * additionally arrange that threads enqueuing unmatched data also |
| * block. Dual Transfer Queues support all of these modes, as |
| * dictated by callers. |
| * |
| * A FIFO dual queue may be implemented using a variation of the |
| * Michael & Scott (M&S) lock-free queue algorithm |
| * (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf). |
| * It maintains two pointer fields, "head", pointing to a |
| * (matched) node that in turn points to the first actual |
| * (unmatched) queue node (or null if empty); and "tail" that |
| * points to the last node on the queue (or again null if |
| * empty). For example, here is a possible queue with four data |
| * elements: |
| * |
| * head tail |
| * | | |
| * v v |
| * M -> U -> U -> U -> U |
| * |
| * The M&S queue algorithm is known to be prone to scalability and |
| * overhead limitations when maintaining (via CAS) these head and |
| * tail pointers. This has led to the development of |
| * contention-reducing variants such as elimination arrays (see |
| * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and |
| * optimistic back pointers (see Ladan-Mozes & Shavit |
| * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). |
| * However, the nature of dual queues enables a simpler tactic for |
| * improving M&S-style implementations when dual-ness is needed. |
| * |
| * In a dual queue, each node must atomically maintain its match |
| * status. While there are other possible variants, we implement |
| * this here as: for a data-mode node, matching entails CASing an |
| * "item" field from a non-null data value to null upon match, and |
| * vice-versa for request nodes, CASing from null to a data |
| * value. (Note that the linearization properties of this style of |
| * queue are easy to verify -- elements are made available by |
| * linking, and unavailable by matching.) Compared to plain M&S |
| * queues, this property of dual queues requires one additional |
| * successful atomic operation per enq/deq pair. But it also |
| * enables lower cost variants of queue maintenance mechanics. (A |
| * variation of this idea applies even for non-dual queues that |
| * support deletion of interior elements, such as |
| * j.u.c.ConcurrentLinkedQueue.) |
| * |
| * Once a node is matched, its match status can never again |
| * change. We may thus arrange that the linked list of them |
| * contain a prefix of zero or more matched nodes, followed by a |
| * suffix of zero or more unmatched nodes. (Note that we allow |
| * both the prefix and suffix to be zero length, which in turn |
| * means that we do not use a dummy header.) If we were not |
| * concerned with either time or space efficiency, we could |
| * correctly perform enqueue and dequeue operations by traversing |
| * from a pointer to the initial node; CASing the item of the |
| * first unmatched node on match and CASing the next field of the |
| * trailing node on appends. While this would be a terrible idea |
| * in itself, it does have the benefit of not requiring ANY atomic |
| * updates on head/tail fields. |
| * |
| * We introduce here an approach that lies between the extremes of |
| * never versus always updating queue (head and tail) pointers. |
| * This offers a tradeoff between sometimes requiring extra |
| * traversal steps to locate the first and/or last unmatched |
| * nodes, versus the reduced overhead and contention of fewer |
| * updates to queue pointers. For example, a possible snapshot of |
| * a queue is: |
| * |
| * head tail |
| * | | |
| * v v |
| * M -> M -> U -> U -> U -> U |
| * |
| * The best value for this "slack" (the targeted maximum distance |
| * between the value of "head" and the first unmatched node, and |
| * similarly for "tail") is an empirical matter. We have found |
| * that using very small constants in the range of 1-3 work best |
| * over a range of platforms. Larger values introduce increasing |
| * costs of cache misses and risks of long traversal chains, while |
| * smaller values increase CAS contention and overhead. |
| * |
| * Dual queues with slack differ from plain M&S dual queues by |
| * virtue of only sometimes updating head or tail pointers when |
| * matching, appending, or even traversing nodes; in order to |
| * maintain a targeted slack. The idea of "sometimes" may be |
| * operationalized in several ways. The simplest is to use a |
| * per-operation counter incremented on each traversal step, and |
| * to try (via CAS) to update the associated queue pointer |
| * whenever the count exceeds a threshold. Another, that requires |
| * more overhead, is to use random number generators to update |
| * with a given probability per traversal step. |
| * |
| * In any strategy along these lines, because CASes updating |
| * fields may fail, the actual slack may exceed targeted slack. |
| * However, they may be retried at any time to maintain targets. |
| * Even when using very small slack values, this approach works |
| * well for dual queues because it allows all operations up to the |
| * point of matching or appending an item (hence potentially |
| * allowing progress by another thread) to be read-only, thus not |
| * introducing any further contention. As described below, we |
| * implement this by performing slack maintenance retries only |
| * after these points. |
| * |
| * As an accompaniment to such techniques, traversal overhead can |
| * be further reduced without increasing contention of head |
| * pointer updates: Threads may sometimes shortcut the "next" link |
| * path from the current "head" node to be closer to the currently |
| * known first unmatched node, and similarly for tail. Again, this |
| * may be triggered with using thresholds or randomization. |
| * |
| * These ideas must be further extended to avoid unbounded amounts |
| * of costly-to-reclaim garbage caused by the sequential "next" |
| * links of nodes starting at old forgotten head nodes: As first |
| * described in detail by Boehm |
| * (http://portal.acm.org/citation.cfm?doid=503272.503282), if a GC |
| * delays noticing that any arbitrarily old node has become |
| * garbage, all newer dead nodes will also be unreclaimed. |
| * (Similar issues arise in non-GC environments.) To cope with |
| * this in our implementation, upon CASing to advance the head |
| * pointer, we set the "next" link of the previous head to point |
| * only to itself; thus limiting the length of chains of dead nodes. |
| * (We also take similar care to wipe out possibly garbage |
| * retaining values held in other Node fields.) However, doing so |
| * adds some further complexity to traversal: If any "next" |
| * pointer links to itself, it indicates that the current thread |
| * has lagged behind a head-update, and so the traversal must |
| * continue from the "head". Traversals trying to find the |
| * current tail starting from "tail" may also encounter |
| * self-links, in which case they also continue at "head". |
| * |
| * It is tempting in slack-based scheme to not even use CAS for |
| * updates (similarly to Ladan-Mozes & Shavit). However, this |
| * cannot be done for head updates under the above link-forgetting |
| * mechanics because an update may leave head at a detached node. |
| * And while direct writes are possible for tail updates, they |
| * increase the risk of long retraversals, and hence long garbage |
| * chains, which can be much more costly than is worthwhile |
| * considering that the cost difference of performing a CAS vs |
| * write is smaller when they are not triggered on each operation |
| * (especially considering that writes and CASes equally require |
| * additional GC bookkeeping ("write barriers") that are sometimes |
| * more costly than the writes themselves because of contention). |
| * |
| * *** Overview of implementation *** |
| * |
| * We use a threshold-based approach to updates, with a slack |
| * threshold of two -- that is, we update head/tail when the |
| * current pointer appears to be two or more steps away from the |
| * first/last node. The slack value is hard-wired: a path greater |
| * than one is naturally implemented by checking equality of |
| * traversal pointers except when the list has only one element, |
| * in which case we keep slack threshold at one. Avoiding tracking |
| * explicit counts across method calls slightly simplifies an |
| * already-messy implementation. Using randomization would |
| * probably work better if there were a low-quality dirt-cheap |
| * per-thread one available, but even ThreadLocalRandom is too |
| * heavy for these purposes. |
| * |
| * With such a small slack threshold value, it is not worthwhile |
| * to augment this with path short-circuiting (i.e., unsplicing |
| * interior nodes) except in the case of cancellation/removal (see |
| * below). |
| * |
| * All enqueue/dequeue operations are handled by the single method |
| * "xfer" with parameters indicating whether to act as some form |
| * of offer, put, poll, take, or transfer (each possibly with |
| * timeout). The relative complexity of using one monolithic |
| * method outweighs the code bulk and maintenance problems of |
| * using separate methods for each case. |
| * |
| * Operation consists of up to two phases. The first is implemented |
| * in method xfer, the second in method awaitMatch. |
| * |
| * 1. Traverse until matching or appending (method xfer) |
| * |
| * Conceptually, we simply traverse all nodes starting from head. |
| * If we encounter an unmatched node of opposite mode, we match |
| * it and return, also updating head (by at least 2 hops) to |
| * one past the matched node (or the node itself if it's the |
| * pinned trailing node). Traversals also check for the |
| * possibility of falling off-list, in which case they restart. |
| * |
| * If the trailing node of the list is reached, a match is not |
| * possible. If this call was untimed poll or tryTransfer |
| * (argument "how" is NOW), return empty-handed immediately. |
| * Else a new node is CAS-appended. On successful append, if |
| * this call was ASYNC (e.g. offer), an element was |
| * successfully added to the end of the queue and we return. |
| * |
| * Of course, this naive traversal is O(n) when no match is |
| * possible. We optimize the traversal by maintaining a tail |
| * pointer, which is expected to be "near" the end of the list. |
| * It is only safe to fast-forward to tail (in the presence of |
| * arbitrary concurrent changes) if it is pointing to a node of |
| * the same mode, even if it is dead (in this case no preceding |
| * node could still be matchable by this traversal). If we |
| * need to restart due to falling off-list, we can again |
| * fast-forward to tail, but only if it has changed since the |
| * last traversal (else we might loop forever). If tail cannot |
| * be used, traversal starts at head (but in this case we |
| * expect to be able to match near head). As with head, we |
| * CAS-advance the tail pointer by at least two hops. |
| * |
| * 2. Await match or cancellation (method awaitMatch) |
| * |
| * Wait for another thread to match node; instead cancelling if |
| * the current thread was interrupted or the wait timed out. On |
| * multiprocessors, we use front-of-queue spinning: If a node |
| * appears to be the first unmatched node in the queue, it |
| * spins a bit before blocking. In either case, before blocking |
| * it tries to unsplice any nodes between the current "head" |
| * and the first unmatched node. |
| * |
| * Front-of-queue spinning vastly improves performance of |
| * heavily contended queues. And so long as it is relatively |
| * brief and "quiet", spinning does not much impact performance |
| * of less-contended queues. During spins threads check their |
| * interrupt status and generate a thread-local random number |
| * to decide to occasionally perform a Thread.yield. While |
| * yield has underdefined specs, we assume that it might help, |
| * and will not hurt, in limiting impact of spinning on busy |
| * systems. We also use smaller (1/2) spins for nodes that are |
| * not known to be front but whose predecessors have not |
| * blocked -- these "chained" spins avoid artifacts of |
| * front-of-queue rules which otherwise lead to alternating |
| * nodes spinning vs blocking. Further, front threads that |
| * represent phase changes (from data to request node or vice |
| * versa) compared to their predecessors receive additional |
| * chained spins, reflecting longer paths typically required to |
| * unblock threads during phase changes. |
| * |
| * |
| * ** Unlinking removed interior nodes ** |
| * |
| * In addition to minimizing garbage retention via self-linking |
| * described above, we also unlink removed interior nodes. These |
| * may arise due to timed out or interrupted waits, or calls to |
| * remove(x) or Iterator.remove. Normally, given a node that was |
| * at one time known to be the predecessor of some node s that is |
| * to be removed, we can unsplice s by CASing the next field of |
| * its predecessor if it still points to s (otherwise s must |
| * already have been removed or is now offlist). But there are two |
| * situations in which we cannot guarantee to make node s |
| * unreachable in this way: (1) If s is the trailing node of list |
| * (i.e., with null next), then it is pinned as the target node |
| * for appends, so can only be removed later after other nodes are |
| * appended. (2) We cannot necessarily unlink s given a |
| * predecessor node that is matched (including the case of being |
| * cancelled): the predecessor may already be unspliced, in which |
| * case some previous reachable node may still point to s. |
| * (For further explanation see Herlihy & Shavit "The Art of |
| * Multiprocessor Programming" chapter 9). Although, in both |
| * cases, we can rule out the need for further action if either s |
| * or its predecessor are (or can be made to be) at, or fall off |
| * from, the head of list. |
| * |
| * Without taking these into account, it would be possible for an |
| * unbounded number of supposedly removed nodes to remain reachable. |
| * Situations leading to such buildup are uncommon but can occur |
| * in practice; for example when a series of short timed calls to |
| * poll repeatedly time out at the trailing node but otherwise |
| * never fall off the list because of an untimed call to take() at |
| * the front of the queue. |
| * |
| * When these cases arise, rather than always retraversing the |
| * entire list to find an actual predecessor to unlink (which |
| * won't help for case (1) anyway), we record a conservative |
| * estimate of possible unsplice failures (in "sweepVotes"). |
| * We trigger a full sweep when the estimate exceeds a threshold |
| * ("SWEEP_THRESHOLD") indicating the maximum number of estimated |
| * removal failures to tolerate before sweeping through, unlinking |
| * cancelled nodes that were not unlinked upon initial removal. |
| * We perform sweeps by the thread hitting threshold (rather than |
| * background threads or by spreading work to other threads) |
| * because in the main contexts in which removal occurs, the |
| * caller is timed-out or cancelled, which are not time-critical |
| * enough to warrant the overhead that alternatives would impose |
| * on other threads. |
| * |
| * Because the sweepVotes estimate is conservative, and because |
| * nodes become unlinked "naturally" as they fall off the head of |
| * the queue, and because we allow votes to accumulate even while |
| * sweeps are in progress, there are typically significantly fewer |
| * such nodes than estimated. Choice of a threshold value |
| * balances the likelihood of wasted effort and contention, versus |
| * providing a worst-case bound on retention of interior nodes in |
| * quiescent queues. The value defined below was chosen |
| * empirically to balance these under various timeout scenarios. |
| * |
| * Because traversal operations on the linked list of nodes are a |
| * natural opportunity to sweep dead nodes, we generally do so, |
| * including all the operations that might remove elements as they |
| * traverse, such as removeIf and Iterator.remove. This largely |
| * eliminates long chains of dead interior nodes, except from |
| * cancelled or timed out blocking operations. |
| * |
| * Note that we cannot self-link unlinked interior nodes during |
| * sweeps. However, the associated garbage chains terminate when |
| * some successor ultimately falls off the head of the list and is |
| * self-linked. |
| */ |
| |
| /** True if on multiprocessor */ |
| private static final boolean MP = |
| Runtime.getRuntime().availableProcessors() > 1; |
| |
| /** |
| * The number of times to spin (with randomly interspersed calls |
| * to Thread.yield) on multiprocessor before blocking when a node |
| * is apparently the first waiter in the queue. See above for |
| * explanation. Must be a power of two. The value is empirically |
| * derived -- it works pretty well across a variety of processors, |
| * numbers of CPUs, and OSes. |
| */ |
| private static final int FRONT_SPINS = 1 << 7; |
| |
| /** |
| * The number of times to spin before blocking when a node is |
| * preceded by another node that is apparently spinning. Also |
| * serves as an increment to FRONT_SPINS on phase changes, and as |
| * base average frequency for yielding during spins. Must be a |
| * power of two. |
| */ |
| private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
| |
| /** |
| * The maximum number of estimated removal failures (sweepVotes) |
| * to tolerate before sweeping through the queue unlinking |
| * cancelled nodes that were not unlinked upon initial |
| * removal. See above for explanation. The value must be at least |
| * two to avoid useless sweeps when removing trailing nodes. |
| */ |
| static final int SWEEP_THRESHOLD = 32; |
| |
| /** |
| * Queue nodes. Uses Object, not E, for items to allow forgetting |
| * them after use. Writes that are intrinsically ordered wrt |
| * other accesses or CASes use simple relaxed forms. |
| */ |
| static final class Node { |
| final boolean isData; // false if this is a request node |
| volatile Object item; // initially non-null if isData; CASed to match |
| volatile Node next; |
| volatile Thread waiter; // null when not waiting for a match |
| |
| /** |
| * Constructs a data node holding item if item is non-null, |
| * else a request node. Uses relaxed write because item can |
| * only be seen after piggy-backing publication via CAS. |
| */ |
| Node(Object item) { |
| ITEM.set(this, item); |
| isData = (item != null); |
| } |
| |
| /** Constructs a (matched data) dummy node. */ |
| Node() { |
| isData = true; |
| } |
| |
| final boolean casNext(Node cmp, Node val) { |
| // assert val != null; |
| return NEXT.compareAndSet(this, cmp, val); |
| } |
| |
| final boolean casItem(Object cmp, Object val) { |
| // assert isData == (cmp != null); |
| // assert isData == (val == null); |
| // assert !(cmp instanceof Node); |
| return ITEM.compareAndSet(this, cmp, val); |
| } |
| |
| /** |
| * Links node to itself to avoid garbage retention. Called |
| * only after CASing head field, so uses relaxed write. |
| */ |
| final void selfLink() { |
| // assert isMatched(); |
| NEXT.setRelease(this, this); |
| } |
| |
| final void appendRelaxed(Node next) { |
| // assert next != null; |
| // assert this.next == null; |
| NEXT.set(this, next); |
| } |
| |
| /** |
| * Sets item (of a request node) to self and waiter to null, |
| * to avoid garbage retention after matching or cancelling. |
| * Uses relaxed writes because order is already constrained in |
| * the only calling contexts: item is forgotten only after |
| * volatile/atomic mechanics that extract items, and visitors |
| * of request nodes only ever check whether item is null. |
| * Similarly, clearing waiter follows either CAS or return |
| * from park (if ever parked; else we don't care). |
| */ |
| final void forgetContents() { |
| // assert isMatched(); |
| if (!isData) |
| ITEM.set(this, this); |
| WAITER.set(this, null); |
| } |
| |
| /** |
| * Returns true if this node has been matched, including the |
| * case of artificial matches due to cancellation. |
| */ |
| final boolean isMatched() { |
| return isData == (item == null); |
| } |
| |
| /** Tries to CAS-match this node; if successful, wakes waiter. */ |
| final boolean tryMatch(Object cmp, Object val) { |
| if (casItem(cmp, val)) { |
| LockSupport.unpark(waiter); |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * Returns true if a node with the given mode cannot be |
| * appended to this node because this node is unmatched and |
| * has opposite data mode. |
| */ |
| final boolean cannotPrecede(boolean haveData) { |
| boolean d = isData; |
| return d != haveData && d != (item == null); |
| } |
| |
| private static final long serialVersionUID = -3375979862319811754L; |
| } |
| |
| /** |
| * A node from which the first live (non-matched) node (if any) |
| * can be reached in O(1) time. |
| * Invariants: |
| * - all live nodes are reachable from head via .next |
| * - head != null |
| * - (tmp = head).next != tmp || tmp != head |
| * Non-invariants: |
| * - head may or may not be live |
| * - it is permitted for tail to lag behind head, that is, for tail |
| * to not be reachable from head! |
| */ |
| transient volatile Node head; |
| |
| /** |
| * A node from which the last node on list (that is, the unique |
| * node with node.next == null) can be reached in O(1) time. |
| * Invariants: |
| * - the last node is always reachable from tail via .next |
| * - tail != null |
| * Non-invariants: |
| * - tail may or may not be live |
| * - it is permitted for tail to lag behind head, that is, for tail |
| * to not be reachable from head! |
| * - tail.next may or may not be self-linked. |
| */ |
| private transient volatile Node tail; |
| |
| /** The number of apparent failures to unsplice cancelled nodes */ |
| private transient volatile int sweepVotes; |
| |
| private boolean casTail(Node cmp, Node val) { |
| // assert cmp != null; |
| // assert val != null; |
| return TAIL.compareAndSet(this, cmp, val); |
| } |
| |
| private boolean casHead(Node cmp, Node val) { |
| return HEAD.compareAndSet(this, cmp, val); |
| } |
| |
| /** Atomic version of ++sweepVotes. */ |
| private int incSweepVotes() { |
| return (int) SWEEPVOTES.getAndAdd(this, 1) + 1; |
| } |
| |
| /** |
| * Tries to CAS pred.next (or head, if pred is null) from c to p. |
| * Caller must ensure that we're not unlinking the trailing node. |
| */ |
| private boolean tryCasSuccessor(Node pred, Node c, Node p) { |
| // assert p != null; |
| // assert c.isData != (c.item != null); |
| // assert c != p; |
| if (pred != null) |
| return pred.casNext(c, p); |
| if (casHead(c, p)) { |
| c.selfLink(); |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * Collapses dead (matched) nodes between pred and q. |
| * @param pred the last known live node, or null if none |
| * @param c the first dead node |
| * @param p the last dead node |
| * @param q p.next: the next live node, or null if at end |
| * @return pred if pred still alive and CAS succeeded; else p |
| */ |
| private Node skipDeadNodes(Node pred, Node c, Node p, Node q) { |
| // assert pred != c; |
| // assert p != q; |
| // assert c.isMatched(); |
| // assert p.isMatched(); |
| if (q == null) { |
| // Never unlink trailing node. |
| if (c == p) return pred; |
| q = p; |
| } |
| return (tryCasSuccessor(pred, c, q) |
| && (pred == null || !pred.isMatched())) |
| ? pred : p; |
| } |
| |
| /** |
| * Collapses dead (matched) nodes from h (which was once head) to p. |
| * Caller ensures all nodes from h up to and including p are dead. |
| */ |
| private void skipDeadNodesNearHead(Node h, Node p) { |
| // assert h != null; |
| // assert h != p; |
| // assert p.isMatched(); |
| for (;;) { |
| final Node q; |
| if ((q = p.next) == null) break; |
| else if (!q.isMatched()) { p = q; break; } |
| else if (p == (p = q)) return; |
| } |
| if (casHead(h, p)) |
| h.selfLink(); |
| } |
| |
| /* Possible values for "how" argument in xfer method. */ |
| |
| private static final int NOW = 0; // for untimed poll, tryTransfer |
| private static final int ASYNC = 1; // for offer, put, add |
| private static final int SYNC = 2; // for transfer, take |
| private static final int TIMED = 3; // for timed poll, tryTransfer |
| |
| /** |
| * Implements all queuing methods. See above for explanation. |
| * |
| * @param e the item or null for take |
| * @param haveData true if this is a put, else a take |
| * @param how NOW, ASYNC, SYNC, or TIMED |
| * @param nanos timeout in nanosecs, used only if mode is TIMED |
| * @return an item if matched, else e |
| * @throws NullPointerException if haveData mode but e is null |
| */ |
| @SuppressWarnings("unchecked") |
| private E xfer(E e, boolean haveData, int how, long nanos) { |
| if (haveData && (e == null)) |
| throw new NullPointerException(); |
| |
| restart: for (Node s = null, t = null, h = null;;) { |
| for (Node p = (t != (t = tail) && t.isData == haveData) ? t |
| : (h = head);; ) { |
| final Node q; final Object item; |
| if (p.isData != haveData |
| && haveData == ((item = p.item) == null)) { |
| if (h == null) h = head; |
| if (p.tryMatch(item, e)) { |
| if (h != p) skipDeadNodesNearHead(h, p); |
| return (E) item; |
| } |
| } |
| if ((q = p.next) == null) { |
| if (how == NOW) return e; |
| if (s == null) s = new Node(e); |
| if (!p.casNext(null, s)) continue; |
| if (p != t) casTail(t, s); |
| if (how == ASYNC) return e; |
| return awaitMatch(s, p, e, (how == TIMED), nanos); |
| } |
| if (p == (p = q)) continue restart; |
| } |
| } |
| } |
| |
| /** |
| * Spins/yields/blocks until node s is matched or caller gives up. |
| * |
| * @param s the waiting node |
| * @param pred the predecessor of s, or null if unknown (the null |
| * case does not occur in any current calls but may in possible |
| * future extensions) |
| * @param e the comparison value for checking match |
| * @param timed if true, wait only until timeout elapses |
| * @param nanos timeout in nanosecs, used only if timed is true |
| * @return matched item, or e if unmatched on interrupt or timeout |
| */ |
| private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { |
| final long deadline = timed ? System.nanoTime() + nanos : 0L; |
| Thread w = Thread.currentThread(); |
| int spins = -1; // initialized after first item and cancel checks |
| ThreadLocalRandom randomYields = null; // bound if needed |
| |
| for (;;) { |
| final Object item; |
| if ((item = s.item) != e) { // matched |
| // assert item != s; |
| s.forgetContents(); // avoid garbage |
| @SuppressWarnings("unchecked") E itemE = (E) item; |
| return itemE; |
| } |
| else if (w.isInterrupted() || (timed && nanos <= 0L)) { |
| // try to cancel and unlink |
| if (s.casItem(e, s.isData ? null : s)) { |
| unsplice(pred, s); |
| return e; |
| } |
| // return normally if lost CAS |
| } |
| else if (spins < 0) { // establish spins at/near front |
| if ((spins = spinsFor(pred, s.isData)) > 0) |
| randomYields = ThreadLocalRandom.current(); |
| } |
| else if (spins > 0) { // spin |
| --spins; |
| if (randomYields.nextInt(CHAINED_SPINS) == 0) |
| Thread.yield(); // occasionally yield |
| } |
| else if (s.waiter == null) { |
| s.waiter = w; // request unpark then recheck |
| } |
| else if (timed) { |
| nanos = deadline - System.nanoTime(); |
| if (nanos > 0L) |
| LockSupport.parkNanos(this, nanos); |
| } |
| else { |
| LockSupport.park(this); |
| } |
| } |
| } |
| |
| /** |
| * Returns spin/yield value for a node with given predecessor and |
| * data mode. See above for explanation. |
| */ |
| private static int spinsFor(Node pred, boolean haveData) { |
| if (MP && pred != null) { |
| if (pred.isData != haveData) // phase change |
| return FRONT_SPINS + CHAINED_SPINS; |
| if (pred.isMatched()) // probably at front |
| return FRONT_SPINS; |
| if (pred.waiter == null) // pred apparently spinning |
| return CHAINED_SPINS; |
| } |
| return 0; |
| } |
| |
| /* -------------- Traversal methods -------------- */ |
| |
| /** |
| * Returns the first unmatched data node, or null if none. |
| * Callers must recheck if the returned node is unmatched |
| * before using. |
| */ |
| final Node firstDataNode() { |
| Node first = null; |
| restartFromHead: for (;;) { |
| Node h = head, p = h; |
| for (; p != null;) { |
| final Object item; |
| if ((item = p.item) != null) { |
| if (p.isData) { |
| first = p; |
| break; |
| } |
| } |
| else if (!p.isData) |
| break; |
| final Node q; |
| if ((q = p.next) == null) |
| break; |
| if (p == (p = q)) |
| continue restartFromHead; |
| } |
| if (p != h && casHead(h, p)) |
| h.selfLink(); |
| return first; |
| } |
| } |
| |
| /** |
| * Traverses and counts unmatched nodes of the given mode. |
| * Used by methods size and getWaitingConsumerCount. |
| */ |
| private int countOfMode(boolean data) { |
| restartFromHead: for (;;) { |
| int count = 0; |
| for (Node p = head; p != null;) { |
| if (!p.isMatched()) { |
| if (p.isData != data) |
| return 0; |
| if (++count == Integer.MAX_VALUE) |
| break; // @see Collection.size() |
| } |
| if (p == (p = p.next)) |
| continue restartFromHead; |
| } |
| return count; |
| } |
| } |
| |
| public String toString() { |
| String[] a = null; |
| restartFromHead: for (;;) { |
| int charLength = 0; |
| int size = 0; |
| for (Node p = head; p != null;) { |
| Object item = p.item; |
| if (p.isData) { |
| if (item != null) { |
| if (a == null) |
| a = new String[4]; |
| else if (size == a.length) |
| a = Arrays.copyOf(a, 2 * size); |
| String s = item.toString(); |
| a[size++] = s; |
| charLength += s.length(); |
| } |
| } else if (item == null) |
| break; |
| if (p == (p = p.next)) |
| continue restartFromHead; |
| } |
| |
| if (size == 0) |
| return "[]"; |
| |
| return Helpers.toString(a, size, charLength); |
| } |
| } |
| |
| private Object[] toArrayInternal(Object[] a) { |
| Object[] x = a; |
| restartFromHead: for (;;) { |
| int size = 0; |
| for (Node p = head; p != null;) { |
| Object item = p.item; |
| if (p.isData) { |
| if (item != null) { |
| if (x == null) |
| x = new Object[4]; |
| else if (size == x.length) |
| x = Arrays.copyOf(x, 2 * (size + 4)); |
| x[size++] = item; |
| } |
| } else if (item == null) |
| break; |
| if (p == (p = p.next)) |
| continue restartFromHead; |
| } |
| if (x == null) |
| return new Object[0]; |
| else if (a != null && size <= a.length) { |
| if (a != x) |
| System.arraycopy(x, 0, a, 0, size); |
| if (size < a.length) |
| a[size] = null; |
| return a; |
| } |
| return (size == x.length) ? x : Arrays.copyOf(x, size); |
| } |
| } |
| |
| /** |
| * Returns an array containing all of the elements in this queue, in |
| * proper sequence. |
| * |
| * <p>The returned array will be "safe" in that no references to it are |
| * maintained by this queue. (In other words, this method must allocate |
| * a new array). The caller is thus free to modify the returned array. |
| * |
| * <p>This method acts as bridge between array-based and collection-based |
| * APIs. |
| * |
| * @return an array containing all of the elements in this queue |
| */ |
| public Object[] toArray() { |
| return toArrayInternal(null); |
| } |
| |
| /** |
| * Returns an array containing all of the elements in this queue, in |
| * proper sequence; the runtime type of the returned array is that of |
| * the specified array. If the queue fits in the specified array, it |
| * is returned therein. Otherwise, a new array is allocated with the |
| * runtime type of the specified array and the size of this queue. |
| * |
| * <p>If this queue fits in the specified array with room to spare |
| * (i.e., the array has more elements than this queue), the element in |
| * the array immediately following the end of the queue is set to |
| * {@code null}. |
| * |
| * <p>Like the {@link #toArray()} method, this method acts as bridge between |
| * array-based and collection-based APIs. Further, this method allows |
| * precise control over the runtime type of the output array, and may, |
| * under certain circumstances, be used to save allocation costs. |
| * |
| * <p>Suppose {@code x} is a queue known to contain only strings. |
| * The following code can be used to dump the queue into a newly |
| * allocated array of {@code String}: |
| * |
| * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
| * |
| * Note that {@code toArray(new Object[0])} is identical in function to |
| * {@code toArray()}. |
| * |
| * @param a the array into which the elements of the queue are to |
| * be stored, if it is big enough; otherwise, a new array of the |
| * same runtime type is allocated for this purpose |
| * @return an array containing all of the elements in this queue |
| * @throws ArrayStoreException if the runtime type of the specified array |
| * is not a supertype of the runtime type of every element in |
| * this queue |
| * @throws NullPointerException if the specified array is null |
| */ |
| @SuppressWarnings("unchecked") |
| public <T> T[] toArray(T[] a) { |
| Objects.requireNonNull(a); |
| return (T[]) toArrayInternal(a); |
| } |
| |
| /** |
| * Weakly-consistent iterator. |
| * |
| * Lazily updated ancestor is expected to be amortized O(1) remove(), |
| * but O(n) in the worst case, when lastRet is concurrently deleted. |
| */ |
| final class Itr implements Iterator<E> { |
| private Node nextNode; // next node to return item for |
| private E nextItem; // the corresponding item |
| private Node lastRet; // last returned node, to support remove |
| private Node ancestor; // Helps unlink lastRet on remove() |
| |
| /** |
| * Moves to next node after pred, or first node if pred null. |
| */ |
| @SuppressWarnings("unchecked") |
| private void advance(Node pred) { |
| for (Node p = (pred == null) ? head : pred.next, c = p; |
| p != null; ) { |
| final Object item; |
| if ((item = p.item) != null && p.isData) { |
| nextNode = p; |
| nextItem = (E) item; |
| if (c != p) |
| tryCasSuccessor(pred, c, p); |
| return; |
| } |
| else if (!p.isData && item == null) |
| break; |
| if (c != p && !tryCasSuccessor(pred, c, c = p)) { |
| pred = p; |
| c = p = p.next; |
| } |
| else if (p == (p = p.next)) { |
| pred = null; |
| c = p = head; |
| } |
| } |
| nextItem = null; |
| nextNode = null; |
| } |
| |
| Itr() { |
| advance(null); |
| } |
| |
| public final boolean hasNext() { |
| return nextNode != null; |
| } |
| |
| public final E next() { |
| final Node p; |
| if ((p = nextNode) == null) throw new NoSuchElementException(); |
| E e = nextItem; |
| advance(lastRet = p); |
| return e; |
| } |
| |
| public void forEachRemaining(Consumer<? super E> action) { |
| Objects.requireNonNull(action); |
| Node q = null; |
| for (Node p; (p = nextNode) != null; advance(q = p)) |
| action.accept(nextItem); |
| if (q != null) |
| lastRet = q; |
| } |
| |
| public final void remove() { |
| final Node lastRet = this.lastRet; |
| if (lastRet == null) |
| throw new IllegalStateException(); |
| this.lastRet = null; |
| if (lastRet.item == null) // already deleted? |
| return; |
| // Advance ancestor, collapsing intervening dead nodes |
| Node pred = ancestor; |
| for (Node p = (pred == null) ? head : pred.next, c = p, q; |
| p != null; ) { |
| if (p == lastRet) { |
| final Object item; |
| if ((item = p.item) != null) |
| p.tryMatch(item, null); |
| if ((q = p.next) == null) q = p; |
| if (c != q) tryCasSuccessor(pred, c, q); |
| ancestor = pred; |
| return; |
| } |
| final Object item; final boolean pAlive; |
| if (pAlive = ((item = p.item) != null && p.isData)) { |
| // exceptionally, nothing to do |
| } |
| else if (!p.isData && item == null) |
| break; |
| if ((c != p && !tryCasSuccessor(pred, c, c = p)) || pAlive) { |
| pred = p; |
| c = p = p.next; |
| } |
| else if (p == (p = p.next)) { |
| pred = null; |
| c = p = head; |
| } |
| } |
| // traversal failed to find lastRet; must have been deleted; |
| // leave ancestor at original location to avoid overshoot; |
| // better luck next time! |
| |
| // assert lastRet.isMatched(); |
| } |
| } |
| |
| /** A customized variant of Spliterators.IteratorSpliterator */ |
| final class LTQSpliterator implements Spliterator<E> { |
| static final int MAX_BATCH = 1 << 25; // max batch array size; |
| Node current; // current node; null until initialized |
| int batch; // batch size for splits |
| boolean exhausted; // true when no more nodes |
| LTQSpliterator() {} |
| |
| public Spliterator<E> trySplit() { |
| Node p, q; |
| if ((p = current()) == null || (q = p.next) == null) |
| return null; |
| int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH); |
| Object[] a = null; |
| do { |
| final Object item = p.item; |
| if (p.isData) { |
| if (item != null) { |
| if (a == null) |
| a = new Object[n]; |
| a[i++] = item; |
| } |
| } else if (item == null) { |
| p = null; |
| break; |
| } |
| if (p == (p = q)) |
| p = firstDataNode(); |
| } while (p != null && (q = p.next) != null && i < n); |
| setCurrent(p); |
| return (i == 0) ? null : |
| Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED | |
| Spliterator.NONNULL | |
| Spliterator.CONCURRENT)); |
| } |
| |
| public void forEachRemaining(Consumer<? super E> action) { |
| Objects.requireNonNull(action); |
| final Node p; |
| if ((p = current()) != null) { |
| current = null; |
| exhausted = true; |
| forEachFrom(action, p); |
| } |
| } |
| |
| @SuppressWarnings("unchecked") |
| public boolean tryAdvance(Consumer<? super E> action) { |
| Objects.requireNonNull(action); |
| Node p; |
| if ((p = current()) != null) { |
| E e = null; |
| do { |
| final Object item = p.item; |
| final boolean isData = p.isData; |
| if (p == (p = p.next)) |
| p = head; |
| if (isData) { |
| if (item != null) { |
| e = (E) item; |
| break; |
| } |
| } |
| else if (item == null) |
| p = null; |
| } while (p != null); |
| setCurrent(p); |
| if (e != null) { |
| action.accept(e); |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| private void setCurrent(Node p) { |
| if ((current = p) == null) |
| exhausted = true; |
| } |
| |
| private Node current() { |
| Node p; |
| if ((p = current) == null && !exhausted) |
| setCurrent(p = firstDataNode()); |
| return p; |
| } |
| |
| public long estimateSize() { return Long.MAX_VALUE; } |
| |
| public int characteristics() { |
| return (Spliterator.ORDERED | |
| Spliterator.NONNULL | |
| Spliterator.CONCURRENT); |
| } |
| } |
| |
| /** |
| * Returns a {@link Spliterator} over the elements in this queue. |
| * |
| * <p>The returned spliterator is |
| * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
| * |
| * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, |
| * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. |
| * |
| * @implNote |
| * The {@code Spliterator} implements {@code trySplit} to permit limited |
| * parallelism. |
| * |
| * @return a {@code Spliterator} over the elements in this queue |
| * @since 1.8 |
| */ |
| public Spliterator<E> spliterator() { |
| return new LTQSpliterator(); |
| } |
| |
| /* -------------- Removal methods -------------- */ |
| |
| /** |
| * Unsplices (now or later) the given deleted/cancelled node with |
| * the given predecessor. |
| * |
| * @param pred a node that was at one time known to be the |
| * predecessor of s |
| * @param s the node to be unspliced |
| */ |
| final void unsplice(Node pred, Node s) { |
| // assert pred != null; |
| // assert pred != s; |
| // assert s != null; |
| // assert s.isMatched(); |
| // assert (SWEEP_THRESHOLD & (SWEEP_THRESHOLD - 1)) == 0; |
| s.waiter = null; // disable signals |
| /* |
| * See above for rationale. Briefly: if pred still points to |
| * s, try to unlink s. If s cannot be unlinked, because it is |
| * trailing node or pred might be unlinked, and neither pred |
| * nor s are head or offlist, add to sweepVotes, and if enough |
| * votes have accumulated, sweep. |
| */ |
| if (pred != null && pred.next == s) { |
| Node n = s.next; |
| if (n == null || |
| (n != s && pred.casNext(s, n) && pred.isMatched())) { |
| for (;;) { // check if at, or could be, head |
| Node h = head; |
| if (h == pred || h == s) |
| return; // at head or list empty |
| if (!h.isMatched()) |
| break; |
| Node hn = h.next; |
| if (hn == null) |
| return; // now empty |
| if (hn != h && casHead(h, hn)) |
| h.selfLink(); // advance head |
| } |
| // sweep every SWEEP_THRESHOLD votes |
| if (pred.next != pred && s.next != s // recheck if offlist |
| && (incSweepVotes() & (SWEEP_THRESHOLD - 1)) == 0) |
| sweep(); |
| } |
| } |
| } |
| |
| /** |
| * Unlinks matched (typically cancelled) nodes encountered in a |
| * traversal from head. |
| */ |
| private void sweep() { |
| for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
| if (!s.isMatched()) |
| // Unmatched nodes are never self-linked |
| p = s; |
| else if ((n = s.next) == null) // trailing node is pinned |
| break; |
| else if (s == n) // stale |
| // No need to also check for p == s, since that implies s == n |
| p = head; |
| else |
| p.casNext(s, n); |
| } |
| } |
| |
| /** |
| * Creates an initially empty {@code LinkedTransferQueue}. |
| */ |
| public LinkedTransferQueue() { |
| head = tail = new Node(); |
| } |
| |
| /** |
| * Creates a {@code LinkedTransferQueue} |
| * initially containing the elements of the given collection, |
| * added in traversal order of the collection's iterator. |
| * |
| * @param c the collection of elements to initially contain |
| * @throws NullPointerException if the specified collection or any |
| * of its elements are null |
| */ |
| public LinkedTransferQueue(Collection<? extends E> c) { |
| Node h = null, t = null; |
| for (E e : c) { |
| Node newNode = new Node(Objects.requireNonNull(e)); |
| if (h == null) |
| h = t = newNode; |
| else |
| t.appendRelaxed(t = newNode); |
| } |
| if (h == null) |
| h = t = new Node(); |
| head = h; |
| tail = t; |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue. |
| * As the queue is unbounded, this method will never block. |
| * |
| * @throws NullPointerException if the specified element is null |
| */ |
| public void put(E e) { |
| xfer(e, true, ASYNC, 0); |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue. |
| * As the queue is unbounded, this method will never block or |
| * return {@code false}. |
| * |
| * @return {@code true} (as specified by |
| * {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit) |
| * BlockingQueue.offer}) |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean offer(E e, long timeout, TimeUnit unit) { |
| xfer(e, true, ASYNC, 0); |
| return true; |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue. |
| * As the queue is unbounded, this method will never return {@code false}. |
| * |
| * @return {@code true} (as specified by {@link Queue#offer}) |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean offer(E e) { |
| xfer(e, true, ASYNC, 0); |
| return true; |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue. |
| * As the queue is unbounded, this method will never throw |
| * {@link IllegalStateException} or return {@code false}. |
| * |
| * @return {@code true} (as specified by {@link Collection#add}) |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean add(E e) { |
| xfer(e, true, ASYNC, 0); |
| return true; |
| } |
| |
| /** |
| * Transfers the element to a waiting consumer immediately, if possible. |
| * |
| * <p>More precisely, transfers the specified element immediately |
| * if there exists a consumer already waiting to receive it (in |
| * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
| * otherwise returning {@code false} without enqueuing the element. |
| * |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean tryTransfer(E e) { |
| return xfer(e, true, NOW, 0) == null; |
| } |
| |
| /** |
| * Transfers the element to a consumer, waiting if necessary to do so. |
| * |
| * <p>More precisely, transfers the specified element immediately |
| * if there exists a consumer already waiting to receive it (in |
| * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
| * else inserts the specified element at the tail of this queue |
| * and waits until the element is received by a consumer. |
| * |
| * @throws NullPointerException if the specified element is null |
| */ |
| public void transfer(E e) throws InterruptedException { |
| if (xfer(e, true, SYNC, 0) != null) { |
| Thread.interrupted(); // failure possible only due to interrupt |
| throw new InterruptedException(); |
| } |
| } |
| |
| /** |
| * Transfers the element to a consumer if it is possible to do so |
| * before the timeout elapses. |
| * |
| * <p>More precisely, transfers the specified element immediately |
| * if there exists a consumer already waiting to receive it (in |
| * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
| * else inserts the specified element at the tail of this queue |
| * and waits until the element is received by a consumer, |
| * returning {@code false} if the specified wait time elapses |
| * before the element can be transferred. |
| * |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
| throws InterruptedException { |
| if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) |
| return true; |
| if (!Thread.interrupted()) |
| return false; |
| throw new InterruptedException(); |
| } |
| |
| public E take() throws InterruptedException { |
| E e = xfer(null, false, SYNC, 0); |
| if (e != null) |
| return e; |
| Thread.interrupted(); |
| throw new InterruptedException(); |
| } |
| |
| public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
| E e = xfer(null, false, TIMED, unit.toNanos(timeout)); |
| if (e != null || !Thread.interrupted()) |
| return e; |
| throw new InterruptedException(); |
| } |
| |
| public E poll() { |
| return xfer(null, false, NOW, 0); |
| } |
| |
| /** |
| * @throws NullPointerException {@inheritDoc} |
| * @throws IllegalArgumentException {@inheritDoc} |
| */ |
| public int drainTo(Collection<? super E> c) { |
| Objects.requireNonNull(c); |
| if (c == this) |
| throw new IllegalArgumentException(); |
| int n = 0; |
| for (E e; (e = poll()) != null; n++) |
| c.add(e); |
| return n; |
| } |
| |
| /** |
| * @throws NullPointerException {@inheritDoc} |
| * @throws IllegalArgumentException {@inheritDoc} |
| */ |
| public int drainTo(Collection<? super E> c, int maxElements) { |
| Objects.requireNonNull(c); |
| if (c == this) |
| throw new IllegalArgumentException(); |
| int n = 0; |
| for (E e; n < maxElements && (e = poll()) != null; n++) |
| c.add(e); |
| return n; |
| } |
| |
| /** |
| * Returns an iterator over the elements in this queue in proper sequence. |
| * The elements will be returned in order from first (head) to last (tail). |
| * |
| * <p>The returned iterator is |
| * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
| * |
| * @return an iterator over the elements in this queue in proper sequence |
| */ |
| public Iterator<E> iterator() { |
| return new Itr(); |
| } |
| |
| public E peek() { |
| restartFromHead: for (;;) { |
| for (Node p = head; p != null;) { |
| Object item = p.item; |
| if (p.isData) { |
| if (item != null) { |
| @SuppressWarnings("unchecked") E e = (E) item; |
| return e; |
| } |
| } |
| else if (item == null) |
| break; |
| if (p == (p = p.next)) |
| continue restartFromHead; |
| } |
| return null; |
| } |
| } |
| |
| /** |
| * Returns {@code true} if this queue contains no elements. |
| * |
| * @return {@code true} if this queue contains no elements |
| */ |
| public boolean isEmpty() { |
| return firstDataNode() == null; |
| } |
| |
| public boolean hasWaitingConsumer() { |
| restartFromHead: for (;;) { |
| for (Node p = head; p != null;) { |
| Object item = p.item; |
| if (p.isData) { |
| if (item != null) |
| break; |
| } |
| else if (item == null) |
| return true; |
| if (p == (p = p.next)) |
| continue restartFromHead; |
| } |
| return false; |
| } |
| } |
| |
| /** |
| * Returns the number of elements in this queue. If this queue |
| * contains more than {@code Integer.MAX_VALUE} elements, returns |
| * {@code Integer.MAX_VALUE}. |
| * |
| * <p>Beware that, unlike in most collections, this method is |
| * <em>NOT</em> a constant-time operation. Because of the |
| * asynchronous nature of these queues, determining the current |
| * number of elements requires an O(n) traversal. |
| * |
| * @return the number of elements in this queue |
| */ |
| public int size() { |
| return countOfMode(true); |
| } |
| |
| public int getWaitingConsumerCount() { |
| return countOfMode(false); |
| } |
| |
| /** |
| * Removes a single instance of the specified element from this queue, |
| * if it is present. More formally, removes an element {@code e} such |
| * that {@code o.equals(e)}, if this queue contains one or more such |
| * elements. |
| * Returns {@code true} if this queue contained the specified element |
| * (or equivalently, if this queue changed as a result of the call). |
| * |
| * @param o element to be removed from this queue, if present |
| * @return {@code true} if this queue changed as a result of the call |
| */ |
| public boolean remove(Object o) { |
| if (o == null) return false; |
| restartFromHead: for (;;) { |
| for (Node p = head, pred = null; p != null; ) { |
| Node q = p.next; |
| final Object item; |
| if ((item = p.item) != null) { |
| if (p.isData) { |
| if (o.equals(item) && p.tryMatch(item, null)) { |
| skipDeadNodes(pred, p, p, q); |
| return true; |
| } |
| pred = p; p = q; continue; |
| } |
| } |
| else if (!p.isData) |
| break; |
| for (Node c = p;; q = p.next) { |
| if (q == null || !q.isMatched()) { |
| pred = skipDeadNodes(pred, c, p, q); p = q; break; |
| } |
| if (p == (p = q)) continue restartFromHead; |
| } |
| } |
| return false; |
| } |
| } |
| |
| /** |
| * Returns {@code true} if this queue contains the specified element. |
| * More formally, returns {@code true} if and only if this queue contains |
| * at least one element {@code e} such that {@code o.equals(e)}. |
| * |
| * @param o object to be checked for containment in this queue |
| * @return {@code true} if this queue contains the specified element |
| */ |
| public boolean contains(Object o) { |
| if (o == null) return false; |
| restartFromHead: for (;;) { |
| for (Node p = head, pred = null; p != null; ) { |
| Node q = p.next; |
| final Object item; |
| if ((item = p.item) != null) { |
| if (p.isData) { |
| if (o.equals(item)) |
| return true; |
| pred = p; p = q; continue; |
| } |
| } |
| else if (!p.isData) |
| break; |
| for (Node c = p;; q = p.next) { |
| if (q == null || !q.isMatched()) { |
| pred = skipDeadNodes(pred, c, p, q); p = q; break; |
| } |
| if (p == (p = q)) continue restartFromHead; |
| } |
| } |
| return false; |
| } |
| } |
| |
| /** |
| * Always returns {@code Integer.MAX_VALUE} because a |
| * {@code LinkedTransferQueue} is not capacity constrained. |
| * |
| * @return {@code Integer.MAX_VALUE} (as specified by |
| * {@link java.util.concurrent.BlockingQueue#remainingCapacity() |
| * BlockingQueue.remainingCapacity}) |
| */ |
| public int remainingCapacity() { |
| return Integer.MAX_VALUE; |
| } |
| |
| /** |
| * Saves this queue to a stream (that is, serializes it). |
| * |
| * @param s the stream |
| * @throws java.io.IOException if an I/O error occurs |
| * @serialData All of the elements (each an {@code E}) in |
| * the proper order, followed by a null |
| */ |
| private void writeObject(java.io.ObjectOutputStream s) |
| throws java.io.IOException { |
| s.defaultWriteObject(); |
| for (E e : this) |
| s.writeObject(e); |
| // Use trailing null as sentinel |
| s.writeObject(null); |
| } |
| |
| /** |
| * Reconstitutes this queue from a stream (that is, deserializes it). |
| * @param s the stream |
| * @throws ClassNotFoundException if the class of a serialized object |
| * could not be found |
| * @throws java.io.IOException if an I/O error occurs |
| */ |
| private void readObject(java.io.ObjectInputStream s) |
| throws java.io.IOException, ClassNotFoundException { |
| |
| // Read in elements until trailing null sentinel found |
| Node h = null, t = null; |
| for (Object item; (item = s.readObject()) != null; ) { |
| @SuppressWarnings("unchecked") |
| Node newNode = new Node((E) item); |
| if (h == null) |
| h = t = newNode; |
| else |
| t.appendRelaxed(t = newNode); |
| } |
| if (h == null) |
| h = t = new Node(); |
| head = h; |
| tail = t; |
| } |
| |
| /** |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public boolean removeIf(Predicate<? super E> filter) { |
| Objects.requireNonNull(filter); |
| return bulkRemove(filter); |
| } |
| |
| /** |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public boolean removeAll(Collection<?> c) { |
| Objects.requireNonNull(c); |
| return bulkRemove(e -> c.contains(e)); |
| } |
| |
| /** |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public boolean retainAll(Collection<?> c) { |
| Objects.requireNonNull(c); |
| return bulkRemove(e -> !c.contains(e)); |
| } |
| |
| public void clear() { |
| bulkRemove(e -> true); |
| } |
| |
| /** |
| * Tolerate this many consecutive dead nodes before CAS-collapsing. |
| * Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element. |
| */ |
| private static final int MAX_HOPS = 8; |
| |
| /** Implementation of bulk remove methods. */ |
| @SuppressWarnings("unchecked") |
| private boolean bulkRemove(Predicate<? super E> filter) { |
| boolean removed = false; |
| restartFromHead: for (;;) { |
| int hops = MAX_HOPS; |
| // c will be CASed to collapse intervening dead nodes between |
| // pred (or head if null) and p. |
| for (Node p = head, c = p, pred = null, q; p != null; p = q) { |
| q = p.next; |
| final Object item; boolean pAlive; |
| if (pAlive = ((item = p.item) != null && p.isData)) { |
| if (filter.test((E) item)) { |
| if (p.tryMatch(item, null)) |
| removed = true; |
| pAlive = false; |
| } |
| } |
| else if (!p.isData && item == null) |
| break; |
| if (pAlive || q == null || --hops == 0) { |
| // p might already be self-linked here, but if so: |
| // - CASing head will surely fail |
| // - CASing pred's next will be useless but harmless. |
| if ((c != p && !tryCasSuccessor(pred, c, c = p)) |
| || pAlive) { |
| // if CAS failed or alive, abandon old pred |
| hops = MAX_HOPS; |
| pred = p; |
| c = q; |
| } |
| } else if (p == q) |
| continue restartFromHead; |
| } |
| return removed; |
| } |
| } |
| |
| /** |
| * Runs action on each element found during a traversal starting at p. |
| * If p is null, the action is not run. |
| */ |
| @SuppressWarnings("unchecked") |
| void forEachFrom(Consumer<? super E> action, Node p) { |
| for (Node pred = null; p != null; ) { |
| Node q = p.next; |
| final Object item; |
| if ((item = p.item) != null) { |
| if (p.isData) { |
| action.accept((E) item); |
| pred = p; p = q; continue; |
| } |
| } |
| else if (!p.isData) |
| break; |
| for (Node c = p;; q = p.next) { |
| if (q == null || !q.isMatched()) { |
| pred = skipDeadNodes(pred, c, p, q); p = q; break; |
| } |
| if (p == (p = q)) { pred = null; p = head; break; } |
| } |
| } |
| } |
| |
| /** |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public void forEach(Consumer<? super E> action) { |
| Objects.requireNonNull(action); |
| forEachFrom(action, head); |
| } |
| |
| // VarHandle mechanics |
| private static final VarHandle HEAD; |
| private static final VarHandle TAIL; |
| private static final VarHandle SWEEPVOTES; |
| static final VarHandle ITEM; |
| static final VarHandle NEXT; |
| static final VarHandle WAITER; |
| static { |
| try { |
| MethodHandles.Lookup l = MethodHandles.lookup(); |
| HEAD = l.findVarHandle(LinkedTransferQueue.class, "head", |
| Node.class); |
| TAIL = l.findVarHandle(LinkedTransferQueue.class, "tail", |
| Node.class); |
| SWEEPVOTES = l.findVarHandle(LinkedTransferQueue.class, "sweepVotes", |
| int.class); |
| ITEM = l.findVarHandle(Node.class, "item", Object.class); |
| NEXT = l.findVarHandle(Node.class, "next", Node.class); |
| WAITER = l.findVarHandle(Node.class, "waiter", Thread.class); |
| } catch (ReflectiveOperationException e) { |
| throw new Error(e); |
| } |
| |
| // Reduce the risk of rare disastrous classloading in first call to |
| // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
| Class<?> ensureLoaded = LockSupport.class; |
| } |
| } |
