/* * 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 head of the queue is that * element that has been on the queue the longest time for some * producer. The tail of the queue is that element that has * been on the queue the shortest time for some producer. * *

Beware that, unlike in most collections, the {@code size} method * is NOT 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. * *

Bulk operations that add, remove, or examine multiple elements, * such as {@link #addAll}, {@link #removeIf} or {@link #forEach}, * are not 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. * *

This class and its iterator implement all of the optional * methods of the {@link Collection} and {@link Iterator} interfaces. * *

Memory consistency effects: As with other concurrent * collections, actions in a thread prior to placing an object into a * {@code LinkedTransferQueue} * happen-before * actions subsequent to the access or removal of that element from * the {@code LinkedTransferQueue} in another thread. * *

This class is a member of the * * Java Collections Framework. * * @since 1.7 * @author Doug Lea * @param the type of elements held in this queue */ public class LinkedTransferQueue extends AbstractQueue implements TransferQueue, 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; while (p != null) { if (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. * *

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. * *

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. * *

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}. * *

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. * *

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}: * * 

 {@code String[] y = x.toArray(new String[0]);}
* * 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[] 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 { 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 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 { 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 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 action) { Objects.requireNonNull(action); final Node p; if ((p = current()) != null) { current = null; exhausted = true; forEachFrom(action, p); } } @SuppressWarnings("unchecked") public boolean tryAdvance(Consumer 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. * *

The returned spliterator is * weakly consistent. * *

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 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 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 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. * *

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. * *

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. * *

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 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 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). * *

The returned iterator is * weakly consistent. * * @return an iterator over the elements in this queue in proper sequence */ public Iterator 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}. * *

Beware that, unlike in most collections, this method is * NOT 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 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; ) { Node newNode = new Node(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 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 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 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 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; } }