diff -r 000000000000 -r 212417b74b72 rt/emul/compact/src/main/java/java/util/concurrent/LinkedTransferQueue.java
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/rt/emul/compact/src/main/java/java/util/concurrent/LinkedTransferQueue.java Sat Mar 19 10:46:31 2016 +0100
@@ -0,0 +1,1351 @@
+/*
+ * 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.util.AbstractQueue;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.Queue;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.LockSupport;
+
+/**
+ * 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.
+ * Additionally, the bulk operations {@code addAll},
+ * {@code removeAll}, {@code retainAll}, {@code containsAll},
+ * {@code equals}, and {@code toArray} are not guaranteed
+ * to be performed atomically. For example, an iterator operating
+ * concurrently with an {@code addAll} operation might view 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 collection
+ */
+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.rice.edu/~wns1/papers/2004-DISC-DDS.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/u/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. (Plus some special-casing when
+ * initially empty). 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 connected dead lists.
+ * (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).
+ *
+ * We allow both the head and tail fields to be null before any
+ * nodes are enqueued; initializing upon first append. This
+ * simplifies some other logic, as well as providing more
+ * efficient explicit control paths instead of letting JVMs insert
+ * implicit NullPointerExceptions when they are null. While not
+ * currently fully implemented, we also leave open the possibility
+ * of re-nulling these fields when empty (which is complicated to
+ * arrange, for little benefit.)
+ *
+ * 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 three phases. The first is
+ * implemented within method xfer, the second in tryAppend, and
+ * the third in method awaitMatch.
+ *
+ * 1. Try to match an existing node
+ *
+ * Starting at head, skip already-matched nodes until finding
+ * an unmatched node of opposite mode, if one exists, in which
+ * case matching it and returning, also if necessary updating
+ * head to one past the matched node (or the node itself if the
+ * list has no other unmatched nodes). If the CAS misses, then
+ * a loop retries advancing head by two steps until either
+ * success or the slack is at most two. By requiring that each
+ * attempt advances head by two (if applicable), we ensure that
+ * the slack does not grow without bound. Traversals also check
+ * if the initial head is now off-list, in which case they
+ * start at the new head.
+ *
+ * If no candidates are found and the call was untimed
+ * poll/offer, (argument "how" is NOW) return.
+ *
+ * 2. Try to append a new node (method tryAppend)
+ *
+ * Starting at current tail pointer, find the actual last node
+ * and try to append a new node (or if head was null, establish
+ * the first node). Nodes can be appended only if their
+ * predecessors are either already matched or are of the same
+ * mode. If we detect otherwise, then a new node with opposite
+ * mode must have been appended during traversal, so we must
+ * restart at phase 1. The traversal and update steps are
+ * otherwise similar to phase 1: Retrying upon CAS misses and
+ * checking for staleness. In particular, if a self-link is
+ * encountered, then we can safely jump to a node on the list
+ * by continuing the traversal at current head.
+ *
+ * On successful append, if the call was ASYNC, return.
+ *
+ * 3. 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 might it 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 but never otherwise 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 already timed-out, cancelled, or performing a
+ * potentially O(n) operation (e.g. remove(x)), none of which are
+ * 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.
+ *
+ * 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. Relies heavily on Unsafe mechanics to minimize
+ * unnecessary ordering constraints: 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 until waiting
+
+ // CAS methods for fields
+ final boolean casNext(Node cmp, Node val) {
+ return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
+ }
+
+ final boolean casItem(Object cmp, Object val) {
+ // assert cmp == null || cmp.getClass() != Node.class;
+ return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
+ }
+
+ /**
+ * Constructs a new node. Uses relaxed write because item can
+ * only be seen after publication via casNext.
+ */
+ Node(Object item, boolean isData) {
+ UNSAFE.putObject(this, itemOffset, item); // relaxed write
+ this.isData = isData;
+ }
+
+ /**
+ * Links node to itself to avoid garbage retention. Called
+ * only after CASing head field, so uses relaxed write.
+ */
+ final void forgetNext() {
+ UNSAFE.putObject(this, nextOffset, this);
+ }
+
+ /**
+ * Sets item 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. Similarly, clearing waiter
+ * follows either CAS or return from park (if ever parked;
+ * else we don't care).
+ */
+ final void forgetContents() {
+ UNSAFE.putObject(this, itemOffset, this);
+ UNSAFE.putObject(this, waiterOffset, null);
+ }
+
+ /**
+ * Returns true if this node has been matched, including the
+ * case of artificial matches due to cancellation.
+ */
+ final boolean isMatched() {
+ Object x = item;
+ return (x == this) || ((x == null) == isData);
+ }
+
+ /**
+ * Returns true if this is an unmatched request node.
+ */
+ final boolean isUnmatchedRequest() {
+ return !isData && item == null;
+ }
+
+ /**
+ * 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;
+ Object x;
+ return d != haveData && (x = item) != this && (x != null) == d;
+ }
+
+ /**
+ * Tries to artificially match a data node -- used by remove.
+ */
+ final boolean tryMatchData() {
+ // assert isData;
+ Object x = item;
+ if (x != null && x != this && casItem(x, null)) {
+ LockSupport.unpark(waiter);
+ return true;
+ }
+ return false;
+ }
+
+ private static final long serialVersionUID = -3375979862319811754L;
+
+ // Unsafe mechanics
+ private static final sun.misc.Unsafe UNSAFE;
+ private static final long itemOffset;
+ private static final long nextOffset;
+ private static final long waiterOffset;
+ static {
+ try {
+ UNSAFE = sun.misc.Unsafe.getUnsafe();
+ Class k = Node.class;
+ itemOffset = UNSAFE.objectFieldOffset
+ (k.getDeclaredField("item"));
+ nextOffset = UNSAFE.objectFieldOffset
+ (k.getDeclaredField("next"));
+ waiterOffset = UNSAFE.objectFieldOffset
+ (k.getDeclaredField("waiter"));
+ } catch (Exception e) {
+ throw new Error(e);
+ }
+ }
+ }
+
+ /** head of the queue; null until first enqueue */
+ transient volatile Node head;
+
+ /** tail of the queue; null until first append */
+ private transient volatile Node tail;
+
+ /** The number of apparent failures to unsplice removed nodes */
+ private transient volatile int sweepVotes;
+
+ // CAS methods for fields
+ private boolean casTail(Node cmp, Node val) {
+ return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
+ }
+
+ private boolean casHead(Node cmp, Node val) {
+ return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
+ }
+
+ private boolean casSweepVotes(int cmp, int val) {
+ return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
+ }
+
+ /*
+ * 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
+
+ @SuppressWarnings("unchecked")
+ static E cast(Object item) {
+ // assert item == null || item.getClass() != Node.class;
+ return (E) item;
+ }
+
+ /**
+ * 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
+ */
+ private E xfer(E e, boolean haveData, int how, long nanos) {
+ if (haveData && (e == null))
+ throw new NullPointerException();
+ Node s = null; // the node to append, if needed
+
+ retry:
+ for (;;) { // restart on append race
+
+ for (Node h = head, p = h; p != null;) { // find & match first node
+ boolean isData = p.isData;
+ Object item = p.item;
+ if (item != p && (item != null) == isData) { // unmatched
+ if (isData == haveData) // can't match
+ break;
+ if (p.casItem(item, e)) { // match
+ for (Node q = p; q != h;) {
+ Node n = q.next; // update by 2 unless singleton
+ if (head == h && casHead(h, n == null ? q : n)) {
+ h.forgetNext();
+ break;
+ } // advance and retry
+ if ((h = head) == null ||
+ (q = h.next) == null || !q.isMatched())
+ break; // unless slack < 2
+ }
+ LockSupport.unpark(p.waiter);
+ return this.cast(item);
+ }
+ }
+ Node n = p.next;
+ p = (p != n) ? n : (h = head); // Use head if p offlist
+ }
+
+ if (how != NOW) { // No matches available
+ if (s == null)
+ s = new Node(e, haveData);
+ Node pred = tryAppend(s, haveData);
+ if (pred == null)
+ continue retry; // lost race vs opposite mode
+ if (how != ASYNC)
+ return awaitMatch(s, pred, e, (how == TIMED), nanos);
+ }
+ return e; // not waiting
+ }
+ }
+
+ /**
+ * Tries to append node s as tail.
+ *
+ * @param s the node to append
+ * @param haveData true if appending in data mode
+ * @return null on failure due to losing race with append in
+ * different mode, else s's predecessor, or s itself if no
+ * predecessor
+ */
+ private Node tryAppend(Node s, boolean haveData) {
+ for (Node t = tail, p = t;;) { // move p to last node and append
+ Node n, u; // temps for reads of next & tail
+ if (p == null && (p = head) == null) {
+ if (casHead(null, s))
+ return s; // initialize
+ }
+ else if (p.cannotPrecede(haveData))
+ return null; // lost race vs opposite mode
+ else if ((n = p.next) != null) // not last; keep traversing
+ p = p != t && t != (u = tail) ? (t = u) : // stale tail
+ (p != n) ? n : null; // restart if off list
+ else if (!p.casNext(null, s))
+ p = p.next; // re-read on CAS failure
+ else {
+ if (p != t) { // update if slack now >= 2
+ while ((tail != t || !casTail(t, s)) &&
+ (t = tail) != null &&
+ (s = t.next) != null && // advance and retry
+ (s = s.next) != null && s != t);
+ }
+ return p;
+ }
+ }
+ }
+
+ /**
+ * Spins/yields/blocks until node s is matched or caller gives up.
+ *
+ * @param s the waiting node
+ * @param pred the predecessor of s, or s itself if it has no
+ * predecessor, 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) {
+ long lastTime = timed ? System.nanoTime() : 0L;
+ Thread w = Thread.currentThread();
+ int spins = -1; // initialized after first item and cancel checks
+ ThreadLocalRandom randomYields = null; // bound if needed
+
+ for (;;) {
+ Object item = s.item;
+ if (item != e) { // matched
+ // assert item != s;
+ s.forgetContents(); // avoid garbage
+ return this.cast(item);
+ }
+ if ((w.isInterrupted() || (timed && nanos <= 0)) &&
+ s.casItem(e, s)) { // cancel
+ unsplice(pred, s);
+ return e;
+ }
+
+ 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) {
+ long now = System.nanoTime();
+ if ((nanos -= now - lastTime) > 0)
+ LockSupport.parkNanos(this, nanos);
+ lastTime = now;
+ }
+ 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 successor of p, or the head node if p.next has been
+ * linked to self, which will only be true if traversing with a
+ * stale pointer that is now off the list.
+ */
+ final Node succ(Node p) {
+ Node next = p.next;
+ return (p == next) ? head : next;
+ }
+
+ /**
+ * Returns the first unmatched node of the given mode, or null if
+ * none. Used by methods isEmpty, hasWaitingConsumer.
+ */
+ private Node firstOfMode(boolean isData) {
+ for (Node p = head; p != null; p = succ(p)) {
+ if (!p.isMatched())
+ return (p.isData == isData) ? p : null;
+ }
+ return null;
+ }
+
+ /**
+ * Returns the item in the first unmatched node with isData; or
+ * null if none. Used by peek.
+ */
+ private E firstDataItem() {
+ for (Node p = head; p != null; p = succ(p)) {
+ Object item = p.item;
+ if (p.isData) {
+ if (item != null && item != p)
+ return this.cast(item);
+ }
+ else if (item == null)
+ return null;
+ }
+ return null;
+ }
+
+ /**
+ * Traverses and counts unmatched nodes of the given mode.
+ * Used by methods size and getWaitingConsumerCount.
+ */
+ private int countOfMode(boolean data) {
+ int count = 0;
+ for (Node p = head; p != null; ) {
+ if (!p.isMatched()) {
+ if (p.isData != data)
+ return 0;
+ if (++count == Integer.MAX_VALUE) // saturated
+ break;
+ }
+ Node n = p.next;
+ if (n != p)
+ p = n;
+ else {
+ count = 0;
+ p = head;
+ }
+ }
+ return count;
+ }
+
+ 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 lastPred; // predecessor to unlink lastRet
+
+ /**
+ * Moves to next node after prev, or first node if prev null.
+ */
+ private void advance(Node prev) {
+ /*
+ * To track and avoid buildup of deleted nodes in the face
+ * of calls to both Queue.remove and Itr.remove, we must
+ * include variants of unsplice and sweep upon each
+ * advance: Upon Itr.remove, we may need to catch up links
+ * from lastPred, and upon other removes, we might need to
+ * skip ahead from stale nodes and unsplice deleted ones
+ * found while advancing.
+ */
+
+ Node r, b; // reset lastPred upon possible deletion of lastRet
+ if ((r = lastRet) != null && !r.isMatched())
+ lastPred = r; // next lastPred is old lastRet
+ else if ((b = lastPred) == null || b.isMatched())
+ lastPred = null; // at start of list
+ else {
+ Node s, n; // help with removal of lastPred.next
+ while ((s = b.next) != null &&
+ s != b && s.isMatched() &&
+ (n = s.next) != null && n != s)
+ b.casNext(s, n);
+ }
+
+ this.lastRet = prev;
+
+ for (Node p = prev, s, n;;) {
+ s = (p == null) ? head : p.next;
+ if (s == null)
+ break;
+ else if (s == p) {
+ p = null;
+ continue;
+ }
+ Object item = s.item;
+ if (s.isData) {
+ if (item != null && item != s) {
+ nextItem = LinkedTransferQueue.cast(item);
+ nextNode = s;
+ return;
+ }
+ }
+ else if (item == null)
+ break;
+ // assert s.isMatched();
+ if (p == null)
+ p = s;
+ else if ((n = s.next) == null)
+ break;
+ else if (s == n)
+ p = null;
+ else
+ p.casNext(s, n);
+ }
+ nextNode = null;
+ nextItem = null;
+ }
+
+ Itr() {
+ advance(null);
+ }
+
+ public final boolean hasNext() {
+ return nextNode != null;
+ }
+
+ public final E next() {
+ Node p = nextNode;
+ if (p == null) throw new NoSuchElementException();
+ E e = nextItem;
+ advance(p);
+ return e;
+ }
+
+ public final void remove() {
+ final Node lastRet = this.lastRet;
+ if (lastRet == null)
+ throw new IllegalStateException();
+ this.lastRet = null;
+ if (lastRet.tryMatchData())
+ unsplice(lastPred, lastRet);
+ }
+ }
+
+ /* -------------- 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, or null or s itself if s is/was at head
+ * @param s the node to be unspliced
+ */
+ final void unsplice(Node pred, Node s) {
+ s.forgetContents(); // forget unneeded fields
+ /*
+ * 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 != s && 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 || h == null)
+ 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.forgetNext(); // advance head
+ }
+ if (pred.next != pred && s.next != s) { // recheck if offlist
+ for (;;) { // sweep now if enough votes
+ int v = sweepVotes;
+ if (v < SWEEP_THRESHOLD) {
+ if (casSweepVotes(v, v + 1))
+ break;
+ }
+ else if (casSweepVotes(v, 0)) {
+ sweep();
+ break;
+ }
+ }
+ }
+ }
+ }
+ }
+
+ /**
+ * 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);
+ }
+ }
+
+ /**
+ * Main implementation of remove(Object)
+ */
+ private boolean findAndRemove(Object e) {
+ if (e != null) {
+ for (Node pred = null, p = head; p != null; ) {
+ Object item = p.item;
+ if (p.isData) {
+ if (item != null && item != p && e.equals(item) &&
+ p.tryMatchData()) {
+ unsplice(pred, p);
+ return true;
+ }
+ }
+ else if (item == null)
+ break;
+ pred = p;
+ if ((p = p.next) == pred) { // stale
+ pred = null;
+ p = head;
+ }
+ }
+ }
+ return false;
+ }
+
+
+ /**
+ * Creates an initially empty {@code LinkedTransferQueue}.
+ */
+ public LinkedTransferQueue() {
+ }
+
+ /**
+ * 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) {
+ this();
+ addAll(c);
+ }
+
+ /**
+ * 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) {
+ if (c == null)
+ throw new NullPointerException();
+ if (c == this)
+ throw new IllegalArgumentException();
+ int n = 0;
+ E e;
+ while ( (e = poll()) != null) {
+ c.add(e);
+ ++n;
+ }
+ return n;
+ }
+
+ /**
+ * @throws NullPointerException {@inheritDoc}
+ * @throws IllegalArgumentException {@inheritDoc}
+ */
+ public int drainTo(Collection c, int maxElements) {
+ if (c == null)
+ throw new NullPointerException();
+ if (c == this)
+ throw new IllegalArgumentException();
+ int n = 0;
+ E e;
+ while (n < maxElements && (e = poll()) != null) {
+ c.add(e);
+ ++n;
+ }
+ 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 a "weakly consistent" iterator that
+ * will never throw {@link java.util.ConcurrentModificationException
+ * ConcurrentModificationException}, and guarantees to traverse
+ * elements as they existed upon construction of the iterator, and
+ * may (but is not guaranteed to) reflect any modifications
+ * subsequent to construction.
+ *
+ * @return an iterator over the elements in this queue in proper sequence
+ */
+ public Iterator iterator() {
+ return new Itr();
+ }
+
+ public E peek() {
+ return firstDataItem();
+ }
+
+ /**
+ * Returns {@code true} if this queue contains no elements.
+ *
+ * @return {@code true} if this queue contains no elements
+ */
+ public boolean isEmpty() {
+ for (Node p = head; p != null; p = succ(p)) {
+ if (!p.isMatched())
+ return !p.isData;
+ }
+ return true;
+ }
+
+ public boolean hasWaitingConsumer() {
+ return firstOfMode(false) != null;
+ }
+
+ /**
+ * 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) {
+ return findAndRemove(o);
+ }
+
+ /**
+ * 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;
+ for (Node p = head; p != null; p = succ(p)) {
+ Object item = p.item;
+ if (p.isData) {
+ if (item != null && item != p && o.equals(item))
+ return true;
+ }
+ else if (item == null)
+ break;
+ }
+ 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 the state to a stream (that is, serializes it).
+ *
+ * @serialData All of the elements (each an {@code E}) in
+ * the proper order, followed by a null
+ * @param s the stream
+ */
+ 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 the Queue instance from a stream (that is,
+ * deserializes it).
+ *
+ * @param s the stream
+ */
+ private void readObject(java.io.ObjectInputStream s)
+ throws java.io.IOException, ClassNotFoundException {
+ s.defaultReadObject();
+ for (;;) {
+ @SuppressWarnings("unchecked") E item = (E) s.readObject();
+ if (item == null)
+ break;
+ else
+ offer(item);
+ }
+ }
+
+ // Unsafe mechanics
+
+ private static final sun.misc.Unsafe UNSAFE;
+ private static final long headOffset;
+ private static final long tailOffset;
+ private static final long sweepVotesOffset;
+ static {
+ try {
+ UNSAFE = sun.misc.Unsafe.getUnsafe();
+ Class k = LinkedTransferQueue.class;
+ headOffset = UNSAFE.objectFieldOffset
+ (k.getDeclaredField("head"));
+ tailOffset = UNSAFE.objectFieldOffset
+ (k.getDeclaredField("tail"));
+ sweepVotesOffset = UNSAFE.objectFieldOffset
+ (k.getDeclaredField("sweepVotes"));
+ } catch (Exception e) {
+ throw new Error(e);
+ }
+ }
+}