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android / platform / prebuilts / fullsdk / sources / dc3f885ebe8ddc75bd9cf2d567eef4d1ed433a09 / . / android-35 / java / util / concurrent / LinkedTransferQueue.java
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/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.AbstractQueue;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Queue;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.locks.LockSupport;
import java.util.function.Consumer;
import java.util.function.Predicate;
/**
* An unbounded {@link TransferQueue} based on linked nodes.
* This queue orders elements FIFO (first-in-first-out) with respect
* to any given producer. The <em>head</em> of the queue is that
* element that has been on the queue the longest time for some
* producer. The <em>tail</em> of the queue is that element that has
* been on the queue the shortest time for some producer.
*
* <p>Beware that, unlike in most collections, the {@code size} method
* is <em>NOT</em> a constant-time operation. Because of the
* asynchronous nature of these queues, determining the current number
* of elements requires a traversal of the elements, and so may report
* inaccurate results if this collection is modified during traversal.
*
* <p>Bulk operations that add, remove, or examine multiple elements,
* such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
* are <em>not</em> guaranteed to be performed atomically.
* For example, a {@code forEach} traversal concurrent with an {@code
* addAll} operation might observe only some of the added elements.
*
* <p>This class and its iterator implement all of the <em>optional</em>
* methods of the {@link Collection} and {@link Iterator} interfaces.
*
* <p>Memory consistency effects: As with other concurrent
* collections, actions in a thread prior to placing an object into a
* {@code LinkedTransferQueue}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions subsequent to the access or removal of that element from
* the {@code LinkedTransferQueue} in another thread.
*
* <p>This class is a member of the
* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.
*
* @since 1.7
* @author Doug Lea
* @param <E> the type of elements held in this queue
*/
public class LinkedTransferQueue<E> extends AbstractQueue<E>
implements TransferQueue<E>, java.io.Serializable {
private static final long serialVersionUID = -3223113410248163686L;
/*
* *** Overview of Dual Queues with Slack ***
*
* Dual Queues, introduced by Scherer and Scott
* (http://www.cs.rochester.edu/~scott/papers/2004_DISC_dual_DS.pdf)
* are (linked) queues in which nodes may represent either data or
* requests. When a thread tries to enqueue a data node, but
* encounters a request node, it instead "matches" and removes it;
* and vice versa for enqueuing requests. Blocking Dual Queues
* arrange that threads enqueuing unmatched requests block until
* other threads provide the match. Dual Synchronous Queues (see
* Scherer, Lea, & Scott
* http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
* additionally arrange that threads enqueuing unmatched data also
* block. Dual Transfer Queues support all of these modes, as
* dictated by callers.
*
* A FIFO dual queue may be implemented using a variation of the
* Michael & Scott (M&S) lock-free queue algorithm
* (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf).
* It maintains two pointer fields, "head", pointing to a
* (matched) node that in turn points to the first actual
* (unmatched) queue node (or null if empty); and "tail" that
* points to the last node on the queue (or again null if
* empty). For example, here is a possible queue with four data
* elements:
*
* head tail
* | |
* v v
* M -> U -> U -> U -> U
*
* The M&S queue algorithm is known to be prone to scalability and
* overhead limitations when maintaining (via CAS) these head and
* tail pointers. This has led to the development of
* contention-reducing variants such as elimination arrays (see
* Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
* optimistic back pointers (see Ladan-Mozes & Shavit
* http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
* However, the nature of dual queues enables a simpler tactic for
* improving M&S-style implementations when dual-ness is needed.
*
* In a dual queue, each node must atomically maintain its match
* status. While there are other possible variants, we implement
* this here as: for a data-mode node, matching entails CASing an
* "item" field from a non-null data value to null upon match, and
* vice-versa for request nodes, CASing from null to a data
* value. (Note that the linearization properties of this style of
* queue are easy to verify -- elements are made available by
* linking, and unavailable by matching.) Compared to plain M&S
* queues, this property of dual queues requires one additional
* successful atomic operation per enq/deq pair. But it also
* enables lower cost variants of queue maintenance mechanics. (A
* variation of this idea applies even for non-dual queues that
* support deletion of interior elements, such as
* j.u.c.ConcurrentLinkedQueue.)
*
* Once a node is matched, its match status can never again
* change. We may thus arrange that the linked list of them
* contain a prefix of zero or more matched nodes, followed by a
* suffix of zero or more unmatched nodes. (Note that we allow
* both the prefix and suffix to be zero length, which in turn
* means that we do not use a dummy header.) If we were not
* concerned with either time or space efficiency, we could
* correctly perform enqueue and dequeue operations by traversing
* from a pointer to the initial node; CASing the item of the
* first unmatched node on match and CASing the next field of the
* trailing node on appends. While this would be a terrible idea
* in itself, it does have the benefit of not requiring ANY atomic
* updates on head/tail fields.
*
* We introduce here an approach that lies between the extremes of
* never versus always updating queue (head and tail) pointers.
* This offers a tradeoff between sometimes requiring extra
* traversal steps to locate the first and/or last unmatched
* nodes, versus the reduced overhead and contention of fewer
* updates to queue pointers. For example, a possible snapshot of
* a queue is:
*
* head tail
* | |
* v v
* M -> M -> U -> U -> U -> U
*
* The best value for this "slack" (the targeted maximum distance
* between the value of "head" and the first unmatched node, and
* similarly for "tail") is an empirical matter. We have found
* that using very small constants in the range of 1-3 work best
* over a range of platforms. Larger values introduce increasing
* costs of cache misses and risks of long traversal chains, while
* smaller values increase CAS contention and overhead.
*
* Dual queues with slack differ from plain M&S dual queues by
* virtue of only sometimes updating head or tail pointers when
* matching, appending, or even traversing nodes; in order to
* maintain a targeted slack. The idea of "sometimes" may be
* operationalized in several ways. The simplest is to use a
* per-operation counter incremented on each traversal step, and
* to try (via CAS) to update the associated queue pointer
* whenever the count exceeds a threshold. Another, that requires
* more overhead, is to use random number generators to update
* with a given probability per traversal step.
*
* In any strategy along these lines, because CASes updating
* fields may fail, the actual slack may exceed targeted slack.
* However, they may be retried at any time to maintain targets.
* Even when using very small slack values, this approach works
* well for dual queues because it allows all operations up to the
* point of matching or appending an item (hence potentially
* allowing progress by another thread) to be read-only, thus not
* introducing any further contention. As described below, we
* implement this by performing slack maintenance retries only
* after these points.
*
* As an accompaniment to such techniques, traversal overhead can
* be further reduced without increasing contention of head
* pointer updates: Threads may sometimes shortcut the "next" link
* path from the current "head" node to be closer to the currently
* known first unmatched node, and similarly for tail. Again, this
* may be triggered with using thresholds or randomization.
*
* These ideas must be further extended to avoid unbounded amounts
* of costly-to-reclaim garbage caused by the sequential "next"
* links of nodes starting at old forgotten head nodes: As first
* described in detail by Boehm
* (http://portal.acm.org/citation.cfm?doid=503272.503282), if a GC
* delays noticing that any arbitrarily old node has become
* garbage, all newer dead nodes will also be unreclaimed.
* (Similar issues arise in non-GC environments.) To cope with
* this in our implementation, upon CASing to advance the head
* pointer, we set the "next" link of the previous head to point
* only to itself; thus limiting the length of chains of dead nodes.
* (We also take similar care to wipe out possibly garbage
* retaining values held in other Node fields.) However, doing so
* adds some further complexity to traversal: If any "next"
* pointer links to itself, it indicates that the current thread
* has lagged behind a head-update, and so the traversal must
* continue from the "head". Traversals trying to find the
* current tail starting from "tail" may also encounter
* self-links, in which case they also continue at "head".
*
* It is tempting in slack-based scheme to not even use CAS for
* updates (similarly to Ladan-Mozes & Shavit). However, this
* cannot be done for head updates under the above link-forgetting
* mechanics because an update may leave head at a detached node.
* And while direct writes are possible for tail updates, they
* increase the risk of long retraversals, and hence long garbage
* chains, which can be much more costly than is worthwhile
* considering that the cost difference of performing a CAS vs
* write is smaller when they are not triggered on each operation
* (especially considering that writes and CASes equally require
* additional GC bookkeeping ("write barriers") that are sometimes
* more costly than the writes themselves because of contention).
*
* *** Overview of implementation ***
*
* We use a threshold-based approach to updates, with a slack
* threshold of two -- that is, we update head/tail when the
* current pointer appears to be two or more steps away from the
* first/last node. The slack value is hard-wired: a path greater
* than one is naturally implemented by checking equality of
* traversal pointers except when the list has only one element,
* in which case we keep slack threshold at one. Avoiding tracking
* explicit counts across method calls slightly simplifies an
* already-messy implementation. Using randomization would
* probably work better if there were a low-quality dirt-cheap
* per-thread one available, but even ThreadLocalRandom is too
* heavy for these purposes.
*
* With such a small slack threshold value, it is not worthwhile
* to augment this with path short-circuiting (i.e., unsplicing
* interior nodes) except in the case of cancellation/removal (see
* below).
*
* All enqueue/dequeue operations are handled by the single method
* "xfer" with parameters indicating whether to act as some form
* of offer, put, poll, take, or transfer (each possibly with
* timeout). The relative complexity of using one monolithic
* method outweighs the code bulk and maintenance problems of
* using separate methods for each case.
*
* Operation consists of up to two phases. The first is implemented
* in method xfer, the second in method awaitMatch.
*
* 1. Traverse until matching or appending (method xfer)
*
* Conceptually, we simply traverse all nodes starting from head.
* If we encounter an unmatched node of opposite mode, we match
* it and return, also updating head (by at least 2 hops) to
* one past the matched node (or the node itself if it's the
* pinned trailing node). Traversals also check for the
* possibility of falling off-list, in which case they restart.
*
* If the trailing node of the list is reached, a match is not
* possible. If this call was untimed poll or tryTransfer
* (argument "how" is NOW), return empty-handed immediately.
* Else a new node is CAS-appended. On successful append, if
* this call was ASYNC (e.g. offer), an element was
* successfully added to the end of the queue and we return.
*
* Of course, this naive traversal is O(n) when no match is
* possible. We optimize the traversal by maintaining a tail
* pointer, which is expected to be "near" the end of the list.
* It is only safe to fast-forward to tail (in the presence of
* arbitrary concurrent changes) if it is pointing to a node of
* the same mode, even if it is dead (in this case no preceding
* node could still be matchable by this traversal). If we
* need to restart due to falling off-list, we can again
* fast-forward to tail, but only if it has changed since the
* last traversal (else we might loop forever). If tail cannot
* be used, traversal starts at head (but in this case we
* expect to be able to match near head). As with head, we
* CAS-advance the tail pointer by at least two hops.
*
* 2. Await match or cancellation (method awaitMatch)
*
* Wait for another thread to match node; instead cancelling if
* the current thread was interrupted or the wait timed out. To
* improve performance in common single-source / single-sink
* usages when there are more tasks that cores, an initial
* Thread.yield is tried when there is apparently only one
* waiter. In other cases, waiters may help with some
* bookkeeping, then park/unpark.
*
* ** 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 the need to sweep the
* next time any thread would otherwise block in awaitMatch. Also,
* 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.
*/
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices.
* Using a power of two minus one simplifies some comparisons.
*/
static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1023L;
/**
* 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 implements ForkJoinPool.ManagedBlocker {
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.setOpaque(this, next);
}
/**
* 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);
}
public final boolean isReleasable() {
return (isData == (item == null)) ||
Thread.currentThread().isInterrupted();
}
public final boolean block() {
while (!isReleasable()) LockSupport.park();
return true;
}
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 boolean needSweep;
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);
}
/**
* 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;
}
}
}
/**
* Possibly 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
*/
@SuppressWarnings("unchecked")
private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
final boolean isData = s.isData;
final long deadline = timed ? System.nanoTime() + nanos : 0L;
final Thread w = Thread.currentThread();
int stat = -1; // -1: may yield, +1: park, else 0
Object item;
while ((item = s.item) == e) {
if (needSweep) // help clean
sweep();
else if ((timed && nanos <= 0L) || w.isInterrupted()) {
if (s.casItem(e, (e == null) ? s : null)) {
unsplice(pred, s); // cancelled
return e;
}
}
else if (stat <= 0) {
if (pred != null && pred.next == s) {
if (stat < 0 &&
(pred.isData != isData || pred.isMatched())) {
stat = 0; // yield once if first
Thread.yield();
}
else {
stat = 1;
s.waiter = w; // enable unpark
}
} // else signal in progress
}
else if ((item = s.item) != e)
break; // recheck
else if (!timed) {
LockSupport.setCurrentBlocker(this);
try {
ForkJoinPool.managedBlock(s);
} catch (InterruptedException cannotHappen) { }
LockSupport.setCurrentBlocker(null);
}
else {
nanos = deadline - System.nanoTime();
if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
LockSupport.parkNanos(this, nanos);
}
}
if (stat == 1)
WAITER.set(s, null);
if (!isData)
ITEM.set(s, s); // self-link to avoid garbage
return (E) item;
}
/* -------------- 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.
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this queue. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this queue
*/
public Object[] toArray() {
return toArrayInternal(null);
}
/**
* Returns an array containing all of the elements in this queue, in
* proper sequence; the runtime type of the returned array is that of
* the specified array. If the queue fits in the specified array, it
* is returned therein. Otherwise, a new array is allocated with the
* runtime type of the specified array and the size of this queue.
*
* <p>If this queue fits in the specified array with room to spare
* (i.e., the array has more elements than this queue), the element in
* the array immediately following the end of the queue is set to
* {@code null}.
*
* <p>Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
*
* <p>Suppose {@code x} is a queue known to contain only strings.
* The following code can be used to dump the queue into a newly
* allocated array of {@code String}:
*
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of the queue are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose
* @return an array containing all of the elements in this queue
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this queue
* @throws NullPointerException if the specified array is null
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
Objects.requireNonNull(a);
return (T[]) toArrayInternal(a);
}
/**
* Weakly-consistent iterator.
*
* Lazily updated ancestor is expected to be amortized O(1) remove(),
* but O(n) in the worst case, when lastRet is concurrently deleted.
*/
final class Itr implements Iterator<E> {
private Node nextNode; // next node to return item for
private E nextItem; // the corresponding item
private Node lastRet; // last returned node, to support remove
private Node ancestor; // Helps unlink lastRet on remove()
/**
* Moves to next node after pred, or first node if pred null.
*/
@SuppressWarnings("unchecked")
private void advance(Node pred) {
for (Node p = (pred == null) ? head : pred.next, c = p;
p != null; ) {
final Object item;
if ((item = p.item) != null && p.isData) {
nextNode = p;
nextItem = (E) item;
if (c != p)
tryCasSuccessor(pred, c, p);
return;
}
else if (!p.isData && item == null)
break;
if (c != p && !tryCasSuccessor(pred, c, c = p)) {
pred = p;
c = p = p.next;
}
else if (p == (p = p.next)) {
pred = null;
c = p = head;
}
}
nextItem = null;
nextNode = null;
}
Itr() {
advance(null);
}
public final boolean hasNext() {
return nextNode != null;
}
public final E next() {
final Node p;
if ((p = nextNode) == null) throw new NoSuchElementException();
E e = nextItem;
advance(lastRet = p);
return e;
}
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
Node q = null;
for (Node p; (p = nextNode) != null; advance(q = p))
action.accept(nextItem);
if (q != null)
lastRet = q;
}
public final void remove() {
final Node lastRet = this.lastRet;
if (lastRet == null)
throw new IllegalStateException();
this.lastRet = null;
if (lastRet.item == null) // already deleted?
return;
// Advance ancestor, collapsing intervening dead nodes
Node pred = ancestor;
for (Node p = (pred == null) ? head : pred.next, c = p, q;
p != null; ) {
if (p == lastRet) {
final Object item;
if ((item = p.item) != null)
p.tryMatch(item, null);
if ((q = p.next) == null) q = p;
if (c != q) tryCasSuccessor(pred, c, q);
ancestor = pred;
return;
}
final Object item; final boolean pAlive;
if (pAlive = ((item = p.item) != null && p.isData)) {
// exceptionally, nothing to do
}
else if (!p.isData && item == null)
break;
if ((c != p && !tryCasSuccessor(pred, c, c = p)) || pAlive) {
pred = p;
c = p = p.next;
}
else if (p == (p = p.next)) {
pred = null;
c = p = head;
}
}
// traversal failed to find lastRet; must have been deleted;
// leave ancestor at original location to avoid overshoot;
// better luck next time!
// assert lastRet.isMatched();
}
}
/** A customized variant of Spliterators.IteratorSpliterator */
final class LTQSpliterator implements Spliterator<E> {
static final int MAX_BATCH = 1 << 25; // max batch array size;
Node current; // current node; null until initialized
int batch; // batch size for splits
boolean exhausted; // true when no more nodes
LTQSpliterator() {}
public Spliterator<E> trySplit() {
Node p, q;
if ((p = current()) == null || (q = p.next) == null)
return null;
int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH);
Object[] a = null;
do {
final Object item = p.item;
if (p.isData) {
if (item != null) {
if (a == null)
a = new Object[n];
a[i++] = item;
}
} else if (item == null) {
p = null;
break;
}
if (p == (p = q))
p = firstDataNode();
} while (p != null && (q = p.next) != null && i < n);
setCurrent(p);
return (i == 0) ? null :
Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED |
Spliterator.NONNULL |
Spliterator.CONCURRENT));
}
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
final Node p;
if ((p = current()) != null) {
current = null;
exhausted = true;
forEachFrom(action, p);
}
}
@SuppressWarnings("unchecked")
public boolean tryAdvance(Consumer<? super E> action) {
Objects.requireNonNull(action);
Node p;
if ((p = current()) != null) {
E e = null;
do {
final Object item = p.item;
final boolean isData = p.isData;
if (p == (p = p.next))
p = head;
if (isData) {
if (item != null) {
e = (E) item;
break;
}
}
else if (item == null)
p = null;
} while (p != null);
setCurrent(p);
if (e != null) {
action.accept(e);
return true;
}
}
return false;
}
private void setCurrent(Node p) {
if ((current = p) == null)
exhausted = true;
}
private Node current() {
Node p;
if ((p = current) == null && !exhausted)
setCurrent(p = firstDataNode());
return p;
}
public long estimateSize() { return Long.MAX_VALUE; }
public int characteristics() {
return (Spliterator.ORDERED |
Spliterator.NONNULL |
Spliterator.CONCURRENT);
}
}
/**
* Returns a {@link Spliterator} over the elements in this queue.
*
* <p>The returned spliterator is
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
*
* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
*
* @implNote
* The {@code Spliterator} implements {@code trySplit} to permit limited
* parallelism.
*
* @return a {@code Spliterator} over the elements in this queue
* @since 1.8
*/
public Spliterator<E> spliterator() {
return new LTQSpliterator();
}
/* -------------- Removal methods -------------- */
/**
* Unsplices (now or later) the given deleted/cancelled node with
* the given predecessor.
*
* @param pred a node that was at one time known to be the
* predecessor of s
* @param s the node to be unspliced
*/
final void unsplice(Node pred, Node s) {
// assert pred != null;
// assert pred != s;
// assert s != null;
// assert s.isMatched();
// assert (SWEEP_THRESHOLD & (SWEEP_THRESHOLD - 1)) == 0;
s.waiter = null; // disable signals
/*
* See above for rationale. Briefly: if pred still points to
* s, try to unlink s. If s cannot be unlinked, because it is
* trailing node or pred might be unlinked, and neither pred
* nor s are head or offlist, set needSweep;
*/
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
}
if (pred.next != pred && s.next != s)
needSweep = true;
}
}
}
/**
* Unlinks matched (typically cancelled) nodes encountered in a
* traversal from head.
*/
private void sweep() {
needSweep = false;
for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
if (!s.isMatched())
// Unmatched nodes are never self-linked
p = s;
else if ((n = s.next) == null) // trailing node is pinned
break;
else if (s == n) // stale
// No need to also check for p == s, since that implies s == n
p = head;
else
p.casNext(s, n);
}
}
/**
* Creates an initially empty {@code LinkedTransferQueue}.
*/
public LinkedTransferQueue() {
head = tail = new Node();
}
/**
* Creates a {@code LinkedTransferQueue}
* initially containing the elements of the given collection,
* added in traversal order of the collection's iterator.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
* of its elements are null
*/
public LinkedTransferQueue(Collection<? extends E> c) {
Node h = null, t = null;
for (E e : c) {
Node newNode = new Node(Objects.requireNonNull(e));
if (h == null)
h = t = newNode;
else
t.appendRelaxed(t = newNode);
}
if (h == null)
h = t = new Node();
head = h;
tail = t;
}
/**
* Inserts the specified element at the tail of this queue.
* As the queue is unbounded, this method will never block.
*
* @throws NullPointerException if the specified element is null
*/
public void put(E e) {
xfer(e, true, ASYNC, 0L);
}
/**
* 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, 0L);
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, 0L);
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, 0L);
return true;
}
/**
* Transfers the element to a waiting consumer immediately, if possible.
*
* <p>More precisely, transfers the specified element immediately
* if there exists a consumer already waiting to receive it (in
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
* otherwise returning {@code false} without enqueuing the element.
*
* @throws NullPointerException if the specified element is null
*/
public boolean tryTransfer(E e) {
return xfer(e, true, NOW, 0L) == null;
}
/**
* Transfers the element to a consumer, waiting if necessary to do so.
*
* <p>More precisely, transfers the specified element immediately
* if there exists a consumer already waiting to receive it (in
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
* else inserts the specified element at the tail of this queue
* and waits until the element is received by a consumer.
*
* @throws NullPointerException if the specified element is null
*/
public void transfer(E e) throws InterruptedException {
if (xfer(e, true, SYNC, 0L) != null) {
Thread.interrupted(); // failure possible only due to interrupt
throw new InterruptedException();
}
}
/**
* Transfers the element to a consumer if it is possible to do so
* before the timeout elapses.
*
* <p>More precisely, transfers the specified element immediately
* if there exists a consumer already waiting to receive it (in
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
* else inserts the specified element at the tail of this queue
* and waits until the element is received by a consumer,
* returning {@code false} if the specified wait time elapses
* before the element can be transferred.
*
* @throws NullPointerException if the specified element is null
*/
public boolean tryTransfer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
return true;
if (!Thread.interrupted())
return false;
throw new InterruptedException();
}
public E take() throws InterruptedException {
E e = xfer(null, false, SYNC, 0L);
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, 0L);
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
public int drainTo(Collection<? super E> c) {
Objects.requireNonNull(c);
if (c == this)
throw new IllegalArgumentException();
int n = 0;
for (E e; (e = poll()) != null; n++)
c.add(e);
return n;
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
public int drainTo(Collection<? super E> c, int maxElements) {
Objects.requireNonNull(c);
if (c == this)
throw new IllegalArgumentException();
int n = 0;
for (E e; n < maxElements && (e = poll()) != null; n++)
c.add(e);
return n;
}
/**
* Returns an iterator over the elements in this queue in proper sequence.
* The elements will be returned in order from first (head) to last (tail).
*
* <p>The returned iterator is
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
*
* @return an iterator over the elements in this queue in proper sequence
*/
public Iterator<E> iterator() {
return new Itr();
}
public E peek() {
restartFromHead: for (;;) {
for (Node p = head; p != null;) {
Object item = p.item;
if (p.isData) {
if (item != null) {
@SuppressWarnings("unchecked") E e = (E) item;
return e;
}
}
else if (item == null)
break;
if (p == (p = p.next))
continue restartFromHead;
}
return null;
}
}
/**
* Returns {@code true} if this queue contains no elements.
*
* @return {@code true} if this queue contains no elements
*/
public boolean isEmpty() {
return firstDataNode() == null;
}
public boolean hasWaitingConsumer() {
restartFromHead: for (;;) {
for (Node p = head; p != null;) {
Object item = p.item;
if (p.isData) {
if (item != null)
break;
}
else if (item == null)
return true;
if (p == (p = p.next))
continue restartFromHead;
}
return false;
}
}
/**
* Returns the number of elements in this queue. If this queue
* contains more than {@code Integer.MAX_VALUE} elements, returns
* {@code Integer.MAX_VALUE}.
*
* <p>Beware that, unlike in most collections, this method is
* <em>NOT</em> a constant-time operation. Because of the
* asynchronous nature of these queues, determining the current
* number of elements requires an O(n) traversal.
*
* @return the number of elements in this queue
*/
public int size() {
return countOfMode(true);
}
public int getWaitingConsumerCount() {
return countOfMode(false);
}
/**
* Removes a single instance of the specified element from this queue,
* if it is present. More formally, removes an element {@code e} such
* that {@code o.equals(e)}, if this queue contains one or more such
* elements.
* Returns {@code true} if this queue contained the specified element
* (or equivalently, if this queue changed as a result of the call).
*
* @param o element to be removed from this queue, if present
* @return {@code true} if this queue changed as a result of the call
*/
public boolean remove(Object o) {
if (o == null) return false;
restartFromHead: for (;;) {
for (Node p = head, pred = null; p != null; ) {
Node q = p.next;
final Object item;
if ((item = p.item) != null) {
if (p.isData) {
if (o.equals(item) && p.tryMatch(item, null)) {
skipDeadNodes(pred, p, p, q);
return true;
}
pred = p; p = q; continue;
}
}
else if (!p.isData)
break;
for (Node c = p;; q = p.next) {
if (q == null || !q.isMatched()) {
pred = skipDeadNodes(pred, c, p, q); p = q; break;
}
if (p == (p = q)) continue restartFromHead;
}
}
return false;
}
}
/**
* Returns {@code true} if this queue contains the specified element.
* More formally, returns {@code true} if and only if this queue contains
* at least one element {@code e} such that {@code o.equals(e)}.
*
* @param o object to be checked for containment in this queue
* @return {@code true} if this queue contains the specified element
*/
public boolean contains(Object o) {
if (o == null) return false;
restartFromHead: for (;;) {
for (Node p = head, pred = null; p != null; ) {
Node q = p.next;
final Object item;
if ((item = p.item) != null) {
if (p.isData) {
if (o.equals(item))
return true;
pred = p; p = q; continue;
}
}
else if (!p.isData)
break;
for (Node c = p;; q = p.next) {
if (q == null || !q.isMatched()) {
pred = skipDeadNodes(pred, c, p, q); p = q; break;
}
if (p == (p = q)) continue restartFromHead;
}
}
return false;
}
}
/**
* Always returns {@code Integer.MAX_VALUE} because a
* {@code LinkedTransferQueue} is not capacity constrained.
*
* @return {@code Integer.MAX_VALUE} (as specified by
* {@link 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<? super E> filter) {
Objects.requireNonNull(filter);
return bulkRemove(filter);
}
/**
* @throws NullPointerException {@inheritDoc}
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return bulkRemove(e -> c.contains(e));
}
/**
* @throws NullPointerException {@inheritDoc}
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return bulkRemove(e -> !c.contains(e));
}
public void clear() {
bulkRemove(e -> true);
}
/**
* Tolerate this many consecutive dead nodes before CAS-collapsing.
* Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element.
*/
private static final int MAX_HOPS = 8;
/** Implementation of bulk remove methods. */
@SuppressWarnings("unchecked")
private boolean bulkRemove(Predicate<? super E> filter) {
boolean removed = false;
restartFromHead: for (;;) {
int hops = MAX_HOPS;
// c will be CASed to collapse intervening dead nodes between
// pred (or head if null) and p.
for (Node p = head, c = p, pred = null, q; p != null; p = q) {
q = p.next;
final Object item; boolean pAlive;
if (pAlive = ((item = p.item) != null && p.isData)) {
if (filter.test((E) item)) {
if (p.tryMatch(item, null))
removed = true;
pAlive = false;
}
}
else if (!p.isData && item == null)
break;
if (pAlive || q == null || --hops == 0) {
// p might already be self-linked here, but if so:
// - CASing head will surely fail
// - CASing pred's next will be useless but harmless.
if ((c != p && !tryCasSuccessor(pred, c, c = p))
|| pAlive) {
// if CAS failed or alive, abandon old pred
hops = MAX_HOPS;
pred = p;
c = q;
}
} else if (p == q)
continue restartFromHead;
}
return removed;
}
}
/**
* Runs action on each element found during a traversal starting at p.
* If p is null, the action is not run.
*/
@SuppressWarnings("unchecked")
void forEachFrom(Consumer<? super E> action, Node p) {
for (Node pred = null; p != null; ) {
Node q = p.next;
final Object item;
if ((item = p.item) != null) {
if (p.isData) {
action.accept((E) item);
pred = p; p = q; continue;
}
}
else if (!p.isData)
break;
for (Node c = p;; q = p.next) {
if (q == null || !q.isMatched()) {
pred = skipDeadNodes(pred, c, p, q); p = q; break;
}
if (p == (p = q)) { pred = null; p = head; break; }
}
}
}
/**
* @throws NullPointerException {@inheritDoc}
*/
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
forEachFrom(action, head);
}
// VarHandle mechanics
private static final VarHandle HEAD;
private static final VarHandle TAIL;
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);
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 ExceptionInInitializerError(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;
}
}