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/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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* This file is available under and governed by the GNU General Public
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* License version 2 only, as published by the Free Software Foundation.
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* However, the following notice accompanied the original version of this
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* file:
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*
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* Written by Doug Lea and Martin Buchholz with assistance from members of
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* JCP JSR-166 Expert Group and released to the public domain, as explained
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* at http://creativecommons.org/publicdomain/zero/1.0/
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*/
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package java.util.concurrent;
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import java.util.AbstractQueue;
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import java.util.ArrayList;
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import java.util.Collection;
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import java.util.Iterator;
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import java.util.NoSuchElementException;
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import java.util.Queue;
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/**
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* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
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* This queue orders elements FIFO (first-in-first-out).
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* The <em>head</em> of the queue is that element that has been on the
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* queue the longest time.
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* The <em>tail</em> of the queue is that element that has been on the
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* queue the shortest time. New elements
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* are inserted at the tail of the queue, and the queue retrieval
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* operations obtain elements at the head of the queue.
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* A {@code ConcurrentLinkedQueue} is an appropriate choice when
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* many threads will share access to a common collection.
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* Like most other concurrent collection implementations, this class
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* does not permit the use of {@code null} elements.
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*
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* <p>This implementation employs an efficient "wait-free"
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* algorithm based on one described in <a
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* href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
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* Fast, and Practical Non-Blocking and Blocking Concurrent Queue
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* Algorithms</a> by Maged M. Michael and Michael L. Scott.
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*
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* <p>Iterators are <i>weakly consistent</i>, returning elements
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* reflecting the state of the queue at some point at or since the
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* creation of the iterator. They do <em>not</em> throw {@link
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* java.util.ConcurrentModificationException}, and may proceed concurrently
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* with other operations. Elements contained in the queue since the creation
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* of the iterator will be returned exactly once.
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*
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* <p>Beware that, unlike in most collections, the {@code size} method
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* is <em>NOT</em> a constant-time operation. Because of the
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* asynchronous nature of these queues, determining the current number
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* of elements requires a traversal of the elements, and so may report
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* inaccurate results if this collection is modified during traversal.
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* Additionally, the bulk operations {@code addAll},
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* {@code removeAll}, {@code retainAll}, {@code containsAll},
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* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
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* to be performed atomically. For example, an iterator operating
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* concurrently with an {@code addAll} operation might view only some
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* of the added elements.
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*
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* <p>This class and its iterator implement all of the <em>optional</em>
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* methods of the {@link Queue} and {@link Iterator} interfaces.
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*
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* <p>Memory consistency effects: As with other concurrent
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* collections, actions in a thread prior to placing an object into a
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* {@code ConcurrentLinkedQueue}
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
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* actions subsequent to the access or removal of that element from
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* the {@code ConcurrentLinkedQueue} in another thread.
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*
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* <p>This class is a member of the
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* <a href="{@docRoot}/../technotes/guides/collections/index.html">
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* Java Collections Framework</a>.
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*
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* @since 1.5
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* @author Doug Lea
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* @param <E> the type of elements held in this collection
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*
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*/
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public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
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implements Queue<E>, java.io.Serializable {
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private static final long serialVersionUID = 196745693267521676L;
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/*
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* This is a modification of the Michael & Scott algorithm,
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* adapted for a garbage-collected environment, with support for
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* interior node deletion (to support remove(Object)). For
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* explanation, read the paper.
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*
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* Note that like most non-blocking algorithms in this package,
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* this implementation relies on the fact that in garbage
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* collected systems, there is no possibility of ABA problems due
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* to recycled nodes, so there is no need to use "counted
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* pointers" or related techniques seen in versions used in
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* non-GC'ed settings.
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*
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* The fundamental invariants are:
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* - There is exactly one (last) Node with a null next reference,
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* which is CASed when enqueueing. This last Node can be
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* reached in O(1) time from tail, but tail is merely an
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* optimization - it can always be reached in O(N) time from
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* head as well.
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* - The elements contained in the queue are the non-null items in
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* Nodes that are reachable from head. CASing the item
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* reference of a Node to null atomically removes it from the
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* queue. Reachability of all elements from head must remain
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* true even in the case of concurrent modifications that cause
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* head to advance. A dequeued Node may remain in use
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* indefinitely due to creation of an Iterator or simply a
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* poll() that has lost its time slice.
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*
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* The above might appear to imply that all Nodes are GC-reachable
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* from a predecessor dequeued Node. That would cause two problems:
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* - allow a rogue Iterator to cause unbounded memory retention
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* - cause cross-generational linking of old Nodes to new Nodes if
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* a Node was tenured while live, which generational GCs have a
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* hard time dealing with, causing repeated major collections.
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* However, only non-deleted Nodes need to be reachable from
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* dequeued Nodes, and reachability does not necessarily have to
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* be of the kind understood by the GC. We use the trick of
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* linking a Node that has just been dequeued to itself. Such a
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* self-link implicitly means to advance to head.
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*
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* Both head and tail are permitted to lag. In fact, failing to
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* update them every time one could is a significant optimization
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* (fewer CASes). As with LinkedTransferQueue (see the internal
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* documentation for that class), we use a slack threshold of two;
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* that is, we update head/tail when the current pointer appears
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* to be two or more steps away from the first/last node.
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*
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* Since head and tail are updated concurrently and independently,
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* it is possible for tail to lag behind head (why not)?
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*
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* CASing a Node's item reference to null atomically removes the
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* element from the queue. Iterators skip over Nodes with null
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* items. Prior implementations of this class had a race between
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* poll() and remove(Object) where the same element would appear
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* to be successfully removed by two concurrent operations. The
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* method remove(Object) also lazily unlinks deleted Nodes, but
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* this is merely an optimization.
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*
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* When constructing a Node (before enqueuing it) we avoid paying
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* for a volatile write to item by using Unsafe.putObject instead
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* of a normal write. This allows the cost of enqueue to be
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* "one-and-a-half" CASes.
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*
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* Both head and tail may or may not point to a Node with a
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* non-null item. If the queue is empty, all items must of course
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* be null. Upon creation, both head and tail refer to a dummy
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* Node with null item. Both head and tail are only updated using
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* CAS, so they never regress, although again this is merely an
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* optimization.
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*/
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private static class Node<E> {
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volatile E item;
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volatile Node<E> next;
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/**
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* Constructs a new node. Uses relaxed write because item can
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* only be seen after publication via casNext.
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*/
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Node(E item) {
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this.item = item;
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}
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boolean casItem(E cmp, E val) {
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if (item == cmp) {
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item = val;
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return true;
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}
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return false;
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}
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void lazySetNext(Node<E> val) {
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this.next = val;
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}
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boolean casNext(Node<E> cmp, Node<E> val) {
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if (next == cmp) {
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next = val;
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return true;
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}
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return false;
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}
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}
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/**
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* A node from which the first live (non-deleted) node (if any)
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* can be reached in O(1) time.
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* Invariants:
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* - all live nodes are reachable from head via succ()
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* - head != null
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* - (tmp = head).next != tmp || tmp != head
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* Non-invariants:
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* - head.item may or may not be null.
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* - it is permitted for tail to lag behind head, that is, for tail
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* to not be reachable from head!
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*/
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private transient volatile Node<E> head;
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/**
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* A node from which the last node on list (that is, the unique
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* node with node.next == null) can be reached in O(1) time.
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* Invariants:
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* - the last node is always reachable from tail via succ()
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* - tail != null
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* Non-invariants:
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* - tail.item may or may not be null.
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* - it is permitted for tail to lag behind head, that is, for tail
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* to not be reachable from head!
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* - tail.next may or may not be self-pointing to tail.
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*/
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private transient volatile Node<E> tail;
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/**
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* Creates a {@code ConcurrentLinkedQueue} that is initially empty.
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*/
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public ConcurrentLinkedQueue() {
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head = tail = new Node<E>(null);
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}
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/**
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* Creates a {@code ConcurrentLinkedQueue}
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* initially containing the elements of the given collection,
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* added in traversal order of the collection's iterator.
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*
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* @param c the collection of elements to initially contain
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* @throws NullPointerException if the specified collection or any
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* of its elements are null
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*/
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public ConcurrentLinkedQueue(Collection<? extends E> c) {
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Node<E> h = null, t = null;
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for (E e : c) {
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checkNotNull(e);
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Node<E> newNode = new Node<E>(e);
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if (h == null)
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h = t = newNode;
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else {
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t.lazySetNext(newNode);
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t = newNode;
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}
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}
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if (h == null)
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h = t = new Node<E>(null);
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head = h;
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tail = t;
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}
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// Have to override just to update the javadoc
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/**
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* Inserts the specified element at the tail of this queue.
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* As the queue is unbounded, this method will never throw
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* {@link IllegalStateException} or return {@code false}.
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*
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* @return {@code true} (as specified by {@link Collection#add})
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* @throws NullPointerException if the specified element is null
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*/
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public boolean add(E e) {
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return offer(e);
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}
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/**
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* Try to CAS head to p. If successful, repoint old head to itself
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* as sentinel for succ(), below.
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*/
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final void updateHead(Node<E> h, Node<E> p) {
|
jaroslav@1890
|
293 |
if (h != p && casHead(h, p))
|
jaroslav@1890
|
294 |
h.lazySetNext(h);
|
jaroslav@1890
|
295 |
}
|
jaroslav@1890
|
296 |
|
jaroslav@1890
|
297 |
/**
|
jaroslav@1890
|
298 |
* Returns the successor of p, or the head node if p.next has been
|
jaroslav@1890
|
299 |
* linked to self, which will only be true if traversing with a
|
jaroslav@1890
|
300 |
* stale pointer that is now off the list.
|
jaroslav@1890
|
301 |
*/
|
jaroslav@1890
|
302 |
final Node<E> succ(Node<E> p) {
|
jaroslav@1890
|
303 |
Node<E> next = p.next;
|
jaroslav@1890
|
304 |
return (p == next) ? head : next;
|
jaroslav@1890
|
305 |
}
|
jaroslav@1890
|
306 |
|
jaroslav@1890
|
307 |
/**
|
jaroslav@1890
|
308 |
* Inserts the specified element at the tail of this queue.
|
jaroslav@1890
|
309 |
* As the queue is unbounded, this method will never return {@code false}.
|
jaroslav@1890
|
310 |
*
|
jaroslav@1890
|
311 |
* @return {@code true} (as specified by {@link Queue#offer})
|
jaroslav@1890
|
312 |
* @throws NullPointerException if the specified element is null
|
jaroslav@1890
|
313 |
*/
|
jaroslav@1890
|
314 |
public boolean offer(E e) {
|
jaroslav@1890
|
315 |
checkNotNull(e);
|
jaroslav@1890
|
316 |
final Node<E> newNode = new Node<E>(e);
|
jaroslav@1890
|
317 |
|
jaroslav@1890
|
318 |
for (Node<E> t = tail, p = t;;) {
|
jaroslav@1890
|
319 |
Node<E> q = p.next;
|
jaroslav@1890
|
320 |
if (q == null) {
|
jaroslav@1890
|
321 |
// p is last node
|
jaroslav@1890
|
322 |
if (p.casNext(null, newNode)) {
|
jaroslav@1890
|
323 |
// Successful CAS is the linearization point
|
jaroslav@1890
|
324 |
// for e to become an element of this queue,
|
jaroslav@1890
|
325 |
// and for newNode to become "live".
|
jaroslav@1890
|
326 |
if (p != t) // hop two nodes at a time
|
jaroslav@1890
|
327 |
casTail(t, newNode); // Failure is OK.
|
jaroslav@1890
|
328 |
return true;
|
jaroslav@1890
|
329 |
}
|
jaroslav@1890
|
330 |
// Lost CAS race to another thread; re-read next
|
jaroslav@1890
|
331 |
}
|
jaroslav@1890
|
332 |
else if (p == q)
|
jaroslav@1890
|
333 |
// We have fallen off list. If tail is unchanged, it
|
jaroslav@1890
|
334 |
// will also be off-list, in which case we need to
|
jaroslav@1890
|
335 |
// jump to head, from which all live nodes are always
|
jaroslav@1890
|
336 |
// reachable. Else the new tail is a better bet.
|
jaroslav@1890
|
337 |
p = (t != (t = tail)) ? t : head;
|
jaroslav@1890
|
338 |
else
|
jaroslav@1890
|
339 |
// Check for tail updates after two hops.
|
jaroslav@1890
|
340 |
p = (p != t && t != (t = tail)) ? t : q;
|
jaroslav@1890
|
341 |
}
|
jaroslav@1890
|
342 |
}
|
jaroslav@1890
|
343 |
|
jaroslav@1890
|
344 |
public E poll() {
|
jaroslav@1890
|
345 |
restartFromHead:
|
jaroslav@1890
|
346 |
for (;;) {
|
jaroslav@1890
|
347 |
for (Node<E> h = head, p = h, q;;) {
|
jaroslav@1890
|
348 |
E item = p.item;
|
jaroslav@1890
|
349 |
|
jaroslav@1890
|
350 |
if (item != null && p.casItem(item, null)) {
|
jaroslav@1890
|
351 |
// Successful CAS is the linearization point
|
jaroslav@1890
|
352 |
// for item to be removed from this queue.
|
jaroslav@1890
|
353 |
if (p != h) // hop two nodes at a time
|
jaroslav@1890
|
354 |
updateHead(h, ((q = p.next) != null) ? q : p);
|
jaroslav@1890
|
355 |
return item;
|
jaroslav@1890
|
356 |
}
|
jaroslav@1890
|
357 |
else if ((q = p.next) == null) {
|
jaroslav@1890
|
358 |
updateHead(h, p);
|
jaroslav@1890
|
359 |
return null;
|
jaroslav@1890
|
360 |
}
|
jaroslav@1890
|
361 |
else if (p == q)
|
jaroslav@1890
|
362 |
continue restartFromHead;
|
jaroslav@1890
|
363 |
else
|
jaroslav@1890
|
364 |
p = q;
|
jaroslav@1890
|
365 |
}
|
jaroslav@1890
|
366 |
}
|
jaroslav@1890
|
367 |
}
|
jaroslav@1890
|
368 |
|
jaroslav@1890
|
369 |
public E peek() {
|
jaroslav@1890
|
370 |
restartFromHead:
|
jaroslav@1890
|
371 |
for (;;) {
|
jaroslav@1890
|
372 |
for (Node<E> h = head, p = h, q;;) {
|
jaroslav@1890
|
373 |
E item = p.item;
|
jaroslav@1890
|
374 |
if (item != null || (q = p.next) == null) {
|
jaroslav@1890
|
375 |
updateHead(h, p);
|
jaroslav@1890
|
376 |
return item;
|
jaroslav@1890
|
377 |
}
|
jaroslav@1890
|
378 |
else if (p == q)
|
jaroslav@1890
|
379 |
continue restartFromHead;
|
jaroslav@1890
|
380 |
else
|
jaroslav@1890
|
381 |
p = q;
|
jaroslav@1890
|
382 |
}
|
jaroslav@1890
|
383 |
}
|
jaroslav@1890
|
384 |
}
|
jaroslav@1890
|
385 |
|
jaroslav@1890
|
386 |
/**
|
jaroslav@1890
|
387 |
* Returns the first live (non-deleted) node on list, or null if none.
|
jaroslav@1890
|
388 |
* This is yet another variant of poll/peek; here returning the
|
jaroslav@1890
|
389 |
* first node, not element. We could make peek() a wrapper around
|
jaroslav@1890
|
390 |
* first(), but that would cost an extra volatile read of item,
|
jaroslav@1890
|
391 |
* and the need to add a retry loop to deal with the possibility
|
jaroslav@1890
|
392 |
* of losing a race to a concurrent poll().
|
jaroslav@1890
|
393 |
*/
|
jaroslav@1890
|
394 |
Node<E> first() {
|
jaroslav@1890
|
395 |
restartFromHead:
|
jaroslav@1890
|
396 |
for (;;) {
|
jaroslav@1890
|
397 |
for (Node<E> h = head, p = h, q;;) {
|
jaroslav@1890
|
398 |
boolean hasItem = (p.item != null);
|
jaroslav@1890
|
399 |
if (hasItem || (q = p.next) == null) {
|
jaroslav@1890
|
400 |
updateHead(h, p);
|
jaroslav@1890
|
401 |
return hasItem ? p : null;
|
jaroslav@1890
|
402 |
}
|
jaroslav@1890
|
403 |
else if (p == q)
|
jaroslav@1890
|
404 |
continue restartFromHead;
|
jaroslav@1890
|
405 |
else
|
jaroslav@1890
|
406 |
p = q;
|
jaroslav@1890
|
407 |
}
|
jaroslav@1890
|
408 |
}
|
jaroslav@1890
|
409 |
}
|
jaroslav@1890
|
410 |
|
jaroslav@1890
|
411 |
/**
|
jaroslav@1890
|
412 |
* Returns {@code true} if this queue contains no elements.
|
jaroslav@1890
|
413 |
*
|
jaroslav@1890
|
414 |
* @return {@code true} if this queue contains no elements
|
jaroslav@1890
|
415 |
*/
|
jaroslav@1890
|
416 |
public boolean isEmpty() {
|
jaroslav@1890
|
417 |
return first() == null;
|
jaroslav@1890
|
418 |
}
|
jaroslav@1890
|
419 |
|
jaroslav@1890
|
420 |
/**
|
jaroslav@1890
|
421 |
* Returns the number of elements in this queue. If this queue
|
jaroslav@1890
|
422 |
* contains more than {@code Integer.MAX_VALUE} elements, returns
|
jaroslav@1890
|
423 |
* {@code Integer.MAX_VALUE}.
|
jaroslav@1890
|
424 |
*
|
jaroslav@1890
|
425 |
* <p>Beware that, unlike in most collections, this method is
|
jaroslav@1890
|
426 |
* <em>NOT</em> a constant-time operation. Because of the
|
jaroslav@1890
|
427 |
* asynchronous nature of these queues, determining the current
|
jaroslav@1890
|
428 |
* number of elements requires an O(n) traversal.
|
jaroslav@1890
|
429 |
* Additionally, if elements are added or removed during execution
|
jaroslav@1890
|
430 |
* of this method, the returned result may be inaccurate. Thus,
|
jaroslav@1890
|
431 |
* this method is typically not very useful in concurrent
|
jaroslav@1890
|
432 |
* applications.
|
jaroslav@1890
|
433 |
*
|
jaroslav@1890
|
434 |
* @return the number of elements in this queue
|
jaroslav@1890
|
435 |
*/
|
jaroslav@1890
|
436 |
public int size() {
|
jaroslav@1890
|
437 |
int count = 0;
|
jaroslav@1890
|
438 |
for (Node<E> p = first(); p != null; p = succ(p))
|
jaroslav@1890
|
439 |
if (p.item != null)
|
jaroslav@1890
|
440 |
// Collection.size() spec says to max out
|
jaroslav@1890
|
441 |
if (++count == Integer.MAX_VALUE)
|
jaroslav@1890
|
442 |
break;
|
jaroslav@1890
|
443 |
return count;
|
jaroslav@1890
|
444 |
}
|
jaroslav@1890
|
445 |
|
jaroslav@1890
|
446 |
/**
|
jaroslav@1890
|
447 |
* Returns {@code true} if this queue contains the specified element.
|
jaroslav@1890
|
448 |
* More formally, returns {@code true} if and only if this queue contains
|
jaroslav@1890
|
449 |
* at least one element {@code e} such that {@code o.equals(e)}.
|
jaroslav@1890
|
450 |
*
|
jaroslav@1890
|
451 |
* @param o object to be checked for containment in this queue
|
jaroslav@1890
|
452 |
* @return {@code true} if this queue contains the specified element
|
jaroslav@1890
|
453 |
*/
|
jaroslav@1890
|
454 |
public boolean contains(Object o) {
|
jaroslav@1890
|
455 |
if (o == null) return false;
|
jaroslav@1890
|
456 |
for (Node<E> p = first(); p != null; p = succ(p)) {
|
jaroslav@1890
|
457 |
E item = p.item;
|
jaroslav@1890
|
458 |
if (item != null && o.equals(item))
|
jaroslav@1890
|
459 |
return true;
|
jaroslav@1890
|
460 |
}
|
jaroslav@1890
|
461 |
return false;
|
jaroslav@1890
|
462 |
}
|
jaroslav@1890
|
463 |
|
jaroslav@1890
|
464 |
/**
|
jaroslav@1890
|
465 |
* Removes a single instance of the specified element from this queue,
|
jaroslav@1890
|
466 |
* if it is present. More formally, removes an element {@code e} such
|
jaroslav@1890
|
467 |
* that {@code o.equals(e)}, if this queue contains one or more such
|
jaroslav@1890
|
468 |
* elements.
|
jaroslav@1890
|
469 |
* Returns {@code true} if this queue contained the specified element
|
jaroslav@1890
|
470 |
* (or equivalently, if this queue changed as a result of the call).
|
jaroslav@1890
|
471 |
*
|
jaroslav@1890
|
472 |
* @param o element to be removed from this queue, if present
|
jaroslav@1890
|
473 |
* @return {@code true} if this queue changed as a result of the call
|
jaroslav@1890
|
474 |
*/
|
jaroslav@1890
|
475 |
public boolean remove(Object o) {
|
jaroslav@1890
|
476 |
if (o == null) return false;
|
jaroslav@1890
|
477 |
Node<E> pred = null;
|
jaroslav@1890
|
478 |
for (Node<E> p = first(); p != null; p = succ(p)) {
|
jaroslav@1890
|
479 |
E item = p.item;
|
jaroslav@1890
|
480 |
if (item != null &&
|
jaroslav@1890
|
481 |
o.equals(item) &&
|
jaroslav@1890
|
482 |
p.casItem(item, null)) {
|
jaroslav@1890
|
483 |
Node<E> next = succ(p);
|
jaroslav@1890
|
484 |
if (pred != null && next != null)
|
jaroslav@1890
|
485 |
pred.casNext(p, next);
|
jaroslav@1890
|
486 |
return true;
|
jaroslav@1890
|
487 |
}
|
jaroslav@1890
|
488 |
pred = p;
|
jaroslav@1890
|
489 |
}
|
jaroslav@1890
|
490 |
return false;
|
jaroslav@1890
|
491 |
}
|
jaroslav@1890
|
492 |
|
jaroslav@1890
|
493 |
/**
|
jaroslav@1890
|
494 |
* Appends all of the elements in the specified collection to the end of
|
jaroslav@1890
|
495 |
* this queue, in the order that they are returned by the specified
|
jaroslav@1890
|
496 |
* collection's iterator. Attempts to {@code addAll} of a queue to
|
jaroslav@1890
|
497 |
* itself result in {@code IllegalArgumentException}.
|
jaroslav@1890
|
498 |
*
|
jaroslav@1890
|
499 |
* @param c the elements to be inserted into this queue
|
jaroslav@1890
|
500 |
* @return {@code true} if this queue changed as a result of the call
|
jaroslav@1890
|
501 |
* @throws NullPointerException if the specified collection or any
|
jaroslav@1890
|
502 |
* of its elements are null
|
jaroslav@1890
|
503 |
* @throws IllegalArgumentException if the collection is this queue
|
jaroslav@1890
|
504 |
*/
|
jaroslav@1890
|
505 |
public boolean addAll(Collection<? extends E> c) {
|
jaroslav@1890
|
506 |
if (c == this)
|
jaroslav@1890
|
507 |
// As historically specified in AbstractQueue#addAll
|
jaroslav@1890
|
508 |
throw new IllegalArgumentException();
|
jaroslav@1890
|
509 |
|
jaroslav@1890
|
510 |
// Copy c into a private chain of Nodes
|
jaroslav@1890
|
511 |
Node<E> beginningOfTheEnd = null, last = null;
|
jaroslav@1890
|
512 |
for (E e : c) {
|
jaroslav@1890
|
513 |
checkNotNull(e);
|
jaroslav@1890
|
514 |
Node<E> newNode = new Node<E>(e);
|
jaroslav@1890
|
515 |
if (beginningOfTheEnd == null)
|
jaroslav@1890
|
516 |
beginningOfTheEnd = last = newNode;
|
jaroslav@1890
|
517 |
else {
|
jaroslav@1890
|
518 |
last.lazySetNext(newNode);
|
jaroslav@1890
|
519 |
last = newNode;
|
jaroslav@1890
|
520 |
}
|
jaroslav@1890
|
521 |
}
|
jaroslav@1890
|
522 |
if (beginningOfTheEnd == null)
|
jaroslav@1890
|
523 |
return false;
|
jaroslav@1890
|
524 |
|
jaroslav@1890
|
525 |
// Atomically append the chain at the tail of this collection
|
jaroslav@1890
|
526 |
for (Node<E> t = tail, p = t;;) {
|
jaroslav@1890
|
527 |
Node<E> q = p.next;
|
jaroslav@1890
|
528 |
if (q == null) {
|
jaroslav@1890
|
529 |
// p is last node
|
jaroslav@1890
|
530 |
if (p.casNext(null, beginningOfTheEnd)) {
|
jaroslav@1890
|
531 |
// Successful CAS is the linearization point
|
jaroslav@1890
|
532 |
// for all elements to be added to this queue.
|
jaroslav@1890
|
533 |
if (!casTail(t, last)) {
|
jaroslav@1890
|
534 |
// Try a little harder to update tail,
|
jaroslav@1890
|
535 |
// since we may be adding many elements.
|
jaroslav@1890
|
536 |
t = tail;
|
jaroslav@1890
|
537 |
if (last.next == null)
|
jaroslav@1890
|
538 |
casTail(t, last);
|
jaroslav@1890
|
539 |
}
|
jaroslav@1890
|
540 |
return true;
|
jaroslav@1890
|
541 |
}
|
jaroslav@1890
|
542 |
// Lost CAS race to another thread; re-read next
|
jaroslav@1890
|
543 |
}
|
jaroslav@1890
|
544 |
else if (p == q)
|
jaroslav@1890
|
545 |
// We have fallen off list. If tail is unchanged, it
|
jaroslav@1890
|
546 |
// will also be off-list, in which case we need to
|
jaroslav@1890
|
547 |
// jump to head, from which all live nodes are always
|
jaroslav@1890
|
548 |
// reachable. Else the new tail is a better bet.
|
jaroslav@1890
|
549 |
p = (t != (t = tail)) ? t : head;
|
jaroslav@1890
|
550 |
else
|
jaroslav@1890
|
551 |
// Check for tail updates after two hops.
|
jaroslav@1890
|
552 |
p = (p != t && t != (t = tail)) ? t : q;
|
jaroslav@1890
|
553 |
}
|
jaroslav@1890
|
554 |
}
|
jaroslav@1890
|
555 |
|
jaroslav@1890
|
556 |
/**
|
jaroslav@1890
|
557 |
* Returns an array containing all of the elements in this queue, in
|
jaroslav@1890
|
558 |
* proper sequence.
|
jaroslav@1890
|
559 |
*
|
jaroslav@1890
|
560 |
* <p>The returned array will be "safe" in that no references to it are
|
jaroslav@1890
|
561 |
* maintained by this queue. (In other words, this method must allocate
|
jaroslav@1890
|
562 |
* a new array). The caller is thus free to modify the returned array.
|
jaroslav@1890
|
563 |
*
|
jaroslav@1890
|
564 |
* <p>This method acts as bridge between array-based and collection-based
|
jaroslav@1890
|
565 |
* APIs.
|
jaroslav@1890
|
566 |
*
|
jaroslav@1890
|
567 |
* @return an array containing all of the elements in this queue
|
jaroslav@1890
|
568 |
*/
|
jaroslav@1890
|
569 |
public Object[] toArray() {
|
jaroslav@1890
|
570 |
// Use ArrayList to deal with resizing.
|
jaroslav@1890
|
571 |
ArrayList<E> al = new ArrayList<E>();
|
jaroslav@1890
|
572 |
for (Node<E> p = first(); p != null; p = succ(p)) {
|
jaroslav@1890
|
573 |
E item = p.item;
|
jaroslav@1890
|
574 |
if (item != null)
|
jaroslav@1890
|
575 |
al.add(item);
|
jaroslav@1890
|
576 |
}
|
jaroslav@1890
|
577 |
return al.toArray();
|
jaroslav@1890
|
578 |
}
|
jaroslav@1890
|
579 |
|
jaroslav@1890
|
580 |
/**
|
jaroslav@1890
|
581 |
* Returns an array containing all of the elements in this queue, in
|
jaroslav@1890
|
582 |
* proper sequence; the runtime type of the returned array is that of
|
jaroslav@1890
|
583 |
* the specified array. If the queue fits in the specified array, it
|
jaroslav@1890
|
584 |
* is returned therein. Otherwise, a new array is allocated with the
|
jaroslav@1890
|
585 |
* runtime type of the specified array and the size of this queue.
|
jaroslav@1890
|
586 |
*
|
jaroslav@1890
|
587 |
* <p>If this queue fits in the specified array with room to spare
|
jaroslav@1890
|
588 |
* (i.e., the array has more elements than this queue), the element in
|
jaroslav@1890
|
589 |
* the array immediately following the end of the queue is set to
|
jaroslav@1890
|
590 |
* {@code null}.
|
jaroslav@1890
|
591 |
*
|
jaroslav@1890
|
592 |
* <p>Like the {@link #toArray()} method, this method acts as bridge between
|
jaroslav@1890
|
593 |
* array-based and collection-based APIs. Further, this method allows
|
jaroslav@1890
|
594 |
* precise control over the runtime type of the output array, and may,
|
jaroslav@1890
|
595 |
* under certain circumstances, be used to save allocation costs.
|
jaroslav@1890
|
596 |
*
|
jaroslav@1890
|
597 |
* <p>Suppose {@code x} is a queue known to contain only strings.
|
jaroslav@1890
|
598 |
* The following code can be used to dump the queue into a newly
|
jaroslav@1890
|
599 |
* allocated array of {@code String}:
|
jaroslav@1890
|
600 |
*
|
jaroslav@1890
|
601 |
* <pre>
|
jaroslav@1890
|
602 |
* String[] y = x.toArray(new String[0]);</pre>
|
jaroslav@1890
|
603 |
*
|
jaroslav@1890
|
604 |
* Note that {@code toArray(new Object[0])} is identical in function to
|
jaroslav@1890
|
605 |
* {@code toArray()}.
|
jaroslav@1890
|
606 |
*
|
jaroslav@1890
|
607 |
* @param a the array into which the elements of the queue are to
|
jaroslav@1890
|
608 |
* be stored, if it is big enough; otherwise, a new array of the
|
jaroslav@1890
|
609 |
* same runtime type is allocated for this purpose
|
jaroslav@1890
|
610 |
* @return an array containing all of the elements in this queue
|
jaroslav@1890
|
611 |
* @throws ArrayStoreException if the runtime type of the specified array
|
jaroslav@1890
|
612 |
* is not a supertype of the runtime type of every element in
|
jaroslav@1890
|
613 |
* this queue
|
jaroslav@1890
|
614 |
* @throws NullPointerException if the specified array is null
|
jaroslav@1890
|
615 |
*/
|
jaroslav@1890
|
616 |
@SuppressWarnings("unchecked")
|
jaroslav@1890
|
617 |
public <T> T[] toArray(T[] a) {
|
jaroslav@1890
|
618 |
// try to use sent-in array
|
jaroslav@1890
|
619 |
int k = 0;
|
jaroslav@1890
|
620 |
Node<E> p;
|
jaroslav@1890
|
621 |
for (p = first(); p != null && k < a.length; p = succ(p)) {
|
jaroslav@1890
|
622 |
E item = p.item;
|
jaroslav@1890
|
623 |
if (item != null)
|
jaroslav@1890
|
624 |
a[k++] = (T)item;
|
jaroslav@1890
|
625 |
}
|
jaroslav@1890
|
626 |
if (p == null) {
|
jaroslav@1890
|
627 |
if (k < a.length)
|
jaroslav@1890
|
628 |
a[k] = null;
|
jaroslav@1890
|
629 |
return a;
|
jaroslav@1890
|
630 |
}
|
jaroslav@1890
|
631 |
|
jaroslav@1890
|
632 |
// If won't fit, use ArrayList version
|
jaroslav@1890
|
633 |
ArrayList<E> al = new ArrayList<E>();
|
jaroslav@1890
|
634 |
for (Node<E> q = first(); q != null; q = succ(q)) {
|
jaroslav@1890
|
635 |
E item = q.item;
|
jaroslav@1890
|
636 |
if (item != null)
|
jaroslav@1890
|
637 |
al.add(item);
|
jaroslav@1890
|
638 |
}
|
jaroslav@1890
|
639 |
return al.toArray(a);
|
jaroslav@1890
|
640 |
}
|
jaroslav@1890
|
641 |
|
jaroslav@1890
|
642 |
/**
|
jaroslav@1890
|
643 |
* Returns an iterator over the elements in this queue in proper sequence.
|
jaroslav@1890
|
644 |
* The elements will be returned in order from first (head) to last (tail).
|
jaroslav@1890
|
645 |
*
|
jaroslav@1890
|
646 |
* <p>The returned iterator is a "weakly consistent" iterator that
|
jaroslav@1890
|
647 |
* will never throw {@link java.util.ConcurrentModificationException
|
jaroslav@1890
|
648 |
* ConcurrentModificationException}, and guarantees to traverse
|
jaroslav@1890
|
649 |
* elements as they existed upon construction of the iterator, and
|
jaroslav@1890
|
650 |
* may (but is not guaranteed to) reflect any modifications
|
jaroslav@1890
|
651 |
* subsequent to construction.
|
jaroslav@1890
|
652 |
*
|
jaroslav@1890
|
653 |
* @return an iterator over the elements in this queue in proper sequence
|
jaroslav@1890
|
654 |
*/
|
jaroslav@1890
|
655 |
public Iterator<E> iterator() {
|
jaroslav@1890
|
656 |
return new Itr();
|
jaroslav@1890
|
657 |
}
|
jaroslav@1890
|
658 |
|
jaroslav@1890
|
659 |
private class Itr implements Iterator<E> {
|
jaroslav@1890
|
660 |
/**
|
jaroslav@1890
|
661 |
* Next node to return item for.
|
jaroslav@1890
|
662 |
*/
|
jaroslav@1890
|
663 |
private Node<E> nextNode;
|
jaroslav@1890
|
664 |
|
jaroslav@1890
|
665 |
/**
|
jaroslav@1890
|
666 |
* nextItem holds on to item fields because once we claim
|
jaroslav@1890
|
667 |
* that an element exists in hasNext(), we must return it in
|
jaroslav@1890
|
668 |
* the following next() call even if it was in the process of
|
jaroslav@1890
|
669 |
* being removed when hasNext() was called.
|
jaroslav@1890
|
670 |
*/
|
jaroslav@1890
|
671 |
private E nextItem;
|
jaroslav@1890
|
672 |
|
jaroslav@1890
|
673 |
/**
|
jaroslav@1890
|
674 |
* Node of the last returned item, to support remove.
|
jaroslav@1890
|
675 |
*/
|
jaroslav@1890
|
676 |
private Node<E> lastRet;
|
jaroslav@1890
|
677 |
|
jaroslav@1890
|
678 |
Itr() {
|
jaroslav@1890
|
679 |
advance();
|
jaroslav@1890
|
680 |
}
|
jaroslav@1890
|
681 |
|
jaroslav@1890
|
682 |
/**
|
jaroslav@1890
|
683 |
* Moves to next valid node and returns item to return for
|
jaroslav@1890
|
684 |
* next(), or null if no such.
|
jaroslav@1890
|
685 |
*/
|
jaroslav@1890
|
686 |
private E advance() {
|
jaroslav@1890
|
687 |
lastRet = nextNode;
|
jaroslav@1890
|
688 |
E x = nextItem;
|
jaroslav@1890
|
689 |
|
jaroslav@1890
|
690 |
Node<E> pred, p;
|
jaroslav@1890
|
691 |
if (nextNode == null) {
|
jaroslav@1890
|
692 |
p = first();
|
jaroslav@1890
|
693 |
pred = null;
|
jaroslav@1890
|
694 |
} else {
|
jaroslav@1890
|
695 |
pred = nextNode;
|
jaroslav@1890
|
696 |
p = succ(nextNode);
|
jaroslav@1890
|
697 |
}
|
jaroslav@1890
|
698 |
|
jaroslav@1890
|
699 |
for (;;) {
|
jaroslav@1890
|
700 |
if (p == null) {
|
jaroslav@1890
|
701 |
nextNode = null;
|
jaroslav@1890
|
702 |
nextItem = null;
|
jaroslav@1890
|
703 |
return x;
|
jaroslav@1890
|
704 |
}
|
jaroslav@1890
|
705 |
E item = p.item;
|
jaroslav@1890
|
706 |
if (item != null) {
|
jaroslav@1890
|
707 |
nextNode = p;
|
jaroslav@1890
|
708 |
nextItem = item;
|
jaroslav@1890
|
709 |
return x;
|
jaroslav@1890
|
710 |
} else {
|
jaroslav@1890
|
711 |
// skip over nulls
|
jaroslav@1890
|
712 |
Node<E> next = succ(p);
|
jaroslav@1890
|
713 |
if (pred != null && next != null)
|
jaroslav@1890
|
714 |
pred.casNext(p, next);
|
jaroslav@1890
|
715 |
p = next;
|
jaroslav@1890
|
716 |
}
|
jaroslav@1890
|
717 |
}
|
jaroslav@1890
|
718 |
}
|
jaroslav@1890
|
719 |
|
jaroslav@1890
|
720 |
public boolean hasNext() {
|
jaroslav@1890
|
721 |
return nextNode != null;
|
jaroslav@1890
|
722 |
}
|
jaroslav@1890
|
723 |
|
jaroslav@1890
|
724 |
public E next() {
|
jaroslav@1890
|
725 |
if (nextNode == null) throw new NoSuchElementException();
|
jaroslav@1890
|
726 |
return advance();
|
jaroslav@1890
|
727 |
}
|
jaroslav@1890
|
728 |
|
jaroslav@1890
|
729 |
public void remove() {
|
jaroslav@1890
|
730 |
Node<E> l = lastRet;
|
jaroslav@1890
|
731 |
if (l == null) throw new IllegalStateException();
|
jaroslav@1890
|
732 |
// rely on a future traversal to relink.
|
jaroslav@1890
|
733 |
l.item = null;
|
jaroslav@1890
|
734 |
lastRet = null;
|
jaroslav@1890
|
735 |
}
|
jaroslav@1890
|
736 |
}
|
jaroslav@1890
|
737 |
|
jaroslav@1890
|
738 |
/**
|
jaroslav@1890
|
739 |
* Saves the state to a stream (that is, serializes it).
|
jaroslav@1890
|
740 |
*
|
jaroslav@1890
|
741 |
* @serialData All of the elements (each an {@code E}) in
|
jaroslav@1890
|
742 |
* the proper order, followed by a null
|
jaroslav@1890
|
743 |
* @param s the stream
|
jaroslav@1890
|
744 |
*/
|
jaroslav@1890
|
745 |
private void writeObject(java.io.ObjectOutputStream s)
|
jaroslav@1890
|
746 |
throws java.io.IOException {
|
jaroslav@1890
|
747 |
|
jaroslav@1890
|
748 |
// Write out any hidden stuff
|
jaroslav@1890
|
749 |
s.defaultWriteObject();
|
jaroslav@1890
|
750 |
|
jaroslav@1890
|
751 |
// Write out all elements in the proper order.
|
jaroslav@1890
|
752 |
for (Node<E> p = first(); p != null; p = succ(p)) {
|
jaroslav@1890
|
753 |
Object item = p.item;
|
jaroslav@1890
|
754 |
if (item != null)
|
jaroslav@1890
|
755 |
s.writeObject(item);
|
jaroslav@1890
|
756 |
}
|
jaroslav@1890
|
757 |
|
jaroslav@1890
|
758 |
// Use trailing null as sentinel
|
jaroslav@1890
|
759 |
s.writeObject(null);
|
jaroslav@1890
|
760 |
}
|
jaroslav@1890
|
761 |
|
jaroslav@1890
|
762 |
/**
|
jaroslav@1890
|
763 |
* Reconstitutes the instance from a stream (that is, deserializes it).
|
jaroslav@1890
|
764 |
* @param s the stream
|
jaroslav@1890
|
765 |
*/
|
jaroslav@1890
|
766 |
private void readObject(java.io.ObjectInputStream s)
|
jaroslav@1890
|
767 |
throws java.io.IOException, ClassNotFoundException {
|
jaroslav@1890
|
768 |
s.defaultReadObject();
|
jaroslav@1890
|
769 |
|
jaroslav@1890
|
770 |
// Read in elements until trailing null sentinel found
|
jaroslav@1890
|
771 |
Node<E> h = null, t = null;
|
jaroslav@1890
|
772 |
Object item;
|
jaroslav@1890
|
773 |
while ((item = s.readObject()) != null) {
|
jaroslav@1890
|
774 |
@SuppressWarnings("unchecked")
|
jaroslav@1890
|
775 |
Node<E> newNode = new Node<E>((E) item);
|
jaroslav@1890
|
776 |
if (h == null)
|
jaroslav@1890
|
777 |
h = t = newNode;
|
jaroslav@1890
|
778 |
else {
|
jaroslav@1890
|
779 |
t.lazySetNext(newNode);
|
jaroslav@1890
|
780 |
t = newNode;
|
jaroslav@1890
|
781 |
}
|
jaroslav@1890
|
782 |
}
|
jaroslav@1890
|
783 |
if (h == null)
|
jaroslav@1890
|
784 |
h = t = new Node<E>(null);
|
jaroslav@1890
|
785 |
head = h;
|
jaroslav@1890
|
786 |
tail = t;
|
jaroslav@1890
|
787 |
}
|
jaroslav@1890
|
788 |
|
jaroslav@1890
|
789 |
/**
|
jaroslav@1890
|
790 |
* Throws NullPointerException if argument is null.
|
jaroslav@1890
|
791 |
*
|
jaroslav@1890
|
792 |
* @param v the element
|
jaroslav@1890
|
793 |
*/
|
jaroslav@1890
|
794 |
private static void checkNotNull(Object v) {
|
jaroslav@1890
|
795 |
if (v == null)
|
jaroslav@1890
|
796 |
throw new NullPointerException();
|
jaroslav@1890
|
797 |
}
|
jaroslav@1890
|
798 |
|
jaroslav@1890
|
799 |
private boolean casTail(Node<E> cmp, Node<E> val) {
|
jaroslav@1895
|
800 |
if (tail == cmp) {
|
jaroslav@1895
|
801 |
tail = val;
|
jaroslav@1895
|
802 |
return true;
|
jaroslav@1895
|
803 |
}
|
jaroslav@1895
|
804 |
return false;
|
jaroslav@1890
|
805 |
}
|
jaroslav@1890
|
806 |
|
jaroslav@1890
|
807 |
private boolean casHead(Node<E> cmp, Node<E> val) {
|
jaroslav@1895
|
808 |
if (head == cmp) {
|
jaroslav@1895
|
809 |
head = val;
|
jaroslav@1895
|
810 |
return true;
|
jaroslav@1895
|
811 |
}
|
jaroslav@1895
|
812 |
return false;
|
jaroslav@1890
|
813 |
}
|
jaroslav@1890
|
814 |
|
jaroslav@1890
|
815 |
}
|