My first initial commit with all of mine experimental changes.

Dmitry Kabak (userdima committed 13 years ago

Commit d37b88d1 parent 3f5b46618352 files · +2598093 −-8690Browse files

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	1	+/*
	2	+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	3	+ *
	4	+ * This code is free software; you can redistribute it and/or modify it
	5	+ * under the terms of the GNU General Public License version 2 only, as
	6	+ * published by the Free Software Foundation. Oracle designates this
	7	+ * particular file as subject to the "Classpath" exception as provided
	8	+ * by Oracle in the LICENSE file that accompanied this code.
	9	+ *
	10	+ * This code is distributed in the hope that it will be useful, but WITHOUT
	11	+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	12	+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	13	+ * version 2 for more details (a copy is included in the LICENSE file that
	14	+ * accompanied this code).
	15	+ *
	16	+ * You should have received a copy of the GNU General Public License version
	17	+ * 2 along with this work; if not, write to the Free Software Foundation,
	18	+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	19	+ *
	20	+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	21	+ * or visit www.oracle.com if you need additional information or have any
	22	+ * questions.
	23	+ */
	24	+
	25	+/*
	26	+ * This file is available under and governed by the GNU General Public
	27	+ * License version 2 only, as published by the Free Software Foundation.
	28	+ * However, the following notice accompanied the original version of this
	29	+ * file:
	30	+ *
	31	+ * Written by Doug Lea and Martin Buchholz with assistance from members of
	32	+ * JCP JSR-166 Expert Group and released to the public domain, as explained
	33	+ * at http://creativecommons.org/publicdomain/zero/1.0/
	34	+ */
	35	+
	36	+package java.util.concurrent;
	37	+
	38	+import java.util.AbstractQueue;
	39	+import java.util.ArrayList;
	40	+import java.util.Collection;
	41	+import java.util.Iterator;
	42	+import java.util.NoSuchElementException;
	43	+import java.util.Queue;
	44	+
	45	+/**
	46	+ * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
	47	+ * This queue orders elements FIFO (first-in-first-out).
	48	+ * The <em>head</em> of the queue is that element that has been on the
	49	+ * queue the longest time.
	50	+ * The <em>tail</em> of the queue is that element that has been on the
	51	+ * queue the shortest time. New elements
	52	+ * are inserted at the tail of the queue, and the queue retrieval
	53	+ * operations obtain elements at the head of the queue.
	54	+ * A {@code ConcurrentLinkedQueue} is an appropriate choice when
	55	+ * many threads will share access to a common collection.
	56	+ * Like most other concurrent collection implementations, this class
	57	+ * does not permit the use of {@code null} elements.
	58	+ *
	59	+ * <p>This implementation employs an efficient "wait-free"
	60	+ * algorithm based on one described in <a
	61	+ * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
	62	+ * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
	63	+ * Algorithms</a> by Maged M. Michael and Michael L. Scott.
	64	+ *
	65	+ * <p>Iterators are <i>weakly consistent</i>, returning elements
	66	+ * reflecting the state of the queue at some point at or since the
	67	+ * creation of the iterator. They do <em>not</em> throw {@link
	68	+ * java.util.ConcurrentModificationException}, and may proceed concurrently
	69	+ * with other operations. Elements contained in the queue since the creation
	70	+ * of the iterator will be returned exactly once.
	71	+ *
	72	+ * <p>Beware that, unlike in most collections, the {@code size} method
	73	+ * is <em>NOT</em> a constant-time operation. Because of the
	74	+ * asynchronous nature of these queues, determining the current number
	75	+ * of elements requires a traversal of the elements, and so may report
	76	+ * inaccurate results if this collection is modified during traversal.
	77	+ * Additionally, the bulk operations {@code addAll},
	78	+ * {@code removeAll}, {@code retainAll}, {@code containsAll},
	79	+ * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
	80	+ * to be performed atomically. For example, an iterator operating
	81	+ * concurrently with an {@code addAll} operation might view only some
	82	+ * of the added elements.
	83	+ *
	84	+ * <p>This class and its iterator implement all of the <em>optional</em>
	85	+ * methods of the {@link Queue} and {@link Iterator} interfaces.
	86	+ *
	87	+ * <p>Memory consistency effects: As with other concurrent
	88	+ * collections, actions in a thread prior to placing an object into a
	89	+ * {@code ConcurrentLinkedQueue}
	90	+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
	91	+ * actions subsequent to the access or removal of that element from
	92	+ * the {@code ConcurrentLinkedQueue} in another thread.
	93	+ *
	94	+ * <p>This class is a member of the
	95	+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
	96	+ * Java Collections Framework</a>.
	97	+ *
	98	+ * @since 1.5
	99	+ * @author Doug Lea
	100	+ * @param <E> the type of elements held in this collection
	101	+ *
	102	+ */
	103	+public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
	104	+ implements Queue<E>, java.io.Serializable {
	105	+ private static final long serialVersionUID = 196745693267521676L;
	106	+
	107	+ /*
	108	+ * This is a modification of the Michael & Scott algorithm,
	109	+ * adapted for a garbage-collected environment, with support for
	110	+ * interior node deletion (to support remove(Object)). For
	111	+ * explanation, read the paper.
	112	+ *
	113	+ * Note that like most non-blocking algorithms in this package,
	114	+ * this implementation relies on the fact that in garbage
	115	+ * collected systems, there is no possibility of ABA problems due
	116	+ * to recycled nodes, so there is no need to use "counted
	117	+ * pointers" or related techniques seen in versions used in
	118	+ * non-GC'ed settings.
	119	+ *
	120	+ * The fundamental invariants are:
	121	+ * - There is exactly one (last) Node with a null next reference,
	122	+ * which is CASed when enqueueing. This last Node can be
	123	+ * reached in O(1) time from tail, but tail is merely an
	124	+ * optimization - it can always be reached in O(N) time from
	125	+ * head as well.
	126	+ * - The elements contained in the queue are the non-null items in
	127	+ * Nodes that are reachable from head. CASing the item
	128	+ * reference of a Node to null atomically removes it from the
	129	+ * queue. Reachability of all elements from head must remain
	130	+ * true even in the case of concurrent modifications that cause
	131	+ * head to advance. A dequeued Node may remain in use
	132	+ * indefinitely due to creation of an Iterator or simply a
	133	+ * poll() that has lost its time slice.
	134	+ *
	135	+ * The above might appear to imply that all Nodes are GC-reachable
	136	+ * from a predecessor dequeued Node. That would cause two problems:
	137	+ * - allow a rogue Iterator to cause unbounded memory retention
	138	+ * - cause cross-generational linking of old Nodes to new Nodes if
	139	+ * a Node was tenured while live, which generational GCs have a
	140	+ * hard time dealing with, causing repeated major collections.
	141	+ * However, only non-deleted Nodes need to be reachable from
	142	+ * dequeued Nodes, and reachability does not necessarily have to
	143	+ * be of the kind understood by the GC. We use the trick of
	144	+ * linking a Node that has just been dequeued to itself. Such a
	145	+ * self-link implicitly means to advance to head.
	146	+ *
	147	+ * Both head and tail are permitted to lag. In fact, failing to
	148	+ * update them every time one could is a significant optimization
	149	+ * (fewer CASes). As with LinkedTransferQueue (see the internal
	150	+ * documentation for that class), we use a slack threshold of two;
	151	+ * that is, we update head/tail when the current pointer appears
	152	+ * to be two or more steps away from the first/last node.
	153	+ *
	154	+ * Since head and tail are updated concurrently and independently,
	155	+ * it is possible for tail to lag behind head (why not)?
	156	+ *
	157	+ * CASing a Node's item reference to null atomically removes the
	158	+ * element from the queue. Iterators skip over Nodes with null
	159	+ * items. Prior implementations of this class had a race between
	160	+ * poll() and remove(Object) where the same element would appear
	161	+ * to be successfully removed by two concurrent operations. The
	162	+ * method remove(Object) also lazily unlinks deleted Nodes, but
	163	+ * this is merely an optimization.
	164	+ *
	165	+ * When constructing a Node (before enqueuing it) we avoid paying
	166	+ * for a volatile write to item by using Unsafe.putObject instead
	167	+ * of a normal write. This allows the cost of enqueue to be
	168	+ * "one-and-a-half" CASes.
	169	+ *
	170	+ * Both head and tail may or may not point to a Node with a
	171	+ * non-null item. If the queue is empty, all items must of course
	172	+ * be null. Upon creation, both head and tail refer to a dummy
	173	+ * Node with null item. Both head and tail are only updated using
	174	+ * CAS, so they never regress, although again this is merely an
	175	+ * optimization.
	176	+ */
	177	+
	178	+ private static class Node<E> {
	179	+ volatile E item;
	180	+ volatile Node<E> next;
	181	+
	182	+ /**
	183	+ * Constructs a new node. Uses relaxed write because item can
	184	+ * only be seen after publication via casNext.
	185	+ */
	186	+ Node(E item) {
	187	+ UNSAFE.putObject(this, itemOffset, item);
	188	+ }
	189	+
	190	+ boolean casItem(E cmp, E val) {
	191	+ return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
	192	+ }
	193	+
	194	+ void lazySetNext(Node<E> val) {
	195	+ UNSAFE.putOrderedObject(this, nextOffset, val);
	196	+ }
	197	+
	198	+ boolean casNext(Node<E> cmp, Node<E> val) {
	199	+ return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
	200	+ }
	201	+
	202	+ // Unsafe mechanics
	203	+
	204	+ private static final sun.misc.Unsafe UNSAFE;
	205	+ private static final long itemOffset;
	206	+ private static final long nextOffset;
	207	+
	208	+ static {
	209	+ try {
	210	+ UNSAFE = sun.misc.Unsafe.getUnsafe();
	211	+ Class k = Node.class;
	212	+ itemOffset = UNSAFE.objectFieldOffset
	213	+ (k.getDeclaredField("item"));
	214	+ nextOffset = UNSAFE.objectFieldOffset
	215	+ (k.getDeclaredField("next"));
	216	+ } catch (Exception e) {
	217	+ throw new Error(e);
	218	+ }
	219	+ }
	220	+ }
	221	+
	222	+ /**
	223	+ * A node from which the first live (non-deleted) node (if any)
	224	+ * can be reached in O(1) time.
	225	+ * Invariants:
	226	+ * - all live nodes are reachable from head via succ()
	227	+ * - head != null
	228	+ * - (tmp = head).next != tmp \|\| tmp != head
	229	+ * Non-invariants:
	230	+ * - head.item may or may not be null.
	231	+ * - it is permitted for tail to lag behind head, that is, for tail
	232	+ * to not be reachable from head!
	233	+ */
	234	+ private transient volatile Node<E> head;
	235	+
	236	+ /**
	237	+ * A node from which the last node on list (that is, the unique
	238	+ * node with node.next == null) can be reached in O(1) time.
	239	+ * Invariants:
	240	+ * - the last node is always reachable from tail via succ()
	241	+ * - tail != null
	242	+ * Non-invariants:
	243	+ * - tail.item may or may not be null.
	244	+ * - it is permitted for tail to lag behind head, that is, for tail
	245	+ * to not be reachable from head!
	246	+ * - tail.next may or may not be self-pointing to tail.
	247	+ */
	248	+ private transient volatile Node<E> tail;
	249	+
	250	+
	251	+ /**
	252	+ * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
	253	+ */
	254	+ public ConcurrentLinkedQueue() {
	255	+ head = tail = new Node<E>(null);
	256	+ }
	257	+
	258	+ /**
	259	+ * Creates a {@code ConcurrentLinkedQueue}
	260	+ * initially containing the elements of the given collection,
	261	+ * added in traversal order of the collection's iterator.
	262	+ *
	263	+ * @param c the collection of elements to initially contain
	264	+ * @throws NullPointerException if the specified collection or any
	265	+ * of its elements are null
	266	+ */
	267	+ public ConcurrentLinkedQueue(Collection<? extends E> c) {
	268	+ Node<E> h = null, t = null;
	269	+ for (E e : c) {
	270	+ checkNotNull(e);
	271	+ Node<E> newNode = new Node<E>(e);
	272	+ if (h == null)
	273	+ h = t = newNode;
	274	+ else {
	275	+ t.lazySetNext(newNode);
	276	+ t = newNode;
	277	+ }
	278	+ }
	279	+ if (h == null)
	280	+ h = t = new Node<E>(null);
	281	+ head = h;
	282	+ tail = t;
	283	+ }
	284	+
	285	+ // Have to override just to update the javadoc
	286	+
	287	+ /**
	288	+ * Inserts the specified element at the tail of this queue.
	289	+ * As the queue is unbounded, this method will never throw
	290	+ * {@link IllegalStateException} or return {@code false}.
	291	+ *
	292	+ * @return {@code true} (as specified by {@link Collection#add})
	293	+ * @throws NullPointerException if the specified element is null
	294	+ */
	295	+ public boolean add(E e) {
	296	+ return offer(e);
	297	+ }
	298	+
	299	+ /**
	300	+ * Try to CAS head to p. If successful, repoint old head to itself
	301	+ * as sentinel for succ(), below.
	302	+ */
	303	+ final void updateHead(Node<E> h, Node<E> p) {
	304	+ if (h != p && casHead(h, p))
	305	+ h.lazySetNext(h);
	306	+ }
	307	+
	308	+ /**
	309	+ * Returns the successor of p, or the head node if p.next has been
	310	+ * linked to self, which will only be true if traversing with a
	311	+ * stale pointer that is now off the list.
	312	+ */
	313	+ final Node<E> succ(Node<E> p) {
	314	+ Node<E> next = p.next;
	315	+ return (p == next) ? head : next;
	316	+ }
	317	+
	318	+ /**
	319	+ * Inserts the specified element at the tail of this queue.
	320	+ * As the queue is unbounded, this method will never return {@code false}.
	321	+ *
	322	+ * @return {@code true} (as specified by {@link Queue#offer})
	323	+ * @throws NullPointerException if the specified element is null
	324	+ */
	325	+ public boolean offer(E e) {
	326	+ checkNotNull(e);
	327	+ final Node<E> newNode = new Node<E>(e);
	328	+
	329	+ for (Node<E> t = tail, p = t;;) {
	330	+ Node<E> q = p.next;
	331	+ if (q == null) {
	332	+ // p is last node
	333	+ if (p.casNext(null, newNode)) {
	334	+ // Successful CAS is the linearization point
	335	+ // for e to become an element of this queue,
	336	+ // and for newNode to become "live".
	337	+ if (p != t) // hop two nodes at a time
	338	+ casTail(t, newNode); // Failure is OK.
	339	+ return true;
	340	+ }
	341	+ // Lost CAS race to another thread; re-read next
	342	+ }
	343	+ else if (p == q)
	344	+ // We have fallen off list. If tail is unchanged, it
	345	+ // will also be off-list, in which case we need to
	346	+ // jump to head, from which all live nodes are always
	347	+ // reachable. Else the new tail is a better bet.
	348	+ p = (t != (t = tail)) ? t : head;
	349	+ else
	350	+ // Check for tail updates after two hops.
	351	+ p = (p != t && t != (t = tail)) ? t : q;
	352	+ }
	353	+ }
	354	+
	355	+ public E poll() {
	356	+ restartFromHead:
	357	+ for (;;) {
	358	+ for (Node<E> h = head, p = h, q;;) {
	359	+ E item = p.item;
	360	+
	361	+ if (item != null && p.casItem(item, null)) {
	362	+ // Successful CAS is the linearization point
	363	+ // for item to be removed from this queue.
	364	+ if (p != h) // hop two nodes at a time
	365	+ updateHead(h, ((q = p.next) != null) ? q : p);
	366	+ return item;
	367	+ }
	368	+ else if ((q = p.next) == null) {
	369	+ updateHead(h, p);
	370	+ return null;
	371	+ }
	372	+ else if (p == q)
	373	+ continue restartFromHead;
	374	+ else
	375	+ p = q;
	376	+ }
	377	+ }
	378	+ }
	379	+
	380	+ public E peek() {
	381	+ restartFromHead:
	382	+ for (;;) {
	383	+ for (Node<E> h = head, p = h, q;;) {
	384	+ E item = p.item;
	385	+ if (item != null \|\| (q = p.next) == null) {
	386	+ updateHead(h, p);
	387	+ return item;
	388	+ }
	389	+ else if (p == q)
	390	+ continue restartFromHead;
	391	+ else
	392	+ p = q;
	393	+ }
	394	+ }
	395	+ }
	396	+
	397	+ /**
	398	+ * Returns the first live (non-deleted) node on list, or null if none.
	399	+ * This is yet another variant of poll/peek; here returning the
	400	+ * first node, not element. We could make peek() a wrapper around
	401	+ * first(), but that would cost an extra volatile read of item,
	402	+ * and the need to add a retry loop to deal with the possibility
	403	+ * of losing a race to a concurrent poll().
	404	+ */
	405	+ Node<E> first() {
	406	+ restartFromHead:
	407	+ for (;;) {
	408	+ for (Node<E> h = head, p = h, q;;) {
	409	+ boolean hasItem = (p.item != null);
	410	+ if (hasItem \|\| (q = p.next) == null) {
	411	+ updateHead(h, p);
	412	+ return hasItem ? p : null;
	413	+ }
	414	+ else if (p == q)
	415	+ continue restartFromHead;
	416	+ else
	417	+ p = q;
	418	+ }
	419	+ }
	420	+ }
	421	+
	422	+ /**
	423	+ * Returns {@code true} if this queue contains no elements.
	424	+ *
	425	+ * @return {@code true} if this queue contains no elements
	426	+ */
	427	+ public boolean isEmpty() {
	428	+ return first() == null;
	429	+ }
	430	+
	431	+ /**
	432	+ * Returns the number of elements in this queue. If this queue
	433	+ * contains more than {@code Integer.MAX_VALUE} elements, returns
	434	+ * {@code Integer.MAX_VALUE}.
	435	+ *
	436	+ * <p>Beware that, unlike in most collections, this method is
	437	+ * <em>NOT</em> a constant-time operation. Because of the
	438	+ * asynchronous nature of these queues, determining the current
	439	+ * number of elements requires an O(n) traversal.
	440	+ * Additionally, if elements are added or removed during execution
	441	+ * of this method, the returned result may be inaccurate. Thus,
	442	+ * this method is typically not very useful in concurrent
	443	+ * applications.
	444	+ *
	445	+ * @return the number of elements in this queue
	446	+ */
	447	+ public int size() {
	448	+ int count = 0;
	449	+ for (Node<E> p = first(); p != null; p = succ(p))
	450	+ if (p.item != null)
	451	+ // Collection.size() spec says to max out
	452	+ if (++count == Integer.MAX_VALUE)
	453	+ break;
	454	+ return count;
	455	+ }
	456	+
	457	+ /**
	458	+ * Returns {@code true} if this queue contains the specified element.
	459	+ * More formally, returns {@code true} if and only if this queue contains
	460	+ * at least one element {@code e} such that {@code o.equals(e)}.
	461	+ *
	462	+ * @param o object to be checked for containment in this queue
	463	+ * @return {@code true} if this queue contains the specified element
	464	+ */
	465	+ public boolean contains(Object o) {
	466	+ if (o == null) return false;
	467	+ for (Node<E> p = first(); p != null; p = succ(p)) {
	468	+ E item = p.item;
	469	+ if (item != null && o.equals(item))
	470	+ return true;
	471	+ }
	472	+ return false;
	473	+ }
	474	+
	475	+ /**
	476	+ * Removes a single instance of the specified element from this queue,
	477	+ * if it is present. More formally, removes an element {@code e} such
	478	+ * that {@code o.equals(e)}, if this queue contains one or more such
	479	+ * elements.
	480	+ * Returns {@code true} if this queue contained the specified element
	481	+ * (or equivalently, if this queue changed as a result of the call).
	482	+ *
	483	+ * @param o element to be removed from this queue, if present
	484	+ * @return {@code true} if this queue changed as a result of the call
	485	+ */
	486	+ public boolean remove(Object o) {
	487	+ if (o == null) return false;
	488	+ Node<E> pred = null;
	489	+ for (Node<E> p = first(); p != null; p = succ(p)) {
	490	+ E item = p.item;
	491	+ if (item != null &&
	492	+ o.equals(item) &&
	493	+ p.casItem(item, null)) {
	494	+ Node<E> next = succ(p);
	495	+ if (pred != null && next != null)
	496	+ pred.casNext(p, next);
	497	+ return true;
	498	+ }
	499	+ pred = p;
	500	+ }
	501	+ return false;
	502	+ }
	503	+
	504	+ /**
	505	+ * Appends all of the elements in the specified collection to the end of
	506	+ * this queue, in the order that they are returned by the specified
	507	+ * collection's iterator. Attempts to {@code addAll} of a queue to
	508	+ * itself result in {@code IllegalArgumentException}.
	509	+ *
	510	+ * @param c the elements to be inserted into this queue
	511	+ * @return {@code true} if this queue changed as a result of the call
	512	+ * @throws NullPointerException if the specified collection or any
	513	+ * of its elements are null
	514	+ * @throws IllegalArgumentException if the collection is this queue
	515	+ */
	516	+ public boolean addAll(Collection<? extends E> c) {
	517	+ if (c == this)
	518	+ // As historically specified in AbstractQueue#addAll
	519	+ throw new IllegalArgumentException();
	520	+
	521	+ // Copy c into a private chain of Nodes
	522	+ Node<E> beginningOfTheEnd = null, last = null;
	523	+ for (E e : c) {
	524	+ checkNotNull(e);
	525	+ Node<E> newNode = new Node<E>(e);
	526	+ if (beginningOfTheEnd == null)
	527	+ beginningOfTheEnd = last = newNode;
	528	+ else {
	529	+ last.lazySetNext(newNode);
	530	+ last = newNode;
	531	+ }
	532	+ }
	533	+ if (beginningOfTheEnd == null)
	534	+ return false;
	535	+
	536	+ // Atomically append the chain at the tail of this collection
	537	+ for (Node<E> t = tail, p = t;;) {
	538	+ Node<E> q = p.next;
	539	+ if (q == null) {
	540	+ // p is last node
	541	+ if (p.casNext(null, beginningOfTheEnd)) {
	542	+ // Successful CAS is the linearization point
	543	+ // for all elements to be added to this queue.
	544	+ if (!casTail(t, last)) {
	545	+ // Try a little harder to update tail,
	546	+ // since we may be adding many elements.
	547	+ t = tail;
	548	+ if (last.next == null)
	549	+ casTail(t, last);
	550	+ }
	551	+ return true;
	552	+ }
	553	+ // Lost CAS race to another thread; re-read next
	554	+ }
	555	+ else if (p == q)
	556	+ // We have fallen off list. If tail is unchanged, it
	557	+ // will also be off-list, in which case we need to
	558	+ // jump to head, from which all live nodes are always
	559	+ // reachable. Else the new tail is a better bet.
	560	+ p = (t != (t = tail)) ? t : head;
	561	+ else
	562	+ // Check for tail updates after two hops.
	563	+ p = (p != t && t != (t = tail)) ? t : q;
	564	+ }
	565	+ }
	566	+
	567	+ /**
	568	+ * Returns an array containing all of the elements in this queue, in
	569	+ * proper sequence.
	570	+ *
	571	+ * <p>The returned array will be "safe" in that no references to it are
	572	+ * maintained by this queue. (In other words, this method must allocate
	573	+ * a new array). The caller is thus free to modify the returned array.
	574	+ *
	575	+ * <p>This method acts as bridge between array-based and collection-based
	576	+ * APIs.
	577	+ *
	578	+ * @return an array containing all of the elements in this queue
	579	+ */
	580	+ public Object[] toArray() {
	581	+ // Use ArrayList to deal with resizing.
	582	+ ArrayList<E> al = new ArrayList<E>();
	583	+ for (Node<E> p = first(); p != null; p = succ(p)) {
	584	+ E item = p.item;
	585	+ if (item != null)
	586	+ al.add(item);
	587	+ }
	588	+ return al.toArray();
	589	+ }
	590	+
	591	+ /**
	592	+ * Returns an array containing all of the elements in this queue, in
	593	+ * proper sequence; the runtime type of the returned array is that of
	594	+ * the specified array. If the queue fits in the specified array, it
	595	+ * is returned therein. Otherwise, a new array is allocated with the
	596	+ * runtime type of the specified array and the size of this queue.
	597	+ *
	598	+ * <p>If this queue fits in the specified array with room to spare
	599	+ * (i.e., the array has more elements than this queue), the element in
	600	+ * the array immediately following the end of the queue is set to
	601	+ * {@code null}.
	602	+ *
	603	+ * <p>Like the {@link #toArray()} method, this method acts as bridge between
	604	+ * array-based and collection-based APIs. Further, this method allows
	605	+ * precise control over the runtime type of the output array, and may,
	606	+ * under certain circumstances, be used to save allocation costs.
	607	+ *
	608	+ * <p>Suppose {@code x} is a queue known to contain only strings.
	609	+ * The following code can be used to dump the queue into a newly
	610	+ * allocated array of {@code String}:
	611	+ *
	612	+ * <pre>
	613	+ * String[] y = x.toArray(new String[0]);</pre>
	614	+ *
	615	+ * Note that {@code toArray(new Object[0])} is identical in function to
	616	+ * {@code toArray()}.
	617	+ *
	618	+ * @param a the array into which the elements of the queue are to
	619	+ * be stored, if it is big enough; otherwise, a new array of the
	620	+ * same runtime type is allocated for this purpose
	621	+ * @return an array containing all of the elements in this queue
	622	+ * @throws ArrayStoreException if the runtime type of the specified array
	623	+ * is not a supertype of the runtime type of every element in
	624	+ * this queue
	625	+ * @throws NullPointerException if the specified array is null
	626	+ */
	627	+ @SuppressWarnings("unchecked")
	628	+ public <T> T[] toArray(T[] a) {
	629	+ // try to use sent-in array
	630	+ int k = 0;
	631	+ Node<E> p;
	632	+ for (p = first(); p != null && k < a.length; p = succ(p)) {
	633	+ E item = p.item;
	634	+ if (item != null)
	635	+ a[k++] = (T)item;
	636	+ }
	637	+ if (p == null) {
	638	+ if (k < a.length)
	639	+ a[k] = null;
	640	+ return a;
	641	+ }
	642	+
	643	+ // If won't fit, use ArrayList version
	644	+ ArrayList<E> al = new ArrayList<E>();
	645	+ for (Node<E> q = first(); q != null; q = succ(q)) {
	646	+ E item = q.item;
	647	+ if (item != null)
	648	+ al.add(item);
	649	+ }
	650	+ return al.toArray(a);
	651	+ }
	652	+
	653	+ /**
	654	+ * Returns an iterator over the elements in this queue in proper sequence.
	655	+ * The elements will be returned in order from first (head) to last (tail).
	656	+ *
	657	+ * <p>The returned iterator is a "weakly consistent" iterator that
	658	+ * will never throw {@link java.util.ConcurrentModificationException
	659	+ * ConcurrentModificationException}, and guarantees to traverse
	660	+ * elements as they existed upon construction of the iterator, and
	661	+ * may (but is not guaranteed to) reflect any modifications
	662	+ * subsequent to construction.
	663	+ *
	664	+ * @return an iterator over the elements in this queue in proper sequence
	665	+ */
	666	+ public Iterator<E> iterator() {
	667	+ return new Itr();
	668	+ }
	669	+
	670	+ private class Itr implements Iterator<E> {
	671	+ /**
	672	+ * Next node to return item for.
	673	+ */
	674	+ private Node<E> nextNode;
	675	+
	676	+ /**
	677	+ * nextItem holds on to item fields because once we claim
	678	+ * that an element exists in hasNext(), we must return it in
	679	+ * the following next() call even if it was in the process of
	680	+ * being removed when hasNext() was called.
	681	+ */
	682	+ private E nextItem;
	683	+
	684	+ /**
	685	+ * Node of the last returned item, to support remove.
	686	+ */
	687	+ private Node<E> lastRet;
	688	+
	689	+ Itr() {
	690	+ advance();
	691	+ }
	692	+
	693	+ /**
	694	+ * Moves to next valid node and returns item to return for
	695	+ * next(), or null if no such.
	696	+ */
	697	+ private E advance() {
	698	+ lastRet = nextNode;
	699	+ E x = nextItem;
	700	+
	701	+ Node<E> pred, p;
	702	+ if (nextNode == null) {
	703	+ p = first();
	704	+ pred = null;
	705	+ } else {
	706	+ pred = nextNode;
	707	+ p = succ(nextNode);
	708	+ }
	709	+
	710	+ for (;;) {
	711	+ if (p == null) {
	712	+ nextNode = null;
	713	+ nextItem = null;
	714	+ return x;
	715	+ }
	716	+ E item = p.item;
	717	+ if (item != null) {
	718	+ nextNode = p;
	719	+ nextItem = item;
	720	+ return x;
	721	+ } else {
	722	+ // skip over nulls
	723	+ Node<E> next = succ(p);
	724	+ if (pred != null && next != null)
	725	+ pred.casNext(p, next);
	726	+ p = next;
	727	+ }
	728	+ }
	729	+ }
	730	+
	731	+ public boolean hasNext() {
	732	+ return nextNode != null;
	733	+ }
	734	+
	735	+ public E next() {
	736	+ if (nextNode == null) throw new NoSuchElementException();
	737	+ return advance();
	738	+ }
	739	+
	740	+ public void remove() {
	741	+ Node<E> l = lastRet;
	742	+ if (l == null) throw new IllegalStateException();
	743	+ // rely on a future traversal to relink.
	744	+ l.item = null;
	745	+ lastRet = null;
	746	+ }
	747	+ }
	748	+
	749	+ /**
	750	+ * Saves the state to a stream (that is, serializes it).
	751	+ *
	752	+ * @serialData All of the elements (each an {@code E}) in
	753	+ * the proper order, followed by a null
	754	+ * @param s the stream
	755	+ */
	756	+ private void writeObject(java.io.ObjectOutputStream s)
	757	+ throws java.io.IOException {
	758	+
	759	+ // Write out any hidden stuff
	760	+ s.defaultWriteObject();
	761	+
	762	+ // Write out all elements in the proper order.
	763	+ for (Node<E> p = first(); p != null; p = succ(p)) {
	764	+ Object item = p.item;
	765	+ if (item != null)
	766	+ s.writeObject(item);
	767	+ }
	768	+
	769	+ // Use trailing null as sentinel
	770	+ s.writeObject(null);
	771	+ }
	772	+
	773	+ /**
	774	+ * Reconstitutes the instance from a stream (that is, deserializes it).
	775	+ * @param s the stream
	776	+ */
	777	+ private void readObject(java.io.ObjectInputStream s)
	778	+ throws java.io.IOException, ClassNotFoundException {
	779	+ s.defaultReadObject();
	780	+
	781	+ // Read in elements until trailing null sentinel found
	782	+ Node<E> h = null, t = null;
	783	+ Object item;
	784	+ while ((item = s.readObject()) != null) {
	785	+ @SuppressWarnings("unchecked")
	786	+ Node<E> newNode = new Node<E>((E) item);
	787	+ if (h == null)
	788	+ h = t = newNode;
	789	+ else {
	790	+ t.lazySetNext(newNode);
	791	+ t = newNode;
	792	+ }
	793	+ }
	794	+ if (h == null)
	795	+ h = t = new Node<E>(null);
	796	+ head = h;
	797	+ tail = t;
	798	+ }
	799	+
	800	+ /**
	801	+ * Throws NullPointerException if argument is null.
	802	+ *
	803	+ * @param v the element
	804	+ */
	805	+ private static void checkNotNull(Object v) {
	806	+ if (v == null)
	807	+ throw new NullPointerException();
	808	+ }
	809	+
	810	+ private boolean casTail(Node<E> cmp, Node<E> val) {
	811	+ return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
	812	+ }
	813	+
	814	+ private boolean casHead(Node<E> cmp, Node<E> val) {
	815	+ return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
	816	+ }
	817	+
	818	+ // Unsafe mechanics
	819	+
	820	+ private static final sun.misc.Unsafe UNSAFE;
	821	+ private static final long headOffset;
	822	+ private static final long tailOffset;
	823	+ static {
	824	+ try {
	825	+ UNSAFE = sun.misc.Unsafe.getUnsafe();
	826	+ Class k = ConcurrentLinkedQueue.class;
	827	+ headOffset = UNSAFE.objectFieldOffset
	828	+ (k.getDeclaredField("head"));
	829	+ tailOffset = UNSAFE.objectFieldOffset
	830	+ (k.getDeclaredField("tail"));
	831	+ } catch (Exception e) {
	832	+ throw new Error(e);
	833	+ }
	834	+ }
	835	+}