Back to index...

	/*
	* 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 and Martin Buchholz 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.function.Consumer;
	import java.util.function.Predicate;

	/**
	* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
	* This queue orders elements FIFO (first-in-first-out).
	* The <em>head</em> of the queue is that element that has been on the
	* queue the longest time.
	* The <em>tail</em> of the queue is that element that has been on the
	* queue the shortest time. New elements
	* are inserted at the tail of the queue, and the queue retrieval
	* operations obtain elements at the head of the queue.
	* A {@code ConcurrentLinkedQueue} is an appropriate choice when
	* many threads will share access to a common collection.
	* Like most other concurrent collection implementations, this class
	* does not permit the use of {@code null} elements.
	*
	* <p>This implementation employs an efficient <em>non-blocking</em>
	* algorithm based on one described in
	* <a href="http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf">
	* Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue
	* Algorithms</a> by Maged M. Michael and Michael L. Scott.
	*
	* <p>Iterators are <i>weakly consistent</i>, returning elements
	* reflecting the state of the queue at some point at or since the
	* creation of the iterator. They do <em>not</em> throw {@link
	* java.util.ConcurrentModificationException}, and may proceed concurrently
	* with other operations. Elements contained in the queue since the creation
	* of the iterator will be returned exactly once.
	*
	* <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 Queue} 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 ConcurrentLinkedQueue}
	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
	* actions subsequent to the access or removal of that element from
	* the {@code ConcurrentLinkedQueue} 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 1.5
	* @author Doug Lea
	* @param <E> the type of elements held in this queue
	*/
	public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
	implements Queue<E>, java.io.Serializable {
	private static final long serialVersionUID = 196745693267521676L;

	/*
	* This is a modification of the Michael & Scott algorithm,
	* adapted for a garbage-collected environment, with support for
	* interior node deletion (to support e.g. remove(Object)). For
	* explanation, read the paper.
	*
	* Note that like most non-blocking algorithms in this package,
	* this implementation relies on the fact that in garbage
	* collected systems, there is no possibility of ABA problems due
	* to recycled nodes, so there is no need to use "counted
	* pointers" or related techniques seen in versions used in
	* non-GC'ed settings.
	*
	* The fundamental invariants are:
	* - There is exactly one (last) Node with a null next reference,
	* which is CASed when enqueueing. This last Node can be
	* reached in O(1) time from tail, but tail is merely an
	* optimization - it can always be reached in O(N) time from
	* head as well.
	* - The elements contained in the queue are the non-null items in
	* Nodes that are reachable from head. CASing the item
	* reference of a Node to null atomically removes it from the
	* queue. Reachability of all elements from head must remain
	* true even in the case of concurrent modifications that cause
	* head to advance. A dequeued Node may remain in use
	* indefinitely due to creation of an Iterator or simply a
	* poll() that has lost its time slice.
	*
	* The above might appear to imply that all Nodes are GC-reachable
	* from a predecessor dequeued Node. That would cause two problems:
	* - allow a rogue Iterator to cause unbounded memory retention
	* - cause cross-generational linking of old Nodes to new Nodes if
	* a Node was tenured while live, which generational GCs have a
	* hard time dealing with, causing repeated major collections.
	* However, only non-deleted Nodes need to be reachable from
	* dequeued Nodes, and reachability does not necessarily have to
	* be of the kind understood by the GC. We use the trick of
	* linking a Node that has just been dequeued to itself. Such a
	* self-link implicitly means to advance to head.
	*
	* Both head and tail are permitted to lag. In fact, failing to
	* update them every time one could is a significant optimization
	* (fewer CASes). As with LinkedTransferQueue (see the internal
	* documentation for that class), we use 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.
	*
	* Since head and tail are updated concurrently and independently,
	* it is possible for tail to lag behind head (why not)?
	*
	* CASing a Node's item reference to null atomically removes the
	* element from the queue, leaving a "dead" node that should later
	* be unlinked (but unlinking is merely an optimization).
	* Interior element removal methods (other than Iterator.remove())
	* keep track of the predecessor node during traversal so that the
	* node can be CAS-unlinked. Some traversal methods try to unlink
	* any deleted nodes encountered during traversal. See comments
	* in bulkRemove.
	*
	* When constructing a Node (before enqueuing it) we avoid paying
	* for a volatile write to item. This allows the cost of enqueue
	* to be "one-and-a-half" CASes.
	*
	* Both head and tail may or may not point to a Node with a
	* non-null item. If the queue is empty, all items must of course
	* be null. Upon creation, both head and tail refer to a dummy
	* Node with null item. Both head and tail are only updated using
	* CAS, so they never regress, although again this is merely an
	* optimization.
	*/

	static final class Node<E> {
	volatile E item;
	volatile Node<E> next;

	/**
	* Constructs a node holding item. Uses relaxed write because
	* item can only be seen after piggy-backing publication via CAS.
	*/
	Node(E item) {
	ITEM.set(this, item);
	}

	/** Constructs a dead dummy node. */
	Node() {}

	void appendRelaxed(Node<E> next) {
	// assert next != null;
	// assert this.next == null;
	NEXT.set(this, next);
	}

	boolean casItem(E cmp, E val) {
	// assert item == cmp \|\| item == null;
	// assert cmp != null;
	// assert val == null;
	return ITEM.compareAndSet(this, cmp, val);
	}
	}

	/**
	* A node from which the first live (non-deleted) node (if any)
	* can be reached in O(1) time.
	* Invariants:
	* - all live nodes are reachable from head via succ()
	* - head != null
	* - (tmp = head).next != tmp \|\| tmp != head
	* Non-invariants:
	* - head.item may or may not be null.
	* - it is permitted for tail to lag behind head, that is, for tail
	* to not be reachable from head!
	*/
	transient volatile Node<E> 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 succ()
	* - tail != null
	* Non-invariants:
	* - tail.item may or may not be null.
	* - 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<E> tail;

	/**
	* Creates a {@code ConcurrentLinkedQueue} that is initially empty.
	*/
	public ConcurrentLinkedQueue() {
	head = tail = new Node<E>();
	}

	/**
	* Creates a {@code ConcurrentLinkedQueue}
	* 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 ConcurrentLinkedQueue(Collection<? extends E> c) {
	Node<E> h = null, t = null;
	for (E e : c) {
	Node<E> newNode = new Node<E>(Objects.requireNonNull(e));
	if (h == null)
	h = t = newNode;
	else
	t.appendRelaxed(t = newNode);
	}
	if (h == null)
	h = t = new Node<E>();
	head = h;
	tail = t;
	}

	// Have to override just to update the javadoc

	/**
	* 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) {
	return offer(e);
	}

	/**
	* Tries to CAS head to p. If successful, repoint old head to itself
	* as sentinel for succ(), below.
	*/
	final void updateHead(Node<E> h, Node<E> p) {
	// assert h != null && p != null && (h == p \|\| h.item == null);
	if (h != p && HEAD.compareAndSet(this, h, p))
	NEXT.setRelease(h, h);
	}

	/**
	* Returns the successor of p, or the head node if p.next has been
	* linked to self, which will only be true if traversing with a
	* stale pointer that is now off the list.
	*/
	final Node<E> succ(Node<E> p) {
	if (p == (p = p.next))
	p = head;
	return p;
	}

	/**
	* 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<E> pred, Node<E> c, Node<E> p) {
	// assert p != null;
	// assert c.item == null;
	// assert c != p;
	if (pred != null)
	return NEXT.compareAndSet(pred, c, p);
	if (HEAD.compareAndSet(this, c, p)) {
	NEXT.setRelease(c, c);
	return true;
	}
	return false;
	}

	/**
	* Collapse dead 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 either old pred or p if pred dead or CAS failed
	*/
	private Node<E> skipDeadNodes(Node<E> pred, Node<E> c, Node<E> p, Node<E> q) {
	// assert pred != c;
	// assert p != q;
	// assert c.item == null;
	// assert p.item == null;
	if (q == null) {
	// Never unlink trailing node.
	if (c == p) return pred;
	q = p;
	}
	return (tryCasSuccessor(pred, c, q)
	&& (pred == null \|\| ITEM.get(pred) != null))
	? pred : p;
	}

	/**
	* 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) {
	final Node<E> newNode = new Node<E>(Objects.requireNonNull(e));

	for (Node<E> t = tail, p = t;;) {
	Node<E> q = p.next;
	if (q == null) {
	// p is last node
	if (NEXT.compareAndSet(p, null, newNode)) {
	// Successful CAS is the linearization point
	// for e to become an element of this queue,
	// and for newNode to become "live".
	if (p != t) // hop two nodes at a time; failure is OK
	TAIL.weakCompareAndSet(this, t, newNode);
	return true;
	}
	// Lost CAS race to another thread; re-read next
	}
	else if (p == q)
	// We have fallen off list. If tail is unchanged, it
	// will also be off-list, in which case we need to
	// jump to head, from which all live nodes are always
	// reachable. Else the new tail is a better bet.
	p = (t != (t = tail)) ? t : head;
	else
	// Check for tail updates after two hops.
	p = (p != t && t != (t = tail)) ? t : q;
	}
	}

	public E poll() {
	restartFromHead: for (;;) {
	for (Node<E> h = head, p = h, q;; p = q) {
	final E item;
	if ((item = p.item) != null && p.casItem(item, null)) {
	// Successful CAS is the linearization point
	// for item to be removed from this queue.
	if (p != h) // hop two nodes at a time
	updateHead(h, ((q = p.next) != null) ? q : p);
	return item;
	}
	else if ((q = p.next) == null) {
	updateHead(h, p);
	return null;
	}
	else if (p == q)
	continue restartFromHead;
	}
	}
	}

	public E peek() {
	restartFromHead: for (;;) {
	for (Node<E> h = head, p = h, q;; p = q) {
	final E item;
	if ((item = p.item) != null
	\|\| (q = p.next) == null) {
	updateHead(h, p);
	return item;
	}
	else if (p == q)
	continue restartFromHead;
	}
	}
	}

	/**
	* Returns the first live (non-deleted) node on list, or null if none.
	* This is yet another variant of poll/peek; here returning the
	* first node, not element. We could make peek() a wrapper around
	* first(), but that would cost an extra volatile read of item,
	* and the need to add a retry loop to deal with the possibility
	* of losing a race to a concurrent poll().
	*/
	Node<E> first() {
	restartFromHead: for (;;) {
	for (Node<E> h = head, p = h, q;; p = q) {
	boolean hasItem = (p.item != null);
	if (hasItem \|\| (q = p.next) == null) {
	updateHead(h, p);
	return hasItem ? p : null;
	}
	else if (p == q)
	continue restartFromHead;
	}
	}
	}

	/**
	* Returns {@code true} if this queue contains no elements.
	*
	* @return {@code true} if this queue contains no elements
	*/
	public boolean isEmpty() {
	return first() == null;
	}

	/**
	* Returns the number of elements in this queue. If this queue
	* contains more than {@code Integer.MAX_VALUE} elements, returns
	* {@code Integer.MAX_VALUE}.
	*
	* <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.
	* Additionally, if elements are added or removed during execution
	* of this method, the returned result may be inaccurate. Thus,
	* this method is typically not very useful in concurrent
	* applications.
	*
	* @return the number of elements in this queue
	*/
	public int size() {
	restartFromHead: for (;;) {
	int count = 0;
	for (Node<E> p = first(); p != null;) {
	if (p.item != null)
	if (++count == Integer.MAX_VALUE)
	break; // @see Collection.size()
	if (p == (p = p.next))
	continue restartFromHead;
	}
	return count;
	}
	}

	/**
	* 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<E> p = head, pred = null; p != null; ) {
	Node<E> q = p.next;
	final E item;
	if ((item = p.item) != null) {
	if (o.equals(item))
	return true;
	pred = p; p = q; continue;
	}
	for (Node<E> c = p;; q = p.next) {
	if (q == null \|\| q.item != null) {
	pred = skipDeadNodes(pred, c, p, q); p = q; break;
	}
	if (p == (p = q)) continue restartFromHead;
	}
	}
	return 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<E> p = head, pred = null; p != null; ) {
	Node<E> q = p.next;
	final E item;
	if ((item = p.item) != null) {
	if (o.equals(item) && p.casItem(item, null)) {
	skipDeadNodes(pred, p, p, q);
	return true;
	}
	pred = p; p = q; continue;
	}
	for (Node<E> c = p;; q = p.next) {
	if (q == null \|\| q.item != null) {
	pred = skipDeadNodes(pred, c, p, q); p = q; break;
	}
	if (p == (p = q)) continue restartFromHead;
	}
	}
	return false;
	}
	}

	/**
	* Appends all of the elements in the specified collection to the end of
	* this queue, in the order that they are returned by the specified
	* collection's iterator. Attempts to {@code addAll} of a queue to
	* itself result in {@code IllegalArgumentException}.
	*
	* @param c the elements to be inserted into this queue
	* @return {@code true} if this queue changed as a result of the call
	* @throws NullPointerException if the specified collection or any
	* of its elements are null
	* @throws IllegalArgumentException if the collection is this queue
	*/
	public boolean addAll(Collection<? extends E> c) {
	if (c == this)
	// As historically specified in AbstractQueue#addAll
	throw new IllegalArgumentException();

	// Copy c into a private chain of Nodes
	Node<E> beginningOfTheEnd = null, last = null;
	for (E e : c) {
	Node<E> newNode = new Node<E>(Objects.requireNonNull(e));
	if (beginningOfTheEnd == null)
	beginningOfTheEnd = last = newNode;
	else
	last.appendRelaxed(last = newNode);
	}
	if (beginningOfTheEnd == null)
	return false;

	// Atomically append the chain at the tail of this collection
	for (Node<E> t = tail, p = t;;) {
	Node<E> q = p.next;
	if (q == null) {
	// p is last node
	if (NEXT.compareAndSet(p, null, beginningOfTheEnd)) {
	// Successful CAS is the linearization point
	// for all elements to be added to this queue.
	if (!TAIL.weakCompareAndSet(this, t, last)) {
	// Try a little harder to update tail,
	// since we may be adding many elements.
	t = tail;
	if (last.next == null)
	TAIL.weakCompareAndSet(this, t, last);
	}
	return true;
	}
	// Lost CAS race to another thread; re-read next
	}
	else if (p == q)
	// We have fallen off list. If tail is unchanged, it
	// will also be off-list, in which case we need to
	// jump to head, from which all live nodes are always
	// reachable. Else the new tail is a better bet.
	p = (t != (t = tail)) ? t : head;
	else
	// Check for tail updates after two hops.
	p = (p != t && t != (t = tail)) ? t : q;
	}
	}

	public String toString() {
	String[] a = null;
	restartFromHead: for (;;) {
	int charLength = 0;
	int size = 0;
	for (Node<E> p = first(); p != null;) {
	final E item;
	if ((item = p.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();
	}
	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<E> p = first(); p != null;) {
	final E item;
	if ((item = p.item) != null) {
	if (x == null)
	x = new Object[4];
	else if (size == x.length)
	x = Arrays.copyOf(x, 2 * (size + 4));
	x[size++] = item;
	}
	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);
	}

	/**
	* 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();
	}

	private class Itr implements Iterator<E> {
	/**
	* Next node to return item for.
	*/
	private Node<E> nextNode;

	/**
	* nextItem holds on to item fields because once we claim
	* that an element exists in hasNext(), we must return it in
	* the following next() call even if it was in the process of
	* being removed when hasNext() was called.
	*/
	private E nextItem;

	/**
	* Node of the last returned item, to support remove.
	*/
	private Node<E> lastRet;

	Itr() {
	restartFromHead: for (;;) {
	Node<E> h, p, q;
	for (p = h = head;; p = q) {
	final E item;
	if ((item = p.item) != null) {
	nextNode = p;
	nextItem = item;
	break;
	}
	else if ((q = p.next) == null)
	break;
	else if (p == q)
	continue restartFromHead;
	}
	updateHead(h, p);
	return;
	}
	}

	public boolean hasNext() {
	return nextItem != null;
	}

	public E next() {
	final Node<E> pred = nextNode;
	if (pred == null) throw new NoSuchElementException();
	// assert nextItem != null;
	lastRet = pred;
	E item = null;

	for (Node<E> p = succ(pred), q;; p = q) {
	if (p == null \|\| (item = p.item) != null) {
	nextNode = p;
	E x = nextItem;
	nextItem = item;
	return x;
	}
	// unlink deleted nodes
	if ((q = succ(p)) != null)
	NEXT.compareAndSet(pred, p, q);
	}
	}

	// Default implementation of forEachRemaining is "good enough".

	public void remove() {
	Node<E> l = lastRet;
	if (l == null) throw new IllegalStateException();
	// rely on a future traversal to relink.
	l.item = null;
	lastRet = null;
	}
	}

	/**
	* 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 {

	// Write out any hidden stuff
	s.defaultWriteObject();

	// Write out all elements in the proper order.
	for (Node<E> p = first(); p != null; p = succ(p)) {
	final E item;
	if ((item = p.item) != null)
	s.writeObject(item);
	}

	// 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 {
	s.defaultReadObject();

	// Read in elements until trailing null sentinel found
	Node<E> h = null, t = null;
	for (Object item; (item = s.readObject()) != null; ) {
	@SuppressWarnings("unchecked")
	Node<E> newNode = new Node<E>((E) item);
	if (h == null)
	h = t = newNode;
	else
	t.appendRelaxed(t = newNode);
	}
	if (h == null)
	h = t = new Node<E>();
	head = h;
	tail = t;
	}

	/** A customized variant of Spliterators.IteratorSpliterator */
	final class CLQSpliterator implements Spliterator<E> {
	static final int MAX_BATCH = 1 << 25; // max batch array size;
	Node<E> current; // current node; null until initialized
	int batch; // batch size for splits
	boolean exhausted; // true when no more nodes

	public Spliterator<E> trySplit() {
	Node<E> 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 E e;
	if ((e = p.item) != null) {
	if (a == null)
	a = new Object[n];
	a[i++] = e;
	}
	if (p == (p = q))
	p = first();
	} 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<E> p;
	if ((p = current()) != null) {
	current = null;
	exhausted = true;
	forEachFrom(action, p);
	}
	}

	public boolean tryAdvance(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	Node<E> p;
	if ((p = current()) != null) {
	E e;
	do {
	e = p.item;
	if (p == (p = p.next))
	p = first();
	} while (e == null && p != null);
	setCurrent(p);
	if (e != null) {
	action.accept(e);
	return true;
	}
	}
	return false;
	}

	private void setCurrent(Node<E> p) {
	if ((current = p) == null)
	exhausted = true;
	}

	private Node<E> current() {
	Node<E> p;
	if ((p = current) == null && !exhausted)
	setCurrent(p = first());
	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
	*/
	@Override
	public Spliterator<E> spliterator() {
	return new CLQSpliterator();
	}

	/**
	* @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. */
	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<E> p = head, c = p, pred = null, q; p != null; p = q) {
	q = p.next;
	final E item; boolean pAlive;
	if (pAlive = ((item = p.item) != null)) {
	if (filter.test(item)) {
	if (p.casItem(item, null))
	removed = true;
	pAlive = false;
	}
	}
	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.
	*/
	void forEachFrom(Consumer<? super E> action, Node<E> p) {
	for (Node<E> pred = null; p != null; ) {
	Node<E> q = p.next;
	final E item;
	if ((item = p.item) != null) {
	action.accept(item);
	pred = p; p = q; continue;
	}
	for (Node<E> c = p;; q = p.next) {
	if (q == null \|\| q.item != null) {
	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 {
	try {
	MethodHandles.Lookup l = MethodHandles.lookup();
	HEAD = l.findVarHandle(ConcurrentLinkedQueue.class, "head",
	Node.class);
	TAIL = l.findVarHandle(ConcurrentLinkedQueue.class, "tail",
	Node.class);
	ITEM = l.findVarHandle(Node.class, "item", Object.class);
	NEXT = l.findVarHandle(Node.class, "next", Node.class);
	} catch (ReflectiveOperationException e) {
	throw new ExceptionInInitializerError(e);
	}
	}
	}

Back to index...