/*
* 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 head of the queue is that element that has been on the
* queue the longest time.
* The tail 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.
*
* This implementation employs an efficient non-blocking
* algorithm based on one described in
*
* Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue
* Algorithms by Maged M. Michael and Michael L. Scott.
*
*
Iterators are weakly consistent, returning elements
* reflecting the state of the queue at some point at or since the
* creation of the iterator. They do not 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.
*
*
Beware that, unlike in most collections, the {@code size} method
* is NOT a constant-time operation. Because of the
* asynchronous nature of these queues, determining the current number
* of elements requires a traversal of the elements, and so may report
* inaccurate results if this collection is modified during traversal.
*
*
Bulk operations that add, remove, or examine multiple elements,
* such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
* are not 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.
*
*
This class and its iterator implement all of the optional
* methods of the {@link Queue} and {@link Iterator} interfaces.
*
*
Memory consistency effects: As with other concurrent
* collections, actions in a thread prior to placing an object into a
* {@code ConcurrentLinkedQueue}
* happen-before
* actions subsequent to the access or removal of that element from
* the {@code ConcurrentLinkedQueue} in another thread.
*
*
This class is a member of the
*
* Java Collections Framework.
*
* @since 1.5
* @author Doug Lea
* @param the type of elements held in this queue
*/
public class ConcurrentLinkedQueue extends AbstractQueue
implements Queue, 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 {
volatile E item;
volatile Node 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 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 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 tail;
/**
* Creates a {@code ConcurrentLinkedQueue} that is initially empty.
*/
public ConcurrentLinkedQueue() {
head = tail = new Node();
}
/**
* 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 c) {
Node h = null, t = null;
for (E e : c) {
Node newNode = new Node(Objects.requireNonNull(e));
if (h == null)
h = t = newNode;
else
t.appendRelaxed(t = newNode);
}
if (h == null)
h = t = new Node();
head = h;
tail = t;
}
// 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 h, Node 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 succ(Node 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 pred, Node c, Node 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 skipDeadNodes(Node pred, Node c, Node p, Node 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 newNode = new Node(Objects.requireNonNull(e));
for (Node t = tail, p = t;;) {
Node 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 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 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 first() {
restartFromHead: for (;;) {
for (Node 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}.
*
* Beware that, unlike in most collections, this method is
* NOT a constant-time operation. Because of the
* asynchronous nature of these queues, determining the current
* number of elements requires an O(n) traversal.
* 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 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 p = head, pred = null; p != null; ) {
Node q = p.next;
final E item;
if ((item = p.item) != null) {
if (o.equals(item))
return true;
pred = p; p = q; continue;
}
for (Node 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 p = head, pred = null; p != null; ) {
Node 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 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 c) {
if (c == this)
// As historically specified in AbstractQueue#addAll
throw new IllegalArgumentException();
// Copy c into a private chain of Nodes
Node beginningOfTheEnd = null, last = null;
for (E e : c) {
Node newNode = new Node(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 t = tail, p = t;;) {
Node 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 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 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.
*
* 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.
*
*
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.
*
*
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}.
*
*
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.
*
*
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}:
*
*
{@code String[] y = x.toArray(new String[0]);}
*
* 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[] 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).
*
* The returned iterator is
* weakly consistent.
*
* @return an iterator over the elements in this queue in proper sequence
*/
public Iterator iterator() {
return new Itr();
}
private class Itr implements Iterator {
/**
* Next node to return item for.
*/
private Node 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 lastRet;
Itr() {
restartFromHead: for (;;) {
Node 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 pred = nextNode;
if (pred == null) throw new NoSuchElementException();
// assert nextItem != null;
lastRet = pred;
E item = null;
for (Node 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 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 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 h = null, t = null;
for (Object item; (item = s.readObject()) != null; ) {
@SuppressWarnings("unchecked")
Node newNode = new Node((E) item);
if (h == null)
h = t = newNode;
else
t.appendRelaxed(t = newNode);
}
if (h == null)
h = t = new Node();
head = h;
tail = t;
}
/** A customized variant of Spliterators.IteratorSpliterator */
final class CLQSpliterator implements Spliterator {
static final int MAX_BATCH = 1 << 25; // max batch array size;
Node current; // current node; null until initialized
int batch; // batch size for splits
boolean exhausted; // true when no more nodes
public Spliterator trySplit() {
Node p, q;
if ((p = current()) == null || (q = p.next) == null)
return null;
int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH);
Object[] a = null;
do {
final 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 action) {
Objects.requireNonNull(action);
final Node p;
if ((p = current()) != null) {
current = null;
exhausted = true;
forEachFrom(action, p);
}
}
public boolean tryAdvance(Consumer action) {
Objects.requireNonNull(action);
Node 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 p) {
if ((current = p) == null)
exhausted = true;
}
private Node current() {
Node 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.
*
* The returned spliterator is
* weakly consistent.
*
*
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 spliterator() {
return new CLQSpliterator();
}
/**
* @throws NullPointerException {@inheritDoc}
*/
public boolean removeIf(Predicate 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 filter) {
boolean removed = false;
restartFromHead: for (;;) {
int hops = MAX_HOPS;
// c will be CASed to collapse intervening dead nodes between
// pred (or head if null) and p.
for (Node p = head, c = p, pred = null, q; p != null; p = q) {
q = p.next;
final 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 action, Node p) {
for (Node pred = null; p != null; ) {
Node q = p.next;
final E item;
if ((item = p.item) != null) {
action.accept(item);
pred = p; p = q; continue;
}
for (Node 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 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 Error(e);
}
}
}