| /* |
| * 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/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 Error(e); |
| } |
| } |
| } |
