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/* |
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* Written by Doug Lea and Martin Buchholz with assistance from members of |
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* JCP JSR-166 Expert Group and released to the public domain, as explained |
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* at http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.util.AbstractQueue; |
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import java.util.ArrayList; |
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import java.util.Arrays; |
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import java.util.Collection; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Queue; |
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import java.util.Spliterator; |
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import java.util.Spliterators; |
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import java.util.function.Consumer; |
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|
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/** |
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* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes. |
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* This queue orders elements FIFO (first-in-first-out). |
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* The <em>head</em> of the queue is that element that has been on the |
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* queue the longest time. |
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* The <em>tail</em> of the queue is that element that has been on the |
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* queue the shortest time. New elements |
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* are inserted at the tail of the queue, and the queue retrieval |
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* operations obtain elements at the head of the queue. |
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* A {@code ConcurrentLinkedQueue} is an appropriate choice when |
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* many threads will share access to a common collection. |
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* Like most other concurrent collection implementations, this class |
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* does not permit the use of {@code null} elements. |
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* |
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* <p>This implementation employs an efficient <em>non-blocking</em> |
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* algorithm based on one described in |
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* <a href="http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf"> |
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* Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue |
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* Algorithms</a> by Maged M. Michael and Michael L. Scott. |
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* |
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* <p>Iterators are <i>weakly consistent</i>, returning elements |
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* reflecting the state of the queue at some point at or since the |
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* creation of the iterator. They do <em>not</em> throw {@link |
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* java.util.ConcurrentModificationException}, and may proceed concurrently |
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* with other operations. Elements contained in the queue since the creation |
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* of the iterator will be returned exactly once. |
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* |
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* <p>Beware that, unlike in most collections, the {@code size} method |
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* is <em>NOT</em> a constant-time operation. Because of the |
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* asynchronous nature of these queues, determining the current number |
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* of elements requires a traversal of the elements, and so may report |
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* inaccurate results if this collection is modified during traversal. |
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* Additionally, the bulk operations {@code addAll}, |
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* {@code removeAll}, {@code retainAll}, {@code containsAll}, |
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* {@code equals}, and {@code toArray} are <em>not</em> guaranteed |
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* to be performed atomically. For example, an iterator operating |
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* concurrently with an {@code addAll} operation might view only some |
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* of the added elements. |
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* |
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* <p>This class and its iterator implement all of the <em>optional</em> |
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* methods of the {@link Queue} and {@link Iterator} interfaces. |
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* |
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* <p>Memory consistency effects: As with other concurrent |
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* collections, actions in a thread prior to placing an object into a |
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* {@code ConcurrentLinkedQueue} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code ConcurrentLinkedQueue} in another thread. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <E> the type of elements held in this queue |
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*/ |
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public class ConcurrentLinkedQueue<E> extends AbstractQueue<E> |
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implements Queue<E>, java.io.Serializable { |
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private static final long serialVersionUID = 196745693267521676L; |
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|
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/* |
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* This is a modification of the Michael & Scott algorithm, |
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* adapted for a garbage-collected environment, with support for |
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* interior node deletion (to support remove(Object)). For |
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* explanation, read the paper. |
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* |
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* Note that like most non-blocking algorithms in this package, |
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* this implementation relies on the fact that in garbage |
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* collected systems, there is no possibility of ABA problems due |
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* to recycled nodes, so there is no need to use "counted |
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* pointers" or related techniques seen in versions used in |
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* non-GC'ed settings. |
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* |
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* The fundamental invariants are: |
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* - There is exactly one (last) Node with a null next reference, |
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* which is CASed when enqueueing. This last Node can be |
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* reached in O(1) time from tail, but tail is merely an |
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* optimization - it can always be reached in O(N) time from |
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* head as well. |
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* - The elements contained in the queue are the non-null items in |
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* Nodes that are reachable from head. CASing the item |
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* reference of a Node to null atomically removes it from the |
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* queue. Reachability of all elements from head must remain |
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* true even in the case of concurrent modifications that cause |
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* head to advance. A dequeued Node may remain in use |
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* indefinitely due to creation of an Iterator or simply a |
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* poll() that has lost its time slice. |
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* |
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* The above might appear to imply that all Nodes are GC-reachable |
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* from a predecessor dequeued Node. That would cause two problems: |
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* - allow a rogue Iterator to cause unbounded memory retention |
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* - cause cross-generational linking of old Nodes to new Nodes if |
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* a Node was tenured while live, which generational GCs have a |
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* hard time dealing with, causing repeated major collections. |
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* However, only non-deleted Nodes need to be reachable from |
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* dequeued Nodes, and reachability does not necessarily have to |
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* be of the kind understood by the GC. We use the trick of |
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* linking a Node that has just been dequeued to itself. Such a |
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* self-link implicitly means to advance to head. |
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* |
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* Both head and tail are permitted to lag. In fact, failing to |
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* update them every time one could is a significant optimization |
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* (fewer CASes). As with LinkedTransferQueue (see the internal |
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* documentation for that class), we use a slack threshold of two; |
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* that is, we update head/tail when the current pointer appears |
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* to be two or more steps away from the first/last node. |
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* |
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* Since head and tail are updated concurrently and independently, |
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* it is possible for tail to lag behind head (why not)? |
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* |
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* CASing a Node's item reference to null atomically removes the |
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* element from the queue. Iterators skip over Nodes with null |
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* items. Prior implementations of this class had a race between |
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* poll() and remove(Object) where the same element would appear |
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* to be successfully removed by two concurrent operations. The |
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* method remove(Object) also lazily unlinks deleted Nodes, but |
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* this is merely an optimization. |
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* |
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* When constructing a Node (before enqueuing it) we avoid paying |
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* for a volatile write to item by using Unsafe.putObject instead |
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* of a normal write. This allows the cost of enqueue to be |
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* "one-and-a-half" CASes. |
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* |
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* Both head and tail may or may not point to a Node with a |
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* non-null item. If the queue is empty, all items must of course |
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* be null. Upon creation, both head and tail refer to a dummy |
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* Node with null item. Both head and tail are only updated using |
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* CAS, so they never regress, although again this is merely an |
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* optimization. |
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*/ |
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|
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private static class Node<E> { |
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volatile E item; |
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volatile Node<E> next; |
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} |
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|
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/** |
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* Returns a new node holding item. Uses relaxed write because item |
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* can only be seen after piggy-backing publication via casNext. |
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*/ |
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static <E> Node<E> newNode(E item) { |
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Node<E> node = new Node<E>(); |
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U.putObject(node, ITEM, item); |
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return node; |
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} |
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|
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static <E> boolean casItem(Node<E> node, E cmp, E val) { |
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return U.compareAndSwapObject(node, ITEM, cmp, val); |
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} |
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|
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static <E> void lazySetNext(Node<E> node, Node<E> val) { |
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U.putOrderedObject(node, NEXT, val); |
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} |
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|
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static <E> boolean casNext(Node<E> node, Node<E> cmp, Node<E> val) { |
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return U.compareAndSwapObject(node, NEXT, cmp, val); |
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} |
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|
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/** |
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* A node from which the first live (non-deleted) node (if any) |
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* can be reached in O(1) time. |
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* Invariants: |
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* - all live nodes are reachable from head via succ() |
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* - head != null |
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* - (tmp = head).next != tmp || tmp != head |
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* Non-invariants: |
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* - head.item may or may not be null. |
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* - it is permitted for tail to lag behind head, that is, for tail |
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* to not be reachable from head! |
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*/ |
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transient volatile Node<E> head; |
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|
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/** |
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* A node from which the last node on list (that is, the unique |
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* node with node.next == null) can be reached in O(1) time. |
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* Invariants: |
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* - the last node is always reachable from tail via succ() |
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* - tail != null |
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* Non-invariants: |
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* - tail.item may or may not be null. |
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* - it is permitted for tail to lag behind head, that is, for tail |
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* to not be reachable from head! |
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* - tail.next may or may not be self-pointing to tail. |
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*/ |
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private transient volatile Node<E> tail; |
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|
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/** |
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* Creates a {@code ConcurrentLinkedQueue} that is initially empty. |
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*/ |
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public ConcurrentLinkedQueue() { |
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head = tail = newNode(null); |
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} |
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|
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/** |
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* Creates a {@code ConcurrentLinkedQueue} |
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* initially containing the elements of the given collection, |
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* added in traversal order of the collection's iterator. |
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* |
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* @param c the collection of elements to initially contain |
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* @throws NullPointerException if the specified collection or any |
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* of its elements are null |
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*/ |
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public ConcurrentLinkedQueue(Collection<? extends E> c) { |
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Node<E> h = null, t = null; |
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for (E e : c) { |
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checkNotNull(e); |
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Node<E> newNode = newNode(e); |
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if (h == null) |
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h = t = newNode; |
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else { |
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lazySetNext(t, newNode); |
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t = newNode; |
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} |
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} |
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if (h == null) |
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h = t = newNode(null); |
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head = h; |
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tail = t; |
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} |
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|
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// Have to override just to update the javadoc |
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|
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/** |
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* Inserts the specified element at the tail of this queue. |
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* As the queue is unbounded, this method will never throw |
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* {@link IllegalStateException} or return {@code false}. |
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* |
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* @return {@code true} (as specified by {@link Collection#add}) |
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* @throws NullPointerException if the specified element is null |
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*/ |
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public boolean add(E e) { |
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return offer(e); |
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} |
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|
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/** |
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* Tries to CAS head to p. If successful, repoint old head to itself |
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* as sentinel for succ(), below. |
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*/ |
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final void updateHead(Node<E> h, Node<E> p) { |
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// assert h != null && p != null && (h == p || h.item == null); |
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if (h != p && casHead(h, p)) |
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lazySetNext(h, h); |
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} |
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|
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/** |
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* Returns the successor of p, or the head node if p.next has been |
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* linked to self, which will only be true if traversing with a |
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* stale pointer that is now off the list. |
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*/ |
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final Node<E> succ(Node<E> p) { |
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Node<E> next = p.next; |
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return (p == next) ? head : next; |
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} |
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|
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/** |
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* Inserts the specified element at the tail of this queue. |
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* As the queue is unbounded, this method will never return {@code false}. |
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* |
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* @return {@code true} (as specified by {@link Queue#offer}) |
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* @throws NullPointerException if the specified element is null |
281 |
*/ |
282 |
public boolean offer(E e) { |
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checkNotNull(e); |
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final Node<E> newNode = newNode(e); |
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|
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for (Node<E> t = tail, p = t;;) { |
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Node<E> q = p.next; |
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if (q == null) { |
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// p is last node |
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if (casNext(p, null, newNode)) { |
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// Successful CAS is the linearization point |
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// for e to become an element of this queue, |
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// and for newNode to become "live". |
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if (p != t) // hop two nodes at a time |
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casTail(t, newNode); // Failure is OK. |
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return true; |
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} |
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// Lost CAS race to another thread; re-read next |
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} |
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else if (p == q) |
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// We have fallen off list. If tail is unchanged, it |
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// will also be off-list, in which case we need to |
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// jump to head, from which all live nodes are always |
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// reachable. Else the new tail is a better bet. |
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p = (t != (t = tail)) ? t : head; |
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else |
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// Check for tail updates after two hops. |
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p = (p != t && t != (t = tail)) ? t : q; |
309 |
} |
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} |
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|
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public E poll() { |
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restartFromHead: |
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for (;;) { |
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for (Node<E> h = head, p = h, q;;) { |
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E item = p.item; |
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|
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if (item != null && casItem(p, item, null)) { |
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// Successful CAS is the linearization point |
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// for item to be removed from this queue. |
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if (p != h) // hop two nodes at a time |
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updateHead(h, ((q = p.next) != null) ? q : p); |
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return item; |
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} |
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else if ((q = p.next) == null) { |
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updateHead(h, p); |
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return null; |
328 |
} |
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else if (p == q) |
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continue restartFromHead; |
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else |
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p = q; |
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} |
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} |
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} |
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|
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public E peek() { |
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restartFromHead: |
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for (;;) { |
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for (Node<E> h = head, p = h, q;;) { |
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E item = p.item; |
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if (item != null || (q = p.next) == null) { |
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updateHead(h, p); |
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return item; |
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} |
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else if (p == q) |
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continue restartFromHead; |
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else |
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p = q; |
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} |
351 |
} |
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} |
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|
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/** |
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* Returns the first live (non-deleted) node on list, or null if none. |
356 |
* This is yet another variant of poll/peek; here returning the |
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* first node, not element. We could make peek() a wrapper around |
358 |
* first(), but that would cost an extra volatile read of item, |
359 |
* and the need to add a retry loop to deal with the possibility |
360 |
* of losing a race to a concurrent poll(). |
361 |
*/ |
362 |
Node<E> first() { |
363 |
restartFromHead: |
364 |
for (;;) { |
365 |
for (Node<E> h = head, p = h, q;;) { |
366 |
boolean hasItem = (p.item != null); |
367 |
if (hasItem || (q = p.next) == null) { |
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updateHead(h, p); |
369 |
return hasItem ? p : null; |
370 |
} |
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else if (p == q) |
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continue restartFromHead; |
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else |
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p = q; |
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} |
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} |
377 |
} |
378 |
|
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/** |
380 |
* Returns {@code true} if this queue contains no elements. |
381 |
* |
382 |
* @return {@code true} if this queue contains no elements |
383 |
*/ |
384 |
public boolean isEmpty() { |
385 |
return first() == null; |
386 |
} |
387 |
|
388 |
/** |
389 |
* Returns the number of elements in this queue. If this queue |
390 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
391 |
* {@code Integer.MAX_VALUE}. |
392 |
* |
393 |
* <p>Beware that, unlike in most collections, this method is |
394 |
* <em>NOT</em> a constant-time operation. Because of the |
395 |
* asynchronous nature of these queues, determining the current |
396 |
* number of elements requires an O(n) traversal. |
397 |
* Additionally, if elements are added or removed during execution |
398 |
* of this method, the returned result may be inaccurate. Thus, |
399 |
* this method is typically not very useful in concurrent |
400 |
* applications. |
401 |
* |
402 |
* @return the number of elements in this queue |
403 |
*/ |
404 |
public int size() { |
405 |
restartFromHead: for (;;) { |
406 |
int count = 0; |
407 |
for (Node<E> p = first(); p != null;) { |
408 |
if (p.item != null) |
409 |
if (++count == Integer.MAX_VALUE) |
410 |
break; // @see Collection.size() |
411 |
if (p == (p = p.next)) |
412 |
continue restartFromHead; |
413 |
} |
414 |
return count; |
415 |
} |
416 |
} |
417 |
|
418 |
/** |
419 |
* Returns {@code true} if this queue contains the specified element. |
420 |
* More formally, returns {@code true} if and only if this queue contains |
421 |
* at least one element {@code e} such that {@code o.equals(e)}. |
422 |
* |
423 |
* @param o object to be checked for containment in this queue |
424 |
* @return {@code true} if this queue contains the specified element |
425 |
*/ |
426 |
public boolean contains(Object o) { |
427 |
if (o != null) { |
428 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
429 |
E item = p.item; |
430 |
if (item != null && o.equals(item)) |
431 |
return true; |
432 |
} |
433 |
} |
434 |
return false; |
435 |
} |
436 |
|
437 |
/** |
438 |
* Removes a single instance of the specified element from this queue, |
439 |
* if it is present. More formally, removes an element {@code e} such |
440 |
* that {@code o.equals(e)}, if this queue contains one or more such |
441 |
* elements. |
442 |
* Returns {@code true} if this queue contained the specified element |
443 |
* (or equivalently, if this queue changed as a result of the call). |
444 |
* |
445 |
* @param o element to be removed from this queue, if present |
446 |
* @return {@code true} if this queue changed as a result of the call |
447 |
*/ |
448 |
public boolean remove(Object o) { |
449 |
if (o != null) { |
450 |
Node<E> next, pred = null; |
451 |
for (Node<E> p = first(); p != null; pred = p, p = next) { |
452 |
boolean removed = false; |
453 |
E item = p.item; |
454 |
if (item != null) { |
455 |
if (!o.equals(item)) { |
456 |
next = succ(p); |
457 |
continue; |
458 |
} |
459 |
removed = casItem(p, item, null); |
460 |
} |
461 |
|
462 |
next = succ(p); |
463 |
if (pred != null && next != null) // unlink |
464 |
casNext(pred, p, next); |
465 |
if (removed) |
466 |
return true; |
467 |
} |
468 |
} |
469 |
return false; |
470 |
} |
471 |
|
472 |
/** |
473 |
* Appends all of the elements in the specified collection to the end of |
474 |
* this queue, in the order that they are returned by the specified |
475 |
* collection's iterator. Attempts to {@code addAll} of a queue to |
476 |
* itself result in {@code IllegalArgumentException}. |
477 |
* |
478 |
* @param c the elements to be inserted into this queue |
479 |
* @return {@code true} if this queue changed as a result of the call |
480 |
* @throws NullPointerException if the specified collection or any |
481 |
* of its elements are null |
482 |
* @throws IllegalArgumentException if the collection is this queue |
483 |
*/ |
484 |
public boolean addAll(Collection<? extends E> c) { |
485 |
if (c == this) |
486 |
// As historically specified in AbstractQueue#addAll |
487 |
throw new IllegalArgumentException(); |
488 |
|
489 |
// Copy c into a private chain of Nodes |
490 |
Node<E> beginningOfTheEnd = null, last = null; |
491 |
for (E e : c) { |
492 |
checkNotNull(e); |
493 |
Node<E> newNode = newNode(e); |
494 |
if (beginningOfTheEnd == null) |
495 |
beginningOfTheEnd = last = newNode; |
496 |
else { |
497 |
lazySetNext(last, newNode); |
498 |
last = newNode; |
499 |
} |
500 |
} |
501 |
if (beginningOfTheEnd == null) |
502 |
return false; |
503 |
|
504 |
// Atomically append the chain at the tail of this collection |
505 |
for (Node<E> t = tail, p = t;;) { |
506 |
Node<E> q = p.next; |
507 |
if (q == null) { |
508 |
// p is last node |
509 |
if (casNext(p, null, beginningOfTheEnd)) { |
510 |
// Successful CAS is the linearization point |
511 |
// for all elements to be added to this queue. |
512 |
if (!casTail(t, last)) { |
513 |
// Try a little harder to update tail, |
514 |
// since we may be adding many elements. |
515 |
t = tail; |
516 |
if (last.next == null) |
517 |
casTail(t, last); |
518 |
} |
519 |
return true; |
520 |
} |
521 |
// Lost CAS race to another thread; re-read next |
522 |
} |
523 |
else if (p == q) |
524 |
// We have fallen off list. If tail is unchanged, it |
525 |
// will also be off-list, in which case we need to |
526 |
// jump to head, from which all live nodes are always |
527 |
// reachable. Else the new tail is a better bet. |
528 |
p = (t != (t = tail)) ? t : head; |
529 |
else |
530 |
// Check for tail updates after two hops. |
531 |
p = (p != t && t != (t = tail)) ? t : q; |
532 |
} |
533 |
} |
534 |
|
535 |
public String toString() { |
536 |
String[] a = null; |
537 |
restartFromHead: for (;;) { |
538 |
int charLength = 0; |
539 |
int size = 0; |
540 |
for (Node<E> p = first(); p != null;) { |
541 |
E item = p.item; |
542 |
if (item != null) { |
543 |
if (a == null) |
544 |
a = new String[4]; |
545 |
else if (size == a.length) |
546 |
a = Arrays.copyOf(a, 2 * size); |
547 |
String s = item.toString(); |
548 |
a[size++] = s; |
549 |
charLength += s.length(); |
550 |
} |
551 |
if (p == (p = p.next)) |
552 |
continue restartFromHead; |
553 |
} |
554 |
|
555 |
if (size == 0) |
556 |
return "[]"; |
557 |
|
558 |
// Copy each string into a perfectly sized char[] |
559 |
final char[] chars = new char[charLength + 2 * size]; |
560 |
chars[0] = '['; |
561 |
int j = 1; |
562 |
for (int i = 0; i < size; i++) { |
563 |
if (i > 0) { |
564 |
chars[j++] = ','; |
565 |
chars[j++] = ' '; |
566 |
} |
567 |
String s = a[i]; |
568 |
int len = s.length(); |
569 |
s.getChars(0, len, chars, j); |
570 |
j += len; |
571 |
} |
572 |
chars[j] = ']'; |
573 |
return new String(chars); |
574 |
} |
575 |
} |
576 |
|
577 |
private Object[] toArrayInternal(Object[] a) { |
578 |
Object[] x = a; |
579 |
restartFromHead: for (;;) { |
580 |
int size = 0; |
581 |
for (Node<E> p = first(); p != null;) { |
582 |
E item = p.item; |
583 |
if (item != null) { |
584 |
if (x == null) |
585 |
x = new Object[4]; |
586 |
else if (size == x.length) |
587 |
x = Arrays.copyOf(x, 2 * (size + 4)); |
588 |
x[size++] = item; |
589 |
} |
590 |
if (p == (p = p.next)) |
591 |
continue restartFromHead; |
592 |
} |
593 |
if (x == null) |
594 |
return new Object[0]; |
595 |
else if (a != null && size <= a.length) { |
596 |
if (a != x) |
597 |
System.arraycopy(x, 0, a, 0, size); |
598 |
if (size < a.length) |
599 |
a[size] = null; |
600 |
return a; |
601 |
} |
602 |
return (size == x.length) ? x : Arrays.copyOf(x, size); |
603 |
} |
604 |
} |
605 |
|
606 |
/** |
607 |
* Returns an array containing all of the elements in this queue, in |
608 |
* proper sequence. |
609 |
* |
610 |
* <p>The returned array will be "safe" in that no references to it are |
611 |
* maintained by this queue. (In other words, this method must allocate |
612 |
* a new array). The caller is thus free to modify the returned array. |
613 |
* |
614 |
* <p>This method acts as bridge between array-based and collection-based |
615 |
* APIs. |
616 |
* |
617 |
* @return an array containing all of the elements in this queue |
618 |
*/ |
619 |
public Object[] toArray() { |
620 |
return toArrayInternal(null); |
621 |
} |
622 |
|
623 |
/** |
624 |
* Returns an array containing all of the elements in this queue, in |
625 |
* proper sequence; the runtime type of the returned array is that of |
626 |
* the specified array. If the queue fits in the specified array, it |
627 |
* is returned therein. Otherwise, a new array is allocated with the |
628 |
* runtime type of the specified array and the size of this queue. |
629 |
* |
630 |
* <p>If this queue fits in the specified array with room to spare |
631 |
* (i.e., the array has more elements than this queue), the element in |
632 |
* the array immediately following the end of the queue is set to |
633 |
* {@code null}. |
634 |
* |
635 |
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
636 |
* array-based and collection-based APIs. Further, this method allows |
637 |
* precise control over the runtime type of the output array, and may, |
638 |
* under certain circumstances, be used to save allocation costs. |
639 |
* |
640 |
* <p>Suppose {@code x} is a queue known to contain only strings. |
641 |
* The following code can be used to dump the queue into a newly |
642 |
* allocated array of {@code String}: |
643 |
* |
644 |
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
645 |
* |
646 |
* Note that {@code toArray(new Object[0])} is identical in function to |
647 |
* {@code toArray()}. |
648 |
* |
649 |
* @param a the array into which the elements of the queue are to |
650 |
* be stored, if it is big enough; otherwise, a new array of the |
651 |
* same runtime type is allocated for this purpose |
652 |
* @return an array containing all of the elements in this queue |
653 |
* @throws ArrayStoreException if the runtime type of the specified array |
654 |
* is not a supertype of the runtime type of every element in |
655 |
* this queue |
656 |
* @throws NullPointerException if the specified array is null |
657 |
*/ |
658 |
@SuppressWarnings("unchecked") |
659 |
public <T> T[] toArray(T[] a) { |
660 |
if (a == null) throw new NullPointerException(); |
661 |
return (T[]) toArrayInternal(a); |
662 |
} |
663 |
|
664 |
/** |
665 |
* Returns an iterator over the elements in this queue in proper sequence. |
666 |
* The elements will be returned in order from first (head) to last (tail). |
667 |
* |
668 |
* <p>The returned iterator is |
669 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
670 |
* |
671 |
* @return an iterator over the elements in this queue in proper sequence |
672 |
*/ |
673 |
public Iterator<E> iterator() { |
674 |
return new Itr(); |
675 |
} |
676 |
|
677 |
private class Itr implements Iterator<E> { |
678 |
/** |
679 |
* Next node to return item for. |
680 |
*/ |
681 |
private Node<E> nextNode; |
682 |
|
683 |
/** |
684 |
* nextItem holds on to item fields because once we claim |
685 |
* that an element exists in hasNext(), we must return it in |
686 |
* the following next() call even if it was in the process of |
687 |
* being removed when hasNext() was called. |
688 |
*/ |
689 |
private E nextItem; |
690 |
|
691 |
/** |
692 |
* Node of the last returned item, to support remove. |
693 |
*/ |
694 |
private Node<E> lastRet; |
695 |
|
696 |
Itr() { |
697 |
restartFromHead: for (;;) { |
698 |
Node<E> h, p, q; |
699 |
for (p = h = head;; p = q) { |
700 |
E item; |
701 |
if ((item = p.item) != null) { |
702 |
nextNode = p; |
703 |
nextItem = item; |
704 |
break; |
705 |
} |
706 |
else if ((q = p.next) == null) |
707 |
break; |
708 |
else if (p == q) |
709 |
continue restartFromHead; |
710 |
} |
711 |
updateHead(h, p); |
712 |
return; |
713 |
} |
714 |
} |
715 |
|
716 |
public boolean hasNext() { |
717 |
return nextItem != null; |
718 |
} |
719 |
|
720 |
public E next() { |
721 |
final Node<E> pred = nextNode; |
722 |
if (pred == null) throw new NoSuchElementException(); |
723 |
// assert nextItem != null; |
724 |
lastRet = pred; |
725 |
E item = null; |
726 |
|
727 |
for (Node<E> p = succ(pred), q;; p = q) { |
728 |
if (p == null || (item = p.item) != null) { |
729 |
nextNode = p; |
730 |
E x = nextItem; |
731 |
nextItem = item; |
732 |
return x; |
733 |
} |
734 |
// unlink deleted nodes |
735 |
if ((q = succ(p)) != null) |
736 |
casNext(pred, p, q); |
737 |
} |
738 |
} |
739 |
|
740 |
public void remove() { |
741 |
Node<E> l = lastRet; |
742 |
if (l == null) throw new IllegalStateException(); |
743 |
// rely on a future traversal to relink. |
744 |
l.item = null; |
745 |
lastRet = null; |
746 |
} |
747 |
} |
748 |
|
749 |
/** |
750 |
* Saves this queue to a stream (that is, serializes it). |
751 |
* |
752 |
* @param s the stream |
753 |
* @throws java.io.IOException if an I/O error occurs |
754 |
* @serialData All of the elements (each an {@code E}) in |
755 |
* the proper order, followed by a null |
756 |
*/ |
757 |
private void writeObject(java.io.ObjectOutputStream s) |
758 |
throws java.io.IOException { |
759 |
|
760 |
// Write out any hidden stuff |
761 |
s.defaultWriteObject(); |
762 |
|
763 |
// Write out all elements in the proper order. |
764 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
765 |
Object item = p.item; |
766 |
if (item != null) |
767 |
s.writeObject(item); |
768 |
} |
769 |
|
770 |
// Use trailing null as sentinel |
771 |
s.writeObject(null); |
772 |
} |
773 |
|
774 |
/** |
775 |
* Reconstitutes this queue from a stream (that is, deserializes it). |
776 |
* @param s the stream |
777 |
* @throws ClassNotFoundException if the class of a serialized object |
778 |
* could not be found |
779 |
* @throws java.io.IOException if an I/O error occurs |
780 |
*/ |
781 |
private void readObject(java.io.ObjectInputStream s) |
782 |
throws java.io.IOException, ClassNotFoundException { |
783 |
s.defaultReadObject(); |
784 |
|
785 |
// Read in elements until trailing null sentinel found |
786 |
Node<E> h = null, t = null; |
787 |
for (Object item; (item = s.readObject()) != null; ) { |
788 |
@SuppressWarnings("unchecked") |
789 |
Node<E> newNode = newNode((E) item); |
790 |
if (h == null) |
791 |
h = t = newNode; |
792 |
else { |
793 |
lazySetNext(t, newNode); |
794 |
t = newNode; |
795 |
} |
796 |
} |
797 |
if (h == null) |
798 |
h = t = newNode(null); |
799 |
head = h; |
800 |
tail = t; |
801 |
} |
802 |
|
803 |
/** A customized variant of Spliterators.IteratorSpliterator */ |
804 |
static final class CLQSpliterator<E> implements Spliterator<E> { |
805 |
static final int MAX_BATCH = 1 << 25; // max batch array size; |
806 |
final ConcurrentLinkedQueue<E> queue; |
807 |
Node<E> current; // current node; null until initialized |
808 |
int batch; // batch size for splits |
809 |
boolean exhausted; // true when no more nodes |
810 |
CLQSpliterator(ConcurrentLinkedQueue<E> queue) { |
811 |
this.queue = queue; |
812 |
} |
813 |
|
814 |
public Spliterator<E> trySplit() { |
815 |
Node<E> p; |
816 |
final ConcurrentLinkedQueue<E> q = this.queue; |
817 |
int b = batch; |
818 |
int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1; |
819 |
if (!exhausted && |
820 |
((p = current) != null || (p = q.first()) != null) && |
821 |
p.next != null) { |
822 |
Object[] a = new Object[n]; |
823 |
int i = 0; |
824 |
do { |
825 |
if ((a[i] = p.item) != null) |
826 |
++i; |
827 |
if (p == (p = p.next)) |
828 |
p = q.first(); |
829 |
} while (p != null && i < n); |
830 |
if ((current = p) == null) |
831 |
exhausted = true; |
832 |
if (i > 0) { |
833 |
batch = i; |
834 |
return Spliterators.spliterator |
835 |
(a, 0, i, Spliterator.ORDERED | Spliterator.NONNULL | |
836 |
Spliterator.CONCURRENT); |
837 |
} |
838 |
} |
839 |
return null; |
840 |
} |
841 |
|
842 |
public void forEachRemaining(Consumer<? super E> action) { |
843 |
Node<E> p; |
844 |
if (action == null) throw new NullPointerException(); |
845 |
final ConcurrentLinkedQueue<E> q = this.queue; |
846 |
if (!exhausted && |
847 |
((p = current) != null || (p = q.first()) != null)) { |
848 |
exhausted = true; |
849 |
do { |
850 |
E e = p.item; |
851 |
if (p == (p = p.next)) |
852 |
p = q.first(); |
853 |
if (e != null) |
854 |
action.accept(e); |
855 |
} while (p != null); |
856 |
} |
857 |
} |
858 |
|
859 |
public boolean tryAdvance(Consumer<? super E> action) { |
860 |
Node<E> p; |
861 |
if (action == null) throw new NullPointerException(); |
862 |
final ConcurrentLinkedQueue<E> q = this.queue; |
863 |
if (!exhausted && |
864 |
((p = current) != null || (p = q.first()) != null)) { |
865 |
E e; |
866 |
do { |
867 |
e = p.item; |
868 |
if (p == (p = p.next)) |
869 |
p = q.first(); |
870 |
} while (e == null && p != null); |
871 |
if ((current = p) == null) |
872 |
exhausted = true; |
873 |
if (e != null) { |
874 |
action.accept(e); |
875 |
return true; |
876 |
} |
877 |
} |
878 |
return false; |
879 |
} |
880 |
|
881 |
public long estimateSize() { return Long.MAX_VALUE; } |
882 |
|
883 |
public int characteristics() { |
884 |
return Spliterator.ORDERED | Spliterator.NONNULL | |
885 |
Spliterator.CONCURRENT; |
886 |
} |
887 |
} |
888 |
|
889 |
/** |
890 |
* Returns a {@link Spliterator} over the elements in this queue. |
891 |
* |
892 |
* <p>The returned spliterator is |
893 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
894 |
* |
895 |
* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, |
896 |
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. |
897 |
* |
898 |
* @implNote |
899 |
* The {@code Spliterator} implements {@code trySplit} to permit limited |
900 |
* parallelism. |
901 |
* |
902 |
* @return a {@code Spliterator} over the elements in this queue |
903 |
* @since 1.8 |
904 |
*/ |
905 |
@Override |
906 |
public Spliterator<E> spliterator() { |
907 |
return new CLQSpliterator<E>(this); |
908 |
} |
909 |
|
910 |
/** |
911 |
* Throws NullPointerException if argument is null. |
912 |
* |
913 |
* @param v the element |
914 |
*/ |
915 |
private static void checkNotNull(Object v) { |
916 |
if (v == null) |
917 |
throw new NullPointerException(); |
918 |
} |
919 |
|
920 |
private boolean casTail(Node<E> cmp, Node<E> val) { |
921 |
return U.compareAndSwapObject(this, TAIL, cmp, val); |
922 |
} |
923 |
|
924 |
private boolean casHead(Node<E> cmp, Node<E> val) { |
925 |
return U.compareAndSwapObject(this, HEAD, cmp, val); |
926 |
} |
927 |
|
928 |
// Unsafe mechanics |
929 |
|
930 |
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
931 |
private static final long HEAD; |
932 |
private static final long TAIL; |
933 |
private static final long ITEM; |
934 |
private static final long NEXT; |
935 |
static { |
936 |
try { |
937 |
HEAD = U.objectFieldOffset |
938 |
(ConcurrentLinkedQueue.class.getDeclaredField("head")); |
939 |
TAIL = U.objectFieldOffset |
940 |
(ConcurrentLinkedQueue.class.getDeclaredField("tail")); |
941 |
ITEM = U.objectFieldOffset |
942 |
(Node.class.getDeclaredField("item")); |
943 |
NEXT = U.objectFieldOffset |
944 |
(Node.class.getDeclaredField("next")); |
945 |
} catch (ReflectiveOperationException e) { |
946 |
throw new Error(e); |
947 |
} |
948 |
} |
949 |
} |