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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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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.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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|
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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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* This queue does not permit {@code null} elements. |
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* |
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* <p>This implementation employs an efficient "wait-free" |
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* algorithm based on one described in <a |
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* href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple, |
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* 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>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. |
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* |
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* <p>This class and its iterator implement all of the |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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* 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 collection |
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* |
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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). This is controlled by local "hops" variables |
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* that only trigger helping-CASes after experiencing multiple |
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* lags. |
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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 lazySet instead of a |
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* 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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private volatile E item; |
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private volatile Node<E> next; |
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|
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Node(E item) { |
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// Piggyback on imminent casNext() |
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lazySetItem(item); |
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} |
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|
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E getItem() { |
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return item; |
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} |
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|
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boolean casItem(E cmp, E val) { |
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return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
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} |
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|
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void setItem(E val) { |
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item = val; |
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} |
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|
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void lazySetItem(E val) { |
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UNSAFE.putOrderedObject(this, itemOffset, val); |
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} |
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|
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void lazySetNext(Node<E> val) { |
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UNSAFE.putOrderedObject(this, nextOffset, val); |
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} |
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|
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Node<E> getNext() { |
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return next; |
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} |
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|
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boolean casNext(Node<E> cmp, Node<E> val) { |
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return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
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} |
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|
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// Unsafe mechanics |
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|
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private static final sun.misc.Unsafe UNSAFE = |
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sun.misc.Unsafe.getUnsafe(); |
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private static final long nextOffset = |
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objectFieldOffset(UNSAFE, "next", Node.class); |
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private static final long itemOffset = |
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objectFieldOffset(UNSAFE, "item", Node.class); |
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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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private transient volatile Node<E> head = new Node<E>(null); |
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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 = head; |
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|
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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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|
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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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* @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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for (Iterator<? extends E> it = c.iterator(); it.hasNext();) |
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add(it.next()); |
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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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* |
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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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* We don't bother to update head or tail pointers if fewer than |
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* HOPS links from "true" location. We assume that volatile |
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* writes are significantly more expensive than volatile reads. |
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*/ |
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private static final int HOPS = 1; |
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|
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/** |
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* Try 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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if (h != p && casHead(h, p)) |
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h.lazySetNext(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.getNext(); |
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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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* |
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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 |
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*/ |
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public boolean offer(E e) { |
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if (e == null) throw new NullPointerException(); |
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Node<E> n = new Node<E>(e); |
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retry: |
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for (;;) { |
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Node<E> t = tail; |
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Node<E> p = t; |
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for (int hops = 0; ; hops++) { |
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Node<E> next = succ(p); |
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if (next != null) { |
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if (hops > HOPS && t != tail) |
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continue retry; |
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p = next; |
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} else if (p.casNext(null, n)) { |
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if (hops >= HOPS) |
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casTail(t, n); // Failure is OK. |
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return true; |
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} else { |
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p = succ(p); |
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} |
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} |
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} |
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} |
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|
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public E poll() { |
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Node<E> h = head; |
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Node<E> p = h; |
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for (int hops = 0; ; hops++) { |
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E item = p.getItem(); |
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|
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if (item != null && p.casItem(item, null)) { |
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if (hops >= HOPS) { |
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Node<E> q = p.getNext(); |
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updateHead(h, (q != null) ? q : p); |
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} |
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return item; |
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} |
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Node<E> next = succ(p); |
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if (next == null) { |
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updateHead(h, p); |
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break; |
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} |
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p = next; |
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} |
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return null; |
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} |
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|
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public E peek() { |
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Node<E> h = head; |
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Node<E> p = h; |
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E item; |
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for (;;) { |
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item = p.getItem(); |
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if (item != null) |
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break; |
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Node<E> next = succ(p); |
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if (next == null) { |
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break; |
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} |
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p = next; |
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} |
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updateHead(h, p); |
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return item; |
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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. |
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* 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 |
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* first(), but that would cost an extra volatile read of item, |
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* and the need to add a retry loop to deal with the possibility |
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* of losing a race to a concurrent poll(). |
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*/ |
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Node<E> first() { |
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Node<E> h = head; |
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Node<E> p = h; |
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Node<E> result; |
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for (;;) { |
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E item = p.getItem(); |
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if (item != null) { |
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result = p; |
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break; |
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} |
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Node<E> next = succ(p); |
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if (next == null) { |
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result = null; |
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break; |
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} |
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p = next; |
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} |
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updateHead(h, p); |
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return result; |
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} |
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|
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/** |
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* Returns {@code true} if this queue contains no elements. |
368 |
* |
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* @return {@code true} if this queue contains no elements |
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*/ |
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public boolean isEmpty() { |
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return first() == null; |
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} |
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|
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/** |
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* Returns the number of elements in this queue. If this queue |
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* contains more than {@code Integer.MAX_VALUE} elements, returns |
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* {@code Integer.MAX_VALUE}. |
379 |
* |
380 |
* <p>Beware that, unlike in most collections, this method is |
381 |
* <em>NOT</em> a constant-time operation. Because of the |
382 |
* asynchronous nature of these queues, determining the current |
383 |
* number of elements requires an O(n) traversal. |
384 |
* |
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* @return the number of elements in this queue |
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*/ |
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public int size() { |
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int count = 0; |
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for (Node<E> p = first(); p != null; p = succ(p)) { |
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if (p.getItem() != null) { |
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// Collections.size() spec says to max out |
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if (++count == Integer.MAX_VALUE) |
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break; |
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} |
395 |
} |
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return count; |
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} |
398 |
|
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/** |
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* Returns {@code true} if this queue contains the specified element. |
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* More formally, returns {@code true} if and only if this queue contains |
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* at least one element {@code e} such that {@code o.equals(e)}. |
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* |
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* @param o object to be checked for containment in this queue |
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* @return {@code true} if this queue contains the specified element |
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*/ |
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public boolean contains(Object o) { |
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if (o == null) return false; |
409 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
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E item = p.getItem(); |
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if (item != null && |
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o.equals(item)) |
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return true; |
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} |
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return false; |
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} |
417 |
|
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/** |
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* Removes a single instance of the specified element from this queue, |
420 |
* if it is present. More formally, removes an element {@code e} such |
421 |
* that {@code o.equals(e)}, if this queue contains one or more such |
422 |
* elements. |
423 |
* Returns {@code true} if this queue contained the specified element |
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* (or equivalently, if this queue changed as a result of the call). |
425 |
* |
426 |
* @param o element to be removed from this queue, if present |
427 |
* @return {@code true} if this queue changed as a result of the call |
428 |
*/ |
429 |
public boolean remove(Object o) { |
430 |
if (o == null) return false; |
431 |
Node<E> pred = null; |
432 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
433 |
E item = p.getItem(); |
434 |
if (item != null && |
435 |
o.equals(item) && |
436 |
p.casItem(item, null)) { |
437 |
Node<E> next = succ(p); |
438 |
if (pred != null && next != null) |
439 |
pred.casNext(p, next); |
440 |
return true; |
441 |
} |
442 |
pred = p; |
443 |
} |
444 |
return false; |
445 |
} |
446 |
|
447 |
/** |
448 |
* Returns an array containing all of the elements in this queue, in |
449 |
* proper sequence. |
450 |
* |
451 |
* <p>The returned array will be "safe" in that no references to it are |
452 |
* maintained by this queue. (In other words, this method must allocate |
453 |
* a new array). The caller is thus free to modify the returned array. |
454 |
* |
455 |
* <p>This method acts as bridge between array-based and collection-based |
456 |
* APIs. |
457 |
* |
458 |
* @return an array containing all of the elements in this queue |
459 |
*/ |
460 |
public Object[] toArray() { |
461 |
// Use ArrayList to deal with resizing. |
462 |
ArrayList<E> al = new ArrayList<E>(); |
463 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
464 |
E item = p.getItem(); |
465 |
if (item != null) |
466 |
al.add(item); |
467 |
} |
468 |
return al.toArray(); |
469 |
} |
470 |
|
471 |
/** |
472 |
* Returns an array containing all of the elements in this queue, in |
473 |
* proper sequence; the runtime type of the returned array is that of |
474 |
* the specified array. If the queue fits in the specified array, it |
475 |
* is returned therein. Otherwise, a new array is allocated with the |
476 |
* runtime type of the specified array and the size of this queue. |
477 |
* |
478 |
* <p>If this queue fits in the specified array with room to spare |
479 |
* (i.e., the array has more elements than this queue), the element in |
480 |
* the array immediately following the end of the queue is set to |
481 |
* {@code null}. |
482 |
* |
483 |
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
484 |
* array-based and collection-based APIs. Further, this method allows |
485 |
* precise control over the runtime type of the output array, and may, |
486 |
* under certain circumstances, be used to save allocation costs. |
487 |
* |
488 |
* <p>Suppose {@code x} is a queue known to contain only strings. |
489 |
* The following code can be used to dump the queue into a newly |
490 |
* allocated array of {@code String}: |
491 |
* |
492 |
* <pre> |
493 |
* String[] y = x.toArray(new String[0]);</pre> |
494 |
* |
495 |
* Note that {@code toArray(new Object[0])} is identical in function to |
496 |
* {@code toArray()}. |
497 |
* |
498 |
* @param a the array into which the elements of the queue are to |
499 |
* be stored, if it is big enough; otherwise, a new array of the |
500 |
* same runtime type is allocated for this purpose |
501 |
* @return an array containing all of the elements in this queue |
502 |
* @throws ArrayStoreException if the runtime type of the specified array |
503 |
* is not a supertype of the runtime type of every element in |
504 |
* this queue |
505 |
* @throws NullPointerException if the specified array is null |
506 |
*/ |
507 |
@SuppressWarnings("unchecked") |
508 |
public <T> T[] toArray(T[] a) { |
509 |
// try to use sent-in array |
510 |
int k = 0; |
511 |
Node<E> p; |
512 |
for (p = first(); p != null && k < a.length; p = succ(p)) { |
513 |
E item = p.getItem(); |
514 |
if (item != null) |
515 |
a[k++] = (T)item; |
516 |
} |
517 |
if (p == null) { |
518 |
if (k < a.length) |
519 |
a[k] = null; |
520 |
return a; |
521 |
} |
522 |
|
523 |
// If won't fit, use ArrayList version |
524 |
ArrayList<E> al = new ArrayList<E>(); |
525 |
for (Node<E> q = first(); q != null; q = succ(q)) { |
526 |
E item = q.getItem(); |
527 |
if (item != null) |
528 |
al.add(item); |
529 |
} |
530 |
return al.toArray(a); |
531 |
} |
532 |
|
533 |
/** |
534 |
* Returns an iterator over the elements in this queue in proper sequence. |
535 |
* The returned iterator is a "weakly consistent" iterator that |
536 |
* will never throw {@link java.util.ConcurrentModificationException |
537 |
* ConcurrentModificationException}, |
538 |
* and guarantees to traverse elements as they existed upon |
539 |
* construction of the iterator, and may (but is not guaranteed to) |
540 |
* reflect any modifications subsequent to construction. |
541 |
* |
542 |
* @return an iterator over the elements in this queue in proper sequence |
543 |
*/ |
544 |
public Iterator<E> iterator() { |
545 |
return new Itr(); |
546 |
} |
547 |
|
548 |
private class Itr implements Iterator<E> { |
549 |
/** |
550 |
* Next node to return item for. |
551 |
*/ |
552 |
private Node<E> nextNode; |
553 |
|
554 |
/** |
555 |
* nextItem holds on to item fields because once we claim |
556 |
* that an element exists in hasNext(), we must return it in |
557 |
* the following next() call even if it was in the process of |
558 |
* being removed when hasNext() was called. |
559 |
*/ |
560 |
private E nextItem; |
561 |
|
562 |
/** |
563 |
* Node of the last returned item, to support remove. |
564 |
*/ |
565 |
private Node<E> lastRet; |
566 |
|
567 |
Itr() { |
568 |
advance(); |
569 |
} |
570 |
|
571 |
/** |
572 |
* Moves to next valid node and returns item to return for |
573 |
* next(), or null if no such. |
574 |
*/ |
575 |
private E advance() { |
576 |
lastRet = nextNode; |
577 |
E x = nextItem; |
578 |
|
579 |
Node<E> pred, p; |
580 |
if (nextNode == null) { |
581 |
p = first(); |
582 |
pred = null; |
583 |
} else { |
584 |
pred = nextNode; |
585 |
p = succ(nextNode); |
586 |
} |
587 |
|
588 |
for (;;) { |
589 |
if (p == null) { |
590 |
nextNode = null; |
591 |
nextItem = null; |
592 |
return x; |
593 |
} |
594 |
E item = p.getItem(); |
595 |
if (item != null) { |
596 |
nextNode = p; |
597 |
nextItem = item; |
598 |
return x; |
599 |
} else { |
600 |
// skip over nulls |
601 |
Node<E> next = succ(p); |
602 |
if (pred != null && next != null) |
603 |
pred.casNext(p, next); |
604 |
p = next; |
605 |
} |
606 |
} |
607 |
} |
608 |
|
609 |
public boolean hasNext() { |
610 |
return nextNode != null; |
611 |
} |
612 |
|
613 |
public E next() { |
614 |
if (nextNode == null) throw new NoSuchElementException(); |
615 |
return advance(); |
616 |
} |
617 |
|
618 |
public void remove() { |
619 |
Node<E> l = lastRet; |
620 |
if (l == null) throw new IllegalStateException(); |
621 |
// rely on a future traversal to relink. |
622 |
l.setItem(null); |
623 |
lastRet = null; |
624 |
} |
625 |
} |
626 |
|
627 |
/** |
628 |
* Save the state to a stream (that is, serialize it). |
629 |
* |
630 |
* @serialData All of the elements (each an {@code E}) in |
631 |
* the proper order, followed by a null |
632 |
* @param s the stream |
633 |
*/ |
634 |
private void writeObject(java.io.ObjectOutputStream s) |
635 |
throws java.io.IOException { |
636 |
|
637 |
// Write out any hidden stuff |
638 |
s.defaultWriteObject(); |
639 |
|
640 |
// Write out all elements in the proper order. |
641 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
642 |
Object item = p.getItem(); |
643 |
if (item != null) |
644 |
s.writeObject(item); |
645 |
} |
646 |
|
647 |
// Use trailing null as sentinel |
648 |
s.writeObject(null); |
649 |
} |
650 |
|
651 |
/** |
652 |
* Reconstitute the Queue instance from a stream (that is, |
653 |
* deserialize it). |
654 |
* @param s the stream |
655 |
*/ |
656 |
private void readObject(java.io.ObjectInputStream s) |
657 |
throws java.io.IOException, ClassNotFoundException { |
658 |
// Read in capacity, and any hidden stuff |
659 |
s.defaultReadObject(); |
660 |
head = new Node<E>(null); |
661 |
tail = head; |
662 |
// Read in all elements and place in queue |
663 |
for (;;) { |
664 |
@SuppressWarnings("unchecked") |
665 |
E item = (E)s.readObject(); |
666 |
if (item == null) |
667 |
break; |
668 |
else |
669 |
offer(item); |
670 |
} |
671 |
} |
672 |
|
673 |
// Unsafe mechanics |
674 |
|
675 |
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
676 |
private static final long headOffset = |
677 |
objectFieldOffset(UNSAFE, "head", ConcurrentLinkedQueue.class); |
678 |
private static final long tailOffset = |
679 |
objectFieldOffset(UNSAFE, "tail", ConcurrentLinkedQueue.class); |
680 |
|
681 |
private boolean casTail(Node<E> cmp, Node<E> val) { |
682 |
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
683 |
} |
684 |
|
685 |
private boolean casHead(Node<E> cmp, Node<E> val) { |
686 |
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
687 |
} |
688 |
|
689 |
private void lazySetHead(Node<E> val) { |
690 |
UNSAFE.putOrderedObject(this, headOffset, val); |
691 |
} |
692 |
|
693 |
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
694 |
String field, Class<?> klazz) { |
695 |
try { |
696 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
697 |
} catch (NoSuchFieldException e) { |
698 |
// Convert Exception to corresponding Error |
699 |
NoSuchFieldError error = new NoSuchFieldError(field); |
700 |
error.initCause(e); |
701 |
throw error; |
702 |
} |
703 |
} |
704 |
} |