1 |
dl |
1.1 |
/* |
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jsr166 |
1.66 |
* Written by Doug Lea and Martin Buchholz with assistance from members of |
3 |
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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/licenses/publicdomain |
5 |
dl |
1.1 |
*/ |
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package java.util.concurrent; |
8 |
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jsr166 |
1.51 |
import java.util.AbstractQueue; |
10 |
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import java.util.ArrayList; |
11 |
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import java.util.Collection; |
12 |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Queue; |
15 |
dl |
1.1 |
|
16 |
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/** |
17 |
jsr166 |
1.29 |
* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes. |
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dholmes |
1.6 |
* 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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dl |
1.17 |
* queue the shortest time. New elements |
23 |
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* are inserted at the tail of the queue, and the queue retrieval |
24 |
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* operations obtain elements at the head of the queue. |
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jsr166 |
1.48 |
* A {@code ConcurrentLinkedQueue} is an appropriate choice when |
26 |
dl |
1.19 |
* many threads will share access to a common collection. |
27 |
jsr166 |
1.55 |
* Like most other concurrent collection implementations, this class |
28 |
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* does not permit the use of {@code null} elements. |
29 |
dl |
1.1 |
* |
30 |
jsr166 |
1.29 |
* <p>This implementation employs an efficient "wait-free" |
31 |
dholmes |
1.6 |
* algorithm based on one described in <a |
32 |
dl |
1.1 |
* href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple, |
33 |
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* Fast, and Practical Non-Blocking and Blocking Concurrent Queue |
34 |
dl |
1.15 |
* Algorithms</a> by Maged M. Michael and Michael L. Scott. |
35 |
dl |
1.1 |
* |
36 |
jsr166 |
1.55 |
* <p>Iterators are <i>weakly consistent</i>, returning elements |
37 |
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* reflecting the state of the queue at some point at or since the |
38 |
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* creation of the iterator. They do <em>not</em> throw {@link |
39 |
dl |
1.68 |
* java.util.ConcurrentModificationException}, and may proceed concurrently |
40 |
jsr166 |
1.69 |
* with other operations. Elements contained in the queue since the creation |
41 |
jsr166 |
1.55 |
* 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 |
44 |
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* is <em>NOT</em> a constant-time operation. Because of the |
45 |
dl |
1.1 |
* asynchronous nature of these queues, determining the current number |
46 |
jsr166 |
1.55 |
* of elements requires a traversal of the elements. |
47 |
dl |
1.18 |
* |
48 |
jsr166 |
1.55 |
* <p>This class and its iterator implement all of the <em>optional</em> |
49 |
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* methods of the {@link Queue} and {@link Iterator} interfaces. |
50 |
dl |
1.18 |
* |
51 |
jsr166 |
1.43 |
* <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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* |
58 |
dl |
1.25 |
* <p>This class is a member of the |
59 |
jsr166 |
1.47 |
* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
60 |
dl |
1.25 |
* Java Collections Framework</a>. |
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* |
62 |
dl |
1.1 |
* @since 1.5 |
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* @author Doug Lea |
64 |
dl |
1.21 |
* @param <E> the type of elements held in this collection |
65 |
tim |
1.2 |
* |
66 |
dl |
1.25 |
*/ |
67 |
dl |
1.1 |
public class ConcurrentLinkedQueue<E> extends AbstractQueue<E> |
68 |
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implements Queue<E>, java.io.Serializable { |
69 |
dl |
1.14 |
private static final long serialVersionUID = 196745693267521676L; |
70 |
dl |
1.1 |
|
71 |
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/* |
72 |
jsr166 |
1.48 |
* This is a modification of the Michael & Scott algorithm, |
73 |
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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 |
75 |
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* explanation, read the paper. |
76 |
dl |
1.44 |
* |
77 |
jsr166 |
1.48 |
* Note that like most non-blocking algorithms in this package, |
78 |
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* this implementation relies on the fact that in garbage |
79 |
dl |
1.44 |
* collected systems, there is no possibility of ABA problems due |
80 |
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* to recycled nodes, so there is no need to use "counted |
81 |
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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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jsr166 |
1.48 |
* |
84 |
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* The fundamental invariants are: |
85 |
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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 |
87 |
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* reached in O(1) time from tail, but tail is merely an |
88 |
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* optimization - it can always be reached in O(N) time from |
89 |
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* head as well. |
90 |
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* - The elements contained in the queue are the non-null items in |
91 |
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* Nodes that are reachable from head. CASing the item |
92 |
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* reference of a Node to null atomically removes it from the |
93 |
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* queue. Reachability of all elements from head must remain |
94 |
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* true even in the case of concurrent modifications that cause |
95 |
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* head to advance. A dequeued Node may remain in use |
96 |
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* indefinitely due to creation of an Iterator or simply a |
97 |
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* poll() that has lost its time slice. |
98 |
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* |
99 |
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* The above might appear to imply that all Nodes are GC-reachable |
100 |
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* from a predecessor dequeued Node. That would cause two problems: |
101 |
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* - allow a rogue Iterator to cause unbounded memory retention |
102 |
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* - cause cross-generational linking of old Nodes to new Nodes if |
103 |
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* a Node was tenured while live, which generational GCs have a |
104 |
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* hard time dealing with, causing repeated major collections. |
105 |
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* However, only non-deleted Nodes need to be reachable from |
106 |
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* dequeued Nodes, and reachability does not necessarily have to |
107 |
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* be of the kind understood by the GC. We use the trick of |
108 |
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* linking a Node that has just been dequeued to itself. Such a |
109 |
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* self-link implicitly means to advance to head. |
110 |
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* |
111 |
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* Both head and tail are permitted to lag. In fact, failing to |
112 |
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* update them every time one could is a significant optimization |
113 |
jsr166 |
1.65 |
* (fewer CASes). As with LinkedTransferQueue (see the internal |
114 |
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* documentation for that class), we use a slack threshold of two; |
115 |
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* that is, we update head/tail when the current pointer appears |
116 |
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* to be two or more steps away from the first/last node. |
117 |
jsr166 |
1.48 |
* |
118 |
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* Since head and tail are updated concurrently and independently, |
119 |
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* it is possible for tail to lag behind head (why not)? |
120 |
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* |
121 |
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* CASing a Node's item reference to null atomically removes the |
122 |
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* element from the queue. Iterators skip over Nodes with null |
123 |
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* items. Prior implementations of this class had a race between |
124 |
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* poll() and remove(Object) where the same element would appear |
125 |
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* to be successfully removed by two concurrent operations. The |
126 |
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* method remove(Object) also lazily unlinks deleted Nodes, but |
127 |
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* this is merely an optimization. |
128 |
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* |
129 |
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* When constructing a Node (before enqueuing it) we avoid paying |
130 |
jsr166 |
1.64 |
* for a volatile write to item by using Unsafe.putObject instead |
131 |
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* of a normal write. This allows the cost of enqueue to be |
132 |
jsr166 |
1.48 |
* "one-and-a-half" CASes. |
133 |
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* |
134 |
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* Both head and tail may or may not point to a Node with a |
135 |
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* non-null item. If the queue is empty, all items must of course |
136 |
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* be null. Upon creation, both head and tail refer to a dummy |
137 |
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* Node with null item. Both head and tail are only updated using |
138 |
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* CAS, so they never regress, although again this is merely an |
139 |
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* optimization. |
140 |
dl |
1.1 |
*/ |
141 |
jsr166 |
1.51 |
|
142 |
dl |
1.23 |
private static class Node<E> { |
143 |
jsr166 |
1.64 |
volatile E item; |
144 |
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volatile Node<E> next; |
145 |
jsr166 |
1.29 |
|
146 |
jsr166 |
1.57 |
/** |
147 |
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* Constructs a new node. Uses relaxed write because item can |
148 |
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* only be seen after publication via casNext. |
149 |
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*/ |
150 |
jsr166 |
1.51 |
Node(E item) { |
151 |
jsr166 |
1.57 |
UNSAFE.putObject(this, itemOffset, item); |
152 |
jsr166 |
1.51 |
} |
153 |
jsr166 |
1.29 |
|
154 |
dl |
1.22 |
boolean casItem(E cmp, E val) { |
155 |
jsr166 |
1.50 |
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
156 |
dl |
1.13 |
} |
157 |
jsr166 |
1.29 |
|
158 |
jsr166 |
1.48 |
void lazySetNext(Node<E> val) { |
159 |
jsr166 |
1.50 |
UNSAFE.putOrderedObject(this, nextOffset, val); |
160 |
dl |
1.13 |
} |
161 |
jsr166 |
1.29 |
|
162 |
dl |
1.23 |
boolean casNext(Node<E> cmp, Node<E> val) { |
163 |
jsr166 |
1.50 |
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
164 |
dl |
1.13 |
} |
165 |
dl |
1.1 |
|
166 |
jsr166 |
1.50 |
// Unsafe mechanics |
167 |
dl |
1.1 |
|
168 |
jsr166 |
1.50 |
private static final sun.misc.Unsafe UNSAFE = |
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sun.misc.Unsafe.getUnsafe(); |
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private static final long nextOffset = |
171 |
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objectFieldOffset(UNSAFE, "next", Node.class); |
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private static final long itemOffset = |
173 |
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objectFieldOffset(UNSAFE, "item", Node.class); |
174 |
jsr166 |
1.48 |
} |
175 |
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176 |
tim |
1.2 |
/** |
177 |
jsr166 |
1.51 |
* A node from which the first live (non-deleted) node (if any) |
178 |
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* can be reached in O(1) time. |
179 |
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* Invariants: |
180 |
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* - all live nodes are reachable from head via succ() |
181 |
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* - head != null |
182 |
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* - (tmp = head).next != tmp || tmp != head |
183 |
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* Non-invariants: |
184 |
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* - head.item may or may not be null. |
185 |
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* - it is permitted for tail to lag behind head, that is, for tail |
186 |
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* to not be reachable from head! |
187 |
dl |
1.1 |
*/ |
188 |
jsr166 |
1.55 |
private transient volatile Node<E> head; |
189 |
dl |
1.1 |
|
190 |
jsr166 |
1.51 |
/** |
191 |
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* A node from which the last node on list (that is, the unique |
192 |
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* node with node.next == null) can be reached in O(1) time. |
193 |
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* Invariants: |
194 |
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* - the last node is always reachable from tail via succ() |
195 |
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* - tail != null |
196 |
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* Non-invariants: |
197 |
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* - tail.item may or may not be null. |
198 |
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* - it is permitted for tail to lag behind head, that is, for tail |
199 |
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* to not be reachable from head! |
200 |
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* - tail.next may or may not be self-pointing to tail. |
201 |
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*/ |
202 |
jsr166 |
1.55 |
private transient volatile Node<E> tail; |
203 |
dl |
1.1 |
|
204 |
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205 |
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/** |
206 |
jsr166 |
1.48 |
* Creates a {@code ConcurrentLinkedQueue} that is initially empty. |
207 |
dl |
1.1 |
*/ |
208 |
jsr166 |
1.55 |
public ConcurrentLinkedQueue() { |
209 |
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head = tail = new Node<E>(null); |
210 |
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} |
211 |
dl |
1.1 |
|
212 |
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/** |
213 |
jsr166 |
1.48 |
* Creates a {@code ConcurrentLinkedQueue} |
214 |
dholmes |
1.7 |
* initially containing the elements of the given collection, |
215 |
dholmes |
1.6 |
* added in traversal order of the collection's iterator. |
216 |
jsr166 |
1.55 |
* |
217 |
dholmes |
1.6 |
* @param c the collection of elements to initially contain |
218 |
jsr166 |
1.34 |
* @throws NullPointerException if the specified collection or any |
219 |
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* of its elements are null |
220 |
dl |
1.1 |
*/ |
221 |
dholmes |
1.6 |
public ConcurrentLinkedQueue(Collection<? extends E> c) { |
222 |
jsr166 |
1.55 |
Node<E> h = null, t = null; |
223 |
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for (E e : c) { |
224 |
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checkNotNull(e); |
225 |
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Node<E> newNode = new Node<E>(e); |
226 |
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if (h == null) |
227 |
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h = t = newNode; |
228 |
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else { |
229 |
jsr166 |
1.62 |
t.lazySetNext(newNode); |
230 |
jsr166 |
1.55 |
t = newNode; |
231 |
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} |
232 |
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} |
233 |
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if (h == null) |
234 |
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h = t = new Node<E>(null); |
235 |
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head = h; |
236 |
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tail = t; |
237 |
dl |
1.1 |
} |
238 |
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|
239 |
jsr166 |
1.29 |
// Have to override just to update the javadoc |
240 |
dholmes |
1.6 |
|
241 |
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/** |
242 |
jsr166 |
1.35 |
* Inserts the specified element at the tail of this queue. |
243 |
jsr166 |
1.67 |
* As the queue is unbounded, this method will never throw |
244 |
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* {@link IllegalStateException} or return {@code false}. |
245 |
dholmes |
1.7 |
* |
246 |
jsr166 |
1.48 |
* @return {@code true} (as specified by {@link Collection#add}) |
247 |
jsr166 |
1.32 |
* @throws NullPointerException if the specified element is null |
248 |
dholmes |
1.6 |
*/ |
249 |
jsr166 |
1.31 |
public boolean add(E e) { |
250 |
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return offer(e); |
251 |
dholmes |
1.6 |
} |
252 |
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253 |
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/** |
254 |
jsr166 |
1.48 |
* Try to CAS head to p. If successful, repoint old head to itself |
255 |
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* as sentinel for succ(), below. |
256 |
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*/ |
257 |
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final void updateHead(Node<E> h, Node<E> p) { |
258 |
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if (h != p && casHead(h, p)) |
259 |
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h.lazySetNext(h); |
260 |
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} |
261 |
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262 |
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/** |
263 |
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* Returns the successor of p, or the head node if p.next has been |
264 |
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* linked to self, which will only be true if traversing with a |
265 |
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* stale pointer that is now off the list. |
266 |
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*/ |
267 |
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final Node<E> succ(Node<E> p) { |
268 |
jsr166 |
1.55 |
Node<E> next = p.next; |
269 |
jsr166 |
1.48 |
return (p == next) ? head : next; |
270 |
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} |
271 |
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|
272 |
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/** |
273 |
jsr166 |
1.32 |
* Inserts the specified element at the tail of this queue. |
274 |
jsr166 |
1.67 |
* As the queue is unbounded, this method will never return {@code false}. |
275 |
dl |
1.17 |
* |
276 |
jsr166 |
1.48 |
* @return {@code true} (as specified by {@link Queue#offer}) |
277 |
jsr166 |
1.32 |
* @throws NullPointerException if the specified element is null |
278 |
dholmes |
1.6 |
*/ |
279 |
jsr166 |
1.31 |
public boolean offer(E e) { |
280 |
jsr166 |
1.55 |
checkNotNull(e); |
281 |
jsr166 |
1.60 |
final Node<E> newNode = new Node<E>(e); |
282 |
jsr166 |
1.58 |
|
283 |
jsr166 |
1.65 |
for (Node<E> t = tail, p = t;;) { |
284 |
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Node<E> q = p.next; |
285 |
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if (q == null) { |
286 |
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// p is last node |
287 |
|
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if (p.casNext(null, newNode)) { |
288 |
jsr166 |
1.63 |
// Successful CAS is the linearization point |
289 |
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// for e to become an element of this queue, |
290 |
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// and for newNode to become "live". |
291 |
jsr166 |
1.65 |
if (p != t) // hop two nodes at a time |
292 |
jsr166 |
1.58 |
casTail(t, newNode); // Failure is OK. |
293 |
jsr166 |
1.48 |
return true; |
294 |
dl |
1.1 |
} |
295 |
jsr166 |
1.65 |
// Lost CAS race to another thread; re-read next |
296 |
dl |
1.1 |
} |
297 |
jsr166 |
1.65 |
else if (p == q) |
298 |
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// We have fallen off list. If tail is unchanged, it |
299 |
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// will also be off-list, in which case we need to |
300 |
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// jump to head, from which all live nodes are always |
301 |
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// reachable. Else the new tail is a better bet. |
302 |
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p = (t != (t = tail)) ? t : head; |
303 |
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else |
304 |
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// Check for tail updates after two hops. |
305 |
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p = (p != t && t != (t = tail)) ? t : q; |
306 |
dl |
1.1 |
} |
307 |
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} |
308 |
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|
309 |
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public E poll() { |
310 |
jsr166 |
1.65 |
restartFromHead: |
311 |
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for (;;) { |
312 |
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for (Node<E> h = head, p = h, q;;) { |
313 |
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E item = p.item; |
314 |
jsr166 |
1.48 |
|
315 |
jsr166 |
1.65 |
if (item != null && p.casItem(item, null)) { |
316 |
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// Successful CAS is the linearization point |
317 |
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// for item to be removed from this queue. |
318 |
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if (p != h) // hop two nodes at a time |
319 |
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updateHead(h, ((q = p.next) != null) ? q : p); |
320 |
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return item; |
321 |
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} |
322 |
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else if ((q = p.next) == null) { |
323 |
|
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updateHead(h, p); |
324 |
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return null; |
325 |
dl |
1.1 |
} |
326 |
jsr166 |
1.65 |
else if (p == q) |
327 |
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continue restartFromHead; |
328 |
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else |
329 |
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p = q; |
330 |
dl |
1.1 |
} |
331 |
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} |
332 |
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} |
333 |
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|
334 |
jsr166 |
1.48 |
public E peek() { |
335 |
jsr166 |
1.65 |
restartFromHead: |
336 |
dl |
1.1 |
for (;;) { |
337 |
jsr166 |
1.65 |
for (Node<E> h = head, p = h, q;;) { |
338 |
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E item = p.item; |
339 |
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if (item != null || (q = p.next) == null) { |
340 |
|
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updateHead(h, p); |
341 |
|
|
return item; |
342 |
|
|
} |
343 |
|
|
else if (p == q) |
344 |
|
|
continue restartFromHead; |
345 |
|
|
else |
346 |
|
|
p = q; |
347 |
dl |
1.1 |
} |
348 |
|
|
} |
349 |
|
|
} |
350 |
|
|
|
351 |
|
|
/** |
352 |
jsr166 |
1.51 |
* Returns the first live (non-deleted) node on list, or null if none. |
353 |
|
|
* This is yet another variant of poll/peek; here returning the |
354 |
|
|
* first node, not element. We could make peek() a wrapper around |
355 |
|
|
* first(), but that would cost an extra volatile read of item, |
356 |
|
|
* and the need to add a retry loop to deal with the possibility |
357 |
|
|
* of losing a race to a concurrent poll(). |
358 |
dl |
1.1 |
*/ |
359 |
dl |
1.23 |
Node<E> first() { |
360 |
jsr166 |
1.65 |
restartFromHead: |
361 |
dl |
1.1 |
for (;;) { |
362 |
jsr166 |
1.65 |
for (Node<E> h = head, p = h, q;;) { |
363 |
|
|
boolean hasItem = (p.item != null); |
364 |
|
|
if (hasItem || (q = p.next) == null) { |
365 |
|
|
updateHead(h, p); |
366 |
|
|
return hasItem ? p : null; |
367 |
|
|
} |
368 |
|
|
else if (p == q) |
369 |
|
|
continue restartFromHead; |
370 |
|
|
else |
371 |
|
|
p = q; |
372 |
dl |
1.1 |
} |
373 |
|
|
} |
374 |
|
|
} |
375 |
|
|
|
376 |
dl |
1.28 |
/** |
377 |
jsr166 |
1.48 |
* Returns {@code true} if this queue contains no elements. |
378 |
dl |
1.28 |
* |
379 |
jsr166 |
1.48 |
* @return {@code true} if this queue contains no elements |
380 |
dl |
1.28 |
*/ |
381 |
dl |
1.1 |
public boolean isEmpty() { |
382 |
|
|
return first() == null; |
383 |
|
|
} |
384 |
|
|
|
385 |
|
|
/** |
386 |
dl |
1.17 |
* Returns the number of elements in this queue. If this queue |
387 |
jsr166 |
1.48 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
388 |
|
|
* {@code Integer.MAX_VALUE}. |
389 |
tim |
1.2 |
* |
390 |
dl |
1.17 |
* <p>Beware that, unlike in most collections, this method is |
391 |
dl |
1.1 |
* <em>NOT</em> a constant-time operation. Because of the |
392 |
|
|
* asynchronous nature of these queues, determining the current |
393 |
jsr166 |
1.55 |
* number of elements requires an O(n) traversal. |
394 |
|
|
* Additionally, if elements are added or removed during execution |
395 |
|
|
* of this method, the returned result may be inaccurate. Thus, |
396 |
|
|
* this method is typically not very useful in concurrent |
397 |
|
|
* applications. |
398 |
dl |
1.17 |
* |
399 |
jsr166 |
1.37 |
* @return the number of elements in this queue |
400 |
tim |
1.2 |
*/ |
401 |
dl |
1.1 |
public int size() { |
402 |
|
|
int count = 0; |
403 |
jsr166 |
1.64 |
for (Node<E> p = first(); p != null; p = succ(p)) |
404 |
|
|
if (p.item != null) |
405 |
|
|
// Collection.size() spec says to max out |
406 |
dl |
1.8 |
if (++count == Integer.MAX_VALUE) |
407 |
|
|
break; |
408 |
dl |
1.1 |
return count; |
409 |
|
|
} |
410 |
|
|
|
411 |
jsr166 |
1.37 |
/** |
412 |
jsr166 |
1.48 |
* Returns {@code true} if this queue contains the specified element. |
413 |
|
|
* More formally, returns {@code true} if and only if this queue contains |
414 |
|
|
* at least one element {@code e} such that {@code o.equals(e)}. |
415 |
jsr166 |
1.37 |
* |
416 |
|
|
* @param o object to be checked for containment in this queue |
417 |
jsr166 |
1.48 |
* @return {@code true} if this queue contains the specified element |
418 |
jsr166 |
1.37 |
*/ |
419 |
dholmes |
1.6 |
public boolean contains(Object o) { |
420 |
|
|
if (o == null) return false; |
421 |
jsr166 |
1.48 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
422 |
jsr166 |
1.64 |
E item = p.item; |
423 |
jsr166 |
1.65 |
if (item != null && o.equals(item)) |
424 |
dl |
1.1 |
return true; |
425 |
|
|
} |
426 |
|
|
return false; |
427 |
|
|
} |
428 |
|
|
|
429 |
jsr166 |
1.37 |
/** |
430 |
|
|
* Removes a single instance of the specified element from this queue, |
431 |
jsr166 |
1.48 |
* if it is present. More formally, removes an element {@code e} such |
432 |
|
|
* that {@code o.equals(e)}, if this queue contains one or more such |
433 |
jsr166 |
1.37 |
* elements. |
434 |
jsr166 |
1.48 |
* Returns {@code true} if this queue contained the specified element |
435 |
jsr166 |
1.37 |
* (or equivalently, if this queue changed as a result of the call). |
436 |
|
|
* |
437 |
|
|
* @param o element to be removed from this queue, if present |
438 |
jsr166 |
1.48 |
* @return {@code true} if this queue changed as a result of the call |
439 |
jsr166 |
1.37 |
*/ |
440 |
dholmes |
1.6 |
public boolean remove(Object o) { |
441 |
|
|
if (o == null) return false; |
442 |
jsr166 |
1.48 |
Node<E> pred = null; |
443 |
|
|
for (Node<E> p = first(); p != null; p = succ(p)) { |
444 |
jsr166 |
1.64 |
E item = p.item; |
445 |
jsr166 |
1.51 |
if (item != null && |
446 |
|
|
o.equals(item) && |
447 |
|
|
p.casItem(item, null)) { |
448 |
jsr166 |
1.48 |
Node<E> next = succ(p); |
449 |
|
|
if (pred != null && next != null) |
450 |
|
|
pred.casNext(p, next); |
451 |
dl |
1.1 |
return true; |
452 |
jsr166 |
1.48 |
} |
453 |
|
|
pred = p; |
454 |
dl |
1.1 |
} |
455 |
|
|
return false; |
456 |
|
|
} |
457 |
tim |
1.2 |
|
458 |
jsr166 |
1.33 |
/** |
459 |
jsr166 |
1.55 |
* Appends all of the elements in the specified collection to the end of |
460 |
|
|
* this queue, in the order that they are returned by the specified |
461 |
jsr166 |
1.56 |
* collection's iterator. Attempts to {@code addAll} of a queue to |
462 |
|
|
* itself result in {@code IllegalArgumentException}. |
463 |
jsr166 |
1.55 |
* |
464 |
|
|
* @param c the elements to be inserted into this queue |
465 |
|
|
* @return {@code true} if this queue changed as a result of the call |
466 |
jsr166 |
1.56 |
* @throws NullPointerException if the specified collection or any |
467 |
|
|
* of its elements are null |
468 |
|
|
* @throws IllegalArgumentException if the collection is this queue |
469 |
jsr166 |
1.55 |
*/ |
470 |
|
|
public boolean addAll(Collection<? extends E> c) { |
471 |
jsr166 |
1.56 |
if (c == this) |
472 |
|
|
// As historically specified in AbstractQueue#addAll |
473 |
|
|
throw new IllegalArgumentException(); |
474 |
|
|
|
475 |
jsr166 |
1.55 |
// Copy c into a private chain of Nodes |
476 |
jsr166 |
1.65 |
Node<E> beginningOfTheEnd = null, last = null; |
477 |
jsr166 |
1.55 |
for (E e : c) { |
478 |
|
|
checkNotNull(e); |
479 |
|
|
Node<E> newNode = new Node<E>(e); |
480 |
jsr166 |
1.65 |
if (beginningOfTheEnd == null) |
481 |
|
|
beginningOfTheEnd = last = newNode; |
482 |
jsr166 |
1.55 |
else { |
483 |
jsr166 |
1.62 |
last.lazySetNext(newNode); |
484 |
jsr166 |
1.55 |
last = newNode; |
485 |
|
|
} |
486 |
|
|
} |
487 |
jsr166 |
1.65 |
if (beginningOfTheEnd == null) |
488 |
jsr166 |
1.55 |
return false; |
489 |
|
|
|
490 |
jsr166 |
1.65 |
// Atomically append the chain at the tail of this collection |
491 |
|
|
for (Node<E> t = tail, p = t;;) { |
492 |
|
|
Node<E> q = p.next; |
493 |
|
|
if (q == null) { |
494 |
|
|
// p is last node |
495 |
|
|
if (p.casNext(null, beginningOfTheEnd)) { |
496 |
|
|
// Successful CAS is the linearization point |
497 |
|
|
// for all elements to be added to this queue. |
498 |
|
|
if (!casTail(t, last)) { |
499 |
jsr166 |
1.55 |
// Try a little harder to update tail, |
500 |
|
|
// since we may be adding many elements. |
501 |
|
|
t = tail; |
502 |
|
|
if (last.next == null) |
503 |
|
|
casTail(t, last); |
504 |
|
|
} |
505 |
|
|
return true; |
506 |
|
|
} |
507 |
jsr166 |
1.65 |
// Lost CAS race to another thread; re-read next |
508 |
jsr166 |
1.55 |
} |
509 |
jsr166 |
1.65 |
else if (p == q) |
510 |
|
|
// We have fallen off list. If tail is unchanged, it |
511 |
|
|
// will also be off-list, in which case we need to |
512 |
|
|
// jump to head, from which all live nodes are always |
513 |
|
|
// reachable. Else the new tail is a better bet. |
514 |
|
|
p = (t != (t = tail)) ? t : head; |
515 |
|
|
else |
516 |
|
|
// Check for tail updates after two hops. |
517 |
|
|
p = (p != t && t != (t = tail)) ? t : q; |
518 |
jsr166 |
1.55 |
} |
519 |
|
|
} |
520 |
|
|
|
521 |
|
|
/** |
522 |
jsr166 |
1.48 |
* Returns an array containing all of the elements in this queue, in |
523 |
|
|
* proper sequence. |
524 |
|
|
* |
525 |
|
|
* <p>The returned array will be "safe" in that no references to it are |
526 |
|
|
* maintained by this queue. (In other words, this method must allocate |
527 |
|
|
* a new array). The caller is thus free to modify the returned array. |
528 |
|
|
* |
529 |
|
|
* <p>This method acts as bridge between array-based and collection-based |
530 |
|
|
* APIs. |
531 |
|
|
* |
532 |
|
|
* @return an array containing all of the elements in this queue |
533 |
|
|
*/ |
534 |
|
|
public Object[] toArray() { |
535 |
|
|
// Use ArrayList to deal with resizing. |
536 |
|
|
ArrayList<E> al = new ArrayList<E>(); |
537 |
|
|
for (Node<E> p = first(); p != null; p = succ(p)) { |
538 |
jsr166 |
1.64 |
E item = p.item; |
539 |
jsr166 |
1.48 |
if (item != null) |
540 |
|
|
al.add(item); |
541 |
|
|
} |
542 |
|
|
return al.toArray(); |
543 |
|
|
} |
544 |
|
|
|
545 |
|
|
/** |
546 |
|
|
* Returns an array containing all of the elements in this queue, in |
547 |
|
|
* proper sequence; the runtime type of the returned array is that of |
548 |
|
|
* the specified array. If the queue fits in the specified array, it |
549 |
|
|
* is returned therein. Otherwise, a new array is allocated with the |
550 |
|
|
* runtime type of the specified array and the size of this queue. |
551 |
|
|
* |
552 |
|
|
* <p>If this queue fits in the specified array with room to spare |
553 |
|
|
* (i.e., the array has more elements than this queue), the element in |
554 |
|
|
* the array immediately following the end of the queue is set to |
555 |
|
|
* {@code null}. |
556 |
|
|
* |
557 |
|
|
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
558 |
|
|
* array-based and collection-based APIs. Further, this method allows |
559 |
|
|
* precise control over the runtime type of the output array, and may, |
560 |
|
|
* under certain circumstances, be used to save allocation costs. |
561 |
|
|
* |
562 |
|
|
* <p>Suppose {@code x} is a queue known to contain only strings. |
563 |
|
|
* The following code can be used to dump the queue into a newly |
564 |
|
|
* allocated array of {@code String}: |
565 |
|
|
* |
566 |
|
|
* <pre> |
567 |
|
|
* String[] y = x.toArray(new String[0]);</pre> |
568 |
|
|
* |
569 |
|
|
* Note that {@code toArray(new Object[0])} is identical in function to |
570 |
|
|
* {@code toArray()}. |
571 |
|
|
* |
572 |
|
|
* @param a the array into which the elements of the queue are to |
573 |
|
|
* be stored, if it is big enough; otherwise, a new array of the |
574 |
|
|
* same runtime type is allocated for this purpose |
575 |
|
|
* @return an array containing all of the elements in this queue |
576 |
|
|
* @throws ArrayStoreException if the runtime type of the specified array |
577 |
|
|
* is not a supertype of the runtime type of every element in |
578 |
|
|
* this queue |
579 |
|
|
* @throws NullPointerException if the specified array is null |
580 |
|
|
*/ |
581 |
|
|
@SuppressWarnings("unchecked") |
582 |
|
|
public <T> T[] toArray(T[] a) { |
583 |
|
|
// try to use sent-in array |
584 |
|
|
int k = 0; |
585 |
|
|
Node<E> p; |
586 |
|
|
for (p = first(); p != null && k < a.length; p = succ(p)) { |
587 |
jsr166 |
1.64 |
E item = p.item; |
588 |
jsr166 |
1.48 |
if (item != null) |
589 |
|
|
a[k++] = (T)item; |
590 |
|
|
} |
591 |
|
|
if (p == null) { |
592 |
|
|
if (k < a.length) |
593 |
|
|
a[k] = null; |
594 |
|
|
return a; |
595 |
|
|
} |
596 |
|
|
|
597 |
|
|
// If won't fit, use ArrayList version |
598 |
|
|
ArrayList<E> al = new ArrayList<E>(); |
599 |
|
|
for (Node<E> q = first(); q != null; q = succ(q)) { |
600 |
jsr166 |
1.64 |
E item = q.item; |
601 |
jsr166 |
1.48 |
if (item != null) |
602 |
|
|
al.add(item); |
603 |
|
|
} |
604 |
|
|
return al.toArray(a); |
605 |
|
|
} |
606 |
|
|
|
607 |
|
|
/** |
608 |
dholmes |
1.7 |
* Returns an iterator over the elements in this queue in proper sequence. |
609 |
jsr166 |
1.55 |
* The elements will be returned in order from first (head) to last (tail). |
610 |
|
|
* |
611 |
|
|
* <p>The returned {@code Iterator} is a "weakly consistent" iterator that |
612 |
jsr166 |
1.52 |
* will never throw {@link java.util.ConcurrentModificationException |
613 |
|
|
* ConcurrentModificationException}, |
614 |
dl |
1.9 |
* and guarantees to traverse elements as they existed upon |
615 |
|
|
* construction of the iterator, and may (but is not guaranteed to) |
616 |
|
|
* reflect any modifications subsequent to construction. |
617 |
dholmes |
1.7 |
* |
618 |
jsr166 |
1.33 |
* @return an iterator over the elements in this queue in proper sequence |
619 |
dholmes |
1.7 |
*/ |
620 |
dl |
1.1 |
public Iterator<E> iterator() { |
621 |
|
|
return new Itr(); |
622 |
|
|
} |
623 |
|
|
|
624 |
|
|
private class Itr implements Iterator<E> { |
625 |
|
|
/** |
626 |
|
|
* Next node to return item for. |
627 |
|
|
*/ |
628 |
dl |
1.23 |
private Node<E> nextNode; |
629 |
dl |
1.1 |
|
630 |
tim |
1.2 |
/** |
631 |
dl |
1.1 |
* nextItem holds on to item fields because once we claim |
632 |
|
|
* that an element exists in hasNext(), we must return it in |
633 |
|
|
* the following next() call even if it was in the process of |
634 |
|
|
* being removed when hasNext() was called. |
635 |
jsr166 |
1.29 |
*/ |
636 |
dl |
1.1 |
private E nextItem; |
637 |
|
|
|
638 |
|
|
/** |
639 |
|
|
* Node of the last returned item, to support remove. |
640 |
|
|
*/ |
641 |
dl |
1.23 |
private Node<E> lastRet; |
642 |
dl |
1.1 |
|
643 |
tim |
1.2 |
Itr() { |
644 |
dl |
1.1 |
advance(); |
645 |
|
|
} |
646 |
tim |
1.2 |
|
647 |
dl |
1.1 |
/** |
648 |
dl |
1.26 |
* Moves to next valid node and returns item to return for |
649 |
|
|
* next(), or null if no such. |
650 |
dl |
1.1 |
*/ |
651 |
tim |
1.2 |
private E advance() { |
652 |
dl |
1.1 |
lastRet = nextNode; |
653 |
dl |
1.22 |
E x = nextItem; |
654 |
dl |
1.1 |
|
655 |
jsr166 |
1.48 |
Node<E> pred, p; |
656 |
|
|
if (nextNode == null) { |
657 |
|
|
p = first(); |
658 |
|
|
pred = null; |
659 |
|
|
} else { |
660 |
|
|
pred = nextNode; |
661 |
|
|
p = succ(nextNode); |
662 |
|
|
} |
663 |
|
|
|
664 |
dl |
1.1 |
for (;;) { |
665 |
|
|
if (p == null) { |
666 |
|
|
nextNode = null; |
667 |
|
|
nextItem = null; |
668 |
|
|
return x; |
669 |
|
|
} |
670 |
jsr166 |
1.64 |
E item = p.item; |
671 |
dl |
1.1 |
if (item != null) { |
672 |
|
|
nextNode = p; |
673 |
|
|
nextItem = item; |
674 |
|
|
return x; |
675 |
jsr166 |
1.48 |
} else { |
676 |
|
|
// skip over nulls |
677 |
|
|
Node<E> next = succ(p); |
678 |
|
|
if (pred != null && next != null) |
679 |
|
|
pred.casNext(p, next); |
680 |
|
|
p = next; |
681 |
|
|
} |
682 |
dl |
1.1 |
} |
683 |
|
|
} |
684 |
tim |
1.2 |
|
685 |
dl |
1.1 |
public boolean hasNext() { |
686 |
|
|
return nextNode != null; |
687 |
|
|
} |
688 |
tim |
1.2 |
|
689 |
dl |
1.1 |
public E next() { |
690 |
|
|
if (nextNode == null) throw new NoSuchElementException(); |
691 |
|
|
return advance(); |
692 |
|
|
} |
693 |
tim |
1.2 |
|
694 |
dl |
1.1 |
public void remove() { |
695 |
dl |
1.23 |
Node<E> l = lastRet; |
696 |
dl |
1.1 |
if (l == null) throw new IllegalStateException(); |
697 |
|
|
// rely on a future traversal to relink. |
698 |
jsr166 |
1.64 |
l.item = null; |
699 |
dl |
1.1 |
lastRet = null; |
700 |
|
|
} |
701 |
|
|
} |
702 |
|
|
|
703 |
|
|
/** |
704 |
jsr166 |
1.55 |
* Saves the state to a stream (that is, serializes it). |
705 |
dl |
1.1 |
* |
706 |
jsr166 |
1.48 |
* @serialData All of the elements (each an {@code E}) in |
707 |
dl |
1.1 |
* the proper order, followed by a null |
708 |
|
|
* @param s the stream |
709 |
|
|
*/ |
710 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
711 |
|
|
throws java.io.IOException { |
712 |
|
|
|
713 |
|
|
// Write out any hidden stuff |
714 |
|
|
s.defaultWriteObject(); |
715 |
tim |
1.2 |
|
716 |
dl |
1.1 |
// Write out all elements in the proper order. |
717 |
jsr166 |
1.48 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
718 |
jsr166 |
1.64 |
Object item = p.item; |
719 |
dl |
1.1 |
if (item != null) |
720 |
|
|
s.writeObject(item); |
721 |
|
|
} |
722 |
|
|
|
723 |
|
|
// Use trailing null as sentinel |
724 |
|
|
s.writeObject(null); |
725 |
|
|
} |
726 |
|
|
|
727 |
|
|
/** |
728 |
jsr166 |
1.55 |
* Reconstitutes the instance from a stream (that is, deserializes it). |
729 |
dl |
1.1 |
* @param s the stream |
730 |
|
|
*/ |
731 |
|
|
private void readObject(java.io.ObjectInputStream s) |
732 |
|
|
throws java.io.IOException, ClassNotFoundException { |
733 |
tim |
1.2 |
s.defaultReadObject(); |
734 |
jsr166 |
1.55 |
|
735 |
|
|
// Read in elements until trailing null sentinel found |
736 |
|
|
Node<E> h = null, t = null; |
737 |
|
|
Object item; |
738 |
|
|
while ((item = s.readObject()) != null) { |
739 |
jsr166 |
1.48 |
@SuppressWarnings("unchecked") |
740 |
jsr166 |
1.55 |
Node<E> newNode = new Node<E>((E) item); |
741 |
|
|
if (h == null) |
742 |
|
|
h = t = newNode; |
743 |
|
|
else { |
744 |
jsr166 |
1.62 |
t.lazySetNext(newNode); |
745 |
jsr166 |
1.55 |
t = newNode; |
746 |
|
|
} |
747 |
dl |
1.1 |
} |
748 |
jsr166 |
1.55 |
if (h == null) |
749 |
|
|
h = t = new Node<E>(null); |
750 |
|
|
head = h; |
751 |
|
|
tail = t; |
752 |
|
|
} |
753 |
|
|
|
754 |
|
|
/** |
755 |
|
|
* Throws NullPointerException if argument is null. |
756 |
|
|
* |
757 |
|
|
* @param v the element |
758 |
|
|
*/ |
759 |
|
|
private static void checkNotNull(Object v) { |
760 |
|
|
if (v == null) |
761 |
|
|
throw new NullPointerException(); |
762 |
dl |
1.1 |
} |
763 |
|
|
|
764 |
jsr166 |
1.48 |
// Unsafe mechanics |
765 |
jsr166 |
1.50 |
|
766 |
|
|
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
767 |
|
|
private static final long headOffset = |
768 |
|
|
objectFieldOffset(UNSAFE, "head", ConcurrentLinkedQueue.class); |
769 |
|
|
private static final long tailOffset = |
770 |
|
|
objectFieldOffset(UNSAFE, "tail", ConcurrentLinkedQueue.class); |
771 |
|
|
|
772 |
|
|
private boolean casTail(Node<E> cmp, Node<E> val) { |
773 |
|
|
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
774 |
|
|
} |
775 |
|
|
|
776 |
|
|
private boolean casHead(Node<E> cmp, Node<E> val) { |
777 |
|
|
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
778 |
jsr166 |
1.48 |
} |
779 |
|
|
|
780 |
jsr166 |
1.50 |
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
781 |
|
|
String field, Class<?> klazz) { |
782 |
jsr166 |
1.48 |
try { |
783 |
jsr166 |
1.50 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
784 |
jsr166 |
1.48 |
} catch (NoSuchFieldException e) { |
785 |
jsr166 |
1.50 |
// Convert Exception to corresponding Error |
786 |
|
|
NoSuchFieldError error = new NoSuchFieldError(field); |
787 |
jsr166 |
1.48 |
error.initCause(e); |
788 |
|
|
throw error; |
789 |
|
|
} |
790 |
|
|
} |
791 |
dl |
1.1 |
} |