1 |
dl |
1.2 |
/* |
2 |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
3 |
dl |
1.29 |
* Expert Group and released to the public domain, as explained at |
4 |
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* http://creativecommons.org/licenses/publicdomain |
5 |
dl |
1.2 |
*/ |
6 |
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7 |
tim |
1.1 |
package java.util.concurrent; |
8 |
dl |
1.8 |
import java.util.concurrent.locks.*; |
9 |
tim |
1.1 |
import java.util.*; |
10 |
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11 |
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/** |
12 |
dl |
1.18 |
* A {@linkplain BlockingQueue blocking queue} in which each |
13 |
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* <tt>put</tt> must wait for a <tt>take</tt>, and vice versa. A |
14 |
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* synchronous queue does not have any internal capacity - in |
15 |
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* particular it does not have a capacity of one. You cannot |
16 |
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* <tt>peek</tt> at a synchronous queue because an element is only |
17 |
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* present when you try to take it; you cannot add an element (using |
18 |
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* any method) unless another thread is trying to remove it; you |
19 |
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* cannot iterate as there is nothing to iterate. The <em>head</em> |
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* of the queue is the element that the first queued thread is trying |
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* to add to the queue; if there are no queued threads then no element |
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* is being added and the head is <tt>null</tt>. For purposes of |
23 |
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* other <tt>Collection</tt> methods (for example <tt>contains</tt>), |
24 |
dl |
1.19 |
* a <tt>SynchronousQueue</tt> acts as an empty collection. This |
25 |
dl |
1.18 |
* queue does not permit <tt>null</tt> elements. |
26 |
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* |
27 |
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* <p>Synchronous queues are similar to rendezvous channels used in |
28 |
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* CSP and Ada. They are well suited for handoff designs, in which an |
29 |
dl |
1.30 |
* object running in one thread must sync up with an object running |
30 |
dl |
1.18 |
* in another thread in order to hand it some information, event, or |
31 |
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* task. |
32 |
dl |
1.19 |
* <p>This class implements all of the <em>optional</em> methods |
33 |
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* of the {@link Collection} and {@link Iterator} interfaces. |
34 |
dl |
1.6 |
* @since 1.5 |
35 |
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* @author Doug Lea |
36 |
dl |
1.24 |
* @param <E> the type of elements held in this collection |
37 |
dl |
1.23 |
*/ |
38 |
dl |
1.2 |
public class SynchronousQueue<E> extends AbstractQueue<E> |
39 |
tim |
1.1 |
implements BlockingQueue<E>, java.io.Serializable { |
40 |
dl |
1.15 |
private static final long serialVersionUID = -3223113410248163686L; |
41 |
tim |
1.1 |
|
42 |
dl |
1.2 |
/* |
43 |
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This implementation divides actions into two cases for puts: |
44 |
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|
45 |
tim |
1.10 |
* An arriving putter that does not already have a waiting taker |
46 |
dl |
1.2 |
creates a node holding item, and then waits for a taker to take it. |
47 |
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* An arriving putter that does already have a waiting taker fills |
48 |
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the slot node created by the taker, and notifies it to continue. |
49 |
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|
50 |
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And symmetrically, two for takes: |
51 |
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|
52 |
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* An arriving taker that does not already have a waiting putter |
53 |
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creates an empty slot node, and then waits for a putter to fill it. |
54 |
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* An arriving taker that does already have a waiting putter takes |
55 |
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item from the node created by the putter, and notifies it to continue. |
56 |
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57 |
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This requires keeping two simple queues: waitingPuts and waitingTakes. |
58 |
tim |
1.10 |
|
59 |
dl |
1.2 |
When a put or take waiting for the actions of its counterpart |
60 |
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aborts due to interruption or timeout, it marks the node |
61 |
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it created as "CANCELLED", which causes its counterpart to retry |
62 |
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the entire put or take sequence. |
63 |
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*/ |
64 |
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65 |
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|
66 |
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/* |
67 |
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* Note that all fields are transient final, so there is |
68 |
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* no explicit serialization code. |
69 |
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*/ |
70 |
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71 |
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private transient final WaitQueue waitingPuts = new WaitQueue(); |
72 |
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private transient final WaitQueue waitingTakes = new WaitQueue(); |
73 |
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private transient final ReentrantLock qlock = new ReentrantLock(); |
74 |
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75 |
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/** |
76 |
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* Nodes each maintain an item and handle waits and signals for |
77 |
dl |
1.31 |
* getting and setting it. The class extends |
78 |
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* AbstractQueuedSynchronizer to manage blocking, using AQS state |
79 |
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* 0 for waiting, 1 for ack, -1 for cancelled. |
80 |
dl |
1.2 |
*/ |
81 |
dl |
1.31 |
private static final class Node extends AbstractQueuedSynchronizer { |
82 |
dl |
1.35 |
/** Synchronization state value representing that node acked */ |
83 |
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private static final int ACK = 1; |
84 |
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/** Synchronization state value representing that node cancelled */ |
85 |
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private static final int CANCEL = -1; |
86 |
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|
87 |
dl |
1.6 |
/** The item being transferred */ |
88 |
dl |
1.2 |
Object item; |
89 |
dl |
1.6 |
/** Next node in wait queue */ |
90 |
dl |
1.2 |
Node next; |
91 |
dl |
1.35 |
|
92 |
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/** Create node with initial item */ |
93 |
dl |
1.31 |
Node(Object x) { item = x; } |
94 |
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95 |
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/** |
96 |
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* Implements AQS base acquire to succeed if not in WAITING state |
97 |
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*/ |
98 |
dl |
1.35 |
protected boolean tryAcquireExclusive(boolean b, int ignore) { |
99 |
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return getState() != 0; |
100 |
dl |
1.31 |
} |
101 |
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102 |
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/** |
103 |
dl |
1.34 |
* Implements AQS base release to signal if state changed |
104 |
dl |
1.31 |
*/ |
105 |
dl |
1.35 |
protected boolean tryReleaseExclusive(int newState) { |
106 |
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return compareAndSetState(0, newState); |
107 |
dl |
1.31 |
} |
108 |
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109 |
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/** |
110 |
dl |
1.35 |
* Take item and null out field (for sake of GC) |
111 |
dl |
1.31 |
*/ |
112 |
dl |
1.35 |
private Object extract() { |
113 |
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Object x = item; |
114 |
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item = null; |
115 |
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return x; |
116 |
dl |
1.31 |
} |
117 |
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118 |
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/** |
119 |
dl |
1.35 |
* Try to cancel on interrupt; if so rethrowing, |
120 |
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* else setting interrupt state |
121 |
dl |
1.31 |
*/ |
122 |
dl |
1.35 |
private void checkCancellationOnInterrupt(InterruptedException ie) |
123 |
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throws InterruptedException { |
124 |
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if (releaseExclusive(CANCEL)) |
125 |
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throw ie; |
126 |
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Thread.currentThread().interrupt(); |
127 |
dl |
1.31 |
} |
128 |
dl |
1.2 |
|
129 |
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/** |
130 |
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* Fill in the slot created by the taker and signal taker to |
131 |
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* continue. |
132 |
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*/ |
133 |
dl |
1.31 |
boolean setItem(Object x) { |
134 |
dl |
1.35 |
item = x; // can place in slot even if cancelled |
135 |
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return releaseExclusive(ACK); |
136 |
dl |
1.2 |
} |
137 |
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138 |
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/** |
139 |
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* Remove item from slot created by putter and signal putter |
140 |
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* to continue. |
141 |
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*/ |
142 |
dl |
1.31 |
Object getItem() { |
143 |
dl |
1.35 |
return (releaseExclusive(ACK))? extract() : null; |
144 |
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} |
145 |
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146 |
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/** |
147 |
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* Wait for a taker to take item placed by putter. |
148 |
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*/ |
149 |
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void waitForTake() throws InterruptedException { |
150 |
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try { |
151 |
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acquireExclusiveInterruptibly(0); |
152 |
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} catch (InterruptedException ie) { |
153 |
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checkCancellationOnInterrupt(ie); |
154 |
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} |
155 |
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} |
156 |
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157 |
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/** |
158 |
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* Wait for a putter to put item placed by taker. |
159 |
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*/ |
160 |
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Object waitForPut() throws InterruptedException { |
161 |
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try { |
162 |
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acquireExclusiveInterruptibly(0); |
163 |
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} catch (InterruptedException ie) { |
164 |
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checkCancellationOnInterrupt(ie); |
165 |
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} |
166 |
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return extract(); |
167 |
dl |
1.31 |
} |
168 |
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169 |
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/** |
170 |
dl |
1.33 |
* Wait for a taker to take item placed by putter or time out. |
171 |
dl |
1.31 |
*/ |
172 |
dl |
1.35 |
boolean waitForTake(long nanos) throws InterruptedException { |
173 |
dl |
1.2 |
try { |
174 |
dl |
1.35 |
if (!acquireExclusiveTimed(0, nanos) && |
175 |
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releaseExclusive(CANCEL)) |
176 |
dl |
1.33 |
return false; |
177 |
dl |
1.31 |
} catch (InterruptedException ie) { |
178 |
dl |
1.35 |
checkCancellationOnInterrupt(ie); |
179 |
dl |
1.2 |
} |
180 |
dl |
1.35 |
return true; |
181 |
dl |
1.2 |
} |
182 |
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183 |
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/** |
184 |
dl |
1.33 |
* Wait for a putter to put item placed by taker, or time out. |
185 |
dl |
1.31 |
*/ |
186 |
dl |
1.35 |
Object waitForPut(long nanos) throws InterruptedException { |
187 |
dl |
1.31 |
try { |
188 |
dl |
1.35 |
if (!acquireExclusiveTimed(0, nanos) && |
189 |
|
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releaseExclusive(CANCEL)) |
190 |
dl |
1.33 |
return null; |
191 |
dl |
1.31 |
} catch (InterruptedException ie) { |
192 |
dl |
1.35 |
checkCancellationOnInterrupt(ie); |
193 |
dl |
1.2 |
} |
194 |
dl |
1.35 |
return extract(); |
195 |
dl |
1.2 |
} |
196 |
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|
197 |
|
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} |
198 |
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|
199 |
|
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/** |
200 |
|
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* Simple FIFO queue class to hold waiting puts/takes. |
201 |
|
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**/ |
202 |
|
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private static class WaitQueue<E> { |
203 |
|
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Node head; |
204 |
|
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Node last; |
205 |
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|
206 |
tim |
1.10 |
Node enq(Object x) { |
207 |
dl |
1.2 |
Node p = new Node(x); |
208 |
tim |
1.10 |
if (last == null) |
209 |
dl |
1.2 |
last = head = p; |
210 |
tim |
1.10 |
else |
211 |
dl |
1.2 |
last = last.next = p; |
212 |
|
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return p; |
213 |
|
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} |
214 |
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|
215 |
|
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Node deq() { |
216 |
|
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Node p = head; |
217 |
dl |
1.35 |
if (p != null) { |
218 |
|
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if ((head = p.next) == null) |
219 |
|
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last = null; |
220 |
|
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p.next = null; |
221 |
|
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} |
222 |
dl |
1.2 |
return p; |
223 |
|
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} |
224 |
|
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} |
225 |
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|
226 |
|
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/** |
227 |
dl |
1.35 |
* Creates a <tt>SynchronousQueue</tt>. |
228 |
tim |
1.10 |
*/ |
229 |
dl |
1.35 |
public SynchronousQueue() {} |
230 |
dl |
1.2 |
|
231 |
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|
232 |
|
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/** |
233 |
dl |
1.35 |
* Adds the specified element to this queue, waiting if necessary for |
234 |
|
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* another thread to receive it. |
235 |
|
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* @param o the element to add |
236 |
|
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* @throws InterruptedException if interrupted while waiting. |
237 |
|
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* @throws NullPointerException if the specified element is <tt>null</tt>. |
238 |
tim |
1.10 |
*/ |
239 |
dl |
1.35 |
public void put(E o) throws InterruptedException { |
240 |
|
|
if (o == null) throw new NullPointerException(); |
241 |
|
|
final ReentrantLock qlock = this.qlock; |
242 |
|
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|
243 |
dl |
1.2 |
for (;;) { |
244 |
|
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Node node; |
245 |
|
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boolean mustWait; |
246 |
|
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qlock.lockInterruptibly(); |
247 |
|
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try { |
248 |
dl |
1.35 |
node = waitingTakes.deq(); |
249 |
dl |
1.2 |
if ( (mustWait = (node == null)) ) |
250 |
dl |
1.35 |
node = waitingPuts.enq(o); |
251 |
tim |
1.14 |
} finally { |
252 |
dl |
1.2 |
qlock.unlock(); |
253 |
|
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} |
254 |
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|
255 |
dl |
1.31 |
if (mustWait) { |
256 |
dl |
1.35 |
node.waitForTake(); |
257 |
|
|
return; |
258 |
dl |
1.2 |
} |
259 |
|
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|
260 |
dl |
1.35 |
else if (node.setItem(o)) |
261 |
|
|
return; |
262 |
dl |
1.2 |
|
263 |
dl |
1.35 |
// else taker cancelled, so retry |
264 |
|
|
} |
265 |
tim |
1.1 |
} |
266 |
|
|
|
267 |
dholmes |
1.11 |
/** |
268 |
dl |
1.20 |
* Inserts the specified element into this queue, waiting if necessary |
269 |
dl |
1.18 |
* up to the specified wait time for another thread to receive it. |
270 |
|
|
* @param o the element to add |
271 |
|
|
* @param timeout how long to wait before giving up, in units of |
272 |
|
|
* <tt>unit</tt> |
273 |
|
|
* @param unit a <tt>TimeUnit</tt> determining how to interpret the |
274 |
|
|
* <tt>timeout</tt> parameter |
275 |
dholmes |
1.11 |
* @return <tt>true</tt> if successful, or <tt>false</tt> if |
276 |
|
|
* the specified waiting time elapses before a taker appears. |
277 |
dl |
1.18 |
* @throws InterruptedException if interrupted while waiting. |
278 |
|
|
* @throws NullPointerException if the specified element is <tt>null</tt>. |
279 |
dholmes |
1.11 |
*/ |
280 |
dl |
1.18 |
public boolean offer(E o, long timeout, TimeUnit unit) throws InterruptedException { |
281 |
dl |
1.35 |
if (o == null) throw new NullPointerException(); |
282 |
|
|
long nanos = unit.toNanos(timeout); |
283 |
|
|
final ReentrantLock qlock = this.qlock; |
284 |
|
|
for (;;) { |
285 |
|
|
Node node; |
286 |
|
|
boolean mustWait; |
287 |
|
|
qlock.lockInterruptibly(); |
288 |
|
|
try { |
289 |
|
|
node = waitingTakes.deq(); |
290 |
|
|
if ( (mustWait = (node == null)) ) |
291 |
|
|
node = waitingPuts.enq(o); |
292 |
|
|
} finally { |
293 |
|
|
qlock.unlock(); |
294 |
|
|
} |
295 |
|
|
|
296 |
|
|
if (mustWait) |
297 |
|
|
return node.waitForTake(nanos); |
298 |
|
|
|
299 |
|
|
else if (node.setItem(o)) |
300 |
|
|
return true; |
301 |
|
|
|
302 |
|
|
// else taker cancelled, so retry |
303 |
|
|
} |
304 |
|
|
|
305 |
tim |
1.1 |
} |
306 |
|
|
|
307 |
dl |
1.2 |
|
308 |
dholmes |
1.11 |
/** |
309 |
|
|
* Retrieves and removes the head of this queue, waiting if necessary |
310 |
|
|
* for another thread to insert it. |
311 |
|
|
* @return the head of this queue |
312 |
|
|
*/ |
313 |
dl |
1.2 |
public E take() throws InterruptedException { |
314 |
dl |
1.35 |
final ReentrantLock qlock = this.qlock; |
315 |
|
|
for (;;) { |
316 |
|
|
Node node; |
317 |
|
|
boolean mustWait; |
318 |
|
|
|
319 |
|
|
qlock.lockInterruptibly(); |
320 |
|
|
try { |
321 |
|
|
node = waitingPuts.deq(); |
322 |
|
|
if ( (mustWait = (node == null)) ) |
323 |
|
|
node = waitingTakes.enq(null); |
324 |
|
|
} finally { |
325 |
|
|
qlock.unlock(); |
326 |
|
|
} |
327 |
|
|
|
328 |
|
|
if (mustWait) |
329 |
|
|
return (E)node.waitForPut(); |
330 |
|
|
|
331 |
|
|
else { |
332 |
|
|
Object x = node.getItem(); |
333 |
|
|
if (x != null) |
334 |
|
|
return (E)x; |
335 |
|
|
// else cancelled, so retry |
336 |
|
|
} |
337 |
|
|
} |
338 |
tim |
1.1 |
} |
339 |
dl |
1.2 |
|
340 |
dholmes |
1.11 |
/** |
341 |
|
|
* Retrieves and removes the head of this queue, waiting |
342 |
|
|
* if necessary up to the specified wait time, for another thread |
343 |
|
|
* to insert it. |
344 |
dl |
1.18 |
* @param timeout how long to wait before giving up, in units of |
345 |
|
|
* <tt>unit</tt> |
346 |
|
|
* @param unit a <tt>TimeUnit</tt> determining how to interpret the |
347 |
|
|
* <tt>timeout</tt> parameter |
348 |
|
|
* @return the head of this queue, or <tt>null</tt> if the |
349 |
|
|
* specified waiting time elapses before an element is present. |
350 |
|
|
* @throws InterruptedException if interrupted while waiting. |
351 |
dholmes |
1.11 |
*/ |
352 |
dl |
1.2 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
353 |
dl |
1.35 |
long nanos = unit.toNanos(timeout); |
354 |
|
|
final ReentrantLock qlock = this.qlock; |
355 |
|
|
|
356 |
|
|
for (;;) { |
357 |
|
|
Node node; |
358 |
|
|
boolean mustWait; |
359 |
|
|
|
360 |
|
|
qlock.lockInterruptibly(); |
361 |
|
|
try { |
362 |
|
|
node = waitingPuts.deq(); |
363 |
|
|
if ( (mustWait = (node == null)) ) |
364 |
|
|
node = waitingTakes.enq(null); |
365 |
|
|
} finally { |
366 |
|
|
qlock.unlock(); |
367 |
|
|
} |
368 |
|
|
|
369 |
|
|
if (mustWait) |
370 |
|
|
return (E) node.waitForPut(nanos); |
371 |
|
|
|
372 |
|
|
else { |
373 |
|
|
Object x = node.getItem(); |
374 |
|
|
if (x != null) |
375 |
|
|
return (E)x; |
376 |
|
|
// else cancelled, so retry |
377 |
|
|
} |
378 |
|
|
} |
379 |
tim |
1.1 |
} |
380 |
dl |
1.2 |
|
381 |
|
|
// Untimed nonblocking versions |
382 |
|
|
|
383 |
dl |
1.18 |
/** |
384 |
dl |
1.20 |
* Inserts the specified element into this queue, if another thread is |
385 |
dl |
1.18 |
* waiting to receive it. |
386 |
|
|
* |
387 |
|
|
* @param o the element to add. |
388 |
|
|
* @return <tt>true</tt> if it was possible to add the element to |
389 |
|
|
* this queue, else <tt>false</tt> |
390 |
|
|
* @throws NullPointerException if the specified element is <tt>null</tt> |
391 |
|
|
*/ |
392 |
dholmes |
1.11 |
public boolean offer(E o) { |
393 |
|
|
if (o == null) throw new NullPointerException(); |
394 |
dl |
1.27 |
final ReentrantLock qlock = this.qlock; |
395 |
tim |
1.10 |
|
396 |
|
|
for (;;) { |
397 |
tim |
1.26 |
Node node; |
398 |
dl |
1.2 |
qlock.lock(); |
399 |
|
|
try { |
400 |
|
|
node = waitingTakes.deq(); |
401 |
tim |
1.14 |
} finally { |
402 |
dl |
1.2 |
qlock.unlock(); |
403 |
|
|
} |
404 |
|
|
if (node == null) |
405 |
|
|
return false; |
406 |
tim |
1.10 |
|
407 |
dl |
1.31 |
else if (node.setItem(o)) |
408 |
dl |
1.2 |
return true; |
409 |
|
|
// else retry |
410 |
|
|
} |
411 |
tim |
1.1 |
} |
412 |
dl |
1.2 |
|
413 |
dl |
1.18 |
/** |
414 |
|
|
* Retrieves and removes the head of this queue, if another thread |
415 |
|
|
* is currently making an element available. |
416 |
|
|
* |
417 |
|
|
* @return the head of this queue, or <tt>null</tt> if no |
418 |
|
|
* element is available. |
419 |
|
|
*/ |
420 |
dl |
1.2 |
public E poll() { |
421 |
dl |
1.27 |
final ReentrantLock qlock = this.qlock; |
422 |
dl |
1.2 |
for (;;) { |
423 |
|
|
Node node; |
424 |
|
|
qlock.lock(); |
425 |
|
|
try { |
426 |
|
|
node = waitingPuts.deq(); |
427 |
tim |
1.14 |
} finally { |
428 |
dl |
1.2 |
qlock.unlock(); |
429 |
|
|
} |
430 |
|
|
if (node == null) |
431 |
|
|
return null; |
432 |
|
|
|
433 |
|
|
else { |
434 |
dl |
1.31 |
Object x = node.getItem(); |
435 |
dl |
1.2 |
if (x != null) |
436 |
|
|
return (E)x; |
437 |
|
|
// else retry |
438 |
|
|
} |
439 |
|
|
} |
440 |
tim |
1.1 |
} |
441 |
dl |
1.2 |
|
442 |
dl |
1.5 |
/** |
443 |
dholmes |
1.11 |
* Always returns <tt>true</tt>. |
444 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
445 |
|
|
* @return <tt>true</tt> |
446 |
dl |
1.5 |
*/ |
447 |
|
|
public boolean isEmpty() { |
448 |
|
|
return true; |
449 |
|
|
} |
450 |
|
|
|
451 |
|
|
/** |
452 |
dholmes |
1.11 |
* Always returns zero. |
453 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
454 |
dl |
1.5 |
* @return zero. |
455 |
|
|
*/ |
456 |
|
|
public int size() { |
457 |
|
|
return 0; |
458 |
tim |
1.1 |
} |
459 |
dl |
1.2 |
|
460 |
dl |
1.5 |
/** |
461 |
dholmes |
1.11 |
* Always returns zero. |
462 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
463 |
dl |
1.5 |
* @return zero. |
464 |
|
|
*/ |
465 |
|
|
public int remainingCapacity() { |
466 |
|
|
return 0; |
467 |
|
|
} |
468 |
|
|
|
469 |
|
|
/** |
470 |
dholmes |
1.11 |
* Does nothing. |
471 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
472 |
|
|
*/ |
473 |
|
|
public void clear() {} |
474 |
|
|
|
475 |
|
|
/** |
476 |
|
|
* Always returns <tt>false</tt>. |
477 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
478 |
dl |
1.18 |
* @param o the element |
479 |
dholmes |
1.11 |
* @return <tt>false</tt> |
480 |
|
|
*/ |
481 |
|
|
public boolean contains(Object o) { |
482 |
|
|
return false; |
483 |
|
|
} |
484 |
|
|
|
485 |
|
|
/** |
486 |
dl |
1.18 |
* Always returns <tt>false</tt>. |
487 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
488 |
|
|
* |
489 |
|
|
* @param o the element to remove |
490 |
|
|
* @return <tt>false</tt> |
491 |
|
|
*/ |
492 |
|
|
public boolean remove(Object o) { |
493 |
|
|
return false; |
494 |
|
|
} |
495 |
|
|
|
496 |
|
|
/** |
497 |
dl |
1.16 |
* Returns <tt>false</tt> unless given collection is empty. |
498 |
dholmes |
1.11 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
499 |
dl |
1.18 |
* @param c the collection |
500 |
dl |
1.16 |
* @return <tt>false</tt> unless given collection is empty |
501 |
dholmes |
1.11 |
*/ |
502 |
dl |
1.12 |
public boolean containsAll(Collection<?> c) { |
503 |
dl |
1.16 |
return c.isEmpty(); |
504 |
dholmes |
1.11 |
} |
505 |
|
|
|
506 |
|
|
/** |
507 |
|
|
* Always returns <tt>false</tt>. |
508 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
509 |
dl |
1.18 |
* @param c the collection |
510 |
dholmes |
1.11 |
* @return <tt>false</tt> |
511 |
|
|
*/ |
512 |
dl |
1.12 |
public boolean removeAll(Collection<?> c) { |
513 |
dholmes |
1.11 |
return false; |
514 |
|
|
} |
515 |
|
|
|
516 |
|
|
/** |
517 |
|
|
* Always returns <tt>false</tt>. |
518 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
519 |
dl |
1.18 |
* @param c the collection |
520 |
dholmes |
1.11 |
* @return <tt>false</tt> |
521 |
|
|
*/ |
522 |
dl |
1.12 |
public boolean retainAll(Collection<?> c) { |
523 |
dholmes |
1.11 |
return false; |
524 |
|
|
} |
525 |
|
|
|
526 |
|
|
/** |
527 |
|
|
* Always returns <tt>null</tt>. |
528 |
|
|
* A <tt>SynchronousQueue</tt> does not return elements |
529 |
dl |
1.5 |
* unless actively waited on. |
530 |
dholmes |
1.11 |
* @return <tt>null</tt> |
531 |
dl |
1.5 |
*/ |
532 |
|
|
public E peek() { |
533 |
|
|
return null; |
534 |
|
|
} |
535 |
|
|
|
536 |
|
|
|
537 |
|
|
static class EmptyIterator<E> implements Iterator<E> { |
538 |
dl |
1.2 |
public boolean hasNext() { |
539 |
|
|
return false; |
540 |
|
|
} |
541 |
|
|
public E next() { |
542 |
|
|
throw new NoSuchElementException(); |
543 |
|
|
} |
544 |
|
|
public void remove() { |
545 |
dl |
1.17 |
throw new IllegalStateException(); |
546 |
dl |
1.2 |
} |
547 |
tim |
1.1 |
} |
548 |
dl |
1.2 |
|
549 |
dl |
1.5 |
/** |
550 |
dl |
1.18 |
* Returns an empty iterator in which <tt>hasNext</tt> always returns |
551 |
tim |
1.13 |
* <tt>false</tt>. |
552 |
|
|
* |
553 |
dholmes |
1.11 |
* @return an empty iterator |
554 |
dl |
1.5 |
*/ |
555 |
dl |
1.2 |
public Iterator<E> iterator() { |
556 |
dl |
1.5 |
return new EmptyIterator<E>(); |
557 |
tim |
1.1 |
} |
558 |
|
|
|
559 |
dl |
1.2 |
|
560 |
dl |
1.5 |
/** |
561 |
dholmes |
1.11 |
* Returns a zero-length array. |
562 |
|
|
* @return a zero-length array |
563 |
dl |
1.5 |
*/ |
564 |
dl |
1.3 |
public Object[] toArray() { |
565 |
dl |
1.25 |
return new Object[0]; |
566 |
tim |
1.1 |
} |
567 |
|
|
|
568 |
dholmes |
1.11 |
/** |
569 |
|
|
* Sets the zeroeth element of the specified array to <tt>null</tt> |
570 |
|
|
* (if the array has non-zero length) and returns it. |
571 |
|
|
* @return the specified array |
572 |
|
|
*/ |
573 |
dl |
1.2 |
public <T> T[] toArray(T[] a) { |
574 |
|
|
if (a.length > 0) |
575 |
|
|
a[0] = null; |
576 |
|
|
return a; |
577 |
|
|
} |
578 |
dl |
1.21 |
|
579 |
|
|
|
580 |
|
|
public int drainTo(Collection<? super E> c) { |
581 |
|
|
if (c == null) |
582 |
|
|
throw new NullPointerException(); |
583 |
|
|
if (c == this) |
584 |
|
|
throw new IllegalArgumentException(); |
585 |
|
|
int n = 0; |
586 |
|
|
E e; |
587 |
|
|
while ( (e = poll()) != null) { |
588 |
|
|
c.add(e); |
589 |
|
|
++n; |
590 |
|
|
} |
591 |
|
|
return n; |
592 |
|
|
} |
593 |
|
|
|
594 |
|
|
public int drainTo(Collection<? super E> c, int maxElements) { |
595 |
|
|
if (c == null) |
596 |
|
|
throw new NullPointerException(); |
597 |
|
|
if (c == this) |
598 |
|
|
throw new IllegalArgumentException(); |
599 |
|
|
int n = 0; |
600 |
|
|
E e; |
601 |
|
|
while (n < maxElements && (e = poll()) != null) { |
602 |
|
|
c.add(e); |
603 |
|
|
++n; |
604 |
|
|
} |
605 |
|
|
return n; |
606 |
|
|
} |
607 |
tim |
1.1 |
} |
608 |
dholmes |
1.11 |
|
609 |
|
|
|
610 |
|
|
|
611 |
|
|
|
612 |
|
|
|