1 |
dl |
1.2 |
/* |
2 |
dl |
1.16 |
* Written by Doug Lea, Bill Scherer, and Michael Scott with |
3 |
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* assistance from members of JCP JSR-166 Expert Group and released to |
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* the public domain, as explained at |
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dl |
1.14 |
* http://creativecommons.org/licenses/publicdomain |
6 |
dl |
1.2 |
*/ |
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|
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tim |
1.1 |
package java.util.concurrent; |
9 |
dl |
1.4 |
import java.util.concurrent.locks.*; |
10 |
dl |
1.16 |
import java.util.concurrent.atomic.*; |
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import java.util.Random; |
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tim |
1.1 |
|
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/** |
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dl |
1.12 |
* A synchronization point at which two threads can exchange objects. |
15 |
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* Each thread presents some object on entry to the {@link #exchange |
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* exchange} method, and receives the object presented by the other |
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* thread on return. |
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tim |
1.1 |
* |
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* <p><b>Sample Usage:</b> |
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* Here are the highlights of a class that uses an <tt>Exchanger</tt> to |
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* swap buffers between threads so that the thread filling the |
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* buffer gets a freshly |
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* emptied one when it needs it, handing off the filled one to |
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* the thread emptying the buffer. |
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* <pre> |
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* class FillAndEmpty { |
27 |
dl |
1.9 |
* Exchanger<DataBuffer> exchanger = new Exchanger(); |
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* DataBuffer initialEmptyBuffer = ... a made-up type |
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* DataBuffer initialFullBuffer = ... |
30 |
tim |
1.1 |
* |
31 |
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* class FillingLoop implements Runnable { |
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* public void run() { |
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dl |
1.9 |
* DataBuffer currentBuffer = initialEmptyBuffer; |
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tim |
1.1 |
* try { |
35 |
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* while (currentBuffer != null) { |
36 |
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* addToBuffer(currentBuffer); |
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* if (currentBuffer.full()) |
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* currentBuffer = exchanger.exchange(currentBuffer); |
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* } |
40 |
tim |
1.7 |
* } catch (InterruptedException ex) { ... handle ... } |
41 |
tim |
1.1 |
* } |
42 |
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* } |
43 |
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* |
44 |
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* class EmptyingLoop implements Runnable { |
45 |
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* public void run() { |
46 |
dl |
1.9 |
* DataBuffer currentBuffer = initialFullBuffer; |
47 |
tim |
1.1 |
* try { |
48 |
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* while (currentBuffer != null) { |
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* takeFromBuffer(currentBuffer); |
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* if (currentBuffer.empty()) |
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* currentBuffer = exchanger.exchange(currentBuffer); |
52 |
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* } |
53 |
tim |
1.7 |
* } catch (InterruptedException ex) { ... handle ...} |
54 |
tim |
1.1 |
* } |
55 |
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* } |
56 |
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* |
57 |
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* void start() { |
58 |
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* new Thread(new FillingLoop()).start(); |
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* new Thread(new EmptyingLoop()).start(); |
60 |
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* } |
61 |
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* } |
62 |
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* </pre> |
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* |
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* @since 1.5 |
65 |
dl |
1.16 |
* @author Doug Lea and Bill Scherer and Michael Scott |
66 |
dl |
1.11 |
* @param <V> The type of objects that may be exchanged |
67 |
tim |
1.1 |
*/ |
68 |
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public class Exchanger<V> { |
69 |
dl |
1.16 |
/* |
70 |
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* The underlying idea is to use a stack to hold nodes containing |
71 |
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* pairs of items to be exchanged. Except that: |
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* |
73 |
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* * Only one element of the pair is known on creation by a |
74 |
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* first-arriving thread; the other is a "hole" waiting to be |
75 |
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* filled in. This is a degenerate form of the dual stacks |
76 |
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* described in "Nonblocking Concurrent Objects with Condition |
77 |
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* Synchronization", by W. N. Scherer III and M. L. Scott. |
78 |
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* 18th Annual Conf. on Distributed Computing, Oct. 2004. |
79 |
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* It is "degenerate" in that both the items and the holes |
80 |
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* are shared in the same nodes. |
81 |
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* |
82 |
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* * There isn't really a stack here! There can't be -- if two |
83 |
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* nodes were both in the stack, they should cancel themselves |
84 |
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* out by combining. So that's what we do. The 0th element of |
85 |
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* the "arena" array serves only as the top of stack. The |
86 |
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* remainder of the array is a form of the elimination backoff |
87 |
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* collision array described in "A Scalable Lock-free Stack |
88 |
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* Algorithm", by D. Hendler, N. Shavit, and L. Yerushalmi. |
89 |
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* 16th ACM Symposium on Parallelism in Algorithms and |
90 |
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* Architectures, June 2004. Here, threads spin (using short |
91 |
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* timed waits with exponential backoff) looking for each |
92 |
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* other. If they fail to find others waiting, they try the |
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* top spot again. As shown in that paper, this always |
94 |
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* converges. |
95 |
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* |
96 |
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* The backoff elimination mechanics never come into play in |
97 |
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* common usages where only two threads ever meet to exchange |
98 |
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* items, but they prevent contention bottlenecks when an |
99 |
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* exchanger is used by a large number of threads. |
100 |
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*/ |
101 |
dl |
1.2 |
|
102 |
dl |
1.16 |
/** |
103 |
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* Size of collision space. Using a size of half the number of |
104 |
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* CPUs provides enough space for threads to find each other but |
105 |
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* not so much that it would always require one or more to time |
106 |
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* out to become unstuck. Note that the arena array holds SIZE+1 |
107 |
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* elements, to include the top-of-stack slot. |
108 |
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*/ |
109 |
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private static final int SIZE = |
110 |
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(Runtime.getRuntime().availableProcessors() + 1) / 2; |
111 |
jsr166 |
1.15 |
|
112 |
dl |
1.2 |
/** |
113 |
dl |
1.16 |
* Base unit in nanoseconds for backoffs. Must be a power of two. |
114 |
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* Should be small because backoffs exponentially increase from |
115 |
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* base. |
116 |
dl |
1.2 |
*/ |
117 |
dl |
1.16 |
private static final long BACKOFF_BASE = 128L; |
118 |
dl |
1.2 |
|
119 |
dl |
1.16 |
/** |
120 |
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* Sentinel item representing cancellation. This value is placed |
121 |
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* in holes on cancellation, and used as a return value from Node |
122 |
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* methods to indicate failure to set or get hole. |
123 |
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**/ |
124 |
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static final Object FAIL = new Object(); |
125 |
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|
126 |
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/** |
127 |
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* The collision arena. arena[0] is used as the top of the stack. |
128 |
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* The remainder is used as the collision elimination space. |
129 |
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* Each slot holds an AtomicReference<Node>, but this cannot be |
130 |
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* expressed for arrays, so elements are casted on each use. |
131 |
dl |
1.3 |
*/ |
132 |
dl |
1.16 |
private final AtomicReference[] arena; |
133 |
dl |
1.5 |
|
134 |
dl |
1.16 |
/** Generator for random backoffs and delays. */ |
135 |
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private final Random random = new Random(); |
136 |
dl |
1.5 |
|
137 |
dl |
1.16 |
/** |
138 |
|
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* Creates a new Exchanger. |
139 |
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*/ |
140 |
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public Exchanger() { |
141 |
|
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arena = new AtomicReference[SIZE + 1]; |
142 |
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for (int i = 0; i < arena.length; ++i) |
143 |
|
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arena[i] = new AtomicReference(); |
144 |
|
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} |
145 |
dl |
1.2 |
|
146 |
dl |
1.16 |
/** |
147 |
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* Main exchange function, handling the different policy variants. |
148 |
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* Uses Object, not "V" as argument and return value to simplify |
149 |
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* handling of internal sentinel values. Callers from public |
150 |
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* methods cast accordingly. |
151 |
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* @param item the item to exchange. |
152 |
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* @param timed true if the wait is timed. |
153 |
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* @param nanos if timed, the maximum wait time. |
154 |
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* @return the other thread's item. |
155 |
|
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*/ |
156 |
|
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private Object doExchange(Object item, boolean timed, long nanos) |
157 |
|
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throws InterruptedException, TimeoutException { |
158 |
|
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Node me = new Node(item); |
159 |
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long lastTime = (timed)? System.nanoTime() : 0; |
160 |
|
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int idx = 0; // start out at slot representing top |
161 |
|
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int backoff = 0; // increases on failure to occupy a slot |
162 |
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|
163 |
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for (;;) { |
164 |
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AtomicReference<Node> slot = (AtomicReference<Node>)arena[idx]; |
165 |
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|
166 |
|
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// If this slot is already occupied, there is a waiting item... |
167 |
|
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Node you = slot.get(); |
168 |
|
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if (you != null) { |
169 |
|
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Object v = you.fillHole(item); |
170 |
|
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slot.compareAndSet(you, null); |
171 |
|
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if (v != FAIL) // ... unless it was cancelled |
172 |
|
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return v; |
173 |
dl |
1.2 |
} |
174 |
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|
175 |
dl |
1.16 |
// Try to occupy this slot |
176 |
|
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if (slot.compareAndSet(null, me)) { |
177 |
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// If this is top slot, use regular wait, else backoff-wait |
178 |
|
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Object v = ((idx == 0)? |
179 |
|
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me.waitForHole(timed, nanos) : |
180 |
|
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me.waitForHole(true, randomDelay(backoff))); |
181 |
|
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slot.compareAndSet(me, null); |
182 |
|
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if (v != FAIL) |
183 |
|
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return v; |
184 |
|
|
if (Thread.interrupted()) |
185 |
|
|
throw new InterruptedException(); |
186 |
|
|
if (timed) { |
187 |
|
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long now = System.nanoTime(); |
188 |
|
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nanos -= now - lastTime; |
189 |
|
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lastTime = now; |
190 |
|
|
if (nanos <= 0) |
191 |
|
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throw new TimeoutException(); |
192 |
|
|
} |
193 |
dl |
1.2 |
|
194 |
dl |
1.16 |
me = new Node(item); // Throw away nodes on failure |
195 |
|
|
if (backoff < SIZE - 1) // Increase or stay saturated |
196 |
|
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++backoff; |
197 |
|
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idx = 0; // Restart at top |
198 |
dl |
1.2 |
} |
199 |
|
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|
200 |
dl |
1.16 |
else // Retry with a random non-top slot <= backoff |
201 |
|
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idx = 1 + random.nextInt(backoff + 1); |
202 |
dl |
1.2 |
|
203 |
|
|
} |
204 |
|
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} |
205 |
tim |
1.1 |
|
206 |
|
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/** |
207 |
dl |
1.16 |
* Returns a random delay less than (base times (2 raised to backoff)) |
208 |
|
|
*/ |
209 |
|
|
private long randomDelay(int backoff) { |
210 |
|
|
return ((BACKOFF_BASE << backoff) - 1) & random.nextInt(); |
211 |
|
|
} |
212 |
|
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|
213 |
|
|
/** |
214 |
|
|
* Nodes hold partially exchanged data. This class |
215 |
|
|
* opportunistically subclasses AtomicReference to represent the |
216 |
|
|
* hole. So get() returns hole, and compareAndSet CAS'es value |
217 |
|
|
* into hole. Note that this class cannot be parameterized as V |
218 |
|
|
* because the sentinel value FAIL is only of type Object. |
219 |
jsr166 |
1.15 |
*/ |
220 |
dl |
1.16 |
static final class Node extends AtomicReference<Object> { |
221 |
|
|
/** The element offered by the Thread creating this node. */ |
222 |
|
|
final Object item; |
223 |
|
|
/** The Thread creating this node. */ |
224 |
|
|
final Thread waiter; |
225 |
|
|
|
226 |
|
|
/** |
227 |
|
|
* Creates node with given item and empty hole. |
228 |
|
|
* @param item the item. |
229 |
|
|
*/ |
230 |
|
|
Node(Object item) { |
231 |
|
|
this.item = item; |
232 |
|
|
waiter = Thread.currentThread(); |
233 |
|
|
} |
234 |
|
|
|
235 |
|
|
/** |
236 |
|
|
* Tries to fill in hole. On success, wakes up the waiter. |
237 |
|
|
* @param val the value to place in hole. |
238 |
|
|
* @return on success, the item; on failure, FAIL. |
239 |
|
|
*/ |
240 |
|
|
Object fillHole(Object val) { |
241 |
|
|
if (compareAndSet(null, val)) { |
242 |
|
|
LockSupport.unpark(waiter); |
243 |
|
|
return item; |
244 |
|
|
} |
245 |
|
|
return FAIL; |
246 |
|
|
} |
247 |
|
|
|
248 |
|
|
/** |
249 |
|
|
* Wait for and get the hole filled in by another thread. |
250 |
|
|
* Fail if timed out or interrupted before hole filled. |
251 |
|
|
* @param timed true if the wait is timed. |
252 |
|
|
* @param nanos if timed, the maximum wait time. |
253 |
|
|
* @return on success, the hole; on failure, FAIL. |
254 |
|
|
*/ |
255 |
|
|
Object waitForHole(boolean timed, long nanos) { |
256 |
|
|
long lastTime = (timed)? System.nanoTime() : 0; |
257 |
|
|
Object h = get(); |
258 |
|
|
while (h == null) { |
259 |
|
|
if (!timed) |
260 |
|
|
LockSupport.park(); |
261 |
|
|
else { |
262 |
|
|
LockSupport.parkNanos(nanos); |
263 |
|
|
long now = System.nanoTime(); |
264 |
|
|
nanos -= now - lastTime; |
265 |
|
|
lastTime = now; |
266 |
|
|
} |
267 |
|
|
// If interrupted or timed out, try to cancel by |
268 |
|
|
// CASing FAIL as hole value. |
269 |
|
|
if ((h = get()) == null && |
270 |
|
|
(Thread.currentThread().isInterrupted() || |
271 |
|
|
(timed && nanos <= 0))) { |
272 |
|
|
compareAndSet(null, FAIL); |
273 |
|
|
h = get(); |
274 |
|
|
} |
275 |
|
|
} |
276 |
|
|
return h; |
277 |
|
|
} |
278 |
tim |
1.1 |
} |
279 |
|
|
|
280 |
|
|
/** |
281 |
|
|
* Waits for another thread to arrive at this exchange point (unless |
282 |
|
|
* it is {@link Thread#interrupt interrupted}), |
283 |
|
|
* and then transfers the given object to it, receiving its object |
284 |
|
|
* in return. |
285 |
|
|
* <p>If another thread is already waiting at the exchange point then |
286 |
|
|
* it is resumed for thread scheduling purposes and receives the object |
287 |
|
|
* passed in by the current thread. The current thread returns immediately, |
288 |
|
|
* receiving the object passed to the exchange by that other thread. |
289 |
jsr166 |
1.15 |
* <p>If no other thread is already waiting at the exchange then the |
290 |
tim |
1.1 |
* current thread is disabled for thread scheduling purposes and lies |
291 |
|
|
* dormant until one of two things happens: |
292 |
|
|
* <ul> |
293 |
|
|
* <li>Some other thread enters the exchange; or |
294 |
|
|
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
295 |
|
|
* thread. |
296 |
|
|
* </ul> |
297 |
|
|
* <p>If the current thread: |
298 |
|
|
* <ul> |
299 |
jsr166 |
1.15 |
* <li>has its interrupted status set on entry to this method; or |
300 |
tim |
1.1 |
* <li>is {@link Thread#interrupt interrupted} while waiting |
301 |
jsr166 |
1.15 |
* for the exchange, |
302 |
tim |
1.1 |
* </ul> |
303 |
jsr166 |
1.15 |
* then {@link InterruptedException} is thrown and the current thread's |
304 |
|
|
* interrupted status is cleared. |
305 |
tim |
1.1 |
* |
306 |
|
|
* @param x the object to exchange |
307 |
|
|
* @return the object provided by the other thread. |
308 |
jsr166 |
1.15 |
* @throws InterruptedException if current thread was interrupted |
309 |
dl |
1.3 |
* while waiting |
310 |
jsr166 |
1.15 |
*/ |
311 |
tim |
1.1 |
public V exchange(V x) throws InterruptedException { |
312 |
dl |
1.2 |
try { |
313 |
dl |
1.16 |
return (V)doExchange(x, false, 0); |
314 |
jsr166 |
1.15 |
} catch (TimeoutException cannotHappen) { |
315 |
dl |
1.2 |
throw new Error(cannotHappen); |
316 |
|
|
} |
317 |
tim |
1.1 |
} |
318 |
|
|
|
319 |
|
|
/** |
320 |
|
|
* Waits for another thread to arrive at this exchange point (unless |
321 |
|
|
* it is {@link Thread#interrupt interrupted}, or the specified waiting |
322 |
|
|
* time elapses), |
323 |
|
|
* and then transfers the given object to it, receiving its object |
324 |
|
|
* in return. |
325 |
|
|
* |
326 |
|
|
* <p>If another thread is already waiting at the exchange point then |
327 |
|
|
* it is resumed for thread scheduling purposes and receives the object |
328 |
|
|
* passed in by the current thread. The current thread returns immediately, |
329 |
|
|
* receiving the object passed to the exchange by that other thread. |
330 |
|
|
* |
331 |
jsr166 |
1.15 |
* <p>If no other thread is already waiting at the exchange then the |
332 |
tim |
1.1 |
* current thread is disabled for thread scheduling purposes and lies |
333 |
|
|
* dormant until one of three things happens: |
334 |
|
|
* <ul> |
335 |
|
|
* <li>Some other thread enters the exchange; or |
336 |
|
|
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
337 |
|
|
* thread; or |
338 |
|
|
* <li>The specified waiting time elapses. |
339 |
|
|
* </ul> |
340 |
|
|
* <p>If the current thread: |
341 |
|
|
* <ul> |
342 |
jsr166 |
1.15 |
* <li>has its interrupted status set on entry to this method; or |
343 |
tim |
1.1 |
* <li>is {@link Thread#interrupt interrupted} while waiting |
344 |
jsr166 |
1.15 |
* for the exchange, |
345 |
tim |
1.1 |
* </ul> |
346 |
jsr166 |
1.15 |
* then {@link InterruptedException} is thrown and the current thread's |
347 |
|
|
* interrupted status is cleared. |
348 |
tim |
1.1 |
* |
349 |
|
|
* <p>If the specified waiting time elapses then {@link TimeoutException} |
350 |
|
|
* is thrown. |
351 |
jsr166 |
1.15 |
* If the time is |
352 |
tim |
1.1 |
* less than or equal to zero, the method will not wait at all. |
353 |
|
|
* |
354 |
|
|
* @param x the object to exchange |
355 |
|
|
* @param timeout the maximum time to wait |
356 |
dl |
1.2 |
* @param unit the time unit of the <tt>timeout</tt> argument. |
357 |
tim |
1.1 |
* @return the object provided by the other thread. |
358 |
dl |
1.3 |
* @throws InterruptedException if current thread was interrupted |
359 |
|
|
* while waiting |
360 |
tim |
1.1 |
* @throws TimeoutException if the specified waiting time elapses before |
361 |
|
|
* another thread enters the exchange. |
362 |
jsr166 |
1.15 |
*/ |
363 |
|
|
public V exchange(V x, long timeout, TimeUnit unit) |
364 |
tim |
1.1 |
throws InterruptedException, TimeoutException { |
365 |
dl |
1.16 |
return (V)doExchange(x, true, unit.toNanos(timeout)); |
366 |
tim |
1.1 |
} |
367 |
|
|
} |