author | smarks |
Tue, 06 Dec 2011 10:14:02 -0800 | |
changeset 11139 | db0c2ff5e1ea |
parent 9242 | ef138d47df58 |
child 14325 | 622c473a21aa |
permissions | -rw-r--r-- |
2 | 1 |
/* |
2 |
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
|
3 |
* |
|
4 |
* This code is free software; you can redistribute it and/or modify it |
|
5 |
* under the terms of the GNU General Public License version 2 only, as |
|
5506 | 6 |
* published by the Free Software Foundation. Oracle designates this |
2 | 7 |
* particular file as subject to the "Classpath" exception as provided |
5506 | 8 |
* by Oracle in the LICENSE file that accompanied this code. |
2 | 9 |
* |
10 |
* This code is distributed in the hope that it will be useful, but WITHOUT |
|
11 |
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
|
12 |
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
|
13 |
* version 2 for more details (a copy is included in the LICENSE file that |
|
14 |
* accompanied this code). |
|
15 |
* |
|
16 |
* You should have received a copy of the GNU General Public License version |
|
17 |
* 2 along with this work; if not, write to the Free Software Foundation, |
|
18 |
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
|
19 |
* |
|
5506 | 20 |
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
21 |
* or visit www.oracle.com if you need additional information or have any |
|
22 |
* questions. |
|
2 | 23 |
*/ |
24 |
||
25 |
/* |
|
26 |
* This file is available under and governed by the GNU General Public |
|
27 |
* License version 2 only, as published by the Free Software Foundation. |
|
28 |
* However, the following notice accompanied the original version of this |
|
29 |
* file: |
|
30 |
* |
|
31 |
* Written by Doug Lea with assistance from members of JCP JSR-166 |
|
32 |
* Expert Group and released to the public domain, as explained at |
|
9242
ef138d47df58
7034657: Update Creative Commons license URL in legal notices
dl
parents:
7518
diff
changeset
|
33 |
* http://creativecommons.org/publicdomain/zero/1.0/ |
2 | 34 |
*/ |
35 |
||
36 |
package java.util.concurrent; |
|
37 |
import java.util.concurrent.locks.*; |
|
38 |
import java.util.concurrent.atomic.*; |
|
39 |
||
40 |
/** |
|
41 |
* A synchronization aid that allows one or more threads to wait until |
|
42 |
* a set of operations being performed in other threads completes. |
|
43 |
* |
|
44 |
* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>. |
|
45 |
* The {@link #await await} methods block until the current count reaches |
|
46 |
* zero due to invocations of the {@link #countDown} method, after which |
|
47 |
* all waiting threads are released and any subsequent invocations of |
|
48 |
* {@link #await await} return immediately. This is a one-shot phenomenon |
|
49 |
* -- the count cannot be reset. If you need a version that resets the |
|
50 |
* count, consider using a {@link CyclicBarrier}. |
|
51 |
* |
|
52 |
* <p>A {@code CountDownLatch} is a versatile synchronization tool |
|
53 |
* and can be used for a number of purposes. A |
|
54 |
* {@code CountDownLatch} initialized with a count of one serves as a |
|
55 |
* simple on/off latch, or gate: all threads invoking {@link #await await} |
|
56 |
* wait at the gate until it is opened by a thread invoking {@link |
|
57 |
* #countDown}. A {@code CountDownLatch} initialized to <em>N</em> |
|
58 |
* can be used to make one thread wait until <em>N</em> threads have |
|
59 |
* completed some action, or some action has been completed N times. |
|
60 |
* |
|
61 |
* <p>A useful property of a {@code CountDownLatch} is that it |
|
62 |
* doesn't require that threads calling {@code countDown} wait for |
|
63 |
* the count to reach zero before proceeding, it simply prevents any |
|
64 |
* thread from proceeding past an {@link #await await} until all |
|
65 |
* threads could pass. |
|
66 |
* |
|
67 |
* <p><b>Sample usage:</b> Here is a pair of classes in which a group |
|
68 |
* of worker threads use two countdown latches: |
|
69 |
* <ul> |
|
70 |
* <li>The first is a start signal that prevents any worker from proceeding |
|
71 |
* until the driver is ready for them to proceed; |
|
72 |
* <li>The second is a completion signal that allows the driver to wait |
|
73 |
* until all workers have completed. |
|
74 |
* </ul> |
|
75 |
* |
|
76 |
* <pre> |
|
77 |
* class Driver { // ... |
|
78 |
* void main() throws InterruptedException { |
|
79 |
* CountDownLatch startSignal = new CountDownLatch(1); |
|
80 |
* CountDownLatch doneSignal = new CountDownLatch(N); |
|
81 |
* |
|
82 |
* for (int i = 0; i < N; ++i) // create and start threads |
|
83 |
* new Thread(new Worker(startSignal, doneSignal)).start(); |
|
84 |
* |
|
85 |
* doSomethingElse(); // don't let run yet |
|
86 |
* startSignal.countDown(); // let all threads proceed |
|
87 |
* doSomethingElse(); |
|
88 |
* doneSignal.await(); // wait for all to finish |
|
89 |
* } |
|
90 |
* } |
|
91 |
* |
|
92 |
* class Worker implements Runnable { |
|
93 |
* private final CountDownLatch startSignal; |
|
94 |
* private final CountDownLatch doneSignal; |
|
95 |
* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) { |
|
96 |
* this.startSignal = startSignal; |
|
97 |
* this.doneSignal = doneSignal; |
|
98 |
* } |
|
99 |
* public void run() { |
|
100 |
* try { |
|
101 |
* startSignal.await(); |
|
102 |
* doWork(); |
|
103 |
* doneSignal.countDown(); |
|
104 |
* } catch (InterruptedException ex) {} // return; |
|
105 |
* } |
|
106 |
* |
|
107 |
* void doWork() { ... } |
|
108 |
* } |
|
109 |
* |
|
110 |
* </pre> |
|
111 |
* |
|
112 |
* <p>Another typical usage would be to divide a problem into N parts, |
|
113 |
* describe each part with a Runnable that executes that portion and |
|
114 |
* counts down on the latch, and queue all the Runnables to an |
|
115 |
* Executor. When all sub-parts are complete, the coordinating thread |
|
116 |
* will be able to pass through await. (When threads must repeatedly |
|
117 |
* count down in this way, instead use a {@link CyclicBarrier}.) |
|
118 |
* |
|
119 |
* <pre> |
|
120 |
* class Driver2 { // ... |
|
121 |
* void main() throws InterruptedException { |
|
122 |
* CountDownLatch doneSignal = new CountDownLatch(N); |
|
123 |
* Executor e = ... |
|
124 |
* |
|
125 |
* for (int i = 0; i < N; ++i) // create and start threads |
|
126 |
* e.execute(new WorkerRunnable(doneSignal, i)); |
|
127 |
* |
|
128 |
* doneSignal.await(); // wait for all to finish |
|
129 |
* } |
|
130 |
* } |
|
131 |
* |
|
132 |
* class WorkerRunnable implements Runnable { |
|
133 |
* private final CountDownLatch doneSignal; |
|
134 |
* private final int i; |
|
135 |
* WorkerRunnable(CountDownLatch doneSignal, int i) { |
|
136 |
* this.doneSignal = doneSignal; |
|
137 |
* this.i = i; |
|
138 |
* } |
|
139 |
* public void run() { |
|
140 |
* try { |
|
141 |
* doWork(i); |
|
142 |
* doneSignal.countDown(); |
|
143 |
* } catch (InterruptedException ex) {} // return; |
|
144 |
* } |
|
145 |
* |
|
146 |
* void doWork() { ... } |
|
147 |
* } |
|
148 |
* |
|
149 |
* </pre> |
|
150 |
* |
|
4110 | 151 |
* <p>Memory consistency effects: Until the count reaches |
152 |
* zero, actions in a thread prior to calling |
|
2 | 153 |
* {@code countDown()} |
154 |
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
|
155 |
* actions following a successful return from a corresponding |
|
156 |
* {@code await()} in another thread. |
|
157 |
* |
|
158 |
* @since 1.5 |
|
159 |
* @author Doug Lea |
|
160 |
*/ |
|
161 |
public class CountDownLatch { |
|
162 |
/** |
|
163 |
* Synchronization control For CountDownLatch. |
|
164 |
* Uses AQS state to represent count. |
|
165 |
*/ |
|
166 |
private static final class Sync extends AbstractQueuedSynchronizer { |
|
167 |
private static final long serialVersionUID = 4982264981922014374L; |
|
168 |
||
169 |
Sync(int count) { |
|
170 |
setState(count); |
|
171 |
} |
|
172 |
||
173 |
int getCount() { |
|
174 |
return getState(); |
|
175 |
} |
|
176 |
||
177 |
protected int tryAcquireShared(int acquires) { |
|
7518 | 178 |
return (getState() == 0) ? 1 : -1; |
2 | 179 |
} |
180 |
||
181 |
protected boolean tryReleaseShared(int releases) { |
|
182 |
// Decrement count; signal when transition to zero |
|
183 |
for (;;) { |
|
184 |
int c = getState(); |
|
185 |
if (c == 0) |
|
186 |
return false; |
|
187 |
int nextc = c-1; |
|
188 |
if (compareAndSetState(c, nextc)) |
|
189 |
return nextc == 0; |
|
190 |
} |
|
191 |
} |
|
192 |
} |
|
193 |
||
194 |
private final Sync sync; |
|
195 |
||
196 |
/** |
|
197 |
* Constructs a {@code CountDownLatch} initialized with the given count. |
|
198 |
* |
|
199 |
* @param count the number of times {@link #countDown} must be invoked |
|
200 |
* before threads can pass through {@link #await} |
|
201 |
* @throws IllegalArgumentException if {@code count} is negative |
|
202 |
*/ |
|
203 |
public CountDownLatch(int count) { |
|
204 |
if (count < 0) throw new IllegalArgumentException("count < 0"); |
|
205 |
this.sync = new Sync(count); |
|
206 |
} |
|
207 |
||
208 |
/** |
|
209 |
* Causes the current thread to wait until the latch has counted down to |
|
210 |
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}. |
|
211 |
* |
|
212 |
* <p>If the current count is zero then this method returns immediately. |
|
213 |
* |
|
214 |
* <p>If the current count is greater than zero then the current |
|
215 |
* thread becomes disabled for thread scheduling purposes and lies |
|
216 |
* dormant until one of two things happen: |
|
217 |
* <ul> |
|
218 |
* <li>The count reaches zero due to invocations of the |
|
219 |
* {@link #countDown} method; or |
|
220 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
|
221 |
* the current thread. |
|
222 |
* </ul> |
|
223 |
* |
|
224 |
* <p>If the current thread: |
|
225 |
* <ul> |
|
226 |
* <li>has its interrupted status set on entry to this method; or |
|
227 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting, |
|
228 |
* </ul> |
|
229 |
* then {@link InterruptedException} is thrown and the current thread's |
|
230 |
* interrupted status is cleared. |
|
231 |
* |
|
232 |
* @throws InterruptedException if the current thread is interrupted |
|
233 |
* while waiting |
|
234 |
*/ |
|
235 |
public void await() throws InterruptedException { |
|
236 |
sync.acquireSharedInterruptibly(1); |
|
237 |
} |
|
238 |
||
239 |
/** |
|
240 |
* Causes the current thread to wait until the latch has counted down to |
|
241 |
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}, |
|
242 |
* or the specified waiting time elapses. |
|
243 |
* |
|
244 |
* <p>If the current count is zero then this method returns immediately |
|
245 |
* with the value {@code true}. |
|
246 |
* |
|
247 |
* <p>If the current count is greater than zero then the current |
|
248 |
* thread becomes disabled for thread scheduling purposes and lies |
|
249 |
* dormant until one of three things happen: |
|
250 |
* <ul> |
|
251 |
* <li>The count reaches zero due to invocations of the |
|
252 |
* {@link #countDown} method; or |
|
253 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
|
254 |
* the current thread; or |
|
255 |
* <li>The specified waiting time elapses. |
|
256 |
* </ul> |
|
257 |
* |
|
258 |
* <p>If the count reaches zero then the method returns with the |
|
259 |
* value {@code true}. |
|
260 |
* |
|
261 |
* <p>If the current thread: |
|
262 |
* <ul> |
|
263 |
* <li>has its interrupted status set on entry to this method; or |
|
264 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting, |
|
265 |
* </ul> |
|
266 |
* then {@link InterruptedException} is thrown and the current thread's |
|
267 |
* interrupted status is cleared. |
|
268 |
* |
|
269 |
* <p>If the specified waiting time elapses then the value {@code false} |
|
270 |
* is returned. If the time is less than or equal to zero, the method |
|
271 |
* will not wait at all. |
|
272 |
* |
|
273 |
* @param timeout the maximum time to wait |
|
274 |
* @param unit the time unit of the {@code timeout} argument |
|
275 |
* @return {@code true} if the count reached zero and {@code false} |
|
276 |
* if the waiting time elapsed before the count reached zero |
|
277 |
* @throws InterruptedException if the current thread is interrupted |
|
278 |
* while waiting |
|
279 |
*/ |
|
280 |
public boolean await(long timeout, TimeUnit unit) |
|
281 |
throws InterruptedException { |
|
282 |
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
|
283 |
} |
|
284 |
||
285 |
/** |
|
286 |
* Decrements the count of the latch, releasing all waiting threads if |
|
287 |
* the count reaches zero. |
|
288 |
* |
|
289 |
* <p>If the current count is greater than zero then it is decremented. |
|
290 |
* If the new count is zero then all waiting threads are re-enabled for |
|
291 |
* thread scheduling purposes. |
|
292 |
* |
|
293 |
* <p>If the current count equals zero then nothing happens. |
|
294 |
*/ |
|
295 |
public void countDown() { |
|
296 |
sync.releaseShared(1); |
|
297 |
} |
|
298 |
||
299 |
/** |
|
300 |
* Returns the current count. |
|
301 |
* |
|
302 |
* <p>This method is typically used for debugging and testing purposes. |
|
303 |
* |
|
304 |
* @return the current count |
|
305 |
*/ |
|
306 |
public long getCount() { |
|
307 |
return sync.getCount(); |
|
308 |
} |
|
309 |
||
310 |
/** |
|
311 |
* Returns a string identifying this latch, as well as its state. |
|
312 |
* The state, in brackets, includes the String {@code "Count ="} |
|
313 |
* followed by the current count. |
|
314 |
* |
|
315 |
* @return a string identifying this latch, as well as its state |
|
316 |
*/ |
|
317 |
public String toString() { |
|
318 |
return super.toString() + "[Count = " + sync.getCount() + "]"; |
|
319 |
} |
|
320 |
} |