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