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 |
|
9 |
import java.util.concurrent.locks.AbstractQueuedSynchronizer; |
10 |
|
11 |
/** |
12 |
* A synchronization aid that allows one or more threads to wait until |
13 |
* a set of operations being performed in other threads completes. |
14 |
* |
15 |
* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>. |
16 |
* The {@link #await await} methods block until the current count reaches |
17 |
* zero due to invocations of the {@link #countDown} method, after which |
18 |
* all waiting threads are released and any subsequent invocations of |
19 |
* {@link #await await} return immediately. This is a one-shot phenomenon |
20 |
* -- the count cannot be reset. If you need a version that resets the |
21 |
* count, consider using a {@link CyclicBarrier}. |
22 |
* |
23 |
* <p>A {@code CountDownLatch} is a versatile synchronization tool |
24 |
* and can be used for a number of purposes. A |
25 |
* {@code CountDownLatch} initialized with a count of one serves as a |
26 |
* simple on/off latch, or gate: all threads invoking {@link #await await} |
27 |
* wait at the gate until it is opened by a thread invoking {@link |
28 |
* #countDown}. A {@code CountDownLatch} initialized to <em>N</em> |
29 |
* can be used to make one thread wait until <em>N</em> threads have |
30 |
* completed some action, or some action has been completed N times. |
31 |
* |
32 |
* <p>A useful property of a {@code CountDownLatch} is that it |
33 |
* doesn't require that threads calling {@code countDown} wait for |
34 |
* the count to reach zero before proceeding, it simply prevents any |
35 |
* thread from proceeding past an {@link #await await} until all |
36 |
* threads could pass. |
37 |
* |
38 |
* <p><b>Sample usage:</b> Here is a pair of classes in which a group |
39 |
* of worker threads use two countdown latches: |
40 |
* <ul> |
41 |
* <li>The first is a start signal that prevents any worker from proceeding |
42 |
* until the driver is ready for them to proceed; |
43 |
* <li>The second is a completion signal that allows the driver to wait |
44 |
* until all workers have completed. |
45 |
* </ul> |
46 |
* |
47 |
* <pre> {@code |
48 |
* class Driver { // ... |
49 |
* void main() throws InterruptedException { |
50 |
* CountDownLatch startSignal = new CountDownLatch(1); |
51 |
* CountDownLatch doneSignal = new CountDownLatch(N); |
52 |
* |
53 |
* for (int i = 0; i < N; ++i) // create and start threads |
54 |
* new Thread(new Worker(startSignal, doneSignal)).start(); |
55 |
* |
56 |
* doSomethingElse(); // don't let run yet |
57 |
* startSignal.countDown(); // let all threads proceed |
58 |
* doSomethingElse(); |
59 |
* doneSignal.await(); // wait for all to finish |
60 |
* } |
61 |
* } |
62 |
* |
63 |
* class Worker implements Runnable { |
64 |
* private final CountDownLatch startSignal; |
65 |
* private final CountDownLatch doneSignal; |
66 |
* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) { |
67 |
* this.startSignal = startSignal; |
68 |
* this.doneSignal = doneSignal; |
69 |
* } |
70 |
* public void run() { |
71 |
* try { |
72 |
* startSignal.await(); |
73 |
* doWork(); |
74 |
* doneSignal.countDown(); |
75 |
* } catch (InterruptedException ex) {} // return; |
76 |
* } |
77 |
* |
78 |
* void doWork() { ... } |
79 |
* }}</pre> |
80 |
* |
81 |
* <p>Another typical usage would be to divide a problem into N parts, |
82 |
* describe each part with a Runnable that executes that portion and |
83 |
* counts down on the latch, and queue all the Runnables to an |
84 |
* Executor. When all sub-parts are complete, the coordinating thread |
85 |
* will be able to pass through await. (When threads must repeatedly |
86 |
* count down in this way, instead use a {@link CyclicBarrier}.) |
87 |
* |
88 |
* <pre> {@code |
89 |
* class Driver2 { // ... |
90 |
* void main() throws InterruptedException { |
91 |
* CountDownLatch doneSignal = new CountDownLatch(N); |
92 |
* Executor e = ... |
93 |
* |
94 |
* for (int i = 0; i < N; ++i) // create and start threads |
95 |
* e.execute(new WorkerRunnable(doneSignal, i)); |
96 |
* |
97 |
* doneSignal.await(); // wait for all to finish |
98 |
* } |
99 |
* } |
100 |
* |
101 |
* class WorkerRunnable implements Runnable { |
102 |
* private final CountDownLatch doneSignal; |
103 |
* private final int i; |
104 |
* WorkerRunnable(CountDownLatch doneSignal, int i) { |
105 |
* this.doneSignal = doneSignal; |
106 |
* this.i = i; |
107 |
* } |
108 |
* public void run() { |
109 |
* try { |
110 |
* doWork(i); |
111 |
* doneSignal.countDown(); |
112 |
* } catch (InterruptedException ex) {} // return; |
113 |
* } |
114 |
* |
115 |
* void doWork() { ... } |
116 |
* }}</pre> |
117 |
* |
118 |
* <p>Memory consistency effects: Until the count reaches |
119 |
* zero, actions in a thread prior to calling |
120 |
* {@code countDown()} |
121 |
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
122 |
* actions following a successful return from a corresponding |
123 |
* {@code await()} in another thread. |
124 |
* |
125 |
* @since 1.5 |
126 |
* @author Doug Lea |
127 |
*/ |
128 |
public class CountDownLatch { |
129 |
/** |
130 |
* Synchronization control For CountDownLatch. |
131 |
* Uses AQS state to represent count. |
132 |
*/ |
133 |
private static final class Sync extends AbstractQueuedSynchronizer { |
134 |
private static final long serialVersionUID = 4982264981922014374L; |
135 |
|
136 |
Sync(int count) { |
137 |
setState(count); |
138 |
} |
139 |
|
140 |
int getCount() { |
141 |
return getState(); |
142 |
} |
143 |
|
144 |
protected int tryAcquireShared(int acquires) { |
145 |
return (getState() == 0) ? 1 : -1; |
146 |
} |
147 |
|
148 |
protected boolean tryReleaseShared(int releases) { |
149 |
// Decrement count; signal when transition to zero |
150 |
for (;;) { |
151 |
int c = getState(); |
152 |
if (c == 0) |
153 |
return false; |
154 |
int nextc = c - 1; |
155 |
if (compareAndSetState(c, nextc)) |
156 |
return nextc == 0; |
157 |
} |
158 |
} |
159 |
} |
160 |
|
161 |
private final Sync sync; |
162 |
|
163 |
/** |
164 |
* Constructs a {@code CountDownLatch} initialized with the given count. |
165 |
* |
166 |
* @param count the number of times {@link #countDown} must be invoked |
167 |
* before threads can pass through {@link #await} |
168 |
* @throws IllegalArgumentException if {@code count} is negative |
169 |
*/ |
170 |
public CountDownLatch(int count) { |
171 |
if (count < 0) throw new IllegalArgumentException("count < 0"); |
172 |
this.sync = new Sync(count); |
173 |
} |
174 |
|
175 |
/** |
176 |
* Causes the current thread to wait until the latch has counted down to |
177 |
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}. |
178 |
* |
179 |
* <p>If the current count is zero then this method returns immediately. |
180 |
* |
181 |
* <p>If the current count is greater than zero then the current |
182 |
* thread becomes disabled for thread scheduling purposes and lies |
183 |
* dormant until one of two things happen: |
184 |
* <ul> |
185 |
* <li>The count reaches zero due to invocations of the |
186 |
* {@link #countDown} method; or |
187 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
188 |
* the current thread. |
189 |
* </ul> |
190 |
* |
191 |
* <p>If the current thread: |
192 |
* <ul> |
193 |
* <li>has its interrupted status set on entry to this method; or |
194 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting, |
195 |
* </ul> |
196 |
* then {@link InterruptedException} is thrown and the current thread's |
197 |
* interrupted status is cleared. |
198 |
* |
199 |
* @throws InterruptedException if the current thread is interrupted |
200 |
* while waiting |
201 |
*/ |
202 |
public void await() throws InterruptedException { |
203 |
sync.acquireSharedInterruptibly(1); |
204 |
} |
205 |
|
206 |
/** |
207 |
* Causes the current thread to wait until the latch has counted down to |
208 |
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}, |
209 |
* or the specified waiting time elapses. |
210 |
* |
211 |
* <p>If the current count is zero then this method returns immediately |
212 |
* with the value {@code true}. |
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 three 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; or |
222 |
* <li>The specified waiting time elapses. |
223 |
* </ul> |
224 |
* |
225 |
* <p>If the count reaches zero then the method returns with the |
226 |
* value {@code true}. |
227 |
* |
228 |
* <p>If the current thread: |
229 |
* <ul> |
230 |
* <li>has its interrupted status set on entry to this method; or |
231 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting, |
232 |
* </ul> |
233 |
* then {@link InterruptedException} is thrown and the current thread's |
234 |
* interrupted status is cleared. |
235 |
* |
236 |
* <p>If the specified waiting time elapses then the value {@code false} |
237 |
* is returned. If the time is less than or equal to zero, the method |
238 |
* will not wait at all. |
239 |
* |
240 |
* @param timeout the maximum time to wait |
241 |
* @param unit the time unit of the {@code timeout} argument |
242 |
* @return {@code true} if the count reached zero and {@code false} |
243 |
* if the waiting time elapsed before the count reached zero |
244 |
* @throws InterruptedException if the current thread is interrupted |
245 |
* while waiting |
246 |
*/ |
247 |
public boolean await(long timeout, TimeUnit unit) |
248 |
throws InterruptedException { |
249 |
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
250 |
} |
251 |
|
252 |
/** |
253 |
* Decrements the count of the latch, releasing all waiting threads if |
254 |
* the count reaches zero. |
255 |
* |
256 |
* <p>If the current count is greater than zero then it is decremented. |
257 |
* If the new count is zero then all waiting threads are re-enabled for |
258 |
* thread scheduling purposes. |
259 |
* |
260 |
* <p>If the current count equals zero then nothing happens. |
261 |
*/ |
262 |
public void countDown() { |
263 |
sync.releaseShared(1); |
264 |
} |
265 |
|
266 |
/** |
267 |
* Returns the current count. |
268 |
* |
269 |
* <p>This method is typically used for debugging and testing purposes. |
270 |
* |
271 |
* @return the current count |
272 |
*/ |
273 |
public long getCount() { |
274 |
return sync.getCount(); |
275 |
} |
276 |
|
277 |
/** |
278 |
* Returns a string identifying this latch, as well as its state. |
279 |
* The state, in brackets, includes the String {@code "Count ="} |
280 |
* followed by the current count. |
281 |
* |
282 |
* @return a string identifying this latch, as well as its state |
283 |
*/ |
284 |
public String toString() { |
285 |
return super.toString() + "[Count = " + sync.getCount() + "]"; |
286 |
} |
287 |
} |