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Java > Open Source Codes > java > util > concurrent > CountDownLatch


1 /*
2  * @(#)CountDownLatch.java 1.5 04/02/09
3  *
4  * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5  * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6  */
7
8 package java.util.concurrent;
9 import java.util.concurrent.locks.*;
10 import java.util.concurrent.atomic.*;
11
12 /**
13  * A synchronization aid that allows one or more threads to wait until
14  * a set of operations being performed in other threads completes.
15  *
16  * <p>A <tt>CountDownLatch</tt> is initialized with a given
17  * <em>count</em>. The {@link #await await} methods block until the current
18  * {@link #getCount count} reaches zero due to invocations of the
19  * {@link #countDown} method, after which all waiting threads are
20  * released and any subsequent invocations of {@link #await await} return
21  * immediately. This is a one-shot phenomenon -- the count cannot be
22  * reset. If you need a version that resets the count, consider using
23  * a {@link CyclicBarrier}.
24  *
25  * <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
26  * and can be used for a number of purposes. A
27  * <tt>CountDownLatch</tt> initialized with a count of one serves as a
28  * simple on/off latch, or gate: all threads invoking {@link #await await}
29  * wait at the gate until it is opened by a thread invoking {@link
30  * #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
31  * can be used to make one thread wait until <em>N</em> threads have
32  * completed some action, or some action has been completed N times.
33  * <p>A useful property of a <tt>CountDownLatch</tt> is that it
34  * doesn't require that threads calling <tt>countDown</tt> wait for
35  * the count to reach zero before proceeding, it simply prevents any
36  * thread from proceeding past an {@link #await await} until all
37  * threads could pass.
38  *
39  * <p><b>Sample usage:</b> Here is a pair of classes in which a group
40  * of worker threads use two countdown latches:
41  * <ul>
42  * <li>The first is a start signal that prevents any worker from proceeding
43  * until the driver is ready for them to proceed;
44  * <li>The second is a completion signal that allows the driver to wait
45  * until all workers have completed.
46  * </ul>
47  *
48  * <pre>
49  * class Driver { // ...
50  * void main() throws InterruptedException {
51  * CountDownLatch startSignal = new CountDownLatch(1);
52  * CountDownLatch doneSignal = new CountDownLatch(N);
53  *
54  * for (int i = 0; i < N; ++i) // create and start threads
55  * new Thread(new Worker(startSignal, doneSignal)).start();
56  *
57  * doSomethingElse(); // don't let run yet
58  * startSignal.countDown(); // let all threads proceed
59  * doSomethingElse();
60  * doneSignal.await(); // wait for all to finish
61  * }
62  * }
63  *
64  * class Worker implements Runnable {
65  * private final CountDownLatch startSignal;
66  * private final CountDownLatch doneSignal;
67  * Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
68  * this.startSignal = startSignal;
69  * this.doneSignal = doneSignal;
70  * }
71  * public void run() {
72  * try {
73  * startSignal.await();
74  * doWork();
75  * doneSignal.countDown();
76  * } catch (InterruptedException ex) {} // return;
77  * }
78  *
79  * void doWork() { ... }
80  * }
81  *
82  * </pre>
83  *
84  * <p>Another typical usage would be to divide a problem into N parts,
85  * describe each part with a Runnable that executes that portion and
86  * counts down on the latch, and queue all the Runnables to an
87  * Executor. When all sub-parts are complete, the coordinating thread
88  * will be able to pass through await. (When threads must repeatedly
89  * count down in this way, instead use a {@link CyclicBarrier}.)
90  *
91  * <pre>
92  * class Driver2 { // ...
93  * void main() throws InterruptedException {
94  * CountDownLatch doneSignal = new CountDownLatch(N);
95  * Executor e = ...
96  *
97  * for (int i = 0; i < N; ++i) // create and start threads
98  * e.execute(new WorkerRunnable(doneSignal, i));
99  *
100  * doneSignal.await(); // wait for all to finish
101  * }
102  * }
103  *
104  * class WorkerRunnable implements Runnable {
105  * private final CountDownLatch doneSignal;
106  * private final int i;
107  * WorkerRunnable(CountDownLatch doneSignal, int i) {
108  * this.doneSignal = doneSignal;
109  * this.i = i;
110  * }
111  * public void run() {
112  * try {
113  * doWork(i);
114  * doneSignal.countDown();
115  * } catch (InterruptedException ex) {} // return;
116  * }
117  *
118  * void doWork() { ... }
119  * }
120  *
121  * </pre>
122  *
123  * @since 1.5
124  * @author Doug Lea
125  */
126 public class CountDownLatch {
127     /**
128      * Synchronization control For CountDownLatch.
129      * Uses AQS state to represent count.
130      */
131     private static final class Sync extends AbstractQueuedSynchronizer {
132         Sync(int count) {
133             setState(count);
134         }
135         
136         int getCount() {
137             return getState();
138         }
139
140         public int tryAcquireShared(int acquires) {
141             return getState() == 0? 1 : -1;
142         }
143         
144         public boolean tryReleaseShared(int releases) {
145             // Decrement count; signal when transition to zero
146 for (;;) {
147                 int c = getState();
148                 if (c == 0)
149                     return false;
150                 int nextc = c-1;
151                 if (compareAndSetState(c, nextc))
152                     return nextc == 0;
153             }
154         }
155     }
156
157     private final Sync sync;
158     /**
159      * Constructs a <tt>CountDownLatch</tt> initialized with the given
160      * count.
161      *
162      * @param count the number of times {@link #countDown} must be invoked
163      * before threads can pass through {@link #await}.
164      *
165      * @throws IllegalArgumentException if <tt>count</tt> is less than zero.
166      */
167     public CountDownLatch(int count) {
168         if (count < 0) throw new IllegalArgumentException JavaDoc("count < 0");
169         this.sync = new Sync(count);
170     }
171
172     /**
173      * Causes the current thread to wait until the latch has counted down to
174      * zero, unless the thread is {@link Thread#interrupt interrupted}.
175      *
176      * <p>If the current {@link #getCount count} is zero then this method
177      * returns immediately.
178      * <p>If the current {@link #getCount count} is greater than zero then
179      * the current thread becomes disabled for thread scheduling
180      * purposes and lies dormant until one of two things happen:
181      * <ul>
182      * <li>The count reaches zero due to invocations of the
183      * {@link #countDown} method; or
184      * <li>Some other thread {@link Thread#interrupt interrupts} the current
185      * thread.
186      * </ul>
187      * <p>If the current thread:
188      * <ul>
189      * <li>has its interrupted status set on entry to this method; or
190      * <li>is {@link Thread#interrupt interrupted} while waiting,
191      * </ul>
192      * then {@link InterruptedException} is thrown and the current thread's
193      * interrupted status is cleared.
194      *
195      * @throws InterruptedException if the current thread is interrupted
196      * while waiting.
197      */
198     public void await() throws InterruptedException JavaDoc {
199         sync.acquireSharedInterruptibly(1);
200     }
201
202     /**
203      * Causes the current thread to wait until the latch has counted down to
204      * zero, unless the thread is {@link Thread#interrupt interrupted},
205      * or the specified waiting time elapses.
206      *
207      * <p>If the current {@link #getCount count} is zero then this method
208      * returns immediately with the value <tt>true</tt>.
209      *
210      * <p>If the current {@link #getCount count} is greater than zero then
211      * the current thread becomes disabled for thread scheduling
212      * purposes and lies dormant until one of three things happen:
213      * <ul>
214      * <li>The count reaches zero due to invocations of the
215      * {@link #countDown} method; or
216      * <li>Some other thread {@link Thread#interrupt interrupts} the current
217      * thread; or
218      * <li>The specified waiting time elapses.
219      * </ul>
220      * <p>If the count reaches zero then the method returns with the
221      * value <tt>true</tt>.
222      * <p>If the current thread:
223      * <ul>
224      * <li>has its interrupted status set on entry to this method; or
225      * <li>is {@link Thread#interrupt interrupted} while waiting,
226      * </ul>
227      * then {@link InterruptedException} is thrown and the current thread's
228      * interrupted status is cleared.
229      *
230      * <p>If the specified waiting time elapses then the value <tt>false</tt>
231      * is returned.
232      * If the time is
233      * less than or equal to zero, the method will not wait at all.
234      *
235      * @param timeout the maximum time to wait
236      * @param unit the time unit of the <tt>timeout</tt> argument.
237      * @return <tt>true</tt> if the count reached zero and <tt>false</tt>
238      * if the waiting time elapsed before the count reached zero.
239      *
240      * @throws InterruptedException if the current thread is interrupted
241      * while waiting.
242      */
243     public boolean await(long timeout, TimeUnit JavaDoc unit)
244         throws InterruptedException JavaDoc {
245         return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
246     }
247
248     /**
249      * Decrements the count of the latch, releasing all waiting threads if
250      * the count reaches zero.
251      * <p>If the current {@link #getCount count} is greater than zero then
252      * it is decremented. If the new count is zero then all waiting threads
253      * are re-enabled for thread scheduling purposes.
254      * <p>If the current {@link #getCount count} equals zero then nothing
255      * happens.
256      */
257     public void countDown() {
258         sync.releaseShared(1);
259     }
260
261     /**
262      * Returns the current count.
263      * <p>This method is typically used for debugging and testing purposes.
264      * @return the current count.
265      */
266     public long getCount() {
267         return sync.getCount();
268     }
269
270     /**
271      * Returns a string identifying this latch, as well as its state.
272      * The state, in brackets, includes the String
273      * &quot;Count =&quot; followed by the current count.
274      * @return a string identifying this latch, as well as its
275      * state
276      */
277     public String JavaDoc toString() {
278         return super.toString() + "[Count = " + sync.getCount() + "]";
279     }
280
281 }
282
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