util/concurrent/CountDownLatch.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;

/**
 * A synchronization aid that allows one or more threads to wait until
 * a set of operations being performed in other threads completes.
 *
 * <p>A <tt>CountDownLatch</tt> is initialized with a given
 * <em>count</em>.  The {@link #await await} methods block until the current
 * {@link #getCount count} reaches zero due to invocations of the
 * {@link #countDown} method, after which all waiting threads are
 * released and any subsequent invocations of {@link #await await} return
 * immediately. This is a one-shot phenomenon -- the count cannot be
 * reset.  If you need a version that resets the count, consider using
 * a {@link CyclicBarrier}.
 *
 * <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
 * and can be used for a number of purposes.  A
 * <tt>CountDownLatch</tt> initialized with a count of one serves as a
 * simple on/off latch, or gate: all threads invoking {@link #await await}
 * wait at the gate until it is opened by a thread invoking {@link
 * #countDown}.  A <tt>CountDownLatch</tt> initialized to <em>N</em>
 * can be used to make one thread wait until <em>N</em> threads have
 * completed some action, or some action has been completed N times.
 * <p>A useful property of a <tt>CountDownLatch</tt> is that it
 * doesn't require that threads calling <tt>countDown</tt> wait for
 * the count to reach zero before proceeding, it simply prevents any
 * thread from proceeding past an {@link #await await} until all
 * threads could pass.
 *
 * <p><b>Sample usage:</b> Here is a pair of classes in which a group
 * of worker threads use two countdown latches:
 * <ul>
 * <li>The first is a start signal that prevents any worker from proceeding
 * until the driver is ready for them to proceed;
 * <li>The second is a completion signal that allows the driver to wait
 * until all workers have completed.
 * </ul>
 *
 * <pre>
 * class Driver { // ...
 *   void main() throws InterruptedException {
 *     CountDownLatch startSignal = new CountDownLatch(1);
 *     CountDownLatch doneSignal = new CountDownLatch(N);
 *
 *     for (int i = 0; i < N; ++i) // create and start threads
 *       new Thread(new Worker(startSignal, doneSignal)).start();
 *
 *     doSomethingElse();            // don't let run yet
 *     startSignal.countDown();      // let all threads proceed
 *     doSomethingElse();
 *     doneSignal.await();           // wait for all to finish
 *   }
 * }
 *
 * class Worker implements Runnable {
 *   private final CountDownLatch startSignal;
 *   private final CountDownLatch doneSignal;
 *   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
 *      this.startSignal = startSignal;
 *      this.doneSignal = doneSignal;
 *   }
 *   public void run() {
 *      try {
 *        startSignal.await();
 *        doWork();
 *        doneSignal.countDown();
 *      } catch (InterruptedException ex) {} // return;
 *   }
 *
 *   void doWork() { ... }
 * }
 *
 * </pre>
 *
 * <p>Another typical usage would be to divide a problem into N parts,
 * describe each part with a Runnable that executes that portion and
 * counts down on the latch, and queue all the Runnables to an
 * Executor.  When all sub-parts are complete, the coordinating thread
 * will be able to pass through await. (When threads must repeatedly
 * count down in this way, instead use a {@link CyclicBarrier}.)
 *
 * <pre>
 * class Driver2 { // ...
 *   void main() throws InterruptedException {
 *     CountDownLatch doneSignal = new CountDownLatch(N);
 *     Executor e = ...
 *
 *     for (int i = 0; i < N; ++i) // create and start threads
 *       e.execute(new WorkerRunnable(doneSignal, i));
 *
 *     doneSignal.await();           // wait for all to finish
 *   }
 * }
 *
 * class WorkerRunnable implements Runnable {
 *   private final CountDownLatch doneSignal;
 *   private final int i;
 *   WorkerRunnable(CountDownLatch doneSignal, int i) {
 *      this.doneSignal = doneSignal;
 *      this.i = i;
 *   }
 *   public void run() {
 *      try {
 *        doWork(i);
 *        doneSignal.countDown();
 *      } catch (InterruptedException ex) {} // return;
 *   }
 *
 *   void doWork() { ... }
 * }
 *
 * </pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CountDownLatch {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        Sync(int count) {
            setState(count); 
        }
        
        int getCount() {
            return getState();
        }

        public int tryAcquireSharedState(boolean isQueued, int acquires) {
            return getState() == 0? 1 : -1;
        }
        
        public boolean releaseSharedState(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                if (compareAndSetState(c, c-1)) 
                    return c == 1;
            }
        }
    }

    private final Sync sync;
    /**
     * Constructs a <tt>CountDownLatch</tt> initialized with the given
     * count.
     * 
     * @param count the number of times {@link #countDown} must be invoked
     * before threads can pass through {@link #await}.
     *
     * @throws IllegalArgumentException if <tt>count</tt> is less than zero.
     */
    public CountDownLatch(int count) { 
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to 
     * zero, unless the thread is {@link Thread#interrupt interrupted}.
     *
     * <p>If the current {@link #getCount count} is zero then this method
     * returns immediately.
     * <p>If the current {@link #getCount count} is greater than zero then
     * the current thread becomes disabled for thread scheduling 
     * purposes and lies dormant until one of two things happen:
     * <ul>
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or 
     * <li>is {@link Thread#interrupt interrupted} while waiting, 
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * @throws InterruptedException if the current thread is interrupted
     * while waiting.
     */
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to 
     * zero, unless the thread is {@link Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     * <p>If the current {@link #getCount count} is zero then this method
     * returns immediately with the value <tt>true</tt>.
     *
     * <p>If the current {@link #getCount count} is greater than zero then
     * the current thread becomes disabled for thread scheduling 
     * purposes and lies dormant until one of three things happen:
     * <ul>
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     * <p>If the count reaches zero then the method returns with the
     * value <tt>true</tt>.
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or 
     * <li>is {@link Thread#interrupt interrupted} while waiting, 
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * <p>If the specified waiting time elapses then the value <tt>false</tt>
     * is returned.
     * If the time is 
     * less than or equal to zero, the method will not wait at all.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the <tt>timeout</tt> argument.
     * @return <tt>true</tt> if the count reached zero  and <tt>false</tt>
     * if the waiting time elapsed before the count reached zero.
     *
     * @throws InterruptedException if the current thread is interrupted
     * while waiting.
     */
    public boolean await(long timeout, TimeUnit unit) 
        throws InterruptedException {
        return sync.acquireSharedTimed(1, unit.toNanos(timeout));
    }

    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     * <p>If the current {@link #getCount count} is greater than zero then
     * it is decremented. If the new count is zero then all waiting threads
     * are re-enabled for thread scheduling purposes.
     * <p>If the current {@link #getCount count} equals zero then nothing
     * happens.
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     * Returns the current count.
     * <p>This method is typically used for debugging and testing purposes.
     * @return the current count.
     */
    public long getCount() {
        return sync.getCount();
    }
}
Revision:	1.17
Committed:	Wed Dec 31 21:30:00 2003 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.16:	+15 -8 lines
Log Message:	AQS API improvements
#	Content
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/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.*;
9	import java.util.concurrent.atomic.*;
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 <tt>CountDownLatch</tt> is initialized with a given
16	* <em>count</em>. The {@link #await await} methods block until the current
17	* {@link #getCount count} reaches zero due to invocations of the
18	* {@link #countDown} method, after which all waiting threads are
19	* released and any subsequent invocations of {@link #await await} return
20	* immediately. This is a one-shot phenomenon -- the count cannot be
21	* reset. If you need a version that resets the count, consider using
22	* a {@link CyclicBarrier}.
23	*
24	* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
25	* and can be used for a number of purposes. A
26	* <tt>CountDownLatch</tt> initialized with a count of one serves as a
27	* simple on/off latch, or gate: all threads invoking {@link #await await}
28	* wait at the gate until it is opened by a thread invoking {@link
29	* #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
30	* can be used to make one thread wait until <em>N</em> threads have
31	* completed some action, or some action has been completed N times.
32	* <p>A useful property of a <tt>CountDownLatch</tt> is that it
33	* doesn't require that threads calling <tt>countDown</tt> 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>
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	* }
80	*
81	* </pre>
82	*
83	* <p>Another typical usage would be to divide a problem into N parts,
84	* describe each part with a Runnable that executes that portion and
85	* counts down on the latch, and queue all the Runnables to an
86	* Executor. When all sub-parts are complete, the coordinating thread
87	* will be able to pass through await. (When threads must repeatedly
88	* count down in this way, instead use a {@link CyclicBarrier}.)
89	*
90	* <pre>
91	* class Driver2 { // ...
92	* void main() throws InterruptedException {
93	* CountDownLatch doneSignal = new CountDownLatch(N);
94	* Executor e = ...
95	*
96	* for (int i = 0; i < N; ++i) // create and start threads
97	* e.execute(new WorkerRunnable(doneSignal, i));
98	*
99	* doneSignal.await(); // wait for all to finish
100	* }
101	* }
102	*
103	* class WorkerRunnable implements Runnable {
104	* private final CountDownLatch doneSignal;
105	* private final int i;
106	* WorkerRunnable(CountDownLatch doneSignal, int i) {
107	* this.doneSignal = doneSignal;
108	* this.i = i;
109	* }
110	* public void run() {
111	* try {
112	* doWork(i);
113	* doneSignal.countDown();
114	* } catch (InterruptedException ex) {} // return;
115	* }
116	*
117	* void doWork() { ... }
118	* }
119	*
120	* </pre>
121	*
122	* @since 1.5
123	* @author Doug Lea
124	*/
125	public class CountDownLatch {
126	/**
127	* Synchronization control For CountDownLatch.
128	* Uses AQS state to represent count.
129	*/
130	private static final class Sync extends AbstractQueuedSynchronizer {
131	Sync(int count) {
132	setState(count);
133	}
134
135	int getCount() {
136	return getState();
137	}
138
139	public int tryAcquireSharedState(boolean isQueued, int acquires) {
140	return getState() == 0? 1 : -1;
141	}
142
143	public boolean releaseSharedState(int releases) {
144	// Decrement count; signal when transition to zero
145	for (;;) {
146	int c = getState();
147	if (c == 0)
148	return false;
149	if (compareAndSetState(c, c-1))
150	return c == 1;
151	}
152	}
153	}
154
155	private final Sync sync;
156	/**
157	* Constructs a <tt>CountDownLatch</tt> initialized with the given
158	* count.
159	*
160	* @param count the number of times {@link #countDown} must be invoked
161	* before threads can pass through {@link #await}.
162	*
163	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
164	*/
165	public CountDownLatch(int count) {
166	if (count < 0) throw new IllegalArgumentException("count < 0");
167	this.sync = new Sync(count);
168	}
169
170	/**
171	* Causes the current thread to wait until the latch has counted down to
172	* zero, unless the thread is {@link Thread#interrupt interrupted}.
173	*
174	* <p>If the current {@link #getCount count} is zero then this method
175	* returns immediately.
176	* <p>If the current {@link #getCount count} is greater than zero then
177	* the current thread becomes disabled for thread scheduling
178	* purposes and lies dormant until one of two things happen:
179	* <ul>
180	* <li>The count reaches zero due to invocations of the
181	* {@link #countDown} method; or
182	* <li>Some other thread {@link Thread#interrupt interrupts} the current
183	* thread.
184	* </ul>
185	* <p>If the current thread:
186	* <ul>
187	* <li>has its interrupted status set on entry to this method; or
188	* <li>is {@link Thread#interrupt interrupted} while waiting,
189	* </ul>
190	* then {@link InterruptedException} is thrown and the current thread's
191	* interrupted status is cleared.
192	*
193	* @throws InterruptedException if the current thread is interrupted
194	* while waiting.
195	*/
196	public void await() throws InterruptedException {
197	sync.acquireSharedInterruptibly(1);
198	}
199
200	/**
201	* Causes the current thread to wait until the latch has counted down to
202	* zero, unless the thread is {@link Thread#interrupt interrupted},
203	* or the specified waiting time elapses.
204	*
205	* <p>If the current {@link #getCount count} is zero then this method
206	* returns immediately with the value <tt>true</tt>.
207	*
208	* <p>If the current {@link #getCount count} is greater than zero then
209	* the current thread becomes disabled for thread scheduling
210	* purposes and lies dormant until one of three things happen:
211	* <ul>
212	* <li>The count reaches zero due to invocations of the
213	* {@link #countDown} method; or
214	* <li>Some other thread {@link Thread#interrupt interrupts} the current
215	* thread; or
216	* <li>The specified waiting time elapses.
217	* </ul>
218	* <p>If the count reaches zero then the method returns with the
219	* value <tt>true</tt>.
220	* <p>If the current thread:
221	* <ul>
222	* <li>has its interrupted status set on entry to this method; or
223	* <li>is {@link Thread#interrupt interrupted} while waiting,
224	* </ul>
225	* then {@link InterruptedException} is thrown and the current thread's
226	* interrupted status is cleared.
227	*
228	* <p>If the specified waiting time elapses then the value <tt>false</tt>
229	* is returned.
230	* If the time is
231	* less than or equal to zero, the method will not wait at all.
232	*
233	* @param timeout the maximum time to wait
234	* @param unit the time unit of the <tt>timeout</tt> argument.
235	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
236	* if the waiting time elapsed before the count reached zero.
237	*
238	* @throws InterruptedException if the current thread is interrupted
239	* while waiting.
240	*/
241	public boolean await(long timeout, TimeUnit unit)
242	throws InterruptedException {
243	return sync.acquireSharedTimed(1, unit.toNanos(timeout));
244	}
245
246	/**
247	* Decrements the count of the latch, releasing all waiting threads if
248	* the count reaches zero.
249	* <p>If the current {@link #getCount count} is greater than zero then
250	* it is decremented. If the new count is zero then all waiting threads
251	* are re-enabled for thread scheduling purposes.
252	* <p>If the current {@link #getCount count} equals zero then nothing
253	* happens.
254	*/
255	public void countDown() {
256	sync.releaseShared(1);
257	}
258
259	/**
260	* Returns the current count.
261	* <p>This method is typically used for debugging and testing purposes.
262	* @return the current count.
263	*/
264	public long getCount() {
265	return sync.getCount();
266	}
267	}