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.*; // for javadoc (till 6280605 is fixed)
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>
 *
 * <p> Memory visibility effects: Actions in a thread prior to calling
 * <tt>countDown()</tt> <a
 * href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a successful return from a corresponding
 * <tt>await()</tt> in another thread.
 *
 * @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 {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        public int tryAcquireShared(int acquires) {
            return getState() == 0? 1 : -1;
        }

        public boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    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.tryAcquireSharedNanos(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();
    }

    /**
     * Returns a string identifying this latch, as well as its state.
     * The state, in brackets, includes the String
     * &quot;Count =&quot; followed by the current count.
     * @return a string identifying this latch, as well as its
     * state
     */
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }

}
Revision:	1.29
Committed:	Fri Sep 2 01:03:08 2005 UTC (18 years, 9 months ago) by brian
Branch:	MAIN
Changes since 1.28:	+6 -0 lines
Log Message:	Happens-before markup
#	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.*; // for javadoc (till 6280605 is fixed)
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	* <p> Memory visibility effects: Actions in a thread prior to calling
124	* <tt>countDown()</tt> <a
125	* href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
126	* actions following a successful return from a corresponding
127	* <tt>await()</tt> in another thread.
128	*
129	* @since 1.5
130	* @author Doug Lea
131	*/
132	public class CountDownLatch {
133	/**
134	* Synchronization control For CountDownLatch.
135	* Uses AQS state to represent count.
136	*/
137	private static final class Sync extends AbstractQueuedSynchronizer {
138	private static final long serialVersionUID = 4982264981922014374L;
139
140	Sync(int count) {
141	setState(count);
142	}
143
144	int getCount() {
145	return getState();
146	}
147
148	public int tryAcquireShared(int acquires) {
149	return getState() == 0? 1 : -1;
150	}
151
152	public boolean tryReleaseShared(int releases) {
153	// Decrement count; signal when transition to zero
154	for (;;) {
155	int c = getState();
156	if (c == 0)
157	return false;
158	int nextc = c-1;
159	if (compareAndSetState(c, nextc))
160	return nextc == 0;
161	}
162	}
163	}
164
165	private final Sync sync;
166	/**
167	* Constructs a <tt>CountDownLatch</tt> initialized with the given
168	* count.
169	*
170	* @param count the number of times {@link #countDown} must be invoked
171	* before threads can pass through {@link #await}.
172	*
173	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
174	*/
175	public CountDownLatch(int count) {
176	if (count < 0) throw new IllegalArgumentException("count < 0");
177	this.sync = new Sync(count);
178	}
179
180	/**
181	* Causes the current thread to wait until the latch has counted down to
182	* zero, unless the thread is {@link Thread#interrupt interrupted}.
183	*
184	* <p>If the current {@link #getCount count} is zero then this method
185	* returns immediately.
186	* <p>If the current {@link #getCount count} is greater than zero then
187	* the current thread becomes disabled for thread scheduling
188	* purposes and lies dormant until one of two things happen:
189	* <ul>
190	* <li>The count reaches zero due to invocations of the
191	* {@link #countDown} method; or
192	* <li>Some other thread {@link Thread#interrupt interrupts} the current
193	* thread.
194	* </ul>
195	* <p>If the current thread:
196	* <ul>
197	* <li>has its interrupted status set on entry to this method; or
198	* <li>is {@link Thread#interrupt interrupted} while waiting,
199	* </ul>
200	* then {@link InterruptedException} is thrown and the current thread's
201	* interrupted status is cleared.
202	*
203	* @throws InterruptedException if the current thread is interrupted
204	* while waiting.
205	*/
206	public void await() throws InterruptedException {
207	sync.acquireSharedInterruptibly(1);
208	}
209
210	/**
211	* Causes the current thread to wait until the latch has counted down to
212	* zero, unless the thread is {@link Thread#interrupt interrupted},
213	* or the specified waiting time elapses.
214	*
215	* <p>If the current {@link #getCount count} is zero then this method
216	* returns immediately with the value <tt>true</tt>.
217	*
218	* <p>If the current {@link #getCount count} is greater than zero then
219	* the current thread becomes disabled for thread scheduling
220	* purposes and lies dormant until one of three things happen:
221	* <ul>
222	* <li>The count reaches zero due to invocations of the
223	* {@link #countDown} method; or
224	* <li>Some other thread {@link Thread#interrupt interrupts} the current
225	* thread; or
226	* <li>The specified waiting time elapses.
227	* </ul>
228	* <p>If the count reaches zero then the method returns with the
229	* value <tt>true</tt>.
230	* <p>If the current thread:
231	* <ul>
232	* <li>has its interrupted status set on entry to this method; or
233	* <li>is {@link Thread#interrupt interrupted} while waiting,
234	* </ul>
235	* then {@link InterruptedException} is thrown and the current thread's
236	* interrupted status is cleared.
237	*
238	* <p>If the specified waiting time elapses then the value <tt>false</tt>
239	* is returned.
240	* If the time is
241	* less than or equal to zero, the method will not wait at all.
242	*
243	* @param timeout the maximum time to wait
244	* @param unit the time unit of the <tt>timeout</tt> argument.
245	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
246	* if the waiting time elapsed before the count reached zero.
247	*
248	* @throws InterruptedException if the current thread is interrupted
249	* while waiting.
250	*/
251	public boolean await(long timeout, TimeUnit unit)
252	throws InterruptedException {
253	return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
254	}
255
256	/**
257	* Decrements the count of the latch, releasing all waiting threads if
258	* the count reaches zero.
259	* <p>If the current {@link #getCount count} is greater than zero then
260	* it is decremented. If the new count is zero then all waiting threads
261	* are re-enabled for thread scheduling purposes.
262	* <p>If the current {@link #getCount count} equals zero then nothing
263	* happens.
264	*/
265	public void countDown() {
266	sync.releaseShared(1);
267	}
268
269	/**
270	* Returns the current count.
271	* <p>This method is typically used for debugging and testing purposes.
272	* @return the current count.
273	*/
274	public long getCount() {
275	return sync.getCount();
276	}
277
278	/**
279	* Returns a string identifying this latch, as well as its state.
280	* The state, in brackets, includes the String
281	* "Count =" followed by the current count.
282	* @return a string identifying this latch, as well as its
283	* state
284	*/
285	public String toString() {
286	return super.toString() + "[Count = " + sync.getCount() + "]";
287	}
288
289	}