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/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.concurrent.locks.AbstractQueuedSynchronizer;

/**
 * A synchronization aid that allows one or more threads to wait until
 * a set of operations being performed in other threads completes.
 *
 * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
 * The {@link #await await} methods block until the current 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 {@code CountDownLatch} is a versatile synchronization tool
 * and can be used for a number of purposes.  A
 * {@code CountDownLatch} 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 {@code CountDownLatch} 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 {@code CountDownLatch} is that it
 * doesn't require that threads calling {@code countDown} 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> {@code
 * 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> {@code
 * 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() {
 *     doWork();
 *     doneSignal.countDown();
 *   }
 *
 *   void doWork() { ... }
 * }}</pre>
 *
 * <p>Memory consistency effects: Until the count reaches
 * zero, actions in a thread prior to calling
 * {@code countDown()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a successful return from a corresponding
 * {@code await()} 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();
        }

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

        protected 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 {@code CountDownLatch} 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 {@code count} is negative
     */
    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 {@linkplain Thread#interrupt interrupted}.
     *
     * <p>If the current count is zero then this method returns immediately.
     *
     * <p>If the current 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 {@linkplain 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 {@linkplain 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 {@linkplain Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     * <p>If the current count is zero then this method returns immediately
     * with the value {@code true}.
     *
     * <p>If the current 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 {@linkplain 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 {@code true}.
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain 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 {@code false}
     * 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 {@code timeout} argument
     * @return {@code true} if the count reached zero and {@code false}
     *         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 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 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 {@code "Count ="}
     * 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:	1.45
Committed:	Fri Nov 27 17:41:59 2020 UTC (3 years, 5 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.44:	+3 -5 lines
Log Message:	Incorporate snippets code improvements from Pavel Rappo
#	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/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	* doWork();
110	* doneSignal.countDown();
111	* }
112	*
113	* void doWork() { ... }
114	* }}</pre>
115	*
116	* <p>Memory consistency effects: Until the count reaches
117	* zero, actions in a thread prior to calling
118	* {@code countDown()}
119	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
120	* actions following a successful return from a corresponding
121	* {@code await()} in another thread.
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	private static final long serialVersionUID = 4982264981922014374L;
133
134	Sync(int count) {
135	setState(count);
136	}
137
138	int getCount() {
139	return getState();
140	}
141
142	protected int tryAcquireShared(int acquires) {
143	return (getState() == 0) ? 1 : -1;
144	}
145
146	protected boolean tryReleaseShared(int releases) {
147	// Decrement count; signal when transition to zero
148	for (;;) {
149	int c = getState();
150	if (c == 0)
151	return false;
152	int nextc = c - 1;
153	if (compareAndSetState(c, nextc))
154	return nextc == 0;
155	}
156	}
157	}
158
159	private final Sync sync;
160
161	/**
162	* Constructs a {@code CountDownLatch} initialized with the given count.
163	*
164	* @param count the number of times {@link #countDown} must be invoked
165	* before threads can pass through {@link #await}
166	* @throws IllegalArgumentException if {@code count} is negative
167	*/
168	public CountDownLatch(int count) {
169	if (count < 0) throw new IllegalArgumentException("count < 0");
170	this.sync = new Sync(count);
171	}
172
173	/**
174	* Causes the current thread to wait until the latch has counted down to
175	* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
176	*
177	* <p>If the current count is zero then this method returns immediately.
178	*
179	* <p>If the current count is greater than zero then the current
180	* thread becomes disabled for thread scheduling purposes and lies
181	* dormant until one of two things happen:
182	* <ul>
183	* <li>The count reaches zero due to invocations of the
184	* {@link #countDown} method; or
185	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
186	* the current thread.
187	* </ul>
188	*
189	* <p>If the current thread:
190	* <ul>
191	* <li>has its interrupted status set on entry to this method; or
192	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
193	* </ul>
194	* then {@link InterruptedException} is thrown and the current thread's
195	* interrupted status is cleared.
196	*
197	* @throws InterruptedException if the current thread is interrupted
198	* while waiting
199	*/
200	public void await() throws InterruptedException {
201	sync.acquireSharedInterruptibly(1);
202	}
203
204	/**
205	* Causes the current thread to wait until the latch has counted down to
206	* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
207	* or the specified waiting time elapses.
208	*
209	* <p>If the current count is zero then this method returns immediately
210	* with the value {@code true}.
211	*
212	* <p>If the current count is greater than zero then the current
213	* thread becomes disabled for thread scheduling purposes and lies
214	* dormant until one of three things happen:
215	* <ul>
216	* <li>The count reaches zero due to invocations of the
217	* {@link #countDown} method; or
218	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
219	* the current thread; or
220	* <li>The specified waiting time elapses.
221	* </ul>
222	*
223	* <p>If the count reaches zero then the method returns with the
224	* value {@code true}.
225	*
226	* <p>If the current thread:
227	* <ul>
228	* <li>has its interrupted status set on entry to this method; or
229	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
230	* </ul>
231	* then {@link InterruptedException} is thrown and the current thread's
232	* interrupted status is cleared.
233	*
234	* <p>If the specified waiting time elapses then the value {@code false}
235	* is returned. If the time is less than or equal to zero, the method
236	* will not wait at all.
237	*
238	* @param timeout the maximum time to wait
239	* @param unit the time unit of the {@code timeout} argument
240	* @return {@code true} if the count reached zero and {@code false}
241	* if the waiting time elapsed before the count reached zero
242	* @throws InterruptedException if the current thread is interrupted
243	* while waiting
244	*/
245	public boolean await(long timeout, TimeUnit unit)
246	throws InterruptedException {
247	return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
248	}
249
250	/**
251	* Decrements the count of the latch, releasing all waiting threads if
252	* the count reaches zero.
253	*
254	* <p>If the current count is greater than zero then it is decremented.
255	* If the new count is zero then all waiting threads are re-enabled for
256	* thread scheduling purposes.
257	*
258	* <p>If the current count equals zero then nothing happens.
259	*/
260	public void countDown() {
261	sync.releaseShared(1);
262	}
263
264	/**
265	* Returns the current count.
266	*
267	* <p>This method is typically used for debugging and testing purposes.
268	*
269	* @return the current count
270	*/
271	public long getCount() {
272	return sync.getCount();
273	}
274
275	/**
276	* Returns a string identifying this latch, as well as its state.
277	* The state, in brackets, includes the String {@code "Count ="}
278	* followed by the current count.
279	*
280	* @return a string identifying this latch, as well as its state
281	*/
282	public String toString() {
283	return super.toString() + "[Count = " + sync.getCount() + "]";
284	}
285	}