util/concurrent/FairSemaphore.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

package java.util.concurrent;

/**
 * A semaphore that guarantees that threads invoking
 * any of the {@link #acquire() acquire} methods 
 * are allocated permits in the order in 
 * which their invocation of those methods was processed (first-in-first-out).
 *
 * <p>This class introduces a fairness guarantee that can be useful
 * in some contexts.
 * However, it needs to be realized that the order in which invocations are
 * processed can be different from the order in which an application perceives
 * those invocations to be processed. Effectively, when no permit is available
 * each thread is stored in a FIFO queue. As permits are released, they
 * are allocated to the thread at the head of the queue, and so the order
 * in which threads enter the queue, is the order in which they come out.  
 * This order need not have any relationship to the order in which requests 
 * were made, nor the order in which requests actually return to the caller as 
 * these depend on the underlying thread scheduling, which is not guaranteed 
 * to be predictable or fair.
 *
 * <p>As permits are allocated in-order, this class also provides convenience
 * methods to {@link #acquire(long) acquire} and 
 * {@link #release(long) release} multiple permits at a time. 
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/01/29 07:08:21 $
 * @editor $Author: dholmes $
 *
 */
public class FairSemaphore extends Semaphore {

    /*
     * This differs from Semaphore only in that it uses a
     * FairReentrantLock.  Because the FairReentrantLock guarantees
     * FIFO queuing, we don't need to do anything fancy to prevent
     * overtaking etc.  for the multiple-permit methods. The only
     * minor downside is that multi-permit acquires will sometimes
     * repeatedly wake up finding that they must re-wait. A custom
     * solution could minimize this, at the expense of being slower
     * and more complex for the typical case.
     */

    /**
     * Construct a <tt>FiFoSemaphore</tt> with the given number of
     * permits.
     * @param permits the initial number of permits available
     */
    public FairSemaphore(long permits) { 
        super(permits, new FairReentrantLock()); 
    }

    /**
     * Acquires the given number of permits from this semaphore, 
     * blocking until all are available, 
     * or the thread is {@link Thread#interrupt interrupted}.
     *
     * <p>Acquires the given number of permits, if they are available,
     * and returns immediately,
     * reducing the number of available permits by the given amount.
     *
     * <p>If insufficient permits are available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * one of two things happens:
     * <ul>
     * <li>Some other thread invokes one of the {@link #release() release} 
     * methods for this semaphore, the current thread is next to be assigned
     * permits and the number of available permits satisfies this request; 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
     * for a permit,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared. 
     * Any available permits are assigned to the next waiting thread as if
     * they had been made available by a call to {@link #release()}.
     *
     * @param permits the number of permits to acquire
     *
     * @throws InterruptedException if the current thread is interrupted
     * @throws IllegalArgumentException if permits less than zero.
     *
     * @see Thread#interrupt
     */
    public void acquire(long permits) throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();

        lock.lockInterruptibly();
        try {
            while (count - permits < 0) 
                available.await();
            count -= permits;
        }
        catch (InterruptedException ie) {
            available.signal();
            throw ie;
        }
        finally {
            lock.unlock();
        }
        
    }

    /**
     * Acquires the given number of permits from this semaphore, 
     * blocking until all are available.
     *
     * <p>Acquires the given number of permits, if they are available,
     * and returns immediately,
     * reducing the number of available permits by the given amount.
     *
     * <p>If insufficient permits are available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * some other thread invokes one of the {@link #release() release} 
     * methods for this semaphore, the current thread is next to be assigned
     * permits and the number of available permits satisfies this request.
     *
     * <p>If the current thread
     * is {@link Thread#interrupt interrupted} while waiting
     * for permits then it will continue to wait and its position in the
     * queue is not affected. When the
     * thread does return from this method its interrupt status will be set.
     *
     * @param permits the number of permits to acquire
     * @throws IllegalArgumentException if permits less than zero.
     *
     */
    public void acquireUninterruptibly(long permits) {
        if (permits < 0) throw new IllegalArgumentException();

        lock.lock();
        try {
            while (count - permits < 0) 
                available.awaitUninterruptibly();
            count -= permits;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Acquires the given number of permits from this semaphore, only if 
     * all are available at the  time of invocation.
     * <p>Acquires the given number of permits, if they are available, and 
     * returns immediately, with the value <tt>true</tt>,
     * reducing the number of available permits by the given amount.
     *
     * <p>If insufficient permits are available then this method will return
     * immediately with the value <tt>false</tt> and the number of available
     * permits is unchanged.
     *
     * @param permits the number of permits to acquire
     *
     * @return <tt>true</tt> if the permits were acquired and <tt>false</tt>
     * otherwise.
     * @throws IllegalArgumentException if permits less than zero.
     */
    public boolean tryAcquire(long permits) {
        if (permits < 0) throw new IllegalArgumentException();

        lock.lock();
        try {
            if (count - permits >= 0) {
                count -= permits;
                return true;
            }
            return false;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Acquires the given number of permits from this semaphore, if all 
     * become available within the given waiting time and the
     * current thread has not been {@link Thread#interrupt interrupted}.
     * <p>Acquires the given number of permits, if they are available and 
     * returns immediately, with the value <tt>true</tt>,
     * reducing the number of available permits by the given amount.
     * <p>If insufficient permits are available then
     * the current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of three things happens:
     * <ul>
     * <li>Some other thread invokes one of the {@link #release() release} 
     * methods for this semaphore, the current thread is next to be assigned
     * permits and the number of available permits satisfies this request; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     * <p>If the permits are acquired then the value <tt>true</tt> is returned.
     * <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 to acquire
     * the permits,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     * Any available permits are assigned to the next waiting thread as if
     * they had been made available by a call to {@link #release()}.
     *
     * <p>If the specified waiting time elapses then the value <tt>false</tt>
     * is returned.
     * The given waiting time is a best-effort lower bound. If the time is
     * less than or equal to zero, the method will not wait at all.
     * Any available permits are assigned to the next waiting thread as if
     * they had been made available by a call to {@link #release()}.
     *
     * @param permits the number of permits to acquire
     * @param timeout the maximum time to wait for the permits
     * @param granularity the time unit of the <tt>timeout</tt> argument.
     * @return <tt>true</tt> if all permits were acquired and <tt>false</tt>
     * if the waiting time elapsed before all permits were acquired.
     *
     * @throws InterruptedException if the current thread is interrupted
     * @throws IllegalArgumentException if permits less than zero.
     *
     * @see Thread#interrupt
     *
     */
    public boolean tryAcquire(long permits, long timeout, TimeUnit granularity)
        throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        lock.lockInterruptibly();
        long nanos = granularity.toNanos(timeout);
        try {
            for (;;) {
                if (count - permits >= 0) {
                    count -= permits;
                    return true;
                }
                if (nanos <= 0)
                    return false;
                nanos = available.awaitNanos(nanos);
            }
        }
        catch (InterruptedException ie) {
            available.signal();
            throw ie;
        }
        finally {
            lock.unlock();
        }

    }


    /**
     * Releases the given number of permits, returning them to the semaphore.
     * <p>Releases the given number of permits, increasing the number of 
     * available permits by that amount.
     * If any threads are blocking trying to acquire permits, then the
     * one that has been waiting the longest
     * is selected and given the permits that were just released.
     * If the number of available permits satisfies that thread's request
     * then that thread is re-enabled for thread scheduling purposes; otherwise
     * the thread continues to wait. If there are still permits available
     * after the first thread's request has been satisfied, then those permits
     * are assigned to the next waiting thread. If it is satisfied then it is
     * re-enabled for thread scheduling purposes. This continues until there
     * are insufficient permits to satisfy the next waiting thread, or there
     * are no more waiting threads.
     *
     * <p>There is no requirement that a thread that releases a permit must
     * have acquired that permit by calling {@link Semaphore#acquire acquire}.
     * Correct usage of a semaphore is established by programming convention
     * in the application.
     *
     * @param permits the number of permits to release
     * @throws IllegalArgumentException if permits less than zero.
     */
    public void release(long permits) {
        if (permits < 0) throw new IllegalArgumentException();
        lock.lock();
        try {
            count += permits;
            for (int i = 0; i < permits; ++i)
                available.signal();
        }
        finally {
            lock.unlock();
        }
    }
        

}


Revision:	1.1
Committed:	Tue May 27 15:50:14 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRERELEASE_0_1
Log Message:	Initial implementations
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain. Use, modify, and
4	* redistribute this code in any way without acknowledgement.
5	*/
6
7	package java.util.concurrent;
8
9	/**
10	* A semaphore that guarantees that threads invoking
11	* any of the {@link #acquire() acquire} methods
12	* are allocated permits in the order in
13	* which their invocation of those methods was processed (first-in-first-out).
14	*
15	* <p>This class introduces a fairness guarantee that can be useful
16	* in some contexts.
17	* However, it needs to be realized that the order in which invocations are
18	* processed can be different from the order in which an application perceives
19	* those invocations to be processed. Effectively, when no permit is available
20	* each thread is stored in a FIFO queue. As permits are released, they
21	* are allocated to the thread at the head of the queue, and so the order
22	* in which threads enter the queue, is the order in which they come out.
23	* This order need not have any relationship to the order in which requests
24	* were made, nor the order in which requests actually return to the caller as
25	* these depend on the underlying thread scheduling, which is not guaranteed
26	* to be predictable or fair.
27	*
28	* <p>As permits are allocated in-order, this class also provides convenience
29	* methods to {@link #acquire(long) acquire} and
30	* {@link #release(long) release} multiple permits at a time.
31	*
32	* @since 1.5
33	* @spec JSR-166
34	* @revised $Date: 2003/01/29 07:08:21 $
35	* @editor $Author: dholmes $
36	*
37	*/
38	public class FairSemaphore extends Semaphore {
39
40	/*
41	* This differs from Semaphore only in that it uses a
42	* FairReentrantLock. Because the FairReentrantLock guarantees
43	* FIFO queuing, we don't need to do anything fancy to prevent
44	* overtaking etc. for the multiple-permit methods. The only
45	* minor downside is that multi-permit acquires will sometimes
46	* repeatedly wake up finding that they must re-wait. A custom
47	* solution could minimize this, at the expense of being slower
48	* and more complex for the typical case.
49	*/
50
51	/**
52	* Construct a <tt>FiFoSemaphore</tt> with the given number of
53	* permits.
54	* @param permits the initial number of permits available
55	*/
56	public FairSemaphore(long permits) {
57	super(permits, new FairReentrantLock());
58	}
59
60	/**
61	* Acquires the given number of permits from this semaphore,
62	* blocking until all are available,
63	* or the thread is {@link Thread#interrupt interrupted}.
64	*
65	* <p>Acquires the given number of permits, if they are available,
66	* and returns immediately,
67	* reducing the number of available permits by the given amount.
68	*
69	* <p>If insufficient permits are available then the current thread becomes
70	* disabled for thread scheduling purposes and lies dormant until
71	* one of two things happens:
72	* <ul>
73	* <li>Some other thread invokes one of the {@link #release() release}
74	* methods for this semaphore, the current thread is next to be assigned
75	* permits and the number of available permits satisfies this request; or
76	* <li>Some other thread {@link Thread#interrupt interrupts} the current
77	* thread.
78	* </ul>
79	*
80	* <p>If the current thread:
81	* <ul>
82	* <li>has its interrupted status set on entry to this method; or
83	* <li>is {@link Thread#interrupt interrupted} while waiting
84	* for a permit,
85	* </ul>
86	* then {@link InterruptedException} is thrown and the current thread's
87	* interrupted status is cleared.
88	* Any available permits are assigned to the next waiting thread as if
89	* they had been made available by a call to {@link #release()}.
90	*
91	* @param permits the number of permits to acquire
92	*
93	* @throws InterruptedException if the current thread is interrupted
94	* @throws IllegalArgumentException if permits less than zero.
95	*
96	* @see Thread#interrupt
97	*/
98	public void acquire(long permits) throws InterruptedException {
99	if (permits < 0) throw new IllegalArgumentException();
100
101	lock.lockInterruptibly();
102	try {
103	while (count - permits < 0)
104	available.await();
105	count -= permits;
106	}
107	catch (InterruptedException ie) {
108	available.signal();
109	throw ie;
110	}
111	finally {
112	lock.unlock();
113	}
114
115	}
116
117	/**
118	* Acquires the given number of permits from this semaphore,
119	* blocking until all are available.
120	*
121	* <p>Acquires the given number of permits, if they are available,
122	* and returns immediately,
123	* reducing the number of available permits by the given amount.
124	*
125	* <p>If insufficient permits are available then the current thread becomes
126	* disabled for thread scheduling purposes and lies dormant until
127	* some other thread invokes one of the {@link #release() release}
128	* methods for this semaphore, the current thread is next to be assigned
129	* permits and the number of available permits satisfies this request.
130	*
131	* <p>If the current thread
132	* is {@link Thread#interrupt interrupted} while waiting
133	* for permits then it will continue to wait and its position in the
134	* queue is not affected. When the
135	* thread does return from this method its interrupt status will be set.
136	*
137	* @param permits the number of permits to acquire
138	* @throws IllegalArgumentException if permits less than zero.
139	*
140	*/
141	public void acquireUninterruptibly(long permits) {
142	if (permits < 0) throw new IllegalArgumentException();
143
144	lock.lock();
145	try {
146	while (count - permits < 0)
147	available.awaitUninterruptibly();
148	count -= permits;
149	}
150	finally {
151	lock.unlock();
152	}
153	}
154
155	/**
156	* Acquires the given number of permits from this semaphore, only if
157	* all are available at the time of invocation.
158	* <p>Acquires the given number of permits, if they are available, and
159	* returns immediately, with the value <tt>true</tt>,
160	* reducing the number of available permits by the given amount.
161	*
162	* <p>If insufficient permits are available then this method will return
163	* immediately with the value <tt>false</tt> and the number of available
164	* permits is unchanged.
165	*
166	* @param permits the number of permits to acquire
167	*
168	* @return <tt>true</tt> if the permits were acquired and <tt>false</tt>
169	* otherwise.
170	* @throws IllegalArgumentException if permits less than zero.
171	*/
172	public boolean tryAcquire(long permits) {
173	if (permits < 0) throw new IllegalArgumentException();
174
175	lock.lock();
176	try {
177	if (count - permits >= 0) {
178	count -= permits;
179	return true;
180	}
181	return false;
182	}
183	finally {
184	lock.unlock();
185	}
186	}
187
188	/**
189	* Acquires the given number of permits from this semaphore, if all
190	* become available within the given waiting time and the
191	* current thread has not been {@link Thread#interrupt interrupted}.
192	* <p>Acquires the given number of permits, if they are available and
193	* returns immediately, with the value <tt>true</tt>,
194	* reducing the number of available permits by the given amount.
195	* <p>If insufficient permits are available then
196	* the current thread becomes disabled for thread scheduling
197	* purposes and lies dormant until one of three things happens:
198	* <ul>
199	* <li>Some other thread invokes one of the {@link #release() release}
200	* methods for this semaphore, the current thread is next to be assigned
201	* permits and the number of available permits satisfies this request; or
202	* <li>Some other thread {@link Thread#interrupt interrupts} the current
203	* thread; or
204	* <li>The specified waiting time elapses.
205	* </ul>
206	* <p>If the permits are acquired then the value <tt>true</tt> is returned.
207	* <p>If the current thread:
208	* <ul>
209	* <li>has its interrupted status set on entry to this method; or
210	* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
211	* the permits,
212	* </ul>
213	* then {@link InterruptedException} is thrown and the current thread's
214	* interrupted status is cleared.
215	* Any available permits are assigned to the next waiting thread as if
216	* they had been made available by a call to {@link #release()}.
217	*
218	* <p>If the specified waiting time elapses then the value <tt>false</tt>
219	* is returned.
220	* The given waiting time is a best-effort lower bound. If the time is
221	* less than or equal to zero, the method will not wait at all.
222	* Any available permits are assigned to the next waiting thread as if
223	* they had been made available by a call to {@link #release()}.
224	*
225	* @param permits the number of permits to acquire
226	* @param timeout the maximum time to wait for the permits
227	* @param granularity the time unit of the <tt>timeout</tt> argument.
228	* @return <tt>true</tt> if all permits were acquired and <tt>false</tt>
229	* if the waiting time elapsed before all permits were acquired.
230	*
231	* @throws InterruptedException if the current thread is interrupted
232	* @throws IllegalArgumentException if permits less than zero.
233	*
234	* @see Thread#interrupt
235	*
236	*/
237	public boolean tryAcquire(long permits, long timeout, TimeUnit granularity)
238	throws InterruptedException {
239	if (permits < 0) throw new IllegalArgumentException();
240	lock.lockInterruptibly();
241	long nanos = granularity.toNanos(timeout);
242	try {
243	for (;;) {
244	if (count - permits >= 0) {
245	count -= permits;
246	return true;
247	}
248	if (nanos <= 0)
249	return false;
250	nanos = available.awaitNanos(nanos);
251	}
252	}
253	catch (InterruptedException ie) {
254	available.signal();
255	throw ie;
256	}
257	finally {
258	lock.unlock();
259	}
260
261	}
262
263
264
265	/**
266	* Releases the given number of permits, returning them to the semaphore.
267	* <p>Releases the given number of permits, increasing the number of
268	* available permits by that amount.
269	* If any threads are blocking trying to acquire permits, then the
270	* one that has been waiting the longest
271	* is selected and given the permits that were just released.
272	* If the number of available permits satisfies that thread's request
273	* then that thread is re-enabled for thread scheduling purposes; otherwise
274	* the thread continues to wait. If there are still permits available
275	* after the first thread's request has been satisfied, then those permits
276	* are assigned to the next waiting thread. If it is satisfied then it is
277	* re-enabled for thread scheduling purposes. This continues until there
278	* are insufficient permits to satisfy the next waiting thread, or there
279	* are no more waiting threads.
280	*
281	* <p>There is no requirement that a thread that releases a permit must
282	* have acquired that permit by calling {@link Semaphore#acquire acquire}.
283	* Correct usage of a semaphore is established by programming convention
284	* in the application.
285	*
286	* @param permits the number of permits to release
287	* @throws IllegalArgumentException if permits less than zero.
288	*/
289	public void release(long permits) {
290	if (permits < 0) throw new IllegalArgumentException();
291	lock.lock();
292	try {
293	count += permits;
294	for (int i = 0; i < permits; ++i)
295	available.signal();
296	}
297	finally {
298	lock.unlock();
299	}
300	}
301
302
303
304	}
305
306
307
308
309