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 guaranteeing 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; FIFO). Note that FIFO ordering necessarily
 * applies to specific internal points of execution within these
 * methods.  So, it is possible for one thread to invoke
 * <tt>acquire</tt> before another, but reach the ordering point after
 * the other, and similarly upon return from the method.
 *
 * <p>Because 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: 2003/05/27 15:50:14 $
 * @editor $Author: dl $
 *
 */
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:	1.2
Committed:	Sat Jun 7 16:07:09 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_1
Changes since 1.1:	+13 -30 lines
Log Message:	Simpified Javadoc account of fairness
#	User	Rev	Content
1	dl	1.1	/*
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	dl	1.2	* A semaphore guaranteeing that threads invoking any of the {@link
11			* #acquire() acquire} methods are allocated permits in the order in
12			* which their invocation of those methods was processed
13			* (first-in-first-out; FIFO). Note that FIFO ordering necessarily
14			* applies to specific internal points of execution within these
15			* methods. So, it is possible for one thread to invoke
16			* <tt>acquire</tt> before another, but reach the ordering point after
17			* the other, and similarly upon return from the method.
18	dl	1.1	*
19	dl	1.2	* <p>Because permits are allocated in-order, this class also provides
20			* convenience methods to {@link #acquire(long) acquire} and {@link
21			* #release(long) release} multiple permits at a time.
22	dl	1.1	*
23			* @since 1.5
24			* @spec JSR-166
25	dl	1.2	* @revised $Date: 2003/05/27 15:50:14 $
26			* @editor $Author: dl $
27	dl	1.1	*
28			*/
29			public class FairSemaphore extends Semaphore {
30
31			/*
32			* This differs from Semaphore only in that it uses a
33			* FairReentrantLock. Because the FairReentrantLock guarantees
34			* FIFO queuing, we don't need to do anything fancy to prevent
35			* overtaking etc. for the multiple-permit methods. The only
36			* minor downside is that multi-permit acquires will sometimes
37			* repeatedly wake up finding that they must re-wait. A custom
38			* solution could minimize this, at the expense of being slower
39			* and more complex for the typical case.
40			*/
41
42			/**
43			* Construct a <tt>FiFoSemaphore</tt> with the given number of
44			* permits.
45			* @param permits the initial number of permits available
46			*/
47			public FairSemaphore(long permits) {
48			super(permits, new FairReentrantLock());
49			}
50
51			/**
52			* Acquires the given number of permits from this semaphore,
53			* blocking until all are available,
54			* or the thread is {@link Thread#interrupt interrupted}.
55			*
56			* <p>Acquires the given number of permits, if they are available,
57			* and returns immediately,
58			* reducing the number of available permits by the given amount.
59			*
60			* <p>If insufficient permits are available then the current thread becomes
61			* disabled for thread scheduling purposes and lies dormant until
62			* one of two things happens:
63			* <ul>
64			* <li>Some other thread invokes one of the {@link #release() release}
65			* methods for this semaphore, the current thread is next to be assigned
66			* permits and the number of available permits satisfies this request; or
67			* <li>Some other thread {@link Thread#interrupt interrupts} the current
68			* thread.
69			* </ul>
70			*
71			* <p>If the current thread:
72			* <ul>
73			* <li>has its interrupted status set on entry to this method; or
74			* <li>is {@link Thread#interrupt interrupted} while waiting
75			* for a permit,
76			* </ul>
77			* then {@link InterruptedException} is thrown and the current thread's
78			* interrupted status is cleared.
79			* Any available permits are assigned to the next waiting thread as if
80			* they had been made available by a call to {@link #release()}.
81			*
82			* @param permits the number of permits to acquire
83			*
84			* @throws InterruptedException if the current thread is interrupted
85			* @throws IllegalArgumentException if permits less than zero.
86			*
87			* @see Thread#interrupt
88			*/
89			public void acquire(long permits) throws InterruptedException {
90			if (permits < 0) throw new IllegalArgumentException();
91
92			lock.lockInterruptibly();
93			try {
94			while (count - permits < 0)
95			available.await();
96			count -= permits;
97			}
98			catch (InterruptedException ie) {
99			available.signal();
100			throw ie;
101			}
102			finally {
103			lock.unlock();
104			}
105
106			}
107
108			/**
109			* Acquires the given number of permits from this semaphore,
110			* blocking until all are available.
111			*
112			* <p>Acquires the given number of permits, if they are available,
113			* and returns immediately,
114			* reducing the number of available permits by the given amount.
115			*
116			* <p>If insufficient permits are available then the current thread becomes
117			* disabled for thread scheduling purposes and lies dormant until
118			* some other thread invokes one of the {@link #release() release}
119			* methods for this semaphore, the current thread is next to be assigned
120			* permits and the number of available permits satisfies this request.
121			*
122			* <p>If the current thread
123			* is {@link Thread#interrupt interrupted} while waiting
124			* for permits then it will continue to wait and its position in the
125			* queue is not affected. When the
126			* thread does return from this method its interrupt status will be set.
127			*
128			* @param permits the number of permits to acquire
129			* @throws IllegalArgumentException if permits less than zero.
130			*
131			*/
132			public void acquireUninterruptibly(long permits) {
133			if (permits < 0) throw new IllegalArgumentException();
134
135			lock.lock();
136			try {
137			while (count - permits < 0)
138			available.awaitUninterruptibly();
139			count -= permits;
140			}
141			finally {
142			lock.unlock();
143			}
144			}
145
146			/**
147			* Acquires the given number of permits from this semaphore, only if
148			* all are available at the time of invocation.
149			* <p>Acquires the given number of permits, if they are available, and
150			* returns immediately, with the value <tt>true</tt>,
151			* reducing the number of available permits by the given amount.
152			*
153			* <p>If insufficient permits are available then this method will return
154			* immediately with the value <tt>false</tt> and the number of available
155			* permits is unchanged.
156			*
157			* @param permits the number of permits to acquire
158			*
159			* @return <tt>true</tt> if the permits were acquired and <tt>false</tt>
160			* otherwise.
161			* @throws IllegalArgumentException if permits less than zero.
162			*/
163			public boolean tryAcquire(long permits) {
164			if (permits < 0) throw new IllegalArgumentException();
165
166			lock.lock();
167			try {
168			if (count - permits >= 0) {
169			count -= permits;
170			return true;
171			}
172			return false;
173			}
174			finally {
175			lock.unlock();
176			}
177			}
178
179			/**
180			* Acquires the given number of permits from this semaphore, if all
181			* become available within the given waiting time and the
182			* current thread has not been {@link Thread#interrupt interrupted}.
183			* <p>Acquires the given number of permits, if they are available and
184			* returns immediately, with the value <tt>true</tt>,
185			* reducing the number of available permits by the given amount.
186			* <p>If insufficient permits are available then
187			* the current thread becomes disabled for thread scheduling
188			* purposes and lies dormant until one of three things happens:
189			* <ul>
190			* <li>Some other thread invokes one of the {@link #release() release}
191			* methods for this semaphore, the current thread is next to be assigned
192			* permits and the number of available permits satisfies this request; or
193			* <li>Some other thread {@link Thread#interrupt interrupts} the current
194			* thread; or
195			* <li>The specified waiting time elapses.
196			* </ul>
197			* <p>If the permits are acquired then the value <tt>true</tt> is returned.
198			* <p>If the current thread:
199			* <ul>
200			* <li>has its interrupted status set on entry to this method; or
201			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
202			* the permits,
203			* </ul>
204			* then {@link InterruptedException} is thrown and the current thread's
205			* interrupted status is cleared.
206			* Any available permits are assigned to the next waiting thread as if
207			* they had been made available by a call to {@link #release()}.
208			*
209			* <p>If the specified waiting time elapses then the value <tt>false</tt>
210			* is returned.
211			* The given waiting time is a best-effort lower bound. If the time is
212			* less than or equal to zero, the method will not wait at all.
213			* Any available permits are assigned to the next waiting thread as if
214			* they had been made available by a call to {@link #release()}.
215			*
216			* @param permits the number of permits to acquire
217			* @param timeout the maximum time to wait for the permits
218			* @param granularity the time unit of the <tt>timeout</tt> argument.
219			* @return <tt>true</tt> if all permits were acquired and <tt>false</tt>
220			* if the waiting time elapsed before all permits were acquired.
221			*
222			* @throws InterruptedException if the current thread is interrupted
223			* @throws IllegalArgumentException if permits less than zero.
224			*
225			* @see Thread#interrupt
226			*
227			*/
228			public boolean tryAcquire(long permits, long timeout, TimeUnit granularity)
229			throws InterruptedException {
230			if (permits < 0) throw new IllegalArgumentException();
231			lock.lockInterruptibly();
232			long nanos = granularity.toNanos(timeout);
233			try {
234			for (;;) {
235			if (count - permits >= 0) {
236			count -= permits;
237			return true;
238			}
239			if (nanos <= 0)
240			return false;
241			nanos = available.awaitNanos(nanos);
242			}
243			}
244			catch (InterruptedException ie) {
245			available.signal();
246			throw ie;
247			}
248			finally {
249			lock.unlock();
250			}
251
252			}
253
254
255
256			/**
257			* Releases the given number of permits, returning them to the semaphore.
258			* <p>Releases the given number of permits, increasing the number of
259			* available permits by that amount.
260			* If any threads are blocking trying to acquire permits, then the
261			* one that has been waiting the longest
262			* is selected and given the permits that were just released.
263			* If the number of available permits satisfies that thread's request
264			* then that thread is re-enabled for thread scheduling purposes; otherwise
265			* the thread continues to wait. If there are still permits available
266			* after the first thread's request has been satisfied, then those permits
267			* are assigned to the next waiting thread. If it is satisfied then it is
268			* re-enabled for thread scheduling purposes. This continues until there
269			* are insufficient permits to satisfy the next waiting thread, or there
270			* are no more waiting threads.
271			*
272			* <p>There is no requirement that a thread that releases a permit must
273			* have acquired that permit by calling {@link Semaphore#acquire acquire}.
274			* Correct usage of a semaphore is established by programming convention
275			* in the application.
276			*
277			* @param permits the number of permits to release
278			* @throws IllegalArgumentException if permits less than zero.
279			*/
280			public void release(long permits) {
281			if (permits < 0) throw new IllegalArgumentException();
282			lock.lock();
283			try {
284			count += permits;
285			for (int i = 0; i < permits; ++i)
286			available.signal();
287			}
288			finally {
289			lock.unlock();
290			}
291			}
292			}