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: 2003/06/07 16:07:09 $
 * @editor $Author: dl $
 * @author Doug Lea
 *
 */
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:	1.3
Committed:	Tue Jun 24 14:34:48 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.2:	+2 -1 lines
Log Message:	Added missing javadoc tags; minor reformatting
#	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	1.3	* @revised $Date: 2003/06/07 16:07:09 $
26	dl	1.2	* @editor $Author: dl $
27	dl	1.3	* @author Doug Lea
28	dl	1.1	*
29			*/
30			public class FairSemaphore extends Semaphore {
31
32			/*
33			* This differs from Semaphore only in that it uses a
34			* FairReentrantLock. Because the FairReentrantLock guarantees
35			* FIFO queuing, we don't need to do anything fancy to prevent
36			* overtaking etc. for the multiple-permit methods. The only
37			* minor downside is that multi-permit acquires will sometimes
38			* repeatedly wake up finding that they must re-wait. A custom
39			* solution could minimize this, at the expense of being slower
40			* and more complex for the typical case.
41			*/
42
43			/**
44			* Construct a <tt>FiFoSemaphore</tt> with the given number of
45			* permits.
46			* @param permits the initial number of permits available
47			*/
48			public FairSemaphore(long permits) {
49			super(permits, new FairReentrantLock());
50			}
51
52			/**
53			* Acquires the given number of permits from this semaphore,
54			* blocking until all are available,
55			* or the thread is {@link Thread#interrupt interrupted}.
56			*
57			* <p>Acquires the given number of permits, if they are available,
58			* and returns immediately,
59			* reducing the number of available permits by the given amount.
60			*
61			* <p>If insufficient permits are available then the current thread becomes
62			* disabled for thread scheduling purposes and lies dormant until
63			* one of two things happens:
64			* <ul>
65			* <li>Some other thread invokes one of the {@link #release() release}
66			* methods for this semaphore, the current thread is next to be assigned
67			* permits and the number of available permits satisfies this request; or
68			* <li>Some other thread {@link Thread#interrupt interrupts} the current
69			* thread.
70			* </ul>
71			*
72			* <p>If the current thread:
73			* <ul>
74			* <li>has its interrupted status set on entry to this method; or
75			* <li>is {@link Thread#interrupt interrupted} while waiting
76			* for a permit,
77			* </ul>
78			* then {@link InterruptedException} is thrown and the current thread's
79			* interrupted status is cleared.
80			* Any available permits are assigned to the next waiting thread as if
81			* they had been made available by a call to {@link #release()}.
82			*
83			* @param permits the number of permits to acquire
84			*
85			* @throws InterruptedException if the current thread is interrupted
86			* @throws IllegalArgumentException if permits less than zero.
87			*
88			* @see Thread#interrupt
89			*/
90			public void acquire(long permits) throws InterruptedException {
91			if (permits < 0) throw new IllegalArgumentException();
92
93			lock.lockInterruptibly();
94			try {
95			while (count - permits < 0)
96			available.await();
97			count -= permits;
98			}
99			catch (InterruptedException ie) {
100			available.signal();
101			throw ie;
102			}
103			finally {
104			lock.unlock();
105			}
106
107			}
108
109			/**
110			* Acquires the given number of permits from this semaphore,
111			* blocking until all are available.
112			*
113			* <p>Acquires the given number of permits, if they are available,
114			* and returns immediately,
115			* reducing the number of available permits by the given amount.
116			*
117			* <p>If insufficient permits are available then the current thread becomes
118			* disabled for thread scheduling purposes and lies dormant until
119			* some other thread invokes one of the {@link #release() release}
120			* methods for this semaphore, the current thread is next to be assigned
121			* permits and the number of available permits satisfies this request.
122			*
123			* <p>If the current thread
124			* is {@link Thread#interrupt interrupted} while waiting
125			* for permits then it will continue to wait and its position in the
126			* queue is not affected. When the
127			* thread does return from this method its interrupt status will be set.
128			*
129			* @param permits the number of permits to acquire
130			* @throws IllegalArgumentException if permits less than zero.
131			*
132			*/
133			public void acquireUninterruptibly(long permits) {
134			if (permits < 0) throw new IllegalArgumentException();
135
136			lock.lock();
137			try {
138			while (count - permits < 0)
139			available.awaitUninterruptibly();
140			count -= permits;
141			}
142			finally {
143			lock.unlock();
144			}
145			}
146
147			/**
148			* Acquires the given number of permits from this semaphore, only if
149			* all are available at the time of invocation.
150			* <p>Acquires the given number of permits, if they are available, and
151			* returns immediately, with the value <tt>true</tt>,
152			* reducing the number of available permits by the given amount.
153			*
154			* <p>If insufficient permits are available then this method will return
155			* immediately with the value <tt>false</tt> and the number of available
156			* permits is unchanged.
157			*
158			* @param permits the number of permits to acquire
159			*
160			* @return <tt>true</tt> if the permits were acquired and <tt>false</tt>
161			* otherwise.
162			* @throws IllegalArgumentException if permits less than zero.
163			*/
164			public boolean tryAcquire(long permits) {
165			if (permits < 0) throw new IllegalArgumentException();
166
167			lock.lock();
168			try {
169			if (count - permits >= 0) {
170			count -= permits;
171			return true;
172			}
173			return false;
174			}
175			finally {
176			lock.unlock();
177			}
178			}
179
180			/**
181			* Acquires the given number of permits from this semaphore, if all
182			* become available within the given waiting time and the
183			* current thread has not been {@link Thread#interrupt interrupted}.
184			* <p>Acquires the given number of permits, if they are available and
185			* returns immediately, with the value <tt>true</tt>,
186			* reducing the number of available permits by the given amount.
187			* <p>If insufficient permits are available then
188			* the current thread becomes disabled for thread scheduling
189			* purposes and lies dormant until one of three things happens:
190			* <ul>
191			* <li>Some other thread invokes one of the {@link #release() release}
192			* methods for this semaphore, the current thread is next to be assigned
193			* permits and the number of available permits satisfies this request; or
194			* <li>Some other thread {@link Thread#interrupt interrupts} the current
195			* thread; or
196			* <li>The specified waiting time elapses.
197			* </ul>
198			* <p>If the permits are acquired then the value <tt>true</tt> is returned.
199			* <p>If the current thread:
200			* <ul>
201			* <li>has its interrupted status set on entry to this method; or
202			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
203			* the permits,
204			* </ul>
205			* then {@link InterruptedException} is thrown and the current thread's
206			* interrupted status is cleared.
207			* Any available permits are assigned to the next waiting thread as if
208			* they had been made available by a call to {@link #release()}.
209			*
210			* <p>If the specified waiting time elapses then the value <tt>false</tt>
211			* is returned.
212			* The given waiting time is a best-effort lower bound. If the time is
213			* less than or equal to zero, the method will not wait at all.
214			* Any available permits are assigned to the next waiting thread as if
215			* they had been made available by a call to {@link #release()}.
216			*
217			* @param permits the number of permits to acquire
218			* @param timeout the maximum time to wait for the permits
219			* @param granularity the time unit of the <tt>timeout</tt> argument.
220			* @return <tt>true</tt> if all permits were acquired and <tt>false</tt>
221			* if the waiting time elapsed before all permits were acquired.
222			*
223			* @throws InterruptedException if the current thread is interrupted
224			* @throws IllegalArgumentException if permits less than zero.
225			*
226			* @see Thread#interrupt
227			*
228			*/
229			public boolean tryAcquire(long permits, long timeout, TimeUnit granularity)
230			throws InterruptedException {
231			if (permits < 0) throw new IllegalArgumentException();
232			lock.lockInterruptibly();
233			long nanos = granularity.toNanos(timeout);
234			try {
235			for (;;) {
236			if (count - permits >= 0) {
237			count -= permits;
238			return true;
239			}
240			if (nanos <= 0)
241			return false;
242			nanos = available.awaitNanos(nanos);
243			}
244			}
245			catch (InterruptedException ie) {
246			available.signal();
247			throw ie;
248			}
249			finally {
250			lock.unlock();
251			}
252
253			}
254
255
256
257			/**
258			* Releases the given number of permits, returning them to the semaphore.
259			* <p>Releases the given number of permits, increasing the number of
260			* available permits by that amount.
261			* If any threads are blocking trying to acquire permits, then the
262			* one that has been waiting the longest
263			* is selected and given the permits that were just released.
264			* If the number of available permits satisfies that thread's request
265			* then that thread is re-enabled for thread scheduling purposes; otherwise
266			* the thread continues to wait. If there are still permits available
267			* after the first thread's request has been satisfied, then those permits
268			* are assigned to the next waiting thread. If it is satisfied then it is
269			* re-enabled for thread scheduling purposes. This continues until there
270			* are insufficient permits to satisfy the next waiting thread, or there
271			* are no more waiting threads.
272			*
273			* <p>There is no requirement that a thread that releases a permit must
274			* have acquired that permit by calling {@link Semaphore#acquire acquire}.
275			* Correct usage of a semaphore is established by programming convention
276			* in the application.
277			*
278			* @param permits the number of permits to release
279			* @throws IllegalArgumentException if permits less than zero.
280			*/
281			public void release(long permits) {
282			if (permits < 0) throw new IllegalArgumentException();
283			lock.lock();
284			try {
285			count += permits;
286			for (int i = 0; i < permits; ++i)
287			available.signal();
288			}
289			finally {
290			lock.unlock();
291			}
292			}
293			}