util/concurrent/Semaphore.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 counting semaphore.  Conceptually, a semaphore maintains a set of
 * permits.  Each {@link #acquire} blocks if necessary until a permit is
 * available, and then takes it.  Each {@link #release} adds a permit,
 * potentially releasing a blocking acquirer.
 * However, no actual permit objects are used; the <tt>Semaphore</tt> just
 * keeps a count of the number available and acts accordingly.
 *
 * <p>Semaphores are used to restrict the number of threads than can
 * access some (physical or logical) resource. For example, here is
 * a class that uses a semaphore to control access to a pool of items:
 * <pre>
 * class Pool {
 *   private static final MAX_AVAILABLE = 100;
 *   private final Semaphore available = new Semaphore(MAX_AVAILABLE);
 *
 *   public Object getItem() throws InterruptedException {
 *     available.acquire();
 *     return getNextAvailableItem();
 *   }
 *
 *   public void putItem(Object x) {
 *     if (markAsUnused(x))
 *       available.release();
 *   }
 *
 *   // Not a particularly efficient data structure; just for demo
 *
 *   protected Object[] items = ... whatever kinds of items being managed
 *   protected boolean[] used = new boolean[MAX_AVAILABLE];
 *
 *   protected synchronized Object getNextAvailableItem() {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (!used[i]) {
 *          used[i] = true;
 *          return items[i];
 *       }
 *     }
 *     return null; // not reached
 *   }
 *
 *   protected synchronized boolean markAsUnused(Object item) {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (item == items[i]) {
 *          if (used[i]) {
 *            used[i] = false;
 *            return true;
 *          }
 *          else
 *            return false;
 *       }
 *     }
 *     return false;
 *   }
 *
 * }
 * </pre>
 * <p>Before obtaining an item each thread must acquire a permit from the
 * semaphore, guaranteeing that an item is available for use. When the
 * thread has finished with the item it is returned back to the pool and
 * a permit is returned to the semaphore, allowing another thread to
 * acquire that item.
 * Note that no synchronization lock is held when {@link #acquire} is
 * called as that would prevent an item from being returned to the pool.
 * The semaphore encapsulates the synchronization needed to restrict access to
 * the pool, separately from any synchronization needed to maintain the
 * consistency of the pool itself.
 *
 * <p>A semaphore initialized to one, and which is used such that it only
 * has at most one permit available, can serve as a mutual exclusion lock.
 * This is more
 * commonly known as a <em>binary semaphore</em>, because it only has two
 * states: one permit available, or zero permits available.
 * When used in this way, the binary semaphore has the property (unlike many
 * {@link Lock} implementations, that the &quot;lock&quot; can be released by
 * a thread other than the owner (as semaphores have no notion of ownership).
 * This can be useful in some specialised contexts, such as deadlock recovery.
 *
 * <p>This class makes no guarantees about the order in which threads
 * acquire permits. In particular, barging is permitted, that is, a thread
 * invoking {@link #acquire} can be allocated a permit ahead of a thread
 * that has been waiting. If you need more deterministic guarantees, consider
 * using {@link FairSemaphore}.
 *
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/05/27 18:14:40 $
 * @editor $Author: dl $
 *
 */
public class Semaphore implements java.io.Serializable {
    // todo SerialID
    // uses default serialization, which happens be fine here.

    // Fields are package-private to allow the FairSemaphore variant
    // to access.

    final ReentrantLock lock;
    final Condition available;
    long count;

    Semaphore(long permits, ReentrantLock lock) {
        this.count = permits;
        this.lock = lock;
        available = lock.newCondition();
    }

    /**
     * Construct a <tt>Semaphore</tt> with the given number of
     * permits.
     * @param permits the initial number of permits available
     */
    public Semaphore(long permits) {
        this(permits, new ReentrantLock());
    }

    /**
     * Acquires a permit from this semaphore, blocking until one is
     * available, or the thread is {@link Thread#interrupt interrupted}.
     *
     * <p>Acquires a permit, if one is available and returns immediately,
     * reducing the number of available permits by one.
     * <p>If no permit is 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 the {@link #release} method for this
     * semaphore and the current thread happens to be chosen as the
     * thread to receive the permit; 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.
     *
     * @throws InterruptedException if the current thread is interrupted
     *
     * @see Thread#interrupt
     */
    public void acquire() throws InterruptedException {
        lock.lockInterruptibly();
        try {
            while (count <= 0) available.await();
            --count;
        }
        catch (InterruptedException ie) {
            available.signal();
            throw ie;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Acquires a permit from this semaphore, blocking until one is
     * available.
     *
     * <p>Acquires a permit, if one is available and returns immediately,
     * reducing the number of available permits by one.
     * <p>If no permit is available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * some other thread invokes the {@link #release} method for this
     * semaphore and the current thread happens to be chosen as the
     * thread to receive the permit.
     *
     * <p>If the current thread
     * is {@link Thread#interrupt interrupted} while waiting
     * for a permit then it will continue to wait, but the time at which
     * the thread is assigned a permit may change compared to the time it
     * would have received the permit had no interruption occurred. When the
     * thread does return from this method its interrupt status will be set.
     *
     */
    public void acquireUninterruptibly() {
        lock.lock();
        try {
            while (count <= 0) available.awaitUninterruptibly();
            --count;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Acquires a permit from this semaphore, only if one is available at the 
     * time of invocation.
     * <p>Acquires a permit, if one is available and returns immediately,
     * with the value <tt>true</tt>,
     * reducing the number of available permits by one.
     *
     * <p>If no permit is available then this method will return
     * immediately with the value <tt>false</tt>.
     *
     * @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
     * otherwise.
     */
    public boolean tryAcquire() {
        lock.lock();
        try {
            if (count > 0) {
                --count;
                return true;
            }
            return false;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Acquires a permit from this semaphore, if one becomes available 
     * within the given waiting time and the
     * current thread has not been {@link Thread#interrupt interrupted}.
     * <p>Acquires a permit, if one is available and returns immediately,
     * with the value <tt>true</tt>,
     * reducing the number of available permits by one.
     * <p>If no permit is 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 the {@link #release} method for this
     * semaphore and the current thread happens to be chosen as the
     * thread to receive the permit; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     * <p>If a permit is 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
     * a permit,
     * </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.
     * 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.
     *
     * @param timeout the maximum time to wait for a permit
     * @param granularity the time unit of the <tt>timeout</tt> argument.
     * @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
     * if the waiting time elapsed before a permit was acquired.
     *
     * @throws InterruptedException if the current thread is interrupted
     *
     * @see Thread#interrupt
     *
     */
    public boolean tryAcquire(long timeout, TimeUnit granularity)
        throws InterruptedException {
        lock.lockInterruptibly();
        long nanos = granularity.toNanos(timeout);
        try {
            for (;;) {
                if (count > 0) {
                    --count;
                    return true;
                }
                if (nanos <= 0)
                    return false;
                nanos = available.awaitNanos(nanos);
            }
        }
        catch (InterruptedException ie) {
            available.signal();
            throw ie;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Releases a permit, returning it to the semaphore.
     * <p>Releases a permit, increasing the number of available permits
     * by one.
     * If any threads are blocking trying to acquire a permit, then one
     * is selected and given the permit that was just released.
     * That thread is re-enabled for thread scheduling purposes.
     * <p>There is no requirement that a thread that releases a permit must
     * have acquired that permit by calling {@link #acquire}.
     * Correct usage of a semaphore is established by programming convention
     * in the application.
     */
    public void release() {
        lock.lock();
        try {
            ++count;
            available.signal();
        }
        finally {
            lock.unlock();
        }
    }
        

    /**
     * Return the current number of permits available in this semaphore.
     * <p>This method is typically used for debugging and testing purposes.
     * @return the number of permits available in this semaphore.
     */
    public long availablePermits() {
        lock.lock();
        try {
            return count;
        }
        finally {
            lock.unlock();
        }
    }
}


Revision:	1.3
Committed:	Fri Jun 6 18:42:17 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_1
Changes since 1.2:	+3 -3 lines
Log Message:	Added to emulation Fixed some javadoc format errors
#	User	Rev	Content
1	dl	1.2	/*
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	tim	1.1	package java.util.concurrent;
8
9			/**
10			* A counting semaphore. Conceptually, a semaphore maintains a set of
11			* permits. Each {@link #acquire} blocks if necessary until a permit is
12			* available, and then takes it. Each {@link #release} adds a permit,
13			* potentially releasing a blocking acquirer.
14			* However, no actual permit objects are used; the <tt>Semaphore</tt> just
15			* keeps a count of the number available and acts accordingly.
16			*
17			* <p>Semaphores are used to restrict the number of threads than can
18			* access some (physical or logical) resource. For example, here is
19			* a class that uses a semaphore to control access to a pool of items:
20			* <pre>
21			* class Pool {
22			* private static final MAX_AVAILABLE = 100;
23			* private final Semaphore available = new Semaphore(MAX_AVAILABLE);
24			*
25			* public Object getItem() throws InterruptedException {
26			* available.acquire();
27			* return getNextAvailableItem();
28			* }
29			*
30			* public void putItem(Object x) {
31			* if (markAsUnused(x))
32			* available.release();
33			* }
34			*
35			* // Not a particularly efficient data structure; just for demo
36			*
37			* protected Object[] items = ... whatever kinds of items being managed
38			* protected boolean[] used = new boolean[MAX_AVAILABLE];
39			*
40			* protected synchronized Object getNextAvailableItem() {
41			* for (int i = 0; i < MAX_AVAILABLE; ++i) {
42			* if (!used[i]) {
43			* used[i] = true;
44			* return items[i];
45			* }
46			* }
47			* return null; // not reached
48			* }
49			*
50			* protected synchronized boolean markAsUnused(Object item) {
51			* for (int i = 0; i < MAX_AVAILABLE; ++i) {
52			* if (item == items[i]) {
53			* if (used[i]) {
54			* used[i] = false;
55			* return true;
56			* }
57			* else
58			* return false;
59			* }
60			* }
61			* return false;
62			* }
63			*
64			* }
65			* </pre>
66			* <p>Before obtaining an item each thread must acquire a permit from the
67			* semaphore, guaranteeing that an item is available for use. When the
68			* thread has finished with the item it is returned back to the pool and
69			* a permit is returned to the semaphore, allowing another thread to
70			* acquire that item.
71			* Note that no synchronization lock is held when {@link #acquire} is
72			* called as that would prevent an item from being returned to the pool.
73			* The semaphore encapsulates the synchronization needed to restrict access to
74			* the pool, separately from any synchronization needed to maintain the
75			* consistency of the pool itself.
76			*
77			* <p>A semaphore initialized to one, and which is used such that it only
78			* has at most one permit available, can serve as a mutual exclusion lock.
79			* This is more
80			* commonly known as a <em>binary semaphore</em>, because it only has two
81			* states: one permit available, or zero permits available.
82			* When used in this way, the binary semaphore has the property (unlike many
83			* {@link Lock} implementations, that the "lock" can be released by
84			* a thread other than the owner (as semaphores have no notion of ownership).
85			* This can be useful in some specialised contexts, such as deadlock recovery.
86			*
87			* <p>This class makes no guarantees about the order in which threads
88			* acquire permits. In particular, barging is permitted, that is, a thread
89			* invoking {@link #acquire} can be allocated a permit ahead of a thread
90			* that has been waiting. If you need more deterministic guarantees, consider
91	dl	1.3	* using {@link FairSemaphore}.
92	tim	1.1	*
93			*
94			* @since 1.5
95			* @spec JSR-166
96	dl	1.3	* @revised $Date: 2003/05/27 18:14:40 $
97			* @editor $Author: dl $
98	tim	1.1	*
99			*/
100	dl	1.2	public class Semaphore implements java.io.Serializable {
101			// todo SerialID
102			// uses default serialization, which happens be fine here.
103
104			// Fields are package-private to allow the FairSemaphore variant
105			// to access.
106
107			final ReentrantLock lock;
108			final Condition available;
109			long count;
110	tim	1.1
111	dl	1.2	Semaphore(long permits, ReentrantLock lock) {
112			this.count = permits;
113			this.lock = lock;
114			available = lock.newCondition();
115			}
116	tim	1.1
117			/**
118			* Construct a <tt>Semaphore</tt> with the given number of
119			* permits.
120			* @param permits the initial number of permits available
121			*/
122			public Semaphore(long permits) {
123	dl	1.2	this(permits, new ReentrantLock());
124	tim	1.1	}
125
126			/**
127			* Acquires a permit from this semaphore, blocking until one is
128			* available, or the thread is {@link Thread#interrupt interrupted}.
129			*
130			* <p>Acquires a permit, if one is available and returns immediately,
131			* reducing the number of available permits by one.
132			* <p>If no permit is available then the current thread becomes
133			* disabled for thread scheduling purposes and lies dormant until
134			* one of two things happens:
135			* <ul>
136			* <li>Some other thread invokes the {@link #release} method for this
137			* semaphore and the current thread happens to be chosen as the
138			* thread to receive the permit; or
139			* <li>Some other thread {@link Thread#interrupt interrupts} the current
140			* thread.
141			* </ul>
142			*
143			* <p>If the current thread:
144			* <ul>
145			* <li>has its interrupted status set on entry to this method; or
146			* <li>is {@link Thread#interrupt interrupted} while waiting
147			* for a permit,
148			* </ul>
149			* then {@link InterruptedException} is thrown and the current thread's
150			* interrupted status is cleared.
151			*
152			* @throws InterruptedException if the current thread is interrupted
153			*
154			* @see Thread#interrupt
155			*/
156	dl	1.2	public void acquire() throws InterruptedException {
157			lock.lockInterruptibly();
158			try {
159			while (count <= 0) available.await();
160			--count;
161			}
162			catch (InterruptedException ie) {
163			available.signal();
164			throw ie;
165			}
166			finally {
167			lock.unlock();
168			}
169			}
170	tim	1.1
171			/**
172			* Acquires a permit from this semaphore, blocking until one is
173			* available.
174			*
175			* <p>Acquires a permit, if one is available and returns immediately,
176			* reducing the number of available permits by one.
177			* <p>If no permit is available then the current thread becomes
178			* disabled for thread scheduling purposes and lies dormant until
179			* some other thread invokes the {@link #release} method for this
180			* semaphore and the current thread happens to be chosen as the
181			* thread to receive the permit.
182			*
183			* <p>If the current thread
184			* is {@link Thread#interrupt interrupted} while waiting
185			* for a permit then it will continue to wait, but the time at which
186			* the thread is assigned a permit may change compared to the time it
187			* would have received the permit had no interruption occurred. When the
188			* thread does return from this method its interrupt status will be set.
189			*
190			*/
191	dl	1.2	public void acquireUninterruptibly() {
192			lock.lock();
193			try {
194			while (count <= 0) available.awaitUninterruptibly();
195			--count;
196			}
197			finally {
198			lock.unlock();
199			}
200			}
201	tim	1.1
202			/**
203			* Acquires a permit from this semaphore, only if one is available at the
204			* time of invocation.
205			* <p>Acquires a permit, if one is available and returns immediately,
206			* with the value <tt>true</tt>,
207			* reducing the number of available permits by one.
208			*
209			* <p>If no permit is available then this method will return
210			* immediately with the value <tt>false</tt>.
211			*
212			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
213			* otherwise.
214			*/
215			public boolean tryAcquire() {
216	dl	1.2	lock.lock();
217			try {
218			if (count > 0) {
219			--count;
220			return true;
221			}
222			return false;
223			}
224			finally {
225			lock.unlock();
226			}
227	tim	1.1	}
228
229			/**
230			* Acquires a permit from this semaphore, if one becomes available
231			* within the given waiting time and the
232			* current thread has not been {@link Thread#interrupt interrupted}.
233			* <p>Acquires a permit, if one is available and returns immediately,
234			* with the value <tt>true</tt>,
235			* reducing the number of available permits by one.
236			* <p>If no permit is available then
237			* the current thread becomes disabled for thread scheduling
238			* purposes and lies dormant until one of three things happens:
239			* <ul>
240			* <li>Some other thread invokes the {@link #release} method for this
241			* semaphore and the current thread happens to be chosen as the
242			* thread to receive the permit; or
243			* <li>Some other thread {@link Thread#interrupt interrupts} the current
244			* thread; or
245			* <li>The specified waiting time elapses.
246			* </ul>
247			* <p>If a permit is acquired then the value <tt>true</tt> is returned.
248			* <p>If the current thread:
249			* <ul>
250			* <li>has its interrupted status set on entry to this method; or
251			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
252			* a permit,
253			* </ul>
254			* then {@link InterruptedException} is thrown and the current thread's
255			* interrupted status is cleared.
256			* <p>If the specified waiting time elapses then the value <tt>false</tt>
257			* is returned.
258			* The given waiting time is a best-effort lower bound. If the time is
259			* less than or equal to zero, the method will not wait at all.
260			*
261			* @param timeout the maximum time to wait for a permit
262			* @param granularity the time unit of the <tt>timeout</tt> argument.
263			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
264			* if the waiting time elapsed before a permit was acquired.
265			*
266			* @throws InterruptedException if the current thread is interrupted
267			*
268			* @see Thread#interrupt
269			*
270			*/
271			public boolean tryAcquire(long timeout, TimeUnit granularity)
272			throws InterruptedException {
273	dl	1.2	lock.lockInterruptibly();
274			long nanos = granularity.toNanos(timeout);
275			try {
276			for (;;) {
277			if (count > 0) {
278			--count;
279			return true;
280			}
281			if (nanos <= 0)
282			return false;
283			nanos = available.awaitNanos(nanos);
284			}
285			}
286			catch (InterruptedException ie) {
287			available.signal();
288			throw ie;
289			}
290			finally {
291			lock.unlock();
292			}
293	tim	1.1	}
294
295			/**
296			* Releases a permit, returning it to the semaphore.
297			* <p>Releases a permit, increasing the number of available permits
298			* by one.
299			* If any threads are blocking trying to acquire a permit, then one
300			* is selected and given the permit that was just released.
301			* That thread is re-enabled for thread scheduling purposes.
302			* <p>There is no requirement that a thread that releases a permit must
303			* have acquired that permit by calling {@link #acquire}.
304			* Correct usage of a semaphore is established by programming convention
305			* in the application.
306			*/
307	dl	1.2	public void release() {
308			lock.lock();
309			try {
310			++count;
311			available.signal();
312			}
313			finally {
314			lock.unlock();
315			}
316			}
317
318	tim	1.1
319			/**
320			* Return the current number of permits available in this semaphore.
321			* <p>This method is typically used for debugging and testing purposes.
322			* @return the number of permits available in this semaphore.
323			*/
324			public long availablePermits() {
325	dl	1.2	lock.lock();
326			try {
327			return count;
328			}
329			finally {
330			lock.unlock();
331			}
332	tim	1.1	}
333			}
334
335
336