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: 2003/06/06 18:42:17 $
 * @editor $Author: dl $
 * @author Doug Lea
 *
 */
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;

    /** 
     * Package-private constructor used by FairSemaphore
     * @param permits the initial number of permits available
     * @param lock the lock to use
     */
    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:	1.4
Committed:	Tue Jun 24 14:34:49 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.3:	+11 -3 lines
Log Message:	Added missing javadoc tags; minor reformatting
#	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	1.4	* @revised $Date: 2003/06/06 18:42:17 $
97	dl	1.3	* @editor $Author: dl $
98	dl	1.4	* @author Doug Lea
99	tim	1.1	*
100			*/
101	dl	1.2	public class Semaphore implements java.io.Serializable {
102			// todo SerialID
103			// uses default serialization, which happens be fine here.
104
105			// Fields are package-private to allow the FairSemaphore variant
106			// to access.
107
108			final ReentrantLock lock;
109			final Condition available;
110			long count;
111	tim	1.1
112	dl	1.4	/**
113			* Package-private constructor used by FairSemaphore
114			* @param permits the initial number of permits available
115			* @param lock the lock to use
116			*/
117	dl	1.2	Semaphore(long permits, ReentrantLock lock) {
118			this.count = permits;
119			this.lock = lock;
120			available = lock.newCondition();
121			}
122	tim	1.1
123			/**
124			* Construct a <tt>Semaphore</tt> with the given number of
125			* permits.
126			* @param permits the initial number of permits available
127			*/
128			public Semaphore(long permits) {
129	dl	1.2	this(permits, new ReentrantLock());
130	tim	1.1	}
131
132			/**
133			* Acquires a permit from this semaphore, blocking until one is
134			* available, or the thread is {@link Thread#interrupt interrupted}.
135			*
136			* <p>Acquires a permit, if one is available and returns immediately,
137			* reducing the number of available permits by one.
138			* <p>If no permit is available then the current thread becomes
139			* disabled for thread scheduling purposes and lies dormant until
140			* one of two things happens:
141			* <ul>
142			* <li>Some other thread invokes the {@link #release} method for this
143			* semaphore and the current thread happens to be chosen as the
144			* thread to receive the permit; or
145			* <li>Some other thread {@link Thread#interrupt interrupts} the current
146			* thread.
147			* </ul>
148			*
149			* <p>If the current thread:
150			* <ul>
151			* <li>has its interrupted status set on entry to this method; or
152			* <li>is {@link Thread#interrupt interrupted} while waiting
153			* for a permit,
154			* </ul>
155			* then {@link InterruptedException} is thrown and the current thread's
156			* interrupted status is cleared.
157			*
158			* @throws InterruptedException if the current thread is interrupted
159			*
160			* @see Thread#interrupt
161			*/
162	dl	1.2	public void acquire() throws InterruptedException {
163			lock.lockInterruptibly();
164			try {
165	dl	1.4	while (count <= 0)
166			available.await();
167	dl	1.2	--count;
168			}
169			catch (InterruptedException ie) {
170			available.signal();
171			throw ie;
172			}
173			finally {
174			lock.unlock();
175			}
176			}
177	tim	1.1
178			/**
179			* Acquires a permit from this semaphore, blocking until one is
180			* available.
181			*
182			* <p>Acquires a permit, if one is available and returns immediately,
183			* reducing the number of available permits by one.
184			* <p>If no permit is available then the current thread becomes
185			* disabled for thread scheduling purposes and lies dormant until
186			* some other thread invokes the {@link #release} method for this
187			* semaphore and the current thread happens to be chosen as the
188			* thread to receive the permit.
189			*
190			* <p>If the current thread
191			* is {@link Thread#interrupt interrupted} while waiting
192			* for a permit then it will continue to wait, but the time at which
193			* the thread is assigned a permit may change compared to the time it
194			* would have received the permit had no interruption occurred. When the
195			* thread does return from this method its interrupt status will be set.
196			*
197			*/
198	dl	1.2	public void acquireUninterruptibly() {
199			lock.lock();
200			try {
201	dl	1.4	while (count <= 0)
202			available.awaitUninterruptibly();
203	dl	1.2	--count;
204			}
205			finally {
206			lock.unlock();
207			}
208			}
209	tim	1.1
210			/**
211			* Acquires a permit from this semaphore, only if one is available at the
212			* time of invocation.
213			* <p>Acquires a permit, if one is available and returns immediately,
214			* with the value <tt>true</tt>,
215			* reducing the number of available permits by one.
216			*
217			* <p>If no permit is available then this method will return
218			* immediately with the value <tt>false</tt>.
219			*
220			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
221			* otherwise.
222			*/
223			public boolean tryAcquire() {
224	dl	1.2	lock.lock();
225			try {
226			if (count > 0) {
227			--count;
228			return true;
229			}
230			return false;
231			}
232			finally {
233			lock.unlock();
234			}
235	tim	1.1	}
236
237			/**
238			* Acquires a permit from this semaphore, if one becomes available
239			* within the given waiting time and the
240			* current thread has not been {@link Thread#interrupt interrupted}.
241			* <p>Acquires a permit, if one is available and returns immediately,
242			* with the value <tt>true</tt>,
243			* reducing the number of available permits by one.
244			* <p>If no permit is available then
245			* the current thread becomes disabled for thread scheduling
246			* purposes and lies dormant until one of three things happens:
247			* <ul>
248			* <li>Some other thread invokes the {@link #release} method for this
249			* semaphore and the current thread happens to be chosen as the
250			* thread to receive the permit; or
251			* <li>Some other thread {@link Thread#interrupt interrupts} the current
252			* thread; or
253			* <li>The specified waiting time elapses.
254			* </ul>
255			* <p>If a permit is acquired then the value <tt>true</tt> is returned.
256			* <p>If the current thread:
257			* <ul>
258			* <li>has its interrupted status set on entry to this method; or
259			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
260			* a permit,
261			* </ul>
262			* then {@link InterruptedException} is thrown and the current thread's
263			* interrupted status is cleared.
264			* <p>If the specified waiting time elapses then the value <tt>false</tt>
265			* is returned.
266			* The given waiting time is a best-effort lower bound. If the time is
267			* less than or equal to zero, the method will not wait at all.
268			*
269			* @param timeout the maximum time to wait for a permit
270			* @param granularity the time unit of the <tt>timeout</tt> argument.
271			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
272			* if the waiting time elapsed before a permit was acquired.
273			*
274			* @throws InterruptedException if the current thread is interrupted
275			*
276			* @see Thread#interrupt
277			*
278			*/
279			public boolean tryAcquire(long timeout, TimeUnit granularity)
280			throws InterruptedException {
281	dl	1.2	lock.lockInterruptibly();
282			long nanos = granularity.toNanos(timeout);
283			try {
284			for (;;) {
285			if (count > 0) {
286			--count;
287			return true;
288			}
289			if (nanos <= 0)
290			return false;
291			nanos = available.awaitNanos(nanos);
292			}
293			}
294			catch (InterruptedException ie) {
295			available.signal();
296			throw ie;
297			}
298			finally {
299			lock.unlock();
300			}
301	tim	1.1	}
302
303			/**
304			* Releases a permit, returning it to the semaphore.
305			* <p>Releases a permit, increasing the number of available permits
306			* by one.
307			* If any threads are blocking trying to acquire a permit, then one
308			* is selected and given the permit that was just released.
309			* That thread is re-enabled for thread scheduling purposes.
310			* <p>There is no requirement that a thread that releases a permit must
311			* have acquired that permit by calling {@link #acquire}.
312			* Correct usage of a semaphore is established by programming convention
313			* in the application.
314			*/
315	dl	1.2	public void release() {
316			lock.lock();
317			try {
318			++count;
319			available.signal();
320			}
321			finally {
322			lock.unlock();
323			}
324			}
325
326	tim	1.1
327			/**
328			* Return the current number of permits available in this semaphore.
329			* <p>This method is typically used for debugging and testing purposes.
330			* @return the number of permits available in this semaphore.
331			*/
332			public long availablePermits() {
333	dl	1.2	lock.lock();
334			try {
335			return count;
336			}
337			finally {
338			lock.unlock();
339			}
340	tim	1.1	}
341			}
342
343
344