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;
import java.util.concurrent.locks.*;

/**
 * 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: 2003/08/25 19:27:58 $
 * @editor $Author: dl $
 * @author Doug Lea
 *
 */
public class Semaphore implements java.io.Serializable {
    private static final long serialVersionUID = 3217036696412297181L;


    // 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.
     * 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:	1.10
Committed:	Tue Aug 26 00:09:18 2003 UTC (20 years, 9 months ago) by dholmes
Branch:	MAIN
Changes since 1.9:	+3 -3 lines
Log Message:	In response to Eamonn's comment that "best effort lower bound" is not defined for waiting times, all such references have been deleted. The preceding text makes it clear that the time must elapse before the method will return, and trying to say anything about the maximum waiting time is pointless. We can still say something to this effect in the package docs if we want.
#	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	dl	1.5	import java.util.concurrent.locks.*;
9	tim	1.1
10			/**
11			* A counting semaphore. Conceptually, a semaphore maintains a set of
12			* permits. Each {@link #acquire} blocks if necessary until a permit is
13			* available, and then takes it. Each {@link #release} adds a permit,
14			* potentially releasing a blocking acquirer.
15			* However, no actual permit objects are used; the <tt>Semaphore</tt> just
16			* keeps a count of the number available and acts accordingly.
17			*
18			* <p>Semaphores are used to restrict the number of threads than can
19			* access some (physical or logical) resource. For example, here is
20			* a class that uses a semaphore to control access to a pool of items:
21			* <pre>
22			* class Pool {
23			* private static final MAX_AVAILABLE = 100;
24			* private final Semaphore available = new Semaphore(MAX_AVAILABLE);
25			*
26			* public Object getItem() throws InterruptedException {
27			* available.acquire();
28			* return getNextAvailableItem();
29			* }
30			*
31			* public void putItem(Object x) {
32			* if (markAsUnused(x))
33			* available.release();
34			* }
35			*
36			* // Not a particularly efficient data structure; just for demo
37			*
38			* protected Object[] items = ... whatever kinds of items being managed
39			* protected boolean[] used = new boolean[MAX_AVAILABLE];
40			*
41			* protected synchronized Object getNextAvailableItem() {
42			* for (int i = 0; i < MAX_AVAILABLE; ++i) {
43			* if (!used[i]) {
44			* used[i] = true;
45			* return items[i];
46			* }
47			* }
48			* return null; // not reached
49			* }
50			*
51			* protected synchronized boolean markAsUnused(Object item) {
52			* for (int i = 0; i < MAX_AVAILABLE; ++i) {
53			* if (item == items[i]) {
54			* if (used[i]) {
55			* used[i] = false;
56			* return true;
57	tim	1.8	* } else
58	tim	1.1	* 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	dholmes	1.10	* @revised $Date: 2003/08/25 19:27:58 $
97			* @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	dl	1.9	private static final long serialVersionUID = 3217036696412297181L;
103
104	dl	1.2
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	tim	1.8	} catch (InterruptedException ie) {
169	dl	1.2	available.signal();
170			throw ie;
171	tim	1.8	} finally {
172	dl	1.2	lock.unlock();
173			}
174			}
175	tim	1.1
176			/**
177			* Acquires a permit from this semaphore, blocking until one is
178			* available.
179			*
180			* <p>Acquires a permit, if one is available and returns immediately,
181			* reducing the number of available permits by one.
182			* <p>If no permit is available then the current thread becomes
183			* disabled for thread scheduling purposes and lies dormant until
184			* some other thread invokes the {@link #release} method for this
185			* semaphore and the current thread happens to be chosen as the
186			* thread to receive the permit.
187			*
188			* <p>If the current thread
189			* is {@link Thread#interrupt interrupted} while waiting
190			* for a permit then it will continue to wait, but the time at which
191			* the thread is assigned a permit may change compared to the time it
192			* would have received the permit had no interruption occurred. When the
193			* thread does return from this method its interrupt status will be set.
194			*
195			*/
196	dl	1.2	public void acquireUninterruptibly() {
197			lock.lock();
198			try {
199	dl	1.4	while (count <= 0)
200			available.awaitUninterruptibly();
201	dl	1.2	--count;
202	tim	1.8	} finally {
203	dl	1.2	lock.unlock();
204			}
205			}
206	tim	1.1
207			/**
208			* Acquires a permit from this semaphore, only if one is available at the
209			* time of invocation.
210			* <p>Acquires a permit, if one is available and returns immediately,
211			* with the value <tt>true</tt>,
212			* reducing the number of available permits by one.
213			*
214			* <p>If no permit is available then this method will return
215			* immediately with the value <tt>false</tt>.
216			*
217			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
218			* otherwise.
219			*/
220			public boolean tryAcquire() {
221	dl	1.2	lock.lock();
222			try {
223			if (count > 0) {
224			--count;
225			return true;
226			}
227			return false;
228	tim	1.8	} finally {
229	dl	1.2	lock.unlock();
230			}
231	tim	1.1	}
232
233			/**
234			* Acquires a permit from this semaphore, if one becomes available
235			* within the given waiting time and the
236			* current thread has not been {@link Thread#interrupt interrupted}.
237			* <p>Acquires a permit, if one is available and returns immediately,
238			* with the value <tt>true</tt>,
239			* reducing the number of available permits by one.
240			* <p>If no permit is available then
241			* the current thread becomes disabled for thread scheduling
242			* purposes and lies dormant until one of three things happens:
243			* <ul>
244			* <li>Some other thread invokes the {@link #release} method for this
245			* semaphore and the current thread happens to be chosen as the
246			* thread to receive the permit; or
247			* <li>Some other thread {@link Thread#interrupt interrupts} the current
248			* thread; or
249			* <li>The specified waiting time elapses.
250			* </ul>
251			* <p>If a permit is acquired then the value <tt>true</tt> is returned.
252			* <p>If the current thread:
253			* <ul>
254			* <li>has its interrupted status set on entry to this method; or
255			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
256			* a permit,
257			* </ul>
258			* then {@link InterruptedException} is thrown and the current thread's
259			* interrupted status is cleared.
260			* <p>If the specified waiting time elapses then the value <tt>false</tt>
261			* is returned.
262	dholmes	1.10	* If the time is
263	tim	1.1	* less than or equal to zero, the method will not wait at all.
264			*
265			* @param timeout the maximum time to wait for a permit
266			* @param granularity the time unit of the <tt>timeout</tt> argument.
267			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
268			* if the waiting time elapsed before a permit was acquired.
269			*
270			* @throws InterruptedException if the current thread is interrupted
271			*
272			* @see Thread#interrupt
273			*
274			*/
275			public boolean tryAcquire(long timeout, TimeUnit granularity)
276			throws InterruptedException {
277	dl	1.2	lock.lockInterruptibly();
278			long nanos = granularity.toNanos(timeout);
279			try {
280			for (;;) {
281			if (count > 0) {
282			--count;
283			return true;
284			}
285			if (nanos <= 0)
286			return false;
287			nanos = available.awaitNanos(nanos);
288			}
289	tim	1.8	} catch (InterruptedException ie) {
290	dl	1.2	available.signal();
291			throw ie;
292	tim	1.8	} finally {
293	dl	1.2	lock.unlock();
294			}
295	tim	1.1	}
296
297			/**
298			* Releases a permit, returning it to the semaphore.
299			* <p>Releases a permit, increasing the number of available permits
300			* by one.
301			* If any threads are blocking trying to acquire a permit, then one
302			* is selected and given the permit that was just released.
303			* That thread is re-enabled for thread scheduling purposes.
304			* <p>There is no requirement that a thread that releases a permit must
305			* have acquired that permit by calling {@link #acquire}.
306			* Correct usage of a semaphore is established by programming convention
307			* in the application.
308			*/
309	dl	1.2	public void release() {
310	dl	1.7	lock.lock();
311	dl	1.2	try {
312			++count;
313			available.signal();
314	tim	1.8	} finally {
315	dl	1.2	lock.unlock();
316			}
317			}
318
319	tim	1.1
320			/**
321			* Return the current number of permits available in this semaphore.
322			* <p>This method is typically used for debugging and testing purposes.
323			* @return the number of permits available in this semaphore.
324			*/
325			public long availablePermits() {
326	dl	1.2	lock.lock();
327			try {
328			return count;
329	tim	1.8	} finally {
330	dl	1.2	lock.unlock();
331			}
332	tim	1.1	}
333			}
334
335
336