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 FifoSemaphore}.
 *
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/01/29 07:08:21 $
 * @editor $Author: dholmes $
 *
 */
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:	1.2
Committed:	Tue May 27 18:14:40 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRERELEASE_0_1
Changes since 1.1:	+95 -9 lines
Log Message:	re-check-in initial implementations
#	Content
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	* 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	* using {@link FifoSemaphore}.
92	*
93	*
94	* @since 1.5
95	* @spec JSR-166
96	* @revised $Date: 2003/01/29 07:08:21 $
97	* @editor $Author: dholmes $
98	*
99	*/
100	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
111	Semaphore(long permits, ReentrantLock lock) {
112	this.count = permits;
113	this.lock = lock;
114	available = lock.newCondition();
115	}
116
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	this(permits, new ReentrantLock());
124	}
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	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
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	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
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	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	}
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	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	}
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	public void release() {
308	lock.lock();
309	try {
310	++count;
311	available.signal();
312	}
313	finally {
314	lock.unlock();
315	}
316	}
317
318
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	lock.lock();
326	try {
327	return count;
328	}
329	finally {
330	lock.unlock();
331	}
332	}
333	}
334
335
336