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
#	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 FairSemaphore}.
92	*
93	*
94	* @since 1.5
95	* @spec JSR-166
96	* @revised $Date: 2003/06/06 18:42:17 $
97	* @editor $Author: dl $
98	* @author Doug Lea
99	*
100	*/
101	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
112	/**
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	Semaphore(long permits, ReentrantLock lock) {
118	this.count = permits;
119	this.lock = lock;
120	available = lock.newCondition();
121	}
122
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	this(permits, new ReentrantLock());
130	}
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	public void acquire() throws InterruptedException {
163	lock.lockInterruptibly();
164	try {
165	while (count <= 0)
166	available.await();
167	--count;
168	}
169	catch (InterruptedException ie) {
170	available.signal();
171	throw ie;
172	}
173	finally {
174	lock.unlock();
175	}
176	}
177
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	public void acquireUninterruptibly() {
199	lock.lock();
200	try {
201	while (count <= 0)
202	available.awaitUninterruptibly();
203	--count;
204	}
205	finally {
206	lock.unlock();
207	}
208	}
209
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	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	}
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	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	}
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	public void release() {
316	lock.lock();
317	try {
318	++count;
319	available.signal();
320	}
321	finally {
322	lock.unlock();
323	}
324	}
325
326
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	lock.lock();
334	try {
335	return count;
336	}
337	finally {
338	lock.unlock();
339	}
340	}
341	}
342
343
344