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 specialized 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
 * @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 unit 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 unit)
        throws InterruptedException {
        lock.lockInterruptibly();
        long nanos = unit.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:	1.13
Committed:	Sat Sep 27 12:22:40 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.12:	+3 -3 lines
Log Message:	Use "unit" instead of "granularity"; add Executors link to first sentence of TPE
#	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	import java.util.concurrent.locks.*;
9
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	* } 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 specialized 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	* @author Doug Lea
96	*
97	*/
98	public class Semaphore implements java.io.Serializable {
99	private static final long serialVersionUID = 3217036696412297181L;
100
101
102	// Fields are package-private to allow the FairSemaphore variant
103	// to access.
104
105	final ReentrantLock lock;
106	final Condition available;
107	long count;
108
109	/**
110	* Package-private constructor used by FairSemaphore
111	* @param permits the initial number of permits available
112	* @param lock the lock to use
113	*/
114	Semaphore(long permits, ReentrantLock lock) {
115	this.count = permits;
116	this.lock = lock;
117	available = lock.newCondition();
118	}
119
120	/**
121	* Construct a <tt>Semaphore</tt> with the given number of
122	* permits.
123	* @param permits the initial number of permits available
124	*/
125	public Semaphore(long permits) {
126	this(permits, new ReentrantLock());
127	}
128
129	/**
130	* Acquires a permit from this semaphore, blocking until one is
131	* available, or the thread is {@link Thread#interrupt interrupted}.
132	*
133	* <p>Acquires a permit, if one is available and returns immediately,
134	* reducing the number of available permits by one.
135	* <p>If no permit is available then the current thread becomes
136	* disabled for thread scheduling purposes and lies dormant until
137	* one of two things happens:
138	* <ul>
139	* <li>Some other thread invokes the {@link #release} method for this
140	* semaphore and the current thread happens to be chosen as the
141	* thread to receive the permit; or
142	* <li>Some other thread {@link Thread#interrupt interrupts} the current
143	* thread.
144	* </ul>
145	*
146	* <p>If the current thread:
147	* <ul>
148	* <li>has its interrupted status set on entry to this method; or
149	* <li>is {@link Thread#interrupt interrupted} while waiting
150	* for a permit,
151	* </ul>
152	* then {@link InterruptedException} is thrown and the current thread's
153	* interrupted status is cleared.
154	*
155	* @throws InterruptedException if the current thread is interrupted
156	*
157	* @see Thread#interrupt
158	*/
159	public void acquire() throws InterruptedException {
160	lock.lockInterruptibly();
161	try {
162	while (count <= 0)
163	available.await();
164	--count;
165	} catch (InterruptedException ie) {
166	available.signal();
167	throw ie;
168	} finally {
169	lock.unlock();
170	}
171	}
172
173	/**
174	* Acquires a permit from this semaphore, blocking until one is
175	* available.
176	*
177	* <p>Acquires a permit, if one is available and returns immediately,
178	* reducing the number of available permits by one.
179	* <p>If no permit is available then the current thread becomes
180	* disabled for thread scheduling purposes and lies dormant until
181	* some other thread invokes the {@link #release} method for this
182	* semaphore and the current thread happens to be chosen as the
183	* thread to receive the permit.
184	*
185	* <p>If the current thread
186	* is {@link Thread#interrupt interrupted} while waiting
187	* for a permit then it will continue to wait, but the time at which
188	* the thread is assigned a permit may change compared to the time it
189	* would have received the permit had no interruption occurred. When the
190	* thread does return from this method its interrupt status will be set.
191	*
192	*/
193	public void acquireUninterruptibly() {
194	lock.lock();
195	try {
196	while (count <= 0)
197	available.awaitUninterruptibly();
198	--count;
199	} finally {
200	lock.unlock();
201	}
202	}
203
204	/**
205	* Acquires a permit from this semaphore, only if one is available at the
206	* time of invocation.
207	* <p>Acquires a permit, if one is available and returns immediately,
208	* with the value <tt>true</tt>,
209	* reducing the number of available permits by one.
210	*
211	* <p>If no permit is available then this method will return
212	* immediately with the value <tt>false</tt>.
213	*
214	* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
215	* otherwise.
216	*/
217	public boolean tryAcquire() {
218	lock.lock();
219	try {
220	if (count > 0) {
221	--count;
222	return true;
223	}
224	return false;
225	} finally {
226	lock.unlock();
227	}
228	}
229
230	/**
231	* Acquires a permit from this semaphore, if one becomes available
232	* within the given waiting time and the
233	* current thread has not been {@link Thread#interrupt interrupted}.
234	* <p>Acquires a permit, if one is available and returns immediately,
235	* with the value <tt>true</tt>,
236	* reducing the number of available permits by one.
237	* <p>If no permit is available then
238	* the current thread becomes disabled for thread scheduling
239	* purposes and lies dormant until one of three things happens:
240	* <ul>
241	* <li>Some other thread invokes the {@link #release} method for this
242	* semaphore and the current thread happens to be chosen as the
243	* thread to receive the permit; or
244	* <li>Some other thread {@link Thread#interrupt interrupts} the current
245	* thread; or
246	* <li>The specified waiting time elapses.
247	* </ul>
248	* <p>If a permit is acquired then the value <tt>true</tt> is returned.
249	* <p>If the current thread:
250	* <ul>
251	* <li>has its interrupted status set on entry to this method; or
252	* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
253	* a permit,
254	* </ul>
255	* then {@link InterruptedException} is thrown and the current thread's
256	* interrupted status is cleared.
257	* <p>If the specified waiting time elapses then the value <tt>false</tt>
258	* is returned.
259	* If the time is
260	* less than or equal to zero, the method will not wait at all.
261	*
262	* @param timeout the maximum time to wait for a permit
263	* @param unit the time unit of the <tt>timeout</tt> argument.
264	* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
265	* if the waiting time elapsed before a permit was acquired.
266	*
267	* @throws InterruptedException if the current thread is interrupted
268	*
269	* @see Thread#interrupt
270	*
271	*/
272	public boolean tryAcquire(long timeout, TimeUnit unit)
273	throws InterruptedException {
274	lock.lockInterruptibly();
275	long nanos = unit.toNanos(timeout);
276	try {
277	for (;;) {
278	if (count > 0) {
279	--count;
280	return true;
281	}
282	if (nanos <= 0)
283	return false;
284	nanos = available.awaitNanos(nanos);
285	}
286	} catch (InterruptedException ie) {
287	available.signal();
288	throw ie;
289	} finally {
290	lock.unlock();
291	}
292	}
293
294	/**
295	* Releases a permit, returning it to the semaphore.
296	* <p>Releases a permit, increasing the number of available permits
297	* by one.
298	* If any threads are blocking trying to acquire a permit, then one
299	* is selected and given the permit that was just released.
300	* That thread is re-enabled for thread scheduling purposes.
301	* <p>There is no requirement that a thread that releases a permit must
302	* have acquired that permit by calling {@link #acquire}.
303	* Correct usage of a semaphore is established by programming convention
304	* in the application.
305	*/
306	public void release() {
307	lock.lock();
308	try {
309	++count;
310	available.signal();
311	} finally {
312	lock.unlock();
313	}
314	}
315
316
317	/**
318	* Return the current number of permits available in this semaphore.
319	* <p>This method is typically used for debugging and testing purposes.
320	* @return the number of permits available in this semaphore.
321	*/
322	public long availablePermits() {
323	lock.lock();
324	try {
325	return count;
326	} finally {
327	lock.unlock();
328	}
329	}
330	}
331
332
333