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 ReentrantLock.ConditionObject 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:	1.14
Committed:	Sat Oct 11 15:37:31 2003 UTC (20 years, 7 months ago) by dl
Branch:	MAIN
Changes since 1.13:	+1 -1 lines
Log Message:	Redeclare some Conditions as ReentrantLock.ConditionObjects
#	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	dl	1.12	* This can be useful in some specialized contexts, such as deadlock recovery.
86	tim	1.1	*
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	dl	1.4	* @author Doug Lea
96	tim	1.1	*
97			*/
98	dl	1.2	public class Semaphore implements java.io.Serializable {
99	dl	1.9	private static final long serialVersionUID = 3217036696412297181L;
100
101	dl	1.2
102			// Fields are package-private to allow the FairSemaphore variant
103			// to access.
104
105			final ReentrantLock lock;
106	dl	1.14	final ReentrantLock.ConditionObject available;
107	dl	1.2	long count;
108	tim	1.1
109	dl	1.4	/**
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	dl	1.2	Semaphore(long permits, ReentrantLock lock) {
115			this.count = permits;
116			this.lock = lock;
117			available = lock.newCondition();
118			}
119	tim	1.1
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	dl	1.2	this(permits, new ReentrantLock());
127	tim	1.1	}
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	dl	1.2	public void acquire() throws InterruptedException {
160			lock.lockInterruptibly();
161			try {
162	dl	1.4	while (count <= 0)
163			available.await();
164	dl	1.2	--count;
165	tim	1.8	} catch (InterruptedException ie) {
166	dl	1.2	available.signal();
167			throw ie;
168	tim	1.8	} finally {
169	dl	1.2	lock.unlock();
170			}
171			}
172	tim	1.1
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	dl	1.2	public void acquireUninterruptibly() {
194			lock.lock();
195			try {
196	dl	1.4	while (count <= 0)
197			available.awaitUninterruptibly();
198	dl	1.2	--count;
199	tim	1.8	} finally {
200	dl	1.2	lock.unlock();
201			}
202			}
203	tim	1.1
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	dl	1.2	lock.lock();
219			try {
220			if (count > 0) {
221			--count;
222			return true;
223			}
224			return false;
225	tim	1.8	} finally {
226	dl	1.2	lock.unlock();
227			}
228	tim	1.1	}
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	dholmes	1.10	* If the time is
260	tim	1.1	* 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	dl	1.13	* @param unit the time unit of the <tt>timeout</tt> argument.
264	tim	1.1	* @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	dl	1.13	public boolean tryAcquire(long timeout, TimeUnit unit)
273	tim	1.1	throws InterruptedException {
274	dl	1.2	lock.lockInterruptibly();
275	dl	1.13	long nanos = unit.toNanos(timeout);
276	dl	1.2	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	tim	1.8	} catch (InterruptedException ie) {
287	dl	1.2	available.signal();
288			throw ie;
289	tim	1.8	} finally {
290	dl	1.2	lock.unlock();
291			}
292	tim	1.1	}
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	dl	1.2	public void release() {
307	dl	1.7	lock.lock();
308	dl	1.2	try {
309			++count;
310			available.signal();
311	tim	1.8	} finally {
312	dl	1.2	lock.unlock();
313			}
314			}
315
316	tim	1.1
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	dl	1.2	lock.lock();
324			try {
325			return count;
326	tim	1.8	} finally {
327	dl	1.2	lock.unlock();
328			}
329	tim	1.1	}
330			}
331
332
333