util/concurrent/Semaphore.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

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

/**
 * A counting semaphore.  Conceptually, a semaphore maintains a set of
 * permits.  Each {@link #acquire} blocks if necessary until a permit is
 * available, and then takes it.  Each {@link #release} adds a permit,
 * potentially releasing a blocking acquirer.
 * However, no actual permit objects are used; the <tt>Semaphore</tt> just
 * keeps a count of the number available and acts accordingly.
 *
 * <p>Semaphores are used to restrict the number of threads than can
 * access some (physical or logical) resource. For example, here is
 * a class that uses a semaphore to control access to a pool of items:
 * <pre>
 * class Pool {
 *   private static final MAX_AVAILABLE = 100;
 *   private final Semaphore available = new Semaphore(MAX_AVAILABLE);
 *
 *   public Object getItem() throws InterruptedException {
 *     available.acquire();
 *     return getNextAvailableItem();
 *   }
 *
 *   public void putItem(Object x) {
 *     if (markAsUnused(x))
 *       available.release();
 *   }
 *
 *   // Not a particularly efficient data structure; just for demo
 *
 *   protected Object[] items = ... whatever kinds of items being managed
 *   protected boolean[] used = new boolean[MAX_AVAILABLE];
 *
 *   protected synchronized Object getNextAvailableItem() {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (!used[i]) {
 *          used[i] = true;
 *          return items[i];
 *       }
 *     }
 *     return null; // not reached
 *   }
 *
 *   protected synchronized boolean markAsUnused(Object item) {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (item == items[i]) {
 *          if (used[i]) {
 *            used[i] = false;
 *            return true;
 *          }
 *          else
 *            return false;
 *       }
 *     }
 *     return false;
 *   }
 *
 * }
 * </pre>
 * <p>Before obtaining an item each thread must acquire a permit from the
 * semaphore, guaranteeing that an item is available for use. When the
 * thread has finished with the item it is returned back to the pool and
 * a permit is returned to the semaphore, allowing another thread to
 * acquire that item.
 * Note that no synchronization lock is held when {@link #acquire} is
 * called as that would prevent an item from being returned to the pool.
 * The semaphore encapsulates the synchronization needed to restrict access to
 * the pool, separately from any synchronization needed to maintain the
 * consistency of the pool itself.
 *
 * <p>A semaphore initialized to one, and which is used such that it only
 * has at most one permit available, can serve as a mutual exclusion lock.
 * This is more
 * commonly known as a <em>binary semaphore</em>, because it only has two
 * states: one permit available, or zero permits available.
 * When used in this way, the binary semaphore has the property (unlike many
 * {@link Lock} implementations, that the &quot;lock&quot; can be released by
 * a thread other than the owner (as semaphores have no notion of ownership).
 * This can be useful in some specialised contexts, such as deadlock recovery.
 *
 * <p>This class makes no guarantees about the order in which threads
 * acquire permits. In particular, barging is permitted, that is, a thread
 * invoking {@link #acquire} can be allocated a permit ahead of a thread
 * that has been waiting. If you need more deterministic guarantees, consider
 * using {@link FairSemaphore}.
 *
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/07/08 00:46:35 $
 * @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() {
        // Even if using fair locks, releases should try to barge in.
        if (!lock.tryLock())
            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:	1.6
Committed:	Wed Jul 9 23:23:17 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.5:	+5 -2 lines
Log Message:	Misc performance tunings
#	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			* }
58			* else
59			* return false;
60			* }
61			* }
62			* return false;
63			* }
64			*
65			* }
66			* </pre>
67			* <p>Before obtaining an item each thread must acquire a permit from the
68			* semaphore, guaranteeing that an item is available for use. When the
69			* thread has finished with the item it is returned back to the pool and
70			* a permit is returned to the semaphore, allowing another thread to
71			* acquire that item.
72			* Note that no synchronization lock is held when {@link #acquire} is
73			* called as that would prevent an item from being returned to the pool.
74			* The semaphore encapsulates the synchronization needed to restrict access to
75			* the pool, separately from any synchronization needed to maintain the
76			* consistency of the pool itself.
77			*
78			* <p>A semaphore initialized to one, and which is used such that it only
79			* has at most one permit available, can serve as a mutual exclusion lock.
80			* This is more
81			* commonly known as a <em>binary semaphore</em>, because it only has two
82			* states: one permit available, or zero permits available.
83			* When used in this way, the binary semaphore has the property (unlike many
84			* {@link Lock} implementations, that the "lock" can be released by
85			* a thread other than the owner (as semaphores have no notion of ownership).
86			* This can be useful in some specialised contexts, such as deadlock recovery.
87			*
88			* <p>This class makes no guarantees about the order in which threads
89			* acquire permits. In particular, barging is permitted, that is, a thread
90			* invoking {@link #acquire} can be allocated a permit ahead of a thread
91			* that has been waiting. If you need more deterministic guarantees, consider
92	dl	1.3	* using {@link FairSemaphore}.
93	tim	1.1	*
94			*
95			* @since 1.5
96			* @spec JSR-166
97	dl	1.6	* @revised $Date: 2003/07/08 00:46:35 $
98	dl	1.3	* @editor $Author: dl $
99	dl	1.4	* @author Doug Lea
100	tim	1.1	*
101			*/
102	dl	1.2	public class Semaphore implements java.io.Serializable {
103			// todo SerialID
104			// uses default serialization, which happens be fine here.
105
106			// Fields are package-private to allow the FairSemaphore variant
107			// to access.
108
109			final ReentrantLock lock;
110			final Condition available;
111			long count;
112	tim	1.1
113	dl	1.4	/**
114			* Package-private constructor used by FairSemaphore
115			* @param permits the initial number of permits available
116			* @param lock the lock to use
117			*/
118	dl	1.2	Semaphore(long permits, ReentrantLock lock) {
119			this.count = permits;
120			this.lock = lock;
121			available = lock.newCondition();
122			}
123	tim	1.1
124			/**
125			* Construct a <tt>Semaphore</tt> with the given number of
126			* permits.
127			* @param permits the initial number of permits available
128			*/
129			public Semaphore(long permits) {
130	dl	1.2	this(permits, new ReentrantLock());
131	tim	1.1	}
132
133			/**
134			* Acquires a permit from this semaphore, blocking until one is
135			* available, or the thread is {@link Thread#interrupt interrupted}.
136			*
137			* <p>Acquires a permit, if one is available and returns immediately,
138			* reducing the number of available permits by one.
139			* <p>If no permit is available then the current thread becomes
140			* disabled for thread scheduling purposes and lies dormant until
141			* one of two things happens:
142			* <ul>
143			* <li>Some other thread invokes the {@link #release} method for this
144			* semaphore and the current thread happens to be chosen as the
145			* thread to receive the permit; or
146			* <li>Some other thread {@link Thread#interrupt interrupts} the current
147			* thread.
148			* </ul>
149			*
150			* <p>If the current thread:
151			* <ul>
152			* <li>has its interrupted status set on entry to this method; or
153			* <li>is {@link Thread#interrupt interrupted} while waiting
154			* for a permit,
155			* </ul>
156			* then {@link InterruptedException} is thrown and the current thread's
157			* interrupted status is cleared.
158			*
159			* @throws InterruptedException if the current thread is interrupted
160			*
161			* @see Thread#interrupt
162			*/
163	dl	1.2	public void acquire() throws InterruptedException {
164			lock.lockInterruptibly();
165			try {
166	dl	1.4	while (count <= 0)
167			available.await();
168	dl	1.2	--count;
169			}
170			catch (InterruptedException ie) {
171			available.signal();
172			throw ie;
173			}
174			finally {
175			lock.unlock();
176			}
177			}
178	tim	1.1
179			/**
180			* Acquires a permit from this semaphore, blocking until one is
181			* available.
182			*
183			* <p>Acquires a permit, if one is available and returns immediately,
184			* reducing the number of available permits by one.
185			* <p>If no permit is available then the current thread becomes
186			* disabled for thread scheduling purposes and lies dormant until
187			* some other thread invokes the {@link #release} method for this
188			* semaphore and the current thread happens to be chosen as the
189			* thread to receive the permit.
190			*
191			* <p>If the current thread
192			* is {@link Thread#interrupt interrupted} while waiting
193			* for a permit then it will continue to wait, but the time at which
194			* the thread is assigned a permit may change compared to the time it
195			* would have received the permit had no interruption occurred. When the
196			* thread does return from this method its interrupt status will be set.
197			*
198			*/
199	dl	1.2	public void acquireUninterruptibly() {
200			lock.lock();
201			try {
202	dl	1.4	while (count <= 0)
203			available.awaitUninterruptibly();
204	dl	1.2	--count;
205			}
206			finally {
207			lock.unlock();
208			}
209			}
210	tim	1.1
211			/**
212			* Acquires a permit from this semaphore, only if one is available at the
213			* time of invocation.
214			* <p>Acquires a permit, if one is available and returns immediately,
215			* with the value <tt>true</tt>,
216			* reducing the number of available permits by one.
217			*
218			* <p>If no permit is available then this method will return
219			* immediately with the value <tt>false</tt>.
220			*
221			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
222			* otherwise.
223			*/
224			public boolean tryAcquire() {
225	dl	1.2	lock.lock();
226			try {
227			if (count > 0) {
228			--count;
229			return true;
230			}
231			return false;
232			}
233			finally {
234			lock.unlock();
235			}
236	tim	1.1	}
237
238			/**
239			* Acquires a permit from this semaphore, if one becomes available
240			* within the given waiting time and the
241			* current thread has not been {@link Thread#interrupt interrupted}.
242			* <p>Acquires a permit, if one is available and returns immediately,
243			* with the value <tt>true</tt>,
244			* reducing the number of available permits by one.
245			* <p>If no permit is available then
246			* the current thread becomes disabled for thread scheduling
247			* purposes and lies dormant until one of three things happens:
248			* <ul>
249			* <li>Some other thread invokes the {@link #release} method for this
250			* semaphore and the current thread happens to be chosen as the
251			* thread to receive the permit; or
252			* <li>Some other thread {@link Thread#interrupt interrupts} the current
253			* thread; or
254			* <li>The specified waiting time elapses.
255			* </ul>
256			* <p>If a permit is acquired then the value <tt>true</tt> is returned.
257			* <p>If the current thread:
258			* <ul>
259			* <li>has its interrupted status set on entry to this method; or
260			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
261			* a permit,
262			* </ul>
263			* then {@link InterruptedException} is thrown and the current thread's
264			* interrupted status is cleared.
265			* <p>If the specified waiting time elapses then the value <tt>false</tt>
266			* is returned.
267			* The given waiting time is a best-effort lower bound. If the time is
268			* less than or equal to zero, the method will not wait at all.
269			*
270			* @param timeout the maximum time to wait for a permit
271			* @param granularity the time unit of the <tt>timeout</tt> argument.
272			* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
273			* if the waiting time elapsed before a permit was acquired.
274			*
275			* @throws InterruptedException if the current thread is interrupted
276			*
277			* @see Thread#interrupt
278			*
279			*/
280			public boolean tryAcquire(long timeout, TimeUnit granularity)
281			throws InterruptedException {
282	dl	1.2	lock.lockInterruptibly();
283			long nanos = granularity.toNanos(timeout);
284			try {
285			for (;;) {
286			if (count > 0) {
287			--count;
288			return true;
289			}
290			if (nanos <= 0)
291			return false;
292			nanos = available.awaitNanos(nanos);
293			}
294			}
295			catch (InterruptedException ie) {
296			available.signal();
297			throw ie;
298			}
299			finally {
300			lock.unlock();
301			}
302	tim	1.1	}
303
304			/**
305			* Releases a permit, returning it to the semaphore.
306			* <p>Releases a permit, increasing the number of available permits
307			* by one.
308			* If any threads are blocking trying to acquire a permit, then one
309			* is selected and given the permit that was just released.
310			* That thread is re-enabled for thread scheduling purposes.
311			* <p>There is no requirement that a thread that releases a permit must
312			* have acquired that permit by calling {@link #acquire}.
313			* Correct usage of a semaphore is established by programming convention
314			* in the application.
315			*/
316	dl	1.2	public void release() {
317	dl	1.6	// Even if using fair locks, releases should try to barge in.
318			if (!lock.tryLock())
319			lock.lock();
320
321	dl	1.2	try {
322			++count;
323			available.signal();
324			}
325			finally {
326			lock.unlock();
327			}
328			}
329
330	tim	1.1
331			/**
332			* Return the current number of permits available in this semaphore.
333			* <p>This method is typically used for debugging and testing purposes.
334			* @return the number of permits available in this semaphore.
335			*/
336			public long availablePermits() {
337	dl	1.2	lock.lock();
338			try {
339			return count;
340			}
341			finally {
342			lock.unlock();
343			}
344	tim	1.1	}
345			}
346
347
348