util/concurrent/Semaphore.java

package java.util.concurrent;

/**
 * A counting semaphore.  Conceptually, a semaphore maintains a set of
 * permits.  Each {@link #acquire} blocks if necessary until a permit is
 * available, and then takes it.  Each {@link #release} adds a permit,
 * potentially releasing a blocking acquirer.
 * However, no actual permit objects are used; the <tt>Semaphore</tt> just
 * keeps a count of the number available and acts accordingly.
 *
 * <p>Semaphores are used to restrict the number of threads than can
 * access some (physical or logical) resource. For example, here is
 * a class that uses a semaphore to control access to a pool of items:
 * <pre>
 * class Pool {
 *   private static final MAX_AVAILABLE = 100;
 *   private final Semaphore available = new Semaphore(MAX_AVAILABLE);
 *
 *   public Object getItem() throws InterruptedException {
 *     available.acquire();
 *     return getNextAvailableItem();
 *   }
 *
 *   public void putItem(Object x) {
 *     if (markAsUnused(x))
 *       available.release();
 *   }
 *
 *   // Not a particularly efficient data structure; just for demo
 *
 *   protected Object[] items = ... whatever kinds of items being managed
 *   protected boolean[] used = new boolean[MAX_AVAILABLE];
 *
 *   protected synchronized Object getNextAvailableItem() {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (!used[i]) {
 *          used[i] = true;
 *          return items[i];
 *       }
 *     }
 *     return null; // not reached
 *   }
 *
 *   protected synchronized boolean markAsUnused(Object item) {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (item == items[i]) {
 *          if (used[i]) {
 *            used[i] = false;
 *            return true;
 *          }
 *          else
 *            return false;
 *       }
 *     }
 *     return false;
 *   }
 *
 * }
 * </pre>
 * <p>Before obtaining an item each thread must acquire a permit from the
 * semaphore, guaranteeing that an item is available for use. When the
 * thread has finished with the item it is returned back to the pool and
 * a permit is returned to the semaphore, allowing another thread to
 * acquire that item.
 * Note that no synchronization lock is held when {@link #acquire} is
 * called as that would prevent an item from being returned to the pool.
 * The semaphore encapsulates the synchronization needed to restrict access to
 * the pool, separately from any synchronization needed to maintain the
 * consistency of the pool itself.
 *
 * <p>A semaphore initialized to one, and which is used such that it only
 * has at most one permit available, can serve as a mutual exclusion lock.
 * This is more
 * commonly known as a <em>binary semaphore</em>, because it only has two
 * states: one permit available, or zero permits available.
 * When used in this way, the binary semaphore has the property (unlike many
 * {@link Lock} implementations, that the &quot;lock&quot; can be released by
 * a thread other than the owner (as semaphores have no notion of ownership).
 * This can be useful in some specialised contexts, such as deadlock recovery.
 *
 * <p>This class makes no guarantees about the order in which threads
 * acquire permits. In particular, barging is permitted, that is, a thread
 * invoking {@link #acquire} can be allocated a permit ahead of a thread
 * that has been waiting. If you need more deterministic guarantees, consider
 * using {@link FifoSemaphore}.
 *
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/01/29 07:08:21 $
 * @editor $Author: dholmes $
 *
 */
public class Semaphore {

    private long permits = 0;

    /**
     * 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 = permits;
    }

    /**
     * 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 {}

    /**
     * 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() {}

    /**
     * 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() {
        return false;
    }

    /**
     * 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 {
        return false;
    }

    /**
     * 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() {}

    /**
     * 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() {
        return permits;
    }
}


Revision:	1.1
Committed:	Wed May 14 21:30:48 2003 UTC (21 years, 1 month ago) by tim
Branch:	MAIN
Log Message:	Moved main source rooted at . to ./src/main Moved test source rooted at ./etc/testcases to ./src/test
#	Content
1	package java.util.concurrent;
2
3	/**
4	* A counting semaphore. Conceptually, a semaphore maintains a set of
5	* permits. Each {@link #acquire} blocks if necessary until a permit is
6	* available, and then takes it. Each {@link #release} adds a permit,
7	* potentially releasing a blocking acquirer.
8	* However, no actual permit objects are used; the <tt>Semaphore</tt> just
9	* keeps a count of the number available and acts accordingly.
10	*
11	* <p>Semaphores are used to restrict the number of threads than can
12	* access some (physical or logical) resource. For example, here is
13	* a class that uses a semaphore to control access to a pool of items:
14	* <pre>
15	* class Pool {
16	* private static final MAX_AVAILABLE = 100;
17	* private final Semaphore available = new Semaphore(MAX_AVAILABLE);
18	*
19	* public Object getItem() throws InterruptedException {
20	* available.acquire();
21	* return getNextAvailableItem();
22	* }
23	*
24	* public void putItem(Object x) {
25	* if (markAsUnused(x))
26	* available.release();
27	* }
28	*
29	* // Not a particularly efficient data structure; just for demo
30	*
31	* protected Object[] items = ... whatever kinds of items being managed
32	* protected boolean[] used = new boolean[MAX_AVAILABLE];
33	*
34	* protected synchronized Object getNextAvailableItem() {
35	* for (int i = 0; i < MAX_AVAILABLE; ++i) {
36	* if (!used[i]) {
37	* used[i] = true;
38	* return items[i];
39	* }
40	* }
41	* return null; // not reached
42	* }
43	*
44	* protected synchronized boolean markAsUnused(Object item) {
45	* for (int i = 0; i < MAX_AVAILABLE; ++i) {
46	* if (item == items[i]) {
47	* if (used[i]) {
48	* used[i] = false;
49	* return true;
50	* }
51	* else
52	* return false;
53	* }
54	* }
55	* return false;
56	* }
57	*
58	* }
59	* </pre>
60	* <p>Before obtaining an item each thread must acquire a permit from the
61	* semaphore, guaranteeing that an item is available for use. When the
62	* thread has finished with the item it is returned back to the pool and
63	* a permit is returned to the semaphore, allowing another thread to
64	* acquire that item.
65	* Note that no synchronization lock is held when {@link #acquire} is
66	* called as that would prevent an item from being returned to the pool.
67	* The semaphore encapsulates the synchronization needed to restrict access to
68	* the pool, separately from any synchronization needed to maintain the
69	* consistency of the pool itself.
70	*
71	* <p>A semaphore initialized to one, and which is used such that it only
72	* has at most one permit available, can serve as a mutual exclusion lock.
73	* This is more
74	* commonly known as a <em>binary semaphore</em>, because it only has two
75	* states: one permit available, or zero permits available.
76	* When used in this way, the binary semaphore has the property (unlike many
77	* {@link Lock} implementations, that the "lock" can be released by
78	* a thread other than the owner (as semaphores have no notion of ownership).
79	* This can be useful in some specialised contexts, such as deadlock recovery.
80	*
81	* <p>This class makes no guarantees about the order in which threads
82	* acquire permits. In particular, barging is permitted, that is, a thread
83	* invoking {@link #acquire} can be allocated a permit ahead of a thread
84	* that has been waiting. If you need more deterministic guarantees, consider
85	* using {@link FifoSemaphore}.
86	*
87	*
88	* @since 1.5
89	* @spec JSR-166
90	* @revised $Date: 2003/01/29 07:08:21 $
91	* @editor $Author: dholmes $
92	*
93	*/
94	public class Semaphore {
95
96	private long permits = 0;
97
98	/**
99	* Construct a <tt>Semaphore</tt> with the given number of
100	* permits.
101	* @param permits the initial number of permits available
102	*/
103	public Semaphore(long permits) {
104	this.permits = permits;
105	}
106
107	/**
108	* Acquires a permit from this semaphore, blocking until one is
109	* available, or the thread is {@link Thread#interrupt interrupted}.
110	*
111	* <p>Acquires a permit, if one is available and returns immediately,
112	* reducing the number of available permits by one.
113	* <p>If no permit is available then the current thread becomes
114	* disabled for thread scheduling purposes and lies dormant until
115	* one of two things happens:
116	* <ul>
117	* <li>Some other thread invokes the {@link #release} method for this
118	* semaphore and the current thread happens to be chosen as the
119	* thread to receive the permit; or
120	* <li>Some other thread {@link Thread#interrupt interrupts} the current
121	* thread.
122	* </ul>
123	*
124	* <p>If the current thread:
125	* <ul>
126	* <li>has its interrupted status set on entry to this method; or
127	* <li>is {@link Thread#interrupt interrupted} while waiting
128	* for a permit,
129	* </ul>
130	* then {@link InterruptedException} is thrown and the current thread's
131	* interrupted status is cleared.
132	*
133	* @throws InterruptedException if the current thread is interrupted
134	*
135	* @see Thread#interrupt
136	*/
137	public void acquire() throws InterruptedException {}
138
139	/**
140	* Acquires a permit from this semaphore, blocking until one is
141	* available.
142	*
143	* <p>Acquires a permit, if one is available and returns immediately,
144	* reducing the number of available permits by one.
145	* <p>If no permit is available then the current thread becomes
146	* disabled for thread scheduling purposes and lies dormant until
147	* some other thread invokes the {@link #release} method for this
148	* semaphore and the current thread happens to be chosen as the
149	* thread to receive the permit.
150	*
151	* <p>If the current thread
152	* is {@link Thread#interrupt interrupted} while waiting
153	* for a permit then it will continue to wait, but the time at which
154	* the thread is assigned a permit may change compared to the time it
155	* would have received the permit had no interruption occurred. When the
156	* thread does return from this method its interrupt status will be set.
157	*
158	*/
159	public void acquireUninterruptibly() {}
160
161	/**
162	* Acquires a permit from this semaphore, only if one is available at the
163	* time of invocation.
164	* <p>Acquires a permit, if one is available and returns immediately,
165	* with the value <tt>true</tt>,
166	* reducing the number of available permits by one.
167	*
168	* <p>If no permit is available then this method will return
169	* immediately with the value <tt>false</tt>.
170	*
171	* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
172	* otherwise.
173	*/
174	public boolean tryAcquire() {
175	return false;
176	}
177
178	/**
179	* Acquires a permit from this semaphore, if one becomes available
180	* within the given waiting time and the
181	* current thread has not been {@link Thread#interrupt interrupted}.
182	* <p>Acquires a permit, if one is available and returns immediately,
183	* with the value <tt>true</tt>,
184	* reducing the number of available permits by one.
185	* <p>If no permit is available then
186	* the current thread becomes disabled for thread scheduling
187	* purposes and lies dormant until one of three things happens:
188	* <ul>
189	* <li>Some other thread invokes the {@link #release} method for this
190	* semaphore and the current thread happens to be chosen as the
191	* thread to receive the permit; or
192	* <li>Some other thread {@link Thread#interrupt interrupts} the current
193	* thread; or
194	* <li>The specified waiting time elapses.
195	* </ul>
196	* <p>If a permit is acquired then the value <tt>true</tt> is returned.
197	* <p>If the current thread:
198	* <ul>
199	* <li>has its interrupted status set on entry to this method; or
200	* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
201	* a permit,
202	* </ul>
203	* then {@link InterruptedException} is thrown and the current thread's
204	* interrupted status is cleared.
205	* <p>If the specified waiting time elapses then the value <tt>false</tt>
206	* is returned.
207	* The given waiting time is a best-effort lower bound. If the time is
208	* less than or equal to zero, the method will not wait at all.
209	*
210	* @param timeout the maximum time to wait for a permit
211	* @param granularity the time unit of the <tt>timeout</tt> argument.
212	* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
213	* if the waiting time elapsed before a permit was acquired.
214	*
215	* @throws InterruptedException if the current thread is interrupted
216	*
217	* @see Thread#interrupt
218	*
219	*/
220	public boolean tryAcquire(long timeout, TimeUnit granularity)
221	throws InterruptedException {
222	return false;
223	}
224
225	/**
226	* Releases a permit, returning it to the semaphore.
227	* <p>Releases a permit, increasing the number of available permits
228	* by one.
229	* If any threads are blocking trying to acquire a permit, then one
230	* is selected and given the permit that was just released.
231	* That thread is re-enabled for thread scheduling purposes.
232	* <p>There is no requirement that a thread that releases a permit must
233	* have acquired that permit by calling {@link #acquire}.
234	* Correct usage of a semaphore is established by programming convention
235	* in the application.
236	*/
237	public void release() {}
238
239	/**
240	* Return the current number of permits available in this semaphore.
241	* <p>This method is typically used for debugging and testing purposes.
242	* @return the number of permits available in this semaphore.
243	*/
244	public long availablePermits() {
245	return permits;
246	}
247	}
248
249
250