util/concurrent/FairSemaphore.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 semaphore guaranteeing that threads invoking any of the {@link
 * #acquire() acquire} methods are allocated permits in the order in
 * which their invocation of those methods was processed
 * (first-in-first-out; FIFO). Note that FIFO ordering necessarily
 * applies to specific internal points of execution within these
 * methods.  So, it is possible for one thread to invoke
 * <tt>acquire</tt> before another, but reach the ordering point after
 * the other, and similarly upon return from the method.
 *
 * <p>Because permits are allocated in-order, this class also provides
 * convenience methods to {@link #acquire(long) acquire} and {@link
 * #release(long) release} multiple permits at a time. 
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/07/11 13:12:06 $
 * @editor $Author: dl $
 * @author Doug Lea
 *
 */
public class FairSemaphore extends Semaphore {

    /*
     * Basic algorithm is a variant of the one described
     * in CPJ book 2nd edition, section 3.7.3
     */

    /**
     * Nodes of the wait queue. Opportunistically subclasses ReentrantLock.
     */
    private static class Node extends ReentrantLock {
        /** The condition to wait on */
        Condition done = newCondition();
        /** True if interrupted before released */
        boolean cancelled;
        /** Number of permits remaining until can release */
        long permitsNeeded;
        /** Next node of queue */
        Node next;

        Node(long n) { permitsNeeded = n; }
    }

    /** Head of the wait queue */
    private Node head;

    /** Pointer to last node of the wait queue */
    private Node last;

    /** Add a new node to wait queue with given number of permits needed */
    private Node enq(long n) { 
        Node p = new Node(n);
        if (last == null) 
            last = head = p;
        else 
            last = last.next = p;
        return p;
    }

    /** Remove first node from wait queue */
    private void removeFirst() {
        Node p = head;
        assert p != null;
        if (p != null && (head = p.next) == null) 
            last = null;
    }

    /**
     * Construct a <tt>FairSemaphore</tt> with the given number of
     * permits.
     * @param permits the initial number of permits available
     */
    public FairSemaphore(long permits) { 
        super(permits, new ReentrantLock(true)); 
    }

    /**
     * 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 {
        acquire(1L);
    }

    /**
     * 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() {
        acquireUninterruptibly(1L);
    }

    /**
     * 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 tryAcquire(1L);
    }

    /**
     * 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 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 {
        return tryAcquire(1L, timeout, unit);
    }

    /**
     * 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() {
        release(1L);
    }

       
    /**
     * Acquires the given number of permits from this semaphore, 
     * blocking until all are available, 
     * or the thread is {@link Thread#interrupt interrupted}.
     *
     * <p>Acquires the given number of permits, if they are available,
     * and returns immediately,
     * reducing the number of available permits by the given amount.
     *
     * <p>If insufficient permits are 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 one of the {@link #release() release} 
     * methods for this semaphore, the current thread is next to be assigned
     * permits and the number of available permits satisfies this request; 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. 
     * Any available permits are assigned to the next waiting thread as if
     * they had been made available by a call to {@link #release()}.
     *
     * @param permits the number of permits to acquire
     *
     * @throws InterruptedException if the current thread is interrupted
     * @throws IllegalArgumentException if permits less than zero.
     *
     * @see Thread#interrupt
     */
    public void acquire(long permits) throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        Node node = null;
        lock.lockInterruptibly();
        try {
            long remaining = count - permits;
            if (remaining >= 0) {
                count = remaining;
                return;
            }
            count = 0;
            node = enq(-remaining);
        } finally {
            lock.unlock();
        }

        node.lock();
        try {
            while (node.permitsNeeded > 0) 
                node.done.await();
        } catch(InterruptedException ie) {
            if (node.permitsNeeded > 0) {
                node.cancelled = true;
                release(permits - node.permitsNeeded);
                throw ie;
            } else { // ignore interrupt
                Thread.currentThread().interrupt();
            }
        } finally {
            node.unlock();
        }
    }

    /**
     * Acquires the given number of permits from this semaphore, 
     * blocking until all are available.
     *
     * <p>Acquires the given number of permits, if they are available,
     * and returns immediately,
     * reducing the number of available permits by the given amount.
     *
     * <p>If insufficient permits are available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * some other thread invokes one of the {@link #release() release} 
     * methods for this semaphore, the current thread is next to be assigned
     * permits and the number of available permits satisfies this request.
     *
     * <p>If the current thread
     * is {@link Thread#interrupt interrupted} while waiting
     * for permits then it will continue to wait and its position in the
     * queue is not affected. When the
     * thread does return from this method its interrupt status will be set.
     *
     * @param permits the number of permits to acquire
     * @throws IllegalArgumentException if permits less than zero.
     *
     */
    public void acquireUninterruptibly(long permits) {
        if (permits < 0) throw new IllegalArgumentException();
        Node node = null;
        lock.lock();
        try {
            long remaining = count - permits;
            if (remaining >= 0) {
                count = remaining;
                return;
            }
            count = 0;
            node = enq(-remaining);
        } finally {
            lock.unlock();
        }

        node.lock();
        try {
            while (node.permitsNeeded > 0) 
                node.done.awaitUninterruptibly();
        } finally {
            node.unlock();
        }
    }

    /**
     * Acquires the given number of permits from this semaphore, only if 
     * all are available at the  time of invocation.
     * <p>Acquires the given number of permits, if they are available, and 
     * returns immediately, with the value <tt>true</tt>,
     * reducing the number of available permits by the given amount.
     *
     * <p>If insufficient permits are available then this method will return
     * immediately with the value <tt>false</tt> and the number of available
     * permits is unchanged.
     *
     * @param permits the number of permits to acquire
     *
     * @return <tt>true</tt> if the permits were acquired and <tt>false</tt>
     * otherwise.
     * @throws IllegalArgumentException if permits less than zero.
     */
    public boolean tryAcquire(long permits) {
        if (permits < 0) throw new IllegalArgumentException();
        lock.lock();
        try {
            long remaining = count - permits;
            if (remaining >= 0) {
                count = remaining;
                return true;
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Acquires the given number of permits from this semaphore, if all 
     * become available within the given waiting time and the
     * current thread has not been {@link Thread#interrupt interrupted}.
     * <p>Acquires the given number of permits, if they are available and 
     * returns immediately, with the value <tt>true</tt>,
     * reducing the number of available permits by the given amount.
     * <p>If insufficient permits are 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 one of the {@link #release() release} 
     * methods for this semaphore, the current thread is next to be assigned
     * permits and the number of available permits satisfies this request; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     * <p>If the permits are 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
     * the permits,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     * Any available permits are assigned to the next waiting thread as if
     * they had been made available by a call to {@link #release()}.
     *
     * <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.
     * Any available permits are assigned to the next waiting thread as if
     * they had been made available by a call to {@link #release()}.
     *
     * @param permits the number of permits to acquire
     * @param timeout the maximum time to wait for the permits
     * @param unit the time unit of the <tt>timeout</tt> argument.
     * @return <tt>true</tt> if all permits were acquired and <tt>false</tt>
     * if the waiting time elapsed before all permits were acquired.
     *
     * @throws InterruptedException if the current thread is interrupted
     * @throws IllegalArgumentException if permits less than zero.
     *
     * @see Thread#interrupt
     *
     */
    public boolean tryAcquire(long permits, long timeout, TimeUnit unit)
        throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        Node node = null;
        long nanos = unit.toNanos(timeout);
        lock.lockInterruptibly();
        try {
            long remaining = count - permits;
            if (remaining >= 0) {
                count = remaining;
                return true;
            }
            if (nanos <= 0)
                return false;
            count = 0;
            node = enq(-remaining);
        } finally {
            lock.unlock();
        }
        
        node.lock();
        try {
            while (node.permitsNeeded > 0) {
                nanos = node.done.awaitNanos(nanos);
                if (nanos <= 0) {
                    release(permits - node.permitsNeeded);
                    return false;
                }
            }
            return true;
        } catch(InterruptedException ie) {
            if (node.permitsNeeded > 0) {
                node.cancelled = true;
                release(permits - node.permitsNeeded);
                throw ie;
            } else { // ignore interrupt
                Thread.currentThread().interrupt();
                return true;
            }
        } finally {
            node.unlock();
        }
    }


    /**
     * Releases the given number of permits, returning them to the semaphore.
     * <p>Releases the given number of permits, increasing the number of 
     * available permits by that amount.
     * If any threads are blocking trying to acquire permits, then the
     * one that has been waiting the longest
     * is selected and given the permits that were just released.
     * If the number of available permits satisfies that thread's request
     * then that thread is re-enabled for thread scheduling purposes; otherwise
     * the thread continues to wait. If there are still permits available
     * after the first thread's request has been satisfied, then those permits
     * are assigned to the next waiting thread. If it is satisfied then it is
     * re-enabled for thread scheduling purposes. This continues until there
     * are insufficient permits to satisfy the next waiting thread, or there
     * are no more waiting threads.
     *
     * <p>There is no requirement that a thread that releases a permit must
     * have acquired that permit by calling {@link Semaphore#acquire acquire}.
     * Correct usage of a semaphore is established by programming convention
     * in the application.
     *
     * @param permits the number of permits to release
     * @throws IllegalArgumentException if permits less than zero.
     */
    public void release(long permits) {
        if (permits < 0) throw new IllegalArgumentException();
        if (permits == 0)
            return;
        // Even when using fair locks, releases should try to barge in.
        if (!lock.tryLock())
            lock.lock();
        try {
            long p = permits;
            while (p > 0) {
                Node node = head;
                if (node == null) {
                    count += p;
                    p = 0;
                } else {
                    node.lock();
                    try {
                        if (node.cancelled)
                            removeFirst();
                        else if (node.permitsNeeded > p) {
                            node.permitsNeeded -= p;
                            p = 0;
                        } else {
                            p -= node.permitsNeeded;
                            node.permitsNeeded = 0;
                            node.done.signal();
                            removeFirst();
                        }
                    } finally {
                        node.unlock();
                    }
                }
            }
        } finally {
            lock.unlock();
        }
    }
}
Revision:	1.7
Committed:	Fri Aug 8 20:05:07 2003 UTC (20 years, 10 months ago) by tim
Branch:	MAIN
Changes since 1.6:	+16 -31 lines
Log Message:	Scrunched catch, finally, else clauses.
#	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 semaphore guaranteeing that threads invoking any of the {@link
12	* #acquire() acquire} methods are allocated permits in the order in
13	* which their invocation of those methods was processed
14	* (first-in-first-out; FIFO). Note that FIFO ordering necessarily
15	* applies to specific internal points of execution within these
16	* methods. So, it is possible for one thread to invoke
17	* <tt>acquire</tt> before another, but reach the ordering point after
18	* the other, and similarly upon return from the method.
19	*
20	* <p>Because permits are allocated in-order, this class also provides
21	* convenience methods to {@link #acquire(long) acquire} and {@link
22	* #release(long) release} multiple permits at a time.
23	*
24	* @since 1.5
25	* @spec JSR-166
26	* @revised $Date: 2003/07/11 13:12:06 $
27	* @editor $Author: dl $
28	* @author Doug Lea
29	*
30	*/
31	public class FairSemaphore extends Semaphore {
32
33	/*
34	* Basic algorithm is a variant of the one described
35	* in CPJ book 2nd edition, section 3.7.3
36	*/
37
38	/**
39	* Nodes of the wait queue. Opportunistically subclasses ReentrantLock.
40	*/
41	private static class Node extends ReentrantLock {
42	/** The condition to wait on */
43	Condition done = newCondition();
44	/** True if interrupted before released */
45	boolean cancelled;
46	/** Number of permits remaining until can release */
47	long permitsNeeded;
48	/** Next node of queue */
49	Node next;
50
51	Node(long n) { permitsNeeded = n; }
52	}
53
54	/** Head of the wait queue */
55	private Node head;
56
57	/** Pointer to last node of the wait queue */
58	private Node last;
59
60	/** Add a new node to wait queue with given number of permits needed */
61	private Node enq(long n) {
62	Node p = new Node(n);
63	if (last == null)
64	last = head = p;
65	else
66	last = last.next = p;
67	return p;
68	}
69
70	/** Remove first node from wait queue */
71	private void removeFirst() {
72	Node p = head;
73	assert p != null;
74	if (p != null && (head = p.next) == null)
75	last = null;
76	}
77
78	/**
79	* Construct a <tt>FairSemaphore</tt> with the given number of
80	* permits.
81	* @param permits the initial number of permits available
82	*/
83	public FairSemaphore(long permits) {
84	super(permits, new ReentrantLock(true));
85	}
86
87	/**
88	* Acquires a permit from this semaphore, blocking until one is
89	* available, or the thread is {@link Thread#interrupt interrupted}.
90	*
91	* <p>Acquires a permit, if one is available and returns immediately,
92	* reducing the number of available permits by one.
93	* <p>If no permit is available then the current thread becomes
94	* disabled for thread scheduling purposes and lies dormant until
95	* one of two things happens:
96	* <ul>
97	* <li>Some other thread invokes the {@link #release} method for this
98	* semaphore and the current thread happens to be chosen as the
99	* thread to receive the permit; or
100	* <li>Some other thread {@link Thread#interrupt interrupts} the current
101	* thread.
102	* </ul>
103	*
104	* <p>If the current thread:
105	* <ul>
106	* <li>has its interrupted status set on entry to this method; or
107	* <li>is {@link Thread#interrupt interrupted} while waiting
108	* for a permit,
109	* </ul>
110	* then {@link InterruptedException} is thrown and the current thread's
111	* interrupted status is cleared.
112	*
113	* @throws InterruptedException if the current thread is interrupted
114	*
115	* @see Thread#interrupt
116	*/
117	public void acquire() throws InterruptedException {
118	acquire(1L);
119	}
120
121	/**
122	* Acquires a permit from this semaphore, blocking until one is
123	* available.
124	*
125	* <p>Acquires a permit, if one is available and returns immediately,
126	* reducing the number of available permits by one.
127	* <p>If no permit is available then the current thread becomes
128	* disabled for thread scheduling purposes and lies dormant until
129	* some other thread invokes the {@link #release} method for this
130	* semaphore and the current thread happens to be chosen as the
131	* thread to receive the permit.
132	*
133	* <p>If the current thread
134	* is {@link Thread#interrupt interrupted} while waiting
135	* for a permit then it will continue to wait, but the time at which
136	* the thread is assigned a permit may change compared to the time it
137	* would have received the permit had no interruption occurred. When the
138	* thread does return from this method its interrupt status will be set.
139	*
140	*/
141	public void acquireUninterruptibly() {
142	acquireUninterruptibly(1L);
143	}
144
145	/**
146	* Acquires a permit from this semaphore, only if one is available at the
147	* time of invocation.
148	* <p>Acquires a permit, if one is available and returns immediately,
149	* with the value <tt>true</tt>,
150	* reducing the number of available permits by one.
151	*
152	* <p>If no permit is available then this method will return
153	* immediately with the value <tt>false</tt>.
154	*
155	* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
156	* otherwise.
157	*/
158	public boolean tryAcquire() {
159	return tryAcquire(1L);
160	}
161
162	/**
163	* Acquires a permit from this semaphore, if one becomes available
164	* within the given waiting time and the
165	* current thread has not been {@link Thread#interrupt interrupted}.
166	* <p>Acquires a permit, if one is available and returns immediately,
167	* with the value <tt>true</tt>,
168	* reducing the number of available permits by one.
169	* <p>If no permit is available then
170	* the current thread becomes disabled for thread scheduling
171	* purposes and lies dormant until one of three things happens:
172	* <ul>
173	* <li>Some other thread invokes the {@link #release} method for this
174	* semaphore and the current thread happens to be chosen as the
175	* thread to receive the permit; or
176	* <li>Some other thread {@link Thread#interrupt interrupts} the current
177	* thread; or
178	* <li>The specified waiting time elapses.
179	* </ul>
180	* <p>If a permit is acquired then the value <tt>true</tt> is returned.
181	* <p>If the current thread:
182	* <ul>
183	* <li>has its interrupted status set on entry to this method; or
184	* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
185	* a permit,
186	* </ul>
187	* then {@link InterruptedException} is thrown and the current thread's
188	* interrupted status is cleared.
189	* <p>If the specified waiting time elapses then the value <tt>false</tt>
190	* is returned.
191	* The given waiting time is a best-effort lower bound. If the time is
192	* less than or equal to zero, the method will not wait at all.
193	*
194	* @param timeout the maximum time to wait for a permit
195	* @param unit the time unit of the <tt>timeout</tt> argument.
196	* @return <tt>true</tt> if a permit was acquired and <tt>false</tt>
197	* if the waiting time elapsed before a permit was acquired.
198	*
199	* @throws InterruptedException if the current thread is interrupted
200	*
201	* @see Thread#interrupt
202	*
203	*/
204	public boolean tryAcquire(long timeout, TimeUnit unit)
205	throws InterruptedException {
206	return tryAcquire(1L, timeout, unit);
207	}
208
209	/**
210	* Releases a permit, returning it to the semaphore.
211	* <p>Releases a permit, increasing the number of available permits
212	* by one.
213	* If any threads are blocking trying to acquire a permit, then one
214	* is selected and given the permit that was just released.
215	* That thread is re-enabled for thread scheduling purposes.
216	* <p>There is no requirement that a thread that releases a permit must
217	* have acquired that permit by calling {@link #acquire}.
218	* Correct usage of a semaphore is established by programming convention
219	* in the application.
220	*/
221	public void release() {
222	release(1L);
223	}
224
225
226	/**
227	* Acquires the given number of permits from this semaphore,
228	* blocking until all are available,
229	* or the thread is {@link Thread#interrupt interrupted}.
230	*
231	* <p>Acquires the given number of permits, if they are available,
232	* and returns immediately,
233	* reducing the number of available permits by the given amount.
234	*
235	* <p>If insufficient permits are available then the current thread becomes
236	* disabled for thread scheduling purposes and lies dormant until
237	* one of two things happens:
238	* <ul>
239	* <li>Some other thread invokes one of the {@link #release() release}
240	* methods for this semaphore, the current thread is next to be assigned
241	* permits and the number of available permits satisfies this request; or
242	* <li>Some other thread {@link Thread#interrupt interrupts} the current
243	* thread.
244	* </ul>
245	*
246	* <p>If the current thread:
247	* <ul>
248	* <li>has its interrupted status set on entry to this method; or
249	* <li>is {@link Thread#interrupt interrupted} while waiting
250	* for a permit,
251	* </ul>
252	* then {@link InterruptedException} is thrown and the current thread's
253	* interrupted status is cleared.
254	* Any available permits are assigned to the next waiting thread as if
255	* they had been made available by a call to {@link #release()}.
256	*
257	* @param permits the number of permits to acquire
258	*
259	* @throws InterruptedException if the current thread is interrupted
260	* @throws IllegalArgumentException if permits less than zero.
261	*
262	* @see Thread#interrupt
263	*/
264	public void acquire(long permits) throws InterruptedException {
265	if (permits < 0) throw new IllegalArgumentException();
266	Node node = null;
267	lock.lockInterruptibly();
268	try {
269	long remaining = count - permits;
270	if (remaining >= 0) {
271	count = remaining;
272	return;
273	}
274	count = 0;
275	node = enq(-remaining);
276	} finally {
277	lock.unlock();
278	}
279
280	node.lock();
281	try {
282	while (node.permitsNeeded > 0)
283	node.done.await();
284	} catch(InterruptedException ie) {
285	if (node.permitsNeeded > 0) {
286	node.cancelled = true;
287	release(permits - node.permitsNeeded);
288	throw ie;
289	} else { // ignore interrupt
290	Thread.currentThread().interrupt();
291	}
292	} finally {
293	node.unlock();
294	}
295	}
296
297	/**
298	* Acquires the given number of permits from this semaphore,
299	* blocking until all are available.
300	*
301	* <p>Acquires the given number of permits, if they are available,
302	* and returns immediately,
303	* reducing the number of available permits by the given amount.
304	*
305	* <p>If insufficient permits are available then the current thread becomes
306	* disabled for thread scheduling purposes and lies dormant until
307	* some other thread invokes one of the {@link #release() release}
308	* methods for this semaphore, the current thread is next to be assigned
309	* permits and the number of available permits satisfies this request.
310	*
311	* <p>If the current thread
312	* is {@link Thread#interrupt interrupted} while waiting
313	* for permits then it will continue to wait and its position in the
314	* queue is not affected. When the
315	* thread does return from this method its interrupt status will be set.
316	*
317	* @param permits the number of permits to acquire
318	* @throws IllegalArgumentException if permits less than zero.
319	*
320	*/
321	public void acquireUninterruptibly(long permits) {
322	if (permits < 0) throw new IllegalArgumentException();
323	Node node = null;
324	lock.lock();
325	try {
326	long remaining = count - permits;
327	if (remaining >= 0) {
328	count = remaining;
329	return;
330	}
331	count = 0;
332	node = enq(-remaining);
333	} finally {
334	lock.unlock();
335	}
336
337	node.lock();
338	try {
339	while (node.permitsNeeded > 0)
340	node.done.awaitUninterruptibly();
341	} finally {
342	node.unlock();
343	}
344	}
345
346	/**
347	* Acquires the given number of permits from this semaphore, only if
348	* all are available at the time of invocation.
349	* <p>Acquires the given number of permits, if they are available, and
350	* returns immediately, with the value <tt>true</tt>,
351	* reducing the number of available permits by the given amount.
352	*
353	* <p>If insufficient permits are available then this method will return
354	* immediately with the value <tt>false</tt> and the number of available
355	* permits is unchanged.
356	*
357	* @param permits the number of permits to acquire
358	*
359	* @return <tt>true</tt> if the permits were acquired and <tt>false</tt>
360	* otherwise.
361	* @throws IllegalArgumentException if permits less than zero.
362	*/
363	public boolean tryAcquire(long permits) {
364	if (permits < 0) throw new IllegalArgumentException();
365	lock.lock();
366	try {
367	long remaining = count - permits;
368	if (remaining >= 0) {
369	count = remaining;
370	return true;
371	}
372	return false;
373	} finally {
374	lock.unlock();
375	}
376	}
377
378	/**
379	* Acquires the given number of permits from this semaphore, if all
380	* become available within the given waiting time and the
381	* current thread has not been {@link Thread#interrupt interrupted}.
382	* <p>Acquires the given number of permits, if they are available and
383	* returns immediately, with the value <tt>true</tt>,
384	* reducing the number of available permits by the given amount.
385	* <p>If insufficient permits are available then
386	* the current thread becomes disabled for thread scheduling
387	* purposes and lies dormant until one of three things happens:
388	* <ul>
389	* <li>Some other thread invokes one of the {@link #release() release}
390	* methods for this semaphore, the current thread is next to be assigned
391	* permits and the number of available permits satisfies this request; or
392	* <li>Some other thread {@link Thread#interrupt interrupts} the current
393	* thread; or
394	* <li>The specified waiting time elapses.
395	* </ul>
396	* <p>If the permits are acquired then the value <tt>true</tt> is returned.
397	* <p>If the current thread:
398	* <ul>
399	* <li>has its interrupted status set on entry to this method; or
400	* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
401	* the permits,
402	* </ul>
403	* then {@link InterruptedException} is thrown and the current thread's
404	* interrupted status is cleared.
405	* Any available permits are assigned to the next waiting thread as if
406	* they had been made available by a call to {@link #release()}.
407	*
408	* <p>If the specified waiting time elapses then the value <tt>false</tt>
409	* is returned.
410	* The given waiting time is a best-effort lower bound. If the time is
411	* less than or equal to zero, the method will not wait at all.
412	* Any available permits are assigned to the next waiting thread as if
413	* they had been made available by a call to {@link #release()}.
414	*
415	* @param permits the number of permits to acquire
416	* @param timeout the maximum time to wait for the permits
417	* @param unit the time unit of the <tt>timeout</tt> argument.
418	* @return <tt>true</tt> if all permits were acquired and <tt>false</tt>
419	* if the waiting time elapsed before all permits were acquired.
420	*
421	* @throws InterruptedException if the current thread is interrupted
422	* @throws IllegalArgumentException if permits less than zero.
423	*
424	* @see Thread#interrupt
425	*
426	*/
427	public boolean tryAcquire(long permits, long timeout, TimeUnit unit)
428	throws InterruptedException {
429	if (permits < 0) throw new IllegalArgumentException();
430	Node node = null;
431	long nanos = unit.toNanos(timeout);
432	lock.lockInterruptibly();
433	try {
434	long remaining = count - permits;
435	if (remaining >= 0) {
436	count = remaining;
437	return true;
438	}
439	if (nanos <= 0)
440	return false;
441	count = 0;
442	node = enq(-remaining);
443	} finally {
444	lock.unlock();
445	}
446
447	node.lock();
448	try {
449	while (node.permitsNeeded > 0) {
450	nanos = node.done.awaitNanos(nanos);
451	if (nanos <= 0) {
452	release(permits - node.permitsNeeded);
453	return false;
454	}
455	}
456	return true;
457	} catch(InterruptedException ie) {
458	if (node.permitsNeeded > 0) {
459	node.cancelled = true;
460	release(permits - node.permitsNeeded);
461	throw ie;
462	} else { // ignore interrupt
463	Thread.currentThread().interrupt();
464	return true;
465	}
466	} finally {
467	node.unlock();
468	}
469	}
470
471
472	/**
473	* Releases the given number of permits, returning them to the semaphore.
474	* <p>Releases the given number of permits, increasing the number of
475	* available permits by that amount.
476	* If any threads are blocking trying to acquire permits, then the
477	* one that has been waiting the longest
478	* is selected and given the permits that were just released.
479	* If the number of available permits satisfies that thread's request
480	* then that thread is re-enabled for thread scheduling purposes; otherwise
481	* the thread continues to wait. If there are still permits available
482	* after the first thread's request has been satisfied, then those permits
483	* are assigned to the next waiting thread. If it is satisfied then it is
484	* re-enabled for thread scheduling purposes. This continues until there
485	* are insufficient permits to satisfy the next waiting thread, or there
486	* are no more waiting threads.
487	*
488	* <p>There is no requirement that a thread that releases a permit must
489	* have acquired that permit by calling {@link Semaphore#acquire acquire}.
490	* Correct usage of a semaphore is established by programming convention
491	* in the application.
492	*
493	* @param permits the number of permits to release
494	* @throws IllegalArgumentException if permits less than zero.
495	*/
496	public void release(long permits) {
497	if (permits < 0) throw new IllegalArgumentException();
498	if (permits == 0)
499	return;
500	// Even when using fair locks, releases should try to barge in.
501	if (!lock.tryLock())
502	lock.lock();
503	try {
504	long p = permits;
505	while (p > 0) {
506	Node node = head;
507	if (node == null) {
508	count += p;
509	p = 0;
510	} else {
511	node.lock();
512	try {
513	if (node.cancelled)
514	removeFirst();
515	else if (node.permitsNeeded > p) {
516	node.permitsNeeded -= p;
517	p = 0;
518	} else {
519	p -= node.permitsNeeded;
520	node.permitsNeeded = 0;
521	node.done.signal();
522	removeFirst();
523	}
524	} finally {
525	node.unlock();
526	}
527	}
528	}
529	} finally {
530	lock.unlock();
531	}
532	}
533	}