src/jsr166e/SequenceLock.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package jsr166e;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.AbstractQueuedLongSynchronizer;
import java.util.Collection;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.io.IOException;

/**
 * A reentrant mutual exclusion {@link Lock} in which each lock
 * acquisition or release advances a sequence number.  When the
 * sequence number (accessible using {@link #getSequence()}) is odd,
 * the lock is held. When it is even (i.e., ({@code lock.getSequence()
 * & 1L) == 0L}), the lock is released. Method {@link
 * #awaitAvailability} can be used to await availability of the lock,
 * returning its current sequence number. Sequence numbers are of type
 * {@code long} to ensure that they will not wrap around until
 * hundreds of years of use under current processor rates.  A
 * SequenceLock can be created with a specified number of
 * spins. Attempts to lock or await release retry at least the given
 * number of times before blocking. If not specified, a default,
 * possibly platform-specific, value is used.
 *
 * <p>Except for the lack of support for specified fairness policies,
 * or {link Condition} objects, a SequenceLock can be used in the same
 * way as {@link ReentrantLock}, and has a nearly identical
 * API. SequenceLocks may be preferable in contexts in which multiple
 * threads invoke read-only methods much more frequently than fully
 * locked methods.
 * 
 * <p> Methods {@code awaitAvailability} and {@code getSequence} can
 * be used together to define (partially) optimistic read-only methods
 * that are usually more efficient than ReadWriteLocks when they
 * apply.  These read-only methods typically read multiple field
 * values into local variables when the lock is not held, retrying if
 * the sequence number changed while doing so.  Alternatively, because
 * {@code awaitAvailability} accommodates reentrancy, a method can
 * retry a bounded number of times before switching to locking mode.
 * While conceptually straightforward, expressing these ideas can be
 * verbose. For example:
 *
 * <pre> {@code
 * class Point {
 *     private float x, y;
 *     private final SequenceLock sl = new SequenceLock();
 *
 *     void move(float deltaX, float deltaY) { // an excluively locked method
 *        sl.lock();
 *        try {
 *            x += deltaX;
 *            y += deltaY;
 *        } finally {
 *          sl.unlock();
 *      }
 *  }
 *
 *  float distanceFromOriginV1() { // A read-only method
 *      float currentX, currentY;
 *      long seq;
 *      do {
 *          seq = sl.awaitAvailability();
 *          currentX = x;
 *          currentY = y;
 *      } while (sl.getSequence() != seq); // retry if sequence changed
 *      return (float)Math.sqrt(currentX * currentX + currentY * currentY);
 *  }
 *
 *  float distanceFromOriginV2() { // Uses bounded retries before locking
 *      float currentX, currentY;
 *      long seq;
 *      int retries = RETRIES_BEFORE_LOCKING; // for example 8
 *      try {
 *        do {
 *           if (--retries < 0)
 *              sl.lock();
 *           seq = sl.awaitAvailability();
 *           currentX = x;
 *           currentY = y;
 *        } while (sl.getSequence() != seq);
 *      } finally {
 *        if (retries < 0)
 *           sl.unlock();
 *      }
 *      return (float)Math.sqrt(currentX * currentX + currentY * currentY);
 *  }
 *}}</pre>
 *
 * @since 1.8
 * @author Doug Lea
 */
public class SequenceLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;

    static final class Sync extends AbstractQueuedLongSynchronizer {
        /**
         * The number of times to spin in lock() and awaitAvailability().
         */
        final int spins;

        /**
         * The number of reentrant holds on this lock. Uses a long for
         * compatibility with other AbstractQueuedLongSynchronizer
         * operations.
         */
        long holds;

        Sync(int spins) { this.spins = spins; }

        // overrides of AQLS methods

        public final boolean isHeldExclusively() {
            return (getState() & 1L) != 0L &&
                getExclusiveOwnerThread() == Thread.currentThread();
        }
        
      public final boolean tryAcquire(long acquires) {
            Thread current = Thread.currentThread();
            long c = getState();
            if ((c & 1L) == 0L) {
                if (compareAndSetState(c, c + 1L)) {
                    holds = acquires;
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                holds += acquires;
                return true;
            }
            return false;
        }
        
        public final boolean tryRelease(long releases) {
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            if ((holds -= releases) == 0L) {
                setExclusiveOwnerThread(null);
                setState(getState() + 1L);
                return true;
            }
            return false;
        }

        public final long tryAcquireShared(long unused) {
            return ((getState() & 1L) == 0L || 
                    getExclusiveOwnerThread() == Thread.currentThread())? 
                1L : -1L; // must return long
        }

        public final boolean tryReleaseShared(long unused) {
            return true;
        }

        public final Condition newCondition() { 
            throw new UnsupportedOperationException();
        }

        // Other methods in support of SequenceLock

        final long getSequence() {
            return getState();
        }
        
        final void lock() {
            int k = spins;
            while (!tryAcquire(1)) {
                if (k == 0) {
                    acquire(1);
                    break;
                }
                --k;
            }
        }
        
        final long awaitAvailability() {
            long s;
            int k = spins;
            while (((s = getState()) & 1L) != 0L &&
                   getExclusiveOwnerThread() != Thread.currentThread()) {
                if (k > 0)
                    --k;
                else {
                    acquireShared(1);
                    releaseShared(1);
                }
            }
            return s;
        }

        final boolean isLocked() {
            return (getState() & 1L) != 0L;
        }

        final Thread getOwner() {
            return (getState() & 1L) == 0L ? null : getExclusiveOwnerThread();
        }

        final long getHoldCount() {
            return isHeldExclusively()? holds : 0;
        }

        private void readObject(ObjectInputStream s)
            throws IOException, ClassNotFoundException {
            s.defaultReadObject();
            holds = 0L;
            setState(0L); // reset to unlocked state
        }
    }

    private final Sync sync;

    /** 
     * The default spin value for constructor. Future versions of this
     * class might choose platform-specific values.  Currently, except
     * on uniprocessors, it is set to a small value that ovecomes near
     * misses between releases and acquires.
     */
    static final int DEFAULT_SPINS = 
        Runtime.getRuntime().availableProcessors() > 1 ? 64 : 0;

    /**
     * Creates an instance of {@code SequenceLock} with the default
     * number of retry attempts to lock or await release before
     * blocking.
     */
    public SequenceLock() { sync = new Sync(DEFAULT_SPINS); }

    /**
     * Creates an instance of {@code SequenceLock} that
     * will retry attempts to lock or await release 
     * at least the given number times before blocking.
     */
    public SequenceLock(int spins) { sync = new Sync(spins); }

    /**
     * Returns the current sequence number of this lock.  The sequence
     * number is advanced upon each lock or unlock action. When this
     * value is odd, the lock is held; when even, it is released.
     *
     * @return the current sequence number
     */
    public long getSequence() { return sync.getSequence(); }

    /**
     * Returns the current sequence number when the lock is, or
     * becomes, available. A lock is available if it is either
     * released, or is held by the current thread.  If the lock is not
     * available, the current thread becomes disabled for thread
     * scheduling purposes and lies dormant until the lock has been
     * released by some other thread.
     *
     * @return the current sequence number
     */
    public long awaitAvailability() { return sync.awaitAvailability(); }
    
    /**
     * Acquires the lock.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds the lock then the hold
     * count is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until the lock has been acquired,
     * at which time the lock hold count is set to one.
     */
    public void lock() { sync.lock(); }

    /**
     * Acquires the lock unless the current thread is
     * {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds this lock then the hold count
     * is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of two things happens:
     *
     * <ul>
     *
     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread.
     *
     * </ul>
     *
     * <p>If the lock is acquired by the current thread then the lock hold
     * count is set to one.
     *
     * <p>If the current thread:
     *
     * <ul>
     *
     * <li>has its interrupted status set on entry to this method; or
     *
     * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
     * the lock,
     *
     * </ul>
     *
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>In this implementation, as this method is an explicit
     * interruption point, preference is given to responding to the
     * interrupt over normal or reentrant acquisition of the lock.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    /**
     * Acquires the lock only if it is not held by another thread at the time
     * of invocation.
     *
     * <p>Acquires the lock if it is not held by another thread and
     * returns immediately with the value {@code true}, setting the
     * lock hold count to one. 
     *
     * <p> If the current thread already holds this lock then the hold
     * count is incremented by one and the method returns {@code true}.
     *
     * <p>If the lock is held by another thread then this method will return
     * immediately with the value {@code false}.
     *
     * @return {@code true} if the lock was free and was acquired by the
     *         current thread, or the lock was already held by the current
     *         thread; and {@code false} otherwise
     */
    public boolean tryLock() { return sync.tryAcquire(1); }

    /**
     * Acquires the lock if it is not held by another thread within the given
     * waiting time and the current thread has not been
     * {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately with the value {@code true}, setting the lock hold count
     * to one. If this lock has been set to use a fair ordering policy then
     * an available lock <em>will not</em> be acquired if any other threads
     * are waiting for the lock. This is in contrast to the {@link #tryLock()}
     * method. If you want a timed {@code tryLock} that does permit barging on
     * a fair lock then combine the timed and un-timed forms together:
     *
     *  <pre> {@code
     * if (lock.tryLock() ||
     *     lock.tryLock(timeout, unit)) {
     *   ...
     * }}</pre>
     *
     * <p>If the current thread
     * already holds this lock then the hold count is incremented by one and
     * the method returns {@code true}.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of three things happens:
     *
     * <ul>
     *
     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The specified waiting time elapses
     *
     * </ul>
     *
     * <p>If the lock is acquired then the value {@code true} is returned and
     * the lock hold count is set to one.
     *
     * <p>If the current thread:
     *
     * <ul>
     *
     * <li>has its interrupted status set on entry to this method; or
     *
     * <li>is {@linkplain Thread#interrupt interrupted} while
     * acquiring the lock,
     *
     * </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 {@code false}
     * is returned.  If the time is less than or equal to zero, the method
     * will not wait at all.
     *
     * <p>In this implementation, as this method is an explicit
     * interruption point, preference is given to responding to the
     * interrupt over normal or reentrant acquisition of the lock, and
     * over reporting the elapse of the waiting time.
     *
     * @param timeout the time to wait for the lock
     * @param unit the time unit of the timeout argument
     * @return {@code true} if the lock was free and was acquired by the
     *         current thread, or the lock was already held by the current
     *         thread; and {@code false} if the waiting time elapsed before
     *         the lock could be acquired
     * @throws InterruptedException if the current thread is interrupted
     * @throws NullPointerException if the time unit is null
     *
     */
    public boolean tryLock(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

    /**
     * Attempts to release this lock.
     *
     * <p>If the current thread is the holder of this lock then the hold
     * count is decremented.  If the hold count is now zero then the lock
     * is released.  If the current thread is not the holder of this
     * lock then {@link IllegalMonitorStateException} is thrown.
     *
     * @throws IllegalMonitorStateException if the current thread does not
     *         hold this lock
     */
    public void unlock()              { sync.release(1); }

    /**
     * Throws UnsupportedOperationException. SequenceLocks
     * do not support Condition objects.
     *
     * @throws UnsupportedOperationException
     */
    public Condition newCondition()   { 
        throw new UnsupportedOperationException();
    }

    /**
     * Queries the number of holds on this lock by the current thread.
     *
     * <p>A thread has a hold on a lock for each lock action that is not
     * matched by an unlock action.
     *
     * <p>The hold count information is typically only used for testing and
     * debugging purposes. 
     *
     * @return the number of holds on this lock by the current thread,
     *         or zero if this lock is not held by the current thread
     */
    public long getHoldCount() { return sync.getHoldCount();  }

    /**
     * Queries if this lock is held by the current thread.
     *
     * @return {@code true} if current thread holds this lock and
     *         {@code false} otherwise
     */
    public boolean isHeldByCurrentThread() { return sync.isHeldExclusively(); }

    /**
     * Queries if this lock is held by any thread. This method is
     * designed for use in monitoring of the system state,
     * not for synchronization control.
     *
     * @return {@code true} if any thread holds this lock and
     *         {@code false} otherwise
     */
    public boolean isLocked() { return sync.isLocked(); }

    /**
     * Returns the thread that currently owns this lock, or
     * {@code null} if not owned. When this method is called by a
     * thread that is not the owner, the return value reflects a
     * best-effort approximation of current lock status. For example,
     * the owner may be momentarily {@code null} even if there are
     * threads trying to acquire the lock but have not yet done so.
     * This method is designed to facilitate construction of
     * subclasses that provide more extensive lock monitoring
     * facilities.
     *
     * @return the owner, or {@code null} if not owned
     */
    protected Thread getOwner() { return sync.getOwner(); }

    /**
     * Queries whether any threads are waiting to acquire this lock. Note that
     * because cancellations may occur at any time, a {@code true}
     * return does not guarantee that any other thread will ever
     * acquire this lock.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @return {@code true} if there may be other threads waiting to
     *         acquire the lock
     */
    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

    /**
     * Queries whether the given thread is waiting to acquire this
     * lock. Note that because cancellations may occur at any time, a
     * {@code true} return does not guarantee that this thread
     * will ever acquire this lock.  This method is designed primarily for use
     * in monitoring of the system state.
     *
     * @param thread the thread
     * @return {@code true} if the given thread is queued waiting for this lock
     * @throws NullPointerException if the thread is null
     */
    public final boolean hasQueuedThread(Thread thread) {
        return sync.isQueued(thread);
    }

    /**
     * Returns an estimate of the number of threads waiting to
     * acquire this lock.  The value is only an estimate because the number of
     * threads may change dynamically while this method traverses
     * internal data structures.  This method is designed for use in
     * monitoring of the system state, not for synchronization
     * control.
     *
     * @return the estimated number of threads waiting for this lock
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire this lock.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }

    /**
     * Returns a string identifying this lock, as well as its lock state.
     * The state, in brackets, includes either the String {@code "Unlocked"}
     * or the String {@code "Locked by"} followed by the
     * {@linkplain Thread#getName name} of the owning thread.
     *
     * @return a string identifying this lock, as well as its lock state
     */
    public String toString() {
        Thread o = sync.getOwner();
        return super.toString() + ((o == null) ?
                                   "[Unlocked]" :
                                   "[Locked by thread " + o.getName() + "]");
    }

}
            
Revision:	1.2
Committed:	Fri Jul 15 15:04:14 2011 UTC (12 years, 10 months ago) by dl
Branch:	MAIN
Changes since 1.1:	+17 -113 lines
Log Message:	Don't support Conditions.
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package jsr166e;
8	import java.util.concurrent.TimeUnit;
9	import java.util.concurrent.locks.Lock;
10	import java.util.concurrent.locks.ReentrantLock;
11	import java.util.concurrent.locks.Condition;
12	import java.util.concurrent.locks.AbstractQueuedLongSynchronizer;
13	import java.util.Collection;
14	import java.io.ObjectOutputStream;
15	import java.io.ObjectInputStream;
16	import java.io.IOException;
17
18	/**
19	* A reentrant mutual exclusion {@link Lock} in which each lock
20	* acquisition or release advances a sequence number. When the
21	* sequence number (accessible using {@link #getSequence()}) is odd,
22	* the lock is held. When it is even (i.e., ({@code lock.getSequence()
23	* & 1L) == 0L}), the lock is released. Method {@link
24	* #awaitAvailability} can be used to await availability of the lock,
25	* returning its current sequence number. Sequence numbers are of type
26	* {@code long} to ensure that they will not wrap around until
27	* hundreds of years of use under current processor rates. A
28	* SequenceLock can be created with a specified number of
29	* spins. Attempts to lock or await release retry at least the given
30	* number of times before blocking. If not specified, a default,
31	* possibly platform-specific, value is used.
32	*
33	* <p>Except for the lack of support for specified fairness policies,
34	* or {link Condition} objects, a SequenceLock can be used in the same
35	* way as {@link ReentrantLock}, and has a nearly identical
36	* API. SequenceLocks may be preferable in contexts in which multiple
37	* threads invoke read-only methods much more frequently than fully
38	* locked methods.
39	*
40	* <p> Methods {@code awaitAvailability} and {@code getSequence} can
41	* be used together to define (partially) optimistic read-only methods
42	* that are usually more efficient than ReadWriteLocks when they
43	* apply. These read-only methods typically read multiple field
44	* values into local variables when the lock is not held, retrying if
45	* the sequence number changed while doing so. Alternatively, because
46	* {@code awaitAvailability} accommodates reentrancy, a method can
47	* retry a bounded number of times before switching to locking mode.
48	* While conceptually straightforward, expressing these ideas can be
49	* verbose. For example:
50	*
51	* <pre> {@code
52	* class Point {
53	* private float x, y;
54	* private final SequenceLock sl = new SequenceLock();
55	*
56	* void move(float deltaX, float deltaY) { // an excluively locked method
57	* sl.lock();
58	* try {
59	* x += deltaX;
60	* y += deltaY;
61	* } finally {
62	* sl.unlock();
63	* }
64	* }
65	*
66	* float distanceFromOriginV1() { // A read-only method
67	* float currentX, currentY;
68	* long seq;
69	* do {
70	* seq = sl.awaitAvailability();
71	* currentX = x;
72	* currentY = y;
73	* } while (sl.getSequence() != seq); // retry if sequence changed
74	* return (float)Math.sqrt(currentX * currentX + currentY * currentY);
75	* }
76	*
77	* float distanceFromOriginV2() { // Uses bounded retries before locking
78	* float currentX, currentY;
79	* long seq;
80	* int retries = RETRIES_BEFORE_LOCKING; // for example 8
81	* try {
82	* do {
83	* if (--retries < 0)
84	* sl.lock();
85	* seq = sl.awaitAvailability();
86	* currentX = x;
87	* currentY = y;
88	* } while (sl.getSequence() != seq);
89	* } finally {
90	* if (retries < 0)
91	* sl.unlock();
92	* }
93	* return (float)Math.sqrt(currentX * currentX + currentY * currentY);
94	* }
95	*}}</pre>
96	*
97	* @since 1.8
98	* @author Doug Lea
99	*/
100	public class SequenceLock implements Lock, java.io.Serializable {
101	private static final long serialVersionUID = 7373984872572414699L;
102
103	static final class Sync extends AbstractQueuedLongSynchronizer {
104	/**
105	* The number of times to spin in lock() and awaitAvailability().
106	*/
107	final int spins;
108
109	/**
110	* The number of reentrant holds on this lock. Uses a long for
111	* compatibility with other AbstractQueuedLongSynchronizer
112	* operations.
113	*/
114	long holds;
115
116	Sync(int spins) { this.spins = spins; }
117
118	// overrides of AQLS methods
119
120	public final boolean isHeldExclusively() {
121	return (getState() & 1L) != 0L &&
122	getExclusiveOwnerThread() == Thread.currentThread();
123	}
124
125	public final boolean tryAcquire(long acquires) {
126	Thread current = Thread.currentThread();
127	long c = getState();
128	if ((c & 1L) == 0L) {
129	if (compareAndSetState(c, c + 1L)) {
130	holds = acquires;
131	setExclusiveOwnerThread(current);
132	return true;
133	}
134	}
135	else if (current == getExclusiveOwnerThread()) {
136	holds += acquires;
137	return true;
138	}
139	return false;
140	}
141
142	public final boolean tryRelease(long releases) {
143	if (Thread.currentThread() != getExclusiveOwnerThread())
144	throw new IllegalMonitorStateException();
145	if ((holds -= releases) == 0L) {
146	setExclusiveOwnerThread(null);
147	setState(getState() + 1L);
148	return true;
149	}
150	return false;
151	}
152
153	public final long tryAcquireShared(long unused) {
154	return ((getState() & 1L) == 0L \|\|
155	getExclusiveOwnerThread() == Thread.currentThread())?
156	1L : -1L; // must return long
157	}
158
159	public final boolean tryReleaseShared(long unused) {
160	return true;
161	}
162
163	public final Condition newCondition() {
164	throw new UnsupportedOperationException();
165	}
166
167	// Other methods in support of SequenceLock
168
169	final long getSequence() {
170	return getState();
171	}
172
173	final void lock() {
174	int k = spins;
175	while (!tryAcquire(1)) {
176	if (k == 0) {
177	acquire(1);
178	break;
179	}
180	--k;
181	}
182	}
183
184	final long awaitAvailability() {
185	long s;
186	int k = spins;
187	while (((s = getState()) & 1L) != 0L &&
188	getExclusiveOwnerThread() != Thread.currentThread()) {
189	if (k > 0)
190	--k;
191	else {
192	acquireShared(1);
193	releaseShared(1);
194	}
195	}
196	return s;
197	}
198
199	final boolean isLocked() {
200	return (getState() & 1L) != 0L;
201	}
202
203	final Thread getOwner() {
204	return (getState() & 1L) == 0L ? null : getExclusiveOwnerThread();
205	}
206
207	final long getHoldCount() {
208	return isHeldExclusively()? holds : 0;
209	}
210
211	private void readObject(ObjectInputStream s)
212	throws IOException, ClassNotFoundException {
213	s.defaultReadObject();
214	holds = 0L;
215	setState(0L); // reset to unlocked state
216	}
217	}
218
219	private final Sync sync;
220
221	/**
222	* The default spin value for constructor. Future versions of this
223	* class might choose platform-specific values. Currently, except
224	* on uniprocessors, it is set to a small value that ovecomes near
225	* misses between releases and acquires.
226	*/
227	static final int DEFAULT_SPINS =
228	Runtime.getRuntime().availableProcessors() > 1 ? 64 : 0;
229
230	/**
231	* Creates an instance of {@code SequenceLock} with the default
232	* number of retry attempts to lock or await release before
233	* blocking.
234	*/
235	public SequenceLock() { sync = new Sync(DEFAULT_SPINS); }
236
237	/**
238	* Creates an instance of {@code SequenceLock} that
239	* will retry attempts to lock or await release
240	* at least the given number times before blocking.
241	*/
242	public SequenceLock(int spins) { sync = new Sync(spins); }
243
244	/**
245	* Returns the current sequence number of this lock. The sequence
246	* number is advanced upon each lock or unlock action. When this
247	* value is odd, the lock is held; when even, it is released.
248	*
249	* @return the current sequence number
250	*/
251	public long getSequence() { return sync.getSequence(); }
252
253	/**
254	* Returns the current sequence number when the lock is, or
255	* becomes, available. A lock is available if it is either
256	* released, or is held by the current thread. If the lock is not
257	* available, the current thread becomes disabled for thread
258	* scheduling purposes and lies dormant until the lock has been
259	* released by some other thread.
260	*
261	* @return the current sequence number
262	*/
263	public long awaitAvailability() { return sync.awaitAvailability(); }
264
265	/**
266	* Acquires the lock.
267	*
268	* <p>Acquires the lock if it is not held by another thread and returns
269	* immediately, setting the lock hold count to one.
270	*
271	* <p>If the current thread already holds the lock then the hold
272	* count is incremented by one and the method returns immediately.
273	*
274	* <p>If the lock is held by another thread then the
275	* current thread becomes disabled for thread scheduling
276	* purposes and lies dormant until the lock has been acquired,
277	* at which time the lock hold count is set to one.
278	*/
279	public void lock() { sync.lock(); }
280
281	/**
282	* Acquires the lock unless the current thread is
283	* {@linkplain Thread#interrupt interrupted}.
284	*
285	* <p>Acquires the lock if it is not held by another thread and returns
286	* immediately, setting the lock hold count to one.
287	*
288	* <p>If the current thread already holds this lock then the hold count
289	* is incremented by one and the method returns immediately.
290	*
291	* <p>If the lock is held by another thread then the
292	* current thread becomes disabled for thread scheduling
293	* purposes and lies dormant until one of two things happens:
294	*
295	* <ul>
296	*
297	* <li>The lock is acquired by the current thread; or
298	*
299	* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
300	* current thread.
301	*
302	* </ul>
303	*
304	* <p>If the lock is acquired by the current thread then the lock hold
305	* count is set to one.
306	*
307	* <p>If the current thread:
308	*
309	* <ul>
310	*
311	* <li>has its interrupted status set on entry to this method; or
312	*
313	* <li>is {@linkplain Thread#interrupt interrupted} while acquiring
314	* the lock,
315	*
316	* </ul>
317	*
318	* then {@link InterruptedException} is thrown and the current thread's
319	* interrupted status is cleared.
320	*
321	* <p>In this implementation, as this method is an explicit
322	* interruption point, preference is given to responding to the
323	* interrupt over normal or reentrant acquisition of the lock.
324	*
325	* @throws InterruptedException if the current thread is interrupted
326	*/
327	public void lockInterruptibly() throws InterruptedException {
328	sync.acquireInterruptibly(1);
329	}
330
331	/**
332	* Acquires the lock only if it is not held by another thread at the time
333	* of invocation.
334	*
335	* <p>Acquires the lock if it is not held by another thread and
336	* returns immediately with the value {@code true}, setting the
337	* lock hold count to one.
338	*
339	* <p> If the current thread already holds this lock then the hold
340	* count is incremented by one and the method returns {@code true}.
341	*
342	* <p>If the lock is held by another thread then this method will return
343	* immediately with the value {@code false}.
344	*
345	* @return {@code true} if the lock was free and was acquired by the
346	* current thread, or the lock was already held by the current
347	* thread; and {@code false} otherwise
348	*/
349	public boolean tryLock() { return sync.tryAcquire(1); }
350
351	/**
352	* Acquires the lock if it is not held by another thread within the given
353	* waiting time and the current thread has not been
354	* {@linkplain Thread#interrupt interrupted}.
355	*
356	* <p>Acquires the lock if it is not held by another thread and returns
357	* immediately with the value {@code true}, setting the lock hold count
358	* to one. If this lock has been set to use a fair ordering policy then
359	* an available lock <em>will not</em> be acquired if any other threads
360	* are waiting for the lock. This is in contrast to the {@link #tryLock()}
361	* method. If you want a timed {@code tryLock} that does permit barging on
362	* a fair lock then combine the timed and un-timed forms together:
363	*
364	* <pre> {@code
365	* if (lock.tryLock() \|\|
366	* lock.tryLock(timeout, unit)) {
367	* ...
368	* }}</pre>
369	*
370	* <p>If the current thread
371	* already holds this lock then the hold count is incremented by one and
372	* the method returns {@code true}.
373	*
374	* <p>If the lock is held by another thread then the
375	* current thread becomes disabled for thread scheduling
376	* purposes and lies dormant until one of three things happens:
377	*
378	* <ul>
379	*
380	* <li>The lock is acquired by the current thread; or
381	*
382	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
383	* the current thread; or
384	*
385	* <li>The specified waiting time elapses
386	*
387	* </ul>
388	*
389	* <p>If the lock is acquired then the value {@code true} is returned and
390	* the lock hold count is set to one.
391	*
392	* <p>If the current thread:
393	*
394	* <ul>
395	*
396	* <li>has its interrupted status set on entry to this method; or
397	*
398	* <li>is {@linkplain Thread#interrupt interrupted} while
399	* acquiring the lock,
400	*
401	* </ul>
402	* then {@link InterruptedException} is thrown and the current thread's
403	* interrupted status is cleared.
404	*
405	* <p>If the specified waiting time elapses then the value {@code false}
406	* is returned. If the time is less than or equal to zero, the method
407	* will not wait at all.
408	*
409	* <p>In this implementation, as this method is an explicit
410	* interruption point, preference is given to responding to the
411	* interrupt over normal or reentrant acquisition of the lock, and
412	* over reporting the elapse of the waiting time.
413	*
414	* @param timeout the time to wait for the lock
415	* @param unit the time unit of the timeout argument
416	* @return {@code true} if the lock was free and was acquired by the
417	* current thread, or the lock was already held by the current
418	* thread; and {@code false} if the waiting time elapsed before
419	* the lock could be acquired
420	* @throws InterruptedException if the current thread is interrupted
421	* @throws NullPointerException if the time unit is null
422	*
423	*/
424	public boolean tryLock(long timeout, TimeUnit unit)
425	throws InterruptedException {
426	return sync.tryAcquireNanos(1, unit.toNanos(timeout));
427	}
428
429	/**
430	* Attempts to release this lock.
431	*
432	* <p>If the current thread is the holder of this lock then the hold
433	* count is decremented. If the hold count is now zero then the lock
434	* is released. If the current thread is not the holder of this
435	* lock then {@link IllegalMonitorStateException} is thrown.
436	*
437	* @throws IllegalMonitorStateException if the current thread does not
438	* hold this lock
439	*/
440	public void unlock() { sync.release(1); }
441
442	/**
443	* Throws UnsupportedOperationException. SequenceLocks
444	* do not support Condition objects.
445	*
446	* @throws UnsupportedOperationException
447	*/
448	public Condition newCondition() {
449	throw new UnsupportedOperationException();
450	}
451
452	/**
453	* Queries the number of holds on this lock by the current thread.
454	*
455	* <p>A thread has a hold on a lock for each lock action that is not
456	* matched by an unlock action.
457	*
458	* <p>The hold count information is typically only used for testing and
459	* debugging purposes.
460	*
461	* @return the number of holds on this lock by the current thread,
462	* or zero if this lock is not held by the current thread
463	*/
464	public long getHoldCount() { return sync.getHoldCount(); }
465
466	/**
467	* Queries if this lock is held by the current thread.
468	*
469	* @return {@code true} if current thread holds this lock and
470	* {@code false} otherwise
471	*/
472	public boolean isHeldByCurrentThread() { return sync.isHeldExclusively(); }
473
474	/**
475	* Queries if this lock is held by any thread. This method is
476	* designed for use in monitoring of the system state,
477	* not for synchronization control.
478	*
479	* @return {@code true} if any thread holds this lock and
480	* {@code false} otherwise
481	*/
482	public boolean isLocked() { return sync.isLocked(); }
483
484	/**
485	* Returns the thread that currently owns this lock, or
486	* {@code null} if not owned. When this method is called by a
487	* thread that is not the owner, the return value reflects a
488	* best-effort approximation of current lock status. For example,
489	* the owner may be momentarily {@code null} even if there are
490	* threads trying to acquire the lock but have not yet done so.
491	* This method is designed to facilitate construction of
492	* subclasses that provide more extensive lock monitoring
493	* facilities.
494	*
495	* @return the owner, or {@code null} if not owned
496	*/
497	protected Thread getOwner() { return sync.getOwner(); }
498
499	/**
500	* Queries whether any threads are waiting to acquire this lock. Note that
501	* because cancellations may occur at any time, a {@code true}
502	* return does not guarantee that any other thread will ever
503	* acquire this lock. This method is designed primarily for use in
504	* monitoring of the system state.
505	*
506	* @return {@code true} if there may be other threads waiting to
507	* acquire the lock
508	*/
509	public final boolean hasQueuedThreads() {
510	return sync.hasQueuedThreads();
511	}
512
513	/**
514	* Queries whether the given thread is waiting to acquire this
515	* lock. Note that because cancellations may occur at any time, a
516	* {@code true} return does not guarantee that this thread
517	* will ever acquire this lock. This method is designed primarily for use
518	* in monitoring of the system state.
519	*
520	* @param thread the thread
521	* @return {@code true} if the given thread is queued waiting for this lock
522	* @throws NullPointerException if the thread is null
523	*/
524	public final boolean hasQueuedThread(Thread thread) {
525	return sync.isQueued(thread);
526	}
527
528	/**
529	* Returns an estimate of the number of threads waiting to
530	* acquire this lock. The value is only an estimate because the number of
531	* threads may change dynamically while this method traverses
532	* internal data structures. This method is designed for use in
533	* monitoring of the system state, not for synchronization
534	* control.
535	*
536	* @return the estimated number of threads waiting for this lock
537	*/
538	public final int getQueueLength() {
539	return sync.getQueueLength();
540	}
541
542	/**
543	* Returns a collection containing threads that may be waiting to
544	* acquire this lock. Because the actual set of threads may change
545	* dynamically while constructing this result, the returned
546	* collection is only a best-effort estimate. The elements of the
547	* returned collection are in no particular order. This method is
548	* designed to facilitate construction of subclasses that provide
549	* more extensive monitoring facilities.
550	*
551	* @return the collection of threads
552	*/
553	protected Collection<Thread> getQueuedThreads() {
554	return sync.getQueuedThreads();
555	}
556
557	/**
558	* Returns a string identifying this lock, as well as its lock state.
559	* The state, in brackets, includes either the String {@code "Unlocked"}
560	* or the String {@code "Locked by"} followed by the
561	* {@linkplain Thread#getName name} of the owning thread.
562	*
563	* @return a string identifying this lock, as well as its lock state
564	*/
565	public String toString() {
566	Thread o = sync.getOwner();
567	return super.toString() + ((o == null) ?
568	"[Unlocked]" :
569	"[Locked by thread " + o.getName() + "]");
570	}
571
572	}
573