util/concurrent/Lock.java

package java.util.concurrent;

/**
 * <tt>Lock</tt> implementations provide more flexible locking operations than
 * can be obtained using <tt>synchronized</tt> methods and statements.
 *
 * <p>A lock is a tool for controlling access to a shared
 * resource by multiple threads. Commonly, a lock provides exclusive access
 * to a shared resource: only one thread at a time can acquire the
 * lock and all access to the shared resource requires that the lock be
 * acquired first. However, some locks may allow concurrent access to a shared
 * resource, such as the read lock of a {@link ReadWriteLock}.
 *
 * <p>The use of <tt>synchronized</tt> methods or statements provides 
 * access to the implicit monitor lock associated with every object, but
 * forces all lock acquisition and release to occur in a block-structured way:
 * when multiple locks are acquired they must be released in the opposite
 * order, and all locks must be released in the same lexical scope in which
 * they were acquired.
 *
 * <p>While the scoping mechanism for <tt>synchronized</tt> methods and 
 * statements makes it much easier to program with monitor locks,
 * and helps avoid many common programming errors involving locks, there are
 * rare occasions where you need to work with locks in a more flexible way. For
 * example, some advanced algorithms for traversing concurrently accessed data
 * structures require the use of what is called &quot;hand-over-hand&quot; or 
 * &quot;chain locking&quot;: you acquire the lock of node A, then node B, 
 * then release A and acquire C, then release B and acquire D and so on. 
 * Implementations of the <tt>Lock</tt> interface facilitate the use of such 
 * advanced algorithms by allowing a lock to be acquired and released in 
 * different scopes, and allowing multiple locks to be acquired and released 
 * in any order. 
 *
 * <p>With this increased flexibilty comes
 * additional responsibility as the absence of block-structured locking
 * removes the automatic release of locks that occurs with 
 * <tt>synchronized</tt> methods and statements. For the simplest usage
 * the following idiom should be used:
 * <pre><tt>     Lock l = ...; 
 *     l.lock();
 *     try {
 *         // access the resource protected by this lock
 *     } catch ( ... ) {
 *         // ensure consistency before releasing lock
 *     } finally {
 *         l.unlock();
 *     }
 * </tt></pre>
 * This mimics the automatic release property of built-in monitor locks, and
 * carries with it the same responsibility for ensuring that if an exception
 * occurs then the resource is left in a consistent state before the lock
 * is released.
 *
 * <p><tt>Lock</tt> implementations provide additional functionality over the 
 * use
 * of <tt>synchronized</tt> methods and statements by providing a non-blocking
 * attempt to acquire a lock ({@link #tryLock()}), an attempt to acquire the
 * lock that can be interrupted ({@link #lockInterruptibly}, and an attempt
 * to acquire the lock that can timeout ({@link #tryLock(long, TimeUnit)}).
 * This additionally functionality is also extended to built-in monitor
 * locks through the methods of the {@link Locks} utility class.
 *
 * <p>A <tt>Lock</tt> class can also provide behavior and semantics that is 
 * quite different from that of the implicit monitor lock, such as guaranteed 
 * ordering,
 * non-reentrant usage, or deadlock detection. If an implementation provides
 * such specialised semantics then the implementation must document those
 * semantics.
 *
 * <p>Note that <tt>Lock</tt> instances are just normal objects and can 
 * themselves be used as the target in a <tt>synchronized</tt> statement.
 * Acquiring the
 * monitor lock of a <tt>Lock</tt> instance has no specified relationship
 * with invoking any of the {@link #lock} methods of that instance. 
 * It is recommended that to avoid confusion you never use <tt>Lock</tt>
 * instances in this way, except within their own implementation.
 *
 * <p>Except where noted, passing a <tt>null</tt> value for any parameter 
 * will result in a {@link NullPointerException} being thrown.
 *
 * <h3>Memory Synchronization</h3>
 * <p>All <tt>Lock</tt> implementations <em>must</em> enforce the same
 * memory synchronization semantics as provided by the built-in monitor lock:
 * <ul>
 * <li>A successful lock operation acts like a successful 
 * <tt>monitorEnter</tt> action
 * <li>A successful <tt>unlock</tt> operation acts like a successful
 * <tt>monitorExit</tt> action
 * </ul>
 * Note that unsuccessful locking and unlocking operations, and reentrant
 * locking/unlocking operations, do not require any memory synchronization
 * effects.
 *
 * <h3>Implementation Considerations</h3>
 * <p>It is recognised that the three forms of lock acquisition (interruptible,
 * non-interruptible, and timed) may differ in their ease of implementation
 * on some platforms and in their performance characteristics.
 * In particular, it may be difficult to provide these features and maintain 
 * specific semantics such as ordering guarantees. 
 * Further, the ability to interrupt the acquisition of a lock may not always
 * be feasible to implement on all platforms.
 * <p>Consequently, an implementation is not required to define exactly the 
 * same 
 * guarantees or semantics for all three forms of lock acquistion, nor is it 
 * required to support interruption of the actual lock acquisition.
 * An implementation is required to clearly
 * document the semantics and guarantees provided by each of the locking 
 * methods. It must also obey the interruption semantics as defined in this
 * interface, to the extent that interruption of lock acquisition is 
 * supported: which is either totally, or only on method entry.
 *
 *
 * @see ReentrantLock
 * @see Condition
 * @see ReadWriteLock
 * @see Locks
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/01/30 22:12:40 $
 * @editor $Author: dholmes $
 *
 */
public interface Lock {

    /**
     * Acquires the lock. 
     * <p>Acquires the lock if it is available and returns immediately.
     * <p>If the lock is not available then
     * the current thread becomes disabled for thread scheduling 
     * purposes and lies dormant until the lock has been acquired.
     * <p><b>Implementation Considerations</b>
     * <p>A <tt>Lock</tt> implementation may be able to detect 
     * erroneous use of the lock, such as an invocation that would cause 
     * deadlock, and may throw an (unchecked) exception in such circumstances. 
     * The circumstances and the exception type must be documented by that 
     * <tt>Lock</tt> implementation.
     *
     */
    void lock();

    /**
     * Acquires the lock unless the current thread is  
     * {@link Thread#interrupt interrupted}. 
     * <p>Acquires the lock if it is available and returns immediately.
     * <p>If the lock is not available 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 {@link Thread#interrupt interrupts} the current
     * thread, and interruption of lock acquisition is supported.
     * </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 to acquire 
     * the lock, and interruption of lock acquisition is supported, 
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * <p><b>Implementation Considerations</b>
     * <p>The ability to interrupt a lock acquisition in some implementations
     * may not be possible, and if possible could reasonably be foreseen to 
     * be an expensive operation. 
     * The programmer should be aware that this may be the case. An
     * implementation should document when this is the case.
     *
     * <p>A <tt>Lock</tt> implementation may be able to detect 
     * erroneous use of the lock, such as an invocation that would cause 
     * deadlock, and may throw an (unchecked) exception in such circumstances. 
     * The circumstances and the exception type must be documented by that
     * <tt>Lock</tt> implementation.
     *
     * @throws InterruptedException if the current thread is interrupted
     * (and interruption of lock acquisition is supported).
     *
     * @see Thread#interrupt
     *
     */
    void lockInterruptibly() throws InterruptedException;


    /**
     * Acquires the lock only if it is free at the time of invocation.
     * <p>Acquires the lock if it is available and returns immediately
     * with the value <tt>true</tt>.
     * <p>If the lock is not available then this method will return 
     * immediately with the value <tt>false</tt>.
     * <p>A typical usage idiom for this method would be:
     * <pre>
     *      Lock lock = ...;
     *      if (lock.tryLock()) {
     *          try {
     *              // manipulate protected state
     *          } finally {
     *              lock.unlock();
     *          }
     *      } else {
     *          // perform alternative actions
     *      }
     * </pre>
     * This usage ensures that the lock is unlocked if it was acquired, and
     * doesn't try to unlock if the lock was not acquired.
     *
     * @return <tt>true</tt> if the lock was acquired and <tt>false</tt>
     * otherwise.
     */
    boolean tryLock();

    /**
     * Acquires the lock if it is free within the given waiting time and the
     * current thread has not been {@link Thread#interrupt interrupted}.
     * <p>Acquires the lock if it is available and returns immediately
     * with the value <tt>true</tt>.
     * <p>If the lock is not available 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 {@link Thread#interrupt interrupts} the current
     * thread, and interruption of lock acquisition is supported; or
     * <li> The specified waiting time elapses
     * </ul>
     * <p>If the lock 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 
     * the lock, and interruption of lock acquisition is supported, 
     * </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.
     *
     * <p><b>Implementation Considerations</b>
     * <p>The ability to interrupt a lock acquisition in some implementations
     * may not be possible, and if possible could reasonably be foreseen to 
     * be an expensive operation. 
     * The programmer should be aware that this may be the case. An
     * implementation should document when this is the case.
     *
     * <p>A <tt>Lock</tt> implementation may be able to detect 
     * erroneous use of the lock, such as an invocation that would cause 
     * deadlock, and may throw an (unchecked) exception in such circumstances. 
     * The circumstances and the exception type must be documented by that 
     * <tt>Lock</tt> implementation.
     *
     * @param timeout the maximum time to wait for the lock
     * @param granularity the time unit of the <tt>timeout</tt> argument.
     * @return <tt>true</tt> if the lock was acquired and <tt>false</tt>
     * if the waiting time elapsed before the lock was acquired.
     *
     * @throws InterruptedException if the current thread is interrupted
     * while trying to acquire the lock (and interruption of lock
     * acquisition is supported).
     *
     * @see Thread#interrupt
     *
     */
    boolean tryLock(long timeout, TimeUnit granularity) 
        throws InterruptedException;

    /**
     * Releases the lock.
     * <p><b>Implementation Considerations</b>
     * <p>A <tt>Lock</tt> implementation will usually impose
     * restrictions on which thread can release a lock (typically only the
     * holder of the lock can release it) and may throw
     * an (unchecked) exception if the restriction is violated.
     * Any restrictions and the exception
     * type must be documented by that <tt>Lock</tt> implementation.
     */
    void unlock();

    /**
     * Returns a {@link Condition} instance that is bound to this <tt>Lock</tt>
     * instance.
     * <p>Conditions are primarily used with the built-in locking provided by
     * <tt>synchronized</tt> methods and statements 
     * (see {@link Locks#newConditionFor}), but in some rare circumstances it 
     * can be useful to wait for a condition when working with a data 
     * structure that is accessed using a stand-alone <tt>Lock</tt> instance 
     * (see {@link ReentrantLock}). 
     * <p>Before waiting on the condition the lock must be held by the 
     * current thread. 
     * A call to {@link Condition#await()} will atomically release the lock 
     * before waiting and re-acquire the lock before the wait returns.
     * <p><b>Implementation Considerations</b>
     * <p>The exact operation of the {@link Condition} instance depends on the
     * <tt>Lock</tt> implementation and must be documented by that
     * implementation.
     * 
     * @return A {@link Condition} instance for this <tt>Lock</tt> instance.
     * @throws UnsupportedOperationException if this <tt>Lock</tt> 
     * implementation does not support conditions.
     */
    Condition newCondition();

}


Revision:	1.1
Committed:	Wed May 14 21:30:47 2003 UTC (21 years 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
#	User	Rev	Content
1	tim	1.1	package java.util.concurrent;
2
3			/**
4			* <tt>Lock</tt> implementations provide more flexible locking operations than
5			* can be obtained using <tt>synchronized</tt> methods and statements.
6			*
7			* <p>A lock is a tool for controlling access to a shared
8			* resource by multiple threads. Commonly, a lock provides exclusive access
9			* to a shared resource: only one thread at a time can acquire the
10			* lock and all access to the shared resource requires that the lock be
11			* acquired first. However, some locks may allow concurrent access to a shared
12			* resource, such as the read lock of a {@link ReadWriteLock}.
13			*
14			* <p>The use of <tt>synchronized</tt> methods or statements provides
15			* access to the implicit monitor lock associated with every object, but
16			* forces all lock acquisition and release to occur in a block-structured way:
17			* when multiple locks are acquired they must be released in the opposite
18			* order, and all locks must be released in the same lexical scope in which
19			* they were acquired.
20			*
21			* <p>While the scoping mechanism for <tt>synchronized</tt> methods and
22			* statements makes it much easier to program with monitor locks,
23			* and helps avoid many common programming errors involving locks, there are
24			* rare occasions where you need to work with locks in a more flexible way. For
25			* example, some advanced algorithms for traversing concurrently accessed data
26			* structures require the use of what is called "hand-over-hand" or
27			* "chain locking": you acquire the lock of node A, then node B,
28			* then release A and acquire C, then release B and acquire D and so on.
29			* Implementations of the <tt>Lock</tt> interface facilitate the use of such
30			* advanced algorithms by allowing a lock to be acquired and released in
31			* different scopes, and allowing multiple locks to be acquired and released
32			* in any order.
33			*
34			* <p>With this increased flexibilty comes
35			* additional responsibility as the absence of block-structured locking
36			* removes the automatic release of locks that occurs with
37			* <tt>synchronized</tt> methods and statements. For the simplest usage
38			* the following idiom should be used:
39			* <pre><tt> Lock l = ...;
40			* l.lock();
41			* try {
42			* // access the resource protected by this lock
43			* } catch ( ... ) {
44			* // ensure consistency before releasing lock
45			* } finally {
46			* l.unlock();
47			* }
48			* </tt></pre>
49			* This mimics the automatic release property of built-in monitor locks, and
50			* carries with it the same responsibility for ensuring that if an exception
51			* occurs then the resource is left in a consistent state before the lock
52			* is released.
53			*
54			* <p><tt>Lock</tt> implementations provide additional functionality over the
55			* use
56			* of <tt>synchronized</tt> methods and statements by providing a non-blocking
57			* attempt to acquire a lock ({@link #tryLock()}), an attempt to acquire the
58			* lock that can be interrupted ({@link #lockInterruptibly}, and an attempt
59			* to acquire the lock that can timeout ({@link #tryLock(long, TimeUnit)}).
60			* This additionally functionality is also extended to built-in monitor
61			* locks through the methods of the {@link Locks} utility class.
62			*
63			* <p>A <tt>Lock</tt> class can also provide behavior and semantics that is
64			* quite different from that of the implicit monitor lock, such as guaranteed
65			* ordering,
66			* non-reentrant usage, or deadlock detection. If an implementation provides
67			* such specialised semantics then the implementation must document those
68			* semantics.
69			*
70			* <p>Note that <tt>Lock</tt> instances are just normal objects and can
71			* themselves be used as the target in a <tt>synchronized</tt> statement.
72			* Acquiring the
73			* monitor lock of a <tt>Lock</tt> instance has no specified relationship
74			* with invoking any of the {@link #lock} methods of that instance.
75			* It is recommended that to avoid confusion you never use <tt>Lock</tt>
76			* instances in this way, except within their own implementation.
77			*
78			* <p>Except where noted, passing a <tt>null</tt> value for any parameter
79			* will result in a {@link NullPointerException} being thrown.
80			*
81			* <h3>Memory Synchronization</h3>
82			* <p>All <tt>Lock</tt> implementations <em>must</em> enforce the same
83			* memory synchronization semantics as provided by the built-in monitor lock:
84			* <ul>
85			* <li>A successful lock operation acts like a successful
86			* <tt>monitorEnter</tt> action
87			* <li>A successful <tt>unlock</tt> operation acts like a successful
88			* <tt>monitorExit</tt> action
89			* </ul>
90			* Note that unsuccessful locking and unlocking operations, and reentrant
91			* locking/unlocking operations, do not require any memory synchronization
92			* effects.
93			*
94			* <h3>Implementation Considerations</h3>
95			* <p>It is recognised that the three forms of lock acquisition (interruptible,
96			* non-interruptible, and timed) may differ in their ease of implementation
97			* on some platforms and in their performance characteristics.
98			* In particular, it may be difficult to provide these features and maintain
99			* specific semantics such as ordering guarantees.
100			* Further, the ability to interrupt the acquisition of a lock may not always
101			* be feasible to implement on all platforms.
102			* <p>Consequently, an implementation is not required to define exactly the
103			* same
104			* guarantees or semantics for all three forms of lock acquistion, nor is it
105			* required to support interruption of the actual lock acquisition.
106			* An implementation is required to clearly
107			* document the semantics and guarantees provided by each of the locking
108			* methods. It must also obey the interruption semantics as defined in this
109			* interface, to the extent that interruption of lock acquisition is
110			* supported: which is either totally, or only on method entry.
111			*
112			*
113			* @see ReentrantLock
114			* @see Condition
115			* @see ReadWriteLock
116			* @see Locks
117			*
118			* @since 1.5
119			* @spec JSR-166
120			* @revised $Date: 2003/01/30 22:12:40 $
121			* @editor $Author: dholmes $
122			*
123			*/
124			public interface Lock {
125
126			/**
127			* Acquires the lock.
128			* <p>Acquires the lock if it is available and returns immediately.
129			* <p>If the lock is not available then
130			* the current thread becomes disabled for thread scheduling
131			* purposes and lies dormant until the lock has been acquired.
132			* <p><b>Implementation Considerations</b>
133			* <p>A <tt>Lock</tt> implementation may be able to detect
134			* erroneous use of the lock, such as an invocation that would cause
135			* deadlock, and may throw an (unchecked) exception in such circumstances.
136			* The circumstances and the exception type must be documented by that
137			* <tt>Lock</tt> implementation.
138			*
139			*/
140			void lock();
141
142			/**
143			* Acquires the lock unless the current thread is
144			* {@link Thread#interrupt interrupted}.
145			* <p>Acquires the lock if it is available and returns immediately.
146			* <p>If the lock is not available then
147			* the current thread becomes disabled for thread scheduling
148			* purposes and lies dormant until one of two things happens:
149			* <ul>
150			* <li> The lock is acquired by the current thread; or
151			* <li> Some other thread {@link Thread#interrupt interrupts} the current
152			* thread, and interruption of lock acquisition is supported.
153			* </ul>
154			* <p>If the current thread:
155			* <ul>
156			* <li>has its interrupted status set on entry to this method; or
157			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
158			* the lock, and interruption of lock acquisition is supported,
159			* </ul>
160			* then {@link InterruptedException} is thrown and the current thread's
161			* interrupted status is cleared.
162			*
163			* <p><b>Implementation Considerations</b>
164			* <p>The ability to interrupt a lock acquisition in some implementations
165			* may not be possible, and if possible could reasonably be foreseen to
166			* be an expensive operation.
167			* The programmer should be aware that this may be the case. An
168			* implementation should document when this is the case.
169			*
170			* <p>A <tt>Lock</tt> implementation may be able to detect
171			* erroneous use of the lock, such as an invocation that would cause
172			* deadlock, and may throw an (unchecked) exception in such circumstances.
173			* The circumstances and the exception type must be documented by that
174			* <tt>Lock</tt> implementation.
175			*
176			* @throws InterruptedException if the current thread is interrupted
177			* (and interruption of lock acquisition is supported).
178			*
179			* @see Thread#interrupt
180			*
181			*/
182			void lockInterruptibly() throws InterruptedException;
183
184
185			/**
186			* Acquires the lock only if it is free at the time of invocation.
187			* <p>Acquires the lock if it is available and returns immediately
188			* with the value <tt>true</tt>.
189			* <p>If the lock is not available then this method will return
190			* immediately with the value <tt>false</tt>.
191			* <p>A typical usage idiom for this method would be:
192			* <pre>
193			* Lock lock = ...;
194			* if (lock.tryLock()) {
195			* try {
196			* // manipulate protected state
197			* } finally {
198			* lock.unlock();
199			* }
200			* } else {
201			* // perform alternative actions
202			* }
203			* </pre>
204			* This usage ensures that the lock is unlocked if it was acquired, and
205			* doesn't try to unlock if the lock was not acquired.
206			*
207			* @return <tt>true</tt> if the lock was acquired and <tt>false</tt>
208			* otherwise.
209			*/
210			boolean tryLock();
211
212			/**
213			* Acquires the lock if it is free within the given waiting time and the
214			* current thread has not been {@link Thread#interrupt interrupted}.
215			* <p>Acquires the lock if it is available and returns immediately
216			* with the value <tt>true</tt>.
217			* <p>If the lock is not available then
218			* the current thread becomes disabled for thread scheduling
219			* purposes and lies dormant until one of three things happens:
220			* <ul>
221			* <li> The lock is acquired by the current thread; or
222			* <li> Some other thread {@link Thread#interrupt interrupts} the current
223			* thread, and interruption of lock acquisition is supported; or
224			* <li> The specified waiting time elapses
225			* </ul>
226			* <p>If the lock is acquired then the value <tt>true</tt> is returned.
227			* <p>If the current thread:
228			* <ul>
229			* <li>has its interrupted status set on entry to this method; or
230			* <li>is {@link Thread#interrupt interrupted} while waiting to acquire
231			* the lock, and interruption of lock acquisition is supported,
232			* </ul>
233			* then {@link InterruptedException} is thrown and the current thread's
234			* interrupted status is cleared.
235			* <p>If the specified waiting time elapses then the value <tt>false</tt>
236			* is returned.
237			* The given waiting time is a best-effort lower bound. If the time is
238			* less than or equal to zero, the method will not wait at all.
239			*
240			* <p><b>Implementation Considerations</b>
241			* <p>The ability to interrupt a lock acquisition in some implementations
242			* may not be possible, and if possible could reasonably be foreseen to
243			* be an expensive operation.
244			* The programmer should be aware that this may be the case. An
245			* implementation should document when this is the case.
246			*
247			* <p>A <tt>Lock</tt> implementation may be able to detect
248			* erroneous use of the lock, such as an invocation that would cause
249			* deadlock, and may throw an (unchecked) exception in such circumstances.
250			* The circumstances and the exception type must be documented by that
251			* <tt>Lock</tt> implementation.
252			*
253			* @param timeout the maximum time to wait for the lock
254			* @param granularity the time unit of the <tt>timeout</tt> argument.
255			* @return <tt>true</tt> if the lock was acquired and <tt>false</tt>
256			* if the waiting time elapsed before the lock was acquired.
257			*
258			* @throws InterruptedException if the current thread is interrupted
259			* while trying to acquire the lock (and interruption of lock
260			* acquisition is supported).
261			*
262			* @see Thread#interrupt
263			*
264			*/
265			boolean tryLock(long timeout, TimeUnit granularity)
266			throws InterruptedException;
267
268			/**
269			* Releases the lock.
270			* <p><b>Implementation Considerations</b>
271			* <p>A <tt>Lock</tt> implementation will usually impose
272			* restrictions on which thread can release a lock (typically only the
273			* holder of the lock can release it) and may throw
274			* an (unchecked) exception if the restriction is violated.
275			* Any restrictions and the exception
276			* type must be documented by that <tt>Lock</tt> implementation.
277			*/
278			void unlock();
279
280			/**
281			* Returns a {@link Condition} instance that is bound to this <tt>Lock</tt>
282			* instance.
283			* <p>Conditions are primarily used with the built-in locking provided by
284			* <tt>synchronized</tt> methods and statements
285			* (see {@link Locks#newConditionFor}), but in some rare circumstances it
286			* can be useful to wait for a condition when working with a data
287			* structure that is accessed using a stand-alone <tt>Lock</tt> instance
288			* (see {@link ReentrantLock}).
289			* <p>Before waiting on the condition the lock must be held by the
290			* current thread.
291			* A call to {@link Condition#await()} will atomically release the lock
292			* before waiting and re-acquire the lock before the wait returns.
293			* <p><b>Implementation Considerations</b>
294			* <p>The exact operation of the {@link Condition} instance depends on the
295			* <tt>Lock</tt> implementation and must be documented by that
296			* implementation.
297			*
298			* @return A {@link Condition} instance for this <tt>Lock</tt> instance.
299			* @throws UnsupportedOperationException if this <tt>Lock</tt>
300			* implementation does not support conditions.
301			*/
302			Condition newCondition();
303
304			}
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