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36 package java.util.concurrent.locks;
39 * A <tt>ReadWriteLock</tt> maintains a pair of associated {@link
40 * Lock locks}, one for read-only operations and one for writing.
41 * The {@link #readLock read lock} may be held simultaneously by
42 * multiple reader threads, so long as there are no writers. The
43 * {@link #writeLock write lock} is exclusive.
45 * <p>All <tt>ReadWriteLock</tt> implementations must guarantee that
46 * the memory synchronization effects of <tt>writeLock</tt> operations
47 * (as specified in the {@link Lock} interface) also hold with respect
48 * to the associated <tt>readLock</tt>. That is, a thread successfully
49 * acquiring the read lock will see all updates made upon previous
50 * release of the write lock.
52 * <p>A read-write lock allows for a greater level of concurrency in
53 * accessing shared data than that permitted by a mutual exclusion lock.
54 * It exploits the fact that while only a single thread at a time (a
55 * <em>writer</em> thread) can modify the shared data, in many cases any
56 * number of threads can concurrently read the data (hence <em>reader</em>
58 * In theory, the increase in concurrency permitted by the use of a read-write
59 * lock will lead to performance improvements over the use of a mutual
60 * exclusion lock. In practice this increase in concurrency will only be fully
61 * realized on a multi-processor, and then only if the access patterns for
62 * the shared data are suitable.
64 * <p>Whether or not a read-write lock will improve performance over the use
65 * of a mutual exclusion lock depends on the frequency that the data is
66 * read compared to being modified, the duration of the read and write
67 * operations, and the contention for the data - that is, the number of
68 * threads that will try to read or write the data at the same time.
69 * For example, a collection that is initially populated with data and
70 * thereafter infrequently modified, while being frequently searched
71 * (such as a directory of some kind) is an ideal candidate for the use of
72 * a read-write lock. However, if updates become frequent then the data
73 * spends most of its time being exclusively locked and there is little, if any
74 * increase in concurrency. Further, if the read operations are too short
75 * the overhead of the read-write lock implementation (which is inherently
76 * more complex than a mutual exclusion lock) can dominate the execution
77 * cost, particularly as many read-write lock implementations still serialize
78 * all threads through a small section of code. Ultimately, only profiling
79 * and measurement will establish whether the use of a read-write lock is
80 * suitable for your application.
83 * <p>Although the basic operation of a read-write lock is straight-forward,
84 * there are many policy decisions that an implementation must make, which
85 * may affect the effectiveness of the read-write lock in a given application.
86 * Examples of these policies include:
88 * <li>Determining whether to grant the read lock or the write lock, when
89 * both readers and writers are waiting, at the time that a writer releases
90 * the write lock. Writer preference is common, as writes are expected to be
91 * short and infrequent. Reader preference is less common as it can lead to
92 * lengthy delays for a write if the readers are frequent and long-lived as
93 * expected. Fair, or "in-order" implementations are also possible.
95 * <li>Determining whether readers that request the read lock while a
96 * reader is active and a writer is waiting, are granted the read lock.
97 * Preference to the reader can delay the writer indefinitely, while
98 * preference to the writer can reduce the potential for concurrency.
100 * <li>Determining whether the locks are reentrant: can a thread with the
101 * write lock reacquire it? Can it acquire a read lock while holding the
102 * write lock? Is the read lock itself reentrant?
104 * <li>Can the write lock be downgraded to a read lock without allowing
105 * an intervening writer? Can a read lock be upgraded to a write lock,
106 * in preference to other waiting readers or writers?
109 * You should consider all of these things when evaluating the suitability
110 * of a given implementation for your application.
112 * @see ReentrantReadWriteLock
119 public interface ReadWriteLock {
121 * Returns the lock used for reading.
123 * @return the lock used for reading.
128 * Returns the lock used for writing.
130 * @return the lock used for writing.