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20 package org.apache.fulcrum.yaafi.framework.locking;
21
22
23 /**
24 * Convenience implementation of a read/write lock based on {@link GenericLock}.
25 * <br>
26 * <br>
27 * Reads are shared which means there can be any number of concurrent read
28 * accesses allowed by this lock. Writes are exclusive. This means when there is
29 * a write access no other access neither read nor write are allowed by this
30 * lock. Additionally, writes are preferred over reads in order to avoid starvation. The idea
31 * is that there are many readers, but few writers and if things work out bad the writer would
32 * never be served at all. That's why it is preferred.<br>
33 * <br>
34 * Calls to both {@link #acquireRead(Object, long)}and
35 * {@link #acquireWrite(Object, long)}are blocking and reentrant. Blocking
36 * means they will wait if they can not acquire the descired access, reentrant means that a lock
37 * request by a specific owner will always be compatible with other accesses on this lock by the
38 * same owner. E.g. if you already have a lock for writing and you try to acquire write access
39 * again you will not be blocked by this first lock, while others of course will be. This is the
40 * natural way you already know from Java monitors and synchronized blocks.
41 *
42 * @version $Revision: 1.1 $
43 * @see GenericLock
44 */
45 public class ReadWriteLock extends GenericLock {
46
47 public static final int NO_LOCK = 0;
48
49 public static final int READ_LOCK = 1;
50
51 public static final int WRITE_LOCK = 2;
52
53 /**
54 * Creates a new read/write lock.
55 *
56 * @param resourceId
57 * identifier for the resource associated to this lock
58 * @param logger
59 * generic logger used for all kind of debug logging
60 */
61 public ReadWriteLock(Object resourceId, LoggerFacade logger) {
62 super(resourceId, WRITE_LOCK, logger);
63 }
64
65 /**
66 * Tries to acquire a blocking, reentrant read lock. A read lock is
67 * compatible with other read locks, but not with a write lock.
68 *
69 * @param ownerId
70 * a unique id identifying the entity that wants to acquire a
71 * certain lock level on this lock
72 * @param timeoutMSecs
73 * if blocking is enabled by the <code>wait</code> parameter
74 * this specifies the maximum wait time in milliseconds
75 * @return <code>true</code> if the lock actually was acquired
76 * @throws InterruptedException
77 * when the thread waiting on this method is interrupted
78 */
79 public boolean acquireRead(Object ownerId, long timeoutMSecs) throws InterruptedException {
80 return acquire(ownerId, READ_LOCK, false, timeoutMSecs);
81 }
82
83 /**
84 * Tries to acquire a blocking, reentrant write lock. A write lock is
85 * incompatible with any another read or write lock and is thus exclusive.
86 *
87 * @param ownerId
88 * a unique id identifying the entity that wants to acquire a
89 * certain lock level on this lock
90 * @param timeoutMSecs
91 * if blocking is enabled by the <code>wait</code> parameter
92 * this specifies the maximum wait time in milliseconds
93 * @return <code>true</code> if the lock actually was acquired
94 * @throws InterruptedException
95 * when the thread waiting on this method is interrupted
96 */
97 public boolean acquireWrite(Object ownerId, long timeoutMSecs) throws InterruptedException {
98 return acquire(ownerId, WRITE_LOCK, true, timeoutMSecs);
99 }
100 }