1 package org.apache.ojb.odmg.locking;
2
3 /* Copyright 2002-2005 The Apache Software Foundation
4 *
5 * Licensed under the Apache License, Version 2.0 (the "License");
6 * you may not use this file except in compliance with the License.
7 * You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17
18 import org.apache.ojb.odmg.TransactionImpl;
19
20 import java.util.Collection;
21
22 /**
23 * The implementation of the Repeatable Reads Locking strategy.
24 * Locks are obtained for reading and modifying the database.
25 * Locks on all modified objects are held until EOT.
26 * Locks obtained for reading data are held until EOT.
27 * Allows:
28 * Phantom Reads
29 *
30 * @author Thomas Mahler & David Dixon-Peugh
31 */
32 public class RepeatableReadStrategy extends AbstractLockStrategy
33 {
34 /**
35 * acquire a read lock on Object obj for Transaction tx.
36 * @param tx the transaction requesting the lock
37 * @param obj the Object to be locked
38 * @return true if successful, else false
39 *
40 */
41 public boolean readLock(TransactionImpl tx, Object obj)
42 {
43 LockEntry writer = getWriter(obj);
44 if (writer == null)
45 {
46 if (addReader(tx, obj))
47 return true;
48 else
49 return readLock(tx, obj);
50 }
51 if (writer.isOwnedBy(tx))
52 {
53 return true; // If I'm the writer, I can read.
54 }
55 else
56 return false;
57
58 }
59
60 /**
61 * acquire a write lock on Object obj for Transaction tx.
62 * @param tx the transaction requesting the lock
63 * @param obj the Object to be locked
64 * @return true if successful, else false
65 *
66 */
67 public boolean writeLock(TransactionImpl tx, Object obj)
68 {
69 LockEntry writer = getWriter(obj);
70 Collection readers = getReaders(obj);
71 if (writer == null)
72 {
73 if (readers.size() == 0)
74 {
75 if (setWriter(tx, obj))
76 return true;
77 else
78 return writeLock(tx, obj);
79 }
80
81 else if (readers.size() == 1)
82 {
83 if (((LockEntry) readers.iterator().next()).isOwnedBy(tx))
84 return upgradeLock(tx, obj);
85 }
86 }
87 else if (writer.isOwnedBy(tx))
88 {
89 return true; // If I'm the writer, then I can write.
90 }
91 return false;
92 }
93
94
95 /**
96 * acquire a lock upgrade (from read to write) lock on Object obj for Transaction tx.
97 * @param tx the transaction requesting the lock
98 * @param obj the Object to be locked
99 * @return true if successful, else false
100 *
101 */
102 public boolean upgradeLock(TransactionImpl tx, Object obj)
103 {
104 LockEntry writer = getWriter(obj);
105 if (writer == null)
106 {
107 Collection readers = this.getReaders(obj);
108 if (readers.size() == 1)
109 {
110 LockEntry reader = (LockEntry) readers.iterator().next();
111 if (reader.isOwnedBy(tx))
112 {
113 if (upgradeLock(reader))
114 return true;
115 else
116 return upgradeLock(tx, obj);
117 }
118 }
119 else if (readers.size() == 0)
120 {
121 if (setWriter(tx, obj))
122 return true;
123 else
124 return upgradeLock(tx, obj);
125 }
126
127
128 }
129 else if (writer.isOwnedBy(tx))
130 {
131 return true; // If I already have Write, then I've upgraded.
132 }
133
134 return false;
135 }
136
137 /**
138 * release a lock on Object obj for Transaction tx.
139 * @param tx the transaction releasing the lock
140 * @param obj the Object to be unlocked
141 * @return true if successful, else false
142 *
143 */
144 public boolean releaseLock(TransactionImpl tx, Object obj)
145 {
146 LockEntry writer = getWriter(obj);
147 if (writer != null && writer.isOwnedBy(tx))
148 {
149 removeWriter(writer);
150 return true;
151 }
152 if (hasReadLock(tx, obj))
153 {
154 removeReader(tx, obj);
155 return true;
156 }
157 return false;
158 }
159
160 /**
161 * checks whether the specified Object obj is read-locked by Transaction tx.
162 * @param tx the transaction
163 * @param obj the Object to be checked
164 * @return true if lock exists, else false
165 */
166 public boolean checkRead(TransactionImpl tx, Object obj)
167 {
168 if (hasReadLock(tx, obj))
169 {
170 return true;
171 }
172 LockEntry writer = getWriter(obj);
173 if (writer != null && writer.isOwnedBy(tx))
174 {
175 return true;
176 }
177 else
178 return false;
179 }
180
181 /**
182 * checks whether the specified Object obj is write-locked by Transaction tx.
183 * @param tx the transaction
184 * @param obj the Object to be checked
185 * @return true if lock exists, else false
186 */
187 public boolean checkWrite(TransactionImpl tx, Object obj)
188 {
189 LockEntry writer = getWriter(obj);
190 return (writer != null && writer.isOwnedBy(tx));
191 }
192 }