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 /**
21 * The implementation of the Commited Reads Locking stra
22 * ReadCommitted - Reads and Writes require locks.
23 *
24 * Locks are acquired for reading and modifying the database.
25 * Locks are released after reading but locks on modified objects
26 * are held until EOT.
27 *
28 * Allows:
29 * Non-Repeatable Reads
30 * Phantom Readstegy.
31 *
32 * @author Thomas Mahler & David Dixon-Peugh
33 */
34 public class ReadCommittedStrategy extends AbstractLockStrategy
35 {
36
37 /**
38 * acquire a read lock on Object obj for Transaction tx.
39 * @param tx the transaction requesting the lock
40 * @param obj the Object to be locked
41 * @return true if successful, else false
42 *
43 */
44 public boolean readLock(TransactionImpl tx, Object obj)
45 {
46
47 LockEntry writer = getWriter(obj);
48 if (writer == null)
49 {
50 addReader(tx, obj);
51 // if there has been a successful write locking, try again
52 if (getWriter(obj) == null)
53 return true;
54 else
55 {
56 removeReader(tx, obj);
57 return readLock(tx, obj);
58 }
59 }
60 if (writer.isOwnedBy(tx))
61 {
62 return true; // If I'm the writer, I can read.
63 }
64 else
65 {
66 return false;
67 }
68 }
69
70 /**
71 * acquire a write lock on Object obj for Transaction tx.
72 * @param tx the transaction requesting the lock
73 * @param obj the Object to be locked
74 * @return true if successful, else false
75 *
76 */
77 public boolean writeLock(TransactionImpl tx, Object obj)
78 {
79 LockEntry writer = getWriter(obj);
80 // if there is no writer yet we can try to get the global write lock
81 if (writer == null)
82 {
83 // if lock could be acquired return true
84 if (setWriter(tx, obj))
85 return true;
86 // else try again
87 else
88 return writeLock(tx, obj);
89 }
90 if (writer.isOwnedBy(tx))
91 {
92 return true; // If I'm the writer, then I can write.
93 }
94
95 return false;
96 }
97
98 /**
99 * acquire a lock upgrade (from read to write) lock on Object obj for Transaction tx.
100 * @param tx the transaction requesting the lock
101 * @param obj the Object to be locked
102 * @return true if successful, else false
103 *
104 */
105 public boolean upgradeLock(TransactionImpl tx, Object obj)
106 {
107 LockEntry writer = getWriter(obj);
108 if (writer == null)
109 {
110 // if lock could be acquired return true
111 if (setWriter(tx, obj))
112 return true;
113 // else try again
114 else
115 return upgradeLock(tx, obj);
116 }
117 if (writer.isOwnedBy(tx))
118 {
119 return true; // If I already have Write, then I've upgraded.
120 }
121
122 return false;
123 }
124
125 /**
126 * release a lock on Object obj for Transaction tx.
127 * @param tx the transaction releasing the lock
128 * @param obj the Object to be unlocked
129 * @return true if successful, else false
130 *
131 */
132 public boolean releaseLock(TransactionImpl tx, Object obj)
133 {
134 LockEntry writer = getWriter(obj);
135
136 if (writer != null && writer.isOwnedBy(tx))
137 {
138 removeWriter(writer);
139 return true;
140 }
141
142 if (hasReadLock(tx, obj))
143 {
144 removeReader(tx, obj);
145 return true;
146 }
147 return false;
148 }
149
150 /**
151 * checks whether the specified Object obj is read-locked by Transaction tx.
152 * @param tx the transaction
153 * @param obj the Object to be checked
154 * @return true if lock exists, else false
155 */
156 public boolean checkRead(TransactionImpl tx, Object obj)
157 {
158 if (hasReadLock(tx, obj))
159 {
160 return true;
161 }
162 LockEntry writer = getWriter(obj);
163 if (writer.isOwnedBy(tx))
164 {
165 return true;
166 }
167 return false;
168 }
169
170 /**
171 * checks whether the specified Object obj is write-locked by Transaction tx.
172 * @param tx the transaction
173 * @param obj the Object to be checked
174 * @return true if lock exists, else false
175 */
176 public boolean checkWrite(TransactionImpl tx, Object obj)
177 {
178 LockEntry writer = getWriter(obj);
179 if (writer == null)
180 return false;
181 else if (writer.isOwnedBy(tx))
182 return true;
183 else
184 return false;
185 }
186 }