001 package org.apache.ojb.odmg.locking;
002
003 /* Copyright 2002-2005 The Apache Software Foundation
004 *
005 * Licensed under the Apache License, Version 2.0 (the "License");
006 * you may not use this file except in compliance with the License.
007 * You may obtain a copy of the License at
008 *
009 * http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017
018 import org.apache.ojb.odmg.TransactionImpl;
019
020 /**
021 * The implementation of the Commited Reads Locking stra
022 * ReadCommitted - Reads and Writes require locks.
023 *
024 * Locks are acquired for reading and modifying the database.
025 * Locks are released after reading but locks on modified objects
026 * are held until EOT.
027 *
028 * Allows:
029 * Non-Repeatable Reads
030 * Phantom Readstegy.
031 *
032 * @author Thomas Mahler & David Dixon-Peugh
033 */
034 public class ReadCommittedStrategy extends AbstractLockStrategy
035 {
036
037 /**
038 * acquire a read lock on Object obj for Transaction tx.
039 * @param tx the transaction requesting the lock
040 * @param obj the Object to be locked
041 * @return true if successful, else false
042 *
043 */
044 public boolean readLock(TransactionImpl tx, Object obj)
045 {
046
047 LockEntry writer = getWriter(obj);
048 if (writer == null)
049 {
050 addReader(tx, obj);
051 // if there has been a successful write locking, try again
052 if (getWriter(obj) == null)
053 return true;
054 else
055 {
056 removeReader(tx, obj);
057 return readLock(tx, obj);
058 }
059 }
060 if (writer.isOwnedBy(tx))
061 {
062 return true; // If I'm the writer, I can read.
063 }
064 else
065 {
066 return false;
067 }
068 }
069
070 /**
071 * acquire a write lock on Object obj for Transaction tx.
072 * @param tx the transaction requesting the lock
073 * @param obj the Object to be locked
074 * @return true if successful, else false
075 *
076 */
077 public boolean writeLock(TransactionImpl tx, Object obj)
078 {
079 LockEntry writer = getWriter(obj);
080 // if there is no writer yet we can try to get the global write lock
081 if (writer == null)
082 {
083 // if lock could be acquired return true
084 if (setWriter(tx, obj))
085 return true;
086 // else try again
087 else
088 return writeLock(tx, obj);
089 }
090 if (writer.isOwnedBy(tx))
091 {
092 return true; // If I'm the writer, then I can write.
093 }
094
095 return false;
096 }
097
098 /**
099 * 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 }