001    package org.apache.ojb.otm.lock.isolation;
002    
003    /* Copyright 2003-2005 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.otm.core.Transaction;
019    import org.apache.ojb.otm.lock.LockingException;
020    import org.apache.ojb.otm.lock.ObjectLock;
021    
022    /**
023     *
024     * READ_COMMITTED IsolationStrategy.
025     *
026     * @author <a href="mailto:rraghuram@hotmail.com">Raghu Rajah</a>
027     * @author Thomas Mahler & David Dixon-Peugh
028     *
029     */
030    public class ReadCommittedIsolation extends AbstractIsolation
031    {
032    
033        /**
034         * @see org.apache.ojb.otm.lock.TransactionIsolation#readLock(Transaction, ObjectLocks)
035         */
036        public void readLock(Transaction tx, ObjectLock lock)
037            throws LockingException
038        {
039            Transaction writer = lock.getWriter();
040            if (writer == null)
041            {
042                lock.readLock(tx);
043                if (lock.getWriter() != null)
044                {
045                    lock.releaseLock(tx);
046                    readLock(tx, lock);
047                }
048            }
049            else if (tx != writer)
050            {
051                lock.waitForTx(writer);
052                readLock(tx, lock);
053            }
054            // else if tx is the writer, it can also read.
055        }
056    
057        /**
058         * @see org.apache.ojb.otm.lock.TransactionIsolation#writeLock(Transaction, ObjectLocks)
059         */
060        public void writeLock(Transaction tx, ObjectLock lock)
061            throws LockingException
062        {
063            lock.writeLock(tx);
064        }
065    
066    }