001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. 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 package org.apache.commons.transaction.locking;
018
019 import org.apache.commons.transaction.util.LoggerFacade;
020
021 /**
022 * Convenience implementation of a read/write lock based on {@link GenericLock}.
023 * <br>
024 * <br>
025 * Reads are shared which means there can be any number of concurrent read
026 * accesses allowed by this lock. Writes are exclusive. This means when there is
027 * a write access no other access neither read nor write are allowed by this
028 * lock. Additionally, writes are preferred over reads in order to avoid starvation. The idea
029 * is that there are many readers, but few writers and if things work out bad the writer would
030 * never be served at all. That's why it is preferred.<br>
031 * <br>
032 * Calls to both {@link #acquireRead(Object, long)}and
033 * {@link #acquireWrite(Object, long)}are blocking and reentrant. Blocking
034 * means they will wait if they can not acquire the descired access, reentrant means that a lock
035 * request by a specific owner will always be compatible with other accesses on this lock by the
036 * same owner. E.g. if you already have a lock for writing and you try to acquire write access
037 * again you will not be blocked by this first lock, while others of course will be. This is the
038 * natural way you already know from Java monitors and synchronized blocks.
039 *
040 * @version $Id: ReadWriteLock.java 493628 2007-01-07 01:42:48Z joerg $
041 * @see GenericLock
042 */
043 public class ReadWriteLock extends GenericLock {
044
045 public static final int NO_LOCK = 0;
046
047 public static final int READ_LOCK = 1;
048
049 public static final int WRITE_LOCK = 2;
050
051 /**
052 * Creates a new read/write lock.
053 *
054 * @param resourceId
055 * identifier for the resource associated to this lock
056 * @param logger
057 * generic logger used for all kind of debug logging
058 */
059 public ReadWriteLock(Object resourceId, LoggerFacade logger) {
060 super(resourceId, WRITE_LOCK, logger);
061 }
062
063 /**
064 * Tries to acquire a blocking, reentrant read lock. A read lock is
065 * compatible with other read locks, but not with a write lock.
066 *
067 * @param ownerId
068 * a unique id identifying the entity that wants to acquire a
069 * certain lock level on this lock
070 * @param timeoutMSecs
071 * if blocking is enabled by the <code>wait</code> parameter
072 * this specifies the maximum wait time in milliseconds
073 * @return <code>true</code> if the lock actually was acquired
074 * @throws InterruptedException
075 * when the thread waiting on this method is interrupted
076 */
077 public boolean acquireRead(Object ownerId, long timeoutMSecs) throws InterruptedException {
078 return acquire(ownerId, READ_LOCK, false, timeoutMSecs);
079 }
080
081 /**
082 * Tries to acquire a blocking, reentrant write lock. A write lock is
083 * incompatible with any another read or write lock and is thus exclusive.
084 *
085 * @param ownerId
086 * a unique id identifying the entity that wants to acquire a
087 * certain lock level on this lock
088 * @param timeoutMSecs
089 * if blocking is enabled by the <code>wait</code> parameter
090 * this specifies the maximum wait time in milliseconds
091 * @return <code>true</code> if the lock actually was acquired
092 * @throws InterruptedException
093 * when the thread waiting on this method is interrupted
094 */
095 public boolean acquireWrite(Object ownerId, long timeoutMSecs) throws InterruptedException {
096 return acquire(ownerId, WRITE_LOCK, true, timeoutMSecs);
097 }
098 }