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 */
017package org.apache.commons.transaction.locking;
018
019import 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 */
043public 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}