001/* $Id: RuleFinder.java 471661 2006-11-06 08:09:25Z skitching $
002 *
003 * Licensed to the Apache Software Foundation (ASF) under one or more
004 * contributor license agreements.  See the NOTICE file distributed with
005 * this work for additional information regarding copyright ownership.
006 * The ASF licenses this file to You under the Apache License, Version 2.0
007 * (the "License"); you may not use this file except in compliance with
008 * the License.  You may obtain a copy of the License at
009 * 
010 *      http://www.apache.org/licenses/LICENSE-2.0
011 * 
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */ 
018 
019package org.apache.commons.digester.plugins;
020
021import java.util.Properties;
022import org.apache.commons.digester.Digester;
023
024/**
025 * Each concrete implementation of RuleFinder is an algorithm for
026 * locating a source of digester rules for a plugin. The algorithm may 
027 * use info explicitly provided by the user as part of the plugin 
028 * declaration, or not (in which case the concrete RuleFinder subclass
029 * typically has Dflt as part of its name).
030 * <p>
031 * Instances of this class can also be regarded as a Factory for RuleLoaders,
032 * except that an instance of a RuleLoader is only created if the particular
033 * finder algorithm can locate a suitable source of rules given the plugin
034 * class and associated properties.
035 * <p>
036 * This is an abstract class rather than an interface in order to make
037 * it possible to enhance this class in future without breaking binary
038 * compatibility; it is possible to add methods to an abstract class, but
039 * not to an interface. 
040 *
041 * @since 1.6
042 */
043
044public abstract class RuleFinder {
045
046    /**
047     * Apply the finder algorithm to attempt to locate a source of
048     * digester rules for the specified plugin class.
049     * <p>
050     * This method is invoked when a plugin is declared by the user, either
051     * via an explicit use of PluginDeclarationRule, or implicitly via an
052     * "inline declaration" where the declaration and use are simultaneous.
053     * <p>
054     * If dynamic rules for the specified plugin class are located, then
055     * the RuleFinder will return a RuleLoader object encapsulating those
056     * rules, and this object will be invoked each time the user actually
057     * requests an instance of the declared plugin class, to load the
058     * custom rules associated with that plugin instance.
059     * <p>
060     * If no dynamic rules can be found, null is returned. This is not an
061     * error; merely an indication that this particular algorithm found
062     * no matches.
063     * <p>
064     * The properties object holds any xml attributes the user may have
065     * specified on the plugin declaration in order to indicate how to locate
066     * the plugin rules.
067     * <p>
068     * @throws PluginConfigurationException if the algorithm finds a source
069     * of rules, but there is something invalid about that source.
070     */
071
072     public abstract RuleLoader findLoader(
073                        Digester d, Class pluginClass, 
074                        Properties p) throws PluginException;
075}
076