001    /* $Id: RuleFinder.java 729120 2008-12-23 21:11:38Z rahul $
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     
019    package org.apache.commons.digester.plugins;
020    
021    import java.util.Properties;
022    import 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    
044    public 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