001    /* $Id: RuleFinder.java 992060 2010-09-02 19:09:47Z simonetripodi $
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    package org.apache.commons.digester.plugins;
019    
020    import java.util.Properties;
021    import org.apache.commons.digester.Digester;
022    
023    /**
024     * Each concrete implementation of RuleFinder is an algorithm for
025     * locating a source of digester rules for a plugin. The algorithm may 
026     * use info explicitly provided by the user as part of the plugin 
027     * declaration, or not (in which case the concrete RuleFinder subclass
028     * typically has Dflt as part of its name).
029     * <p>
030     * Instances of this class can also be regarded as a Factory for RuleLoaders,
031     * except that an instance of a RuleLoader is only created if the particular
032     * finder algorithm can locate a suitable source of rules given the plugin
033     * class and associated properties.
034     * <p>
035     * This is an abstract class rather than an interface in order to make
036     * it possible to enhance this class in future without breaking binary
037     * compatibility; it is possible to add methods to an abstract class, but
038     * not to an interface. 
039     *
040     * @since 1.6
041     */
042    
043    public abstract class RuleFinder {
044    
045        /**
046         * Apply the finder algorithm to attempt to locate a source of
047         * digester rules for the specified plugin class.
048         * <p>
049         * This method is invoked when a plugin is declared by the user, either
050         * via an explicit use of PluginDeclarationRule, or implicitly via an
051         * "inline declaration" where the declaration and use are simultaneous.
052         * <p>
053         * If dynamic rules for the specified plugin class are located, then
054         * the RuleFinder will return a RuleLoader object encapsulating those
055         * rules, and this object will be invoked each time the user actually
056         * requests an instance of the declared plugin class, to load the
057         * custom rules associated with that plugin instance.
058         * <p>
059         * If no dynamic rules can be found, null is returned. This is not an
060         * error; merely an indication that this particular algorithm found
061         * no matches.
062         * <p>
063         * The properties object holds any xml attributes the user may have
064         * specified on the plugin declaration in order to indicate how to locate
065         * the plugin rules.
066         * <p>
067         * @throws PluginConfigurationException if the algorithm finds a source
068         * of rules, but there is something invalid about that source.
069         */
070    
071         public abstract RuleLoader findLoader(
072                            Digester d, Class<?> pluginClass, 
073                            Properties p) throws PluginException;
074    }
075