001package org.apache.commons.digester3.plugins;
002
003/*
004 * Licensed to the Apache Software Foundation (ASF) under one
005 * or more contributor license agreements.  See the NOTICE file
006 * distributed with this work for additional information
007 * regarding copyright ownership.  The ASF licenses this file
008 * to you under the Apache License, Version 2.0 (the
009 * "License"); you may not use this file except in compliance
010 * with the License.  You may obtain a copy of the License at
011 *
012 *   http://www.apache.org/licenses/LICENSE-2.0
013 *
014 * Unless required by applicable law or agreed to in writing,
015 * software distributed under the License is distributed on an
016 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
017 * KIND, either express or implied.  See the License for the
018 * specific language governing permissions and limitations
019 * under the License.
020 */
021
022import java.util.Properties;
023
024import org.apache.commons.digester3.Digester;
025
026/**
027 * Each concrete implementation of RuleFinder is an algorithm for locating a source of digester rules for a plugin. The
028 * algorithm may use info explicitly provided by the user as part of the plugin declaration, or not (in which case the
029 * concrete RuleFinder subclass typically has Dflt as part of its name).
030 * <p>
031 * Instances of this class can also be regarded as a Factory for RuleLoaders, except that an instance of a RuleLoader is
032 * only created if the particular finder algorithm can locate a suitable source of rules given the plugin class and
033 * associated properties.
034 * <p>
035 * This is an abstract class rather than an interface in order to make it possible to enhance this class in future
036 * without breaking binary compatibility; it is possible to add methods to an abstract class, but not to an interface.
037 * 
038 * @since 1.6
039 */
040public abstract class RuleFinder
041{
042
043    /**
044     * Apply the finder algorithm to attempt to locate a source of digester rules for the specified plugin class.
045     * <p>
046     * This method is invoked when a plugin is declared by the user, either via an explicit use of
047     * PluginDeclarationRule, or implicitly via an "inline declaration" where the declaration and use are simultaneous.
048     * <p>
049     * If dynamic rules for the specified plugin class are located, then the RuleFinder will return a RuleLoader object
050     * encapsulating those rules, and this object will be invoked each time the user actually requests an instance of
051     * the declared plugin class, to load the custom rules associated with that plugin instance.
052     * <p>
053     * If no dynamic rules can be found, null is returned. This is not an error; merely an indication that this
054     * particular algorithm found no matches.
055     * <p>
056     * The properties object holds any xml attributes the user may have specified on the plugin declaration in order to
057     * indicate how to locate the plugin rules.
058     * <p>
059     *
060     * @param d The digester instance where locating plugin classes
061     * @param pluginClass The plugin Java class
062     * @param p The properties object that holds any xml attributes the user may have specified on the plugin
063     *          declaration in order to indicate how to locate the plugin rules.
064     * @return a source of digester rules for the specified plugin class.
065     * @throws PluginException if the algorithm finds a source of rules, but there is something invalid
066     *         about that source.
067     */
068    public abstract RuleLoader findLoader( Digester d, Class<?> pluginClass, Properties p )
069        throws PluginException;
070
071}