    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