/* $Id: RuleFinder.java,v 1.5 2004/05/10 06:36:38 skitching Exp $
 *
 * Copyright 2004 The Apache Software Foundation.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */ 
 
package org.apache.commons.digester.plugins;

import java.util.Properties;
import org.apache.commons.digester.Digester;

/**
 * Each concrete implementation of RuleFinder is an algorithm for
 * locating a source of digester rules for a plugin. The algorithm may 
 * use info explicitly provided by the user as part of the plugin 
 * declaration, or not (in which case the concrete RuleFinder subclass
 * typically has Dflt as part of its name).
 * <p>
 * Instances of this class can also be regarded as a Factory for RuleLoaders,
 * except that an instance of a RuleLoader is only created if the particular
 * finder algorithm can locate a suitable source of rules given the plugin
 * class and associated properties.
 * <p>
 * This is an abstract class rather than an interface in order to make
 * it possible to enhance this class in future without breaking binary
 * compatibility; it is possible to add methods to an abstract class, but
 * not to an interface. 
 *
 * @since 1.6
 */

public abstract class RuleFinder {

    /**
     * Apply the finder algorithm to attempt to locate a source of
     * digester rules for the specified plugin class.
     * <p>
     * This method is invoked when a plugin is declared by the user, either
     * via an explicit use of PluginDeclarationRule, or implicitly via an
     * "inline declaration" where the declaration and use are simultaneous.
     * <p>
     * If dynamic rules for the specified plugin class are located, then
     * the RuleFinder will return a RuleLoader object encapsulating those
     * rules, and this object will be invoked each time the user actually
     * requests an instance of the declared plugin class, to load the
     * custom rules associated with that plugin instance.
     * <p>
     * If no dynamic rules can be found, null is returned. This is not an
     * error; merely an indication that this particular algorithm found
     * no matches.
     * <p>
     * The properties object holds any xml attributes the user may have
     * specified on the plugin declaration in order to indicate how to locate
     * the plugin rules.
     * <p>
     * @throws PluginConfigurationException if the algorithm finds a source
     * of rules, but there is something invalid about that source.
     */

     public abstract RuleLoader findLoader(
                        Digester d, Class pluginClass, 
                        Properties p) throws PluginException;
}