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The Apache Commons Digester package lets you configure an XML to Java object mapping module which triggers certain actions called rules whenever a particular pattern of nested XML elements is recognized.

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package org.apache.commons.digester3.plugins.strategies;

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */

import java.util.Properties;

import org.apache.commons.digester3.Digester;
import org.apache.commons.digester3.plugins.PluginException;
import org.apache.commons.digester3.plugins.RuleFinder;
import org.apache.commons.digester3.plugins.RuleLoader;

/**
 * A rule-finding algorithm which expects the caller to specify a methodname as a plugin property, where the method
 * exists on the plugin class.
 * 
 * @since 1.6
 */
public class FinderFromMethod
    extends RuleFinder
{

    /**
     * Default XML attribute that needs to be present on a plugin declaration in order to specify the method
     * to load rules from.
     */
    private static final String DFLT_METHOD_ATTR = "method";

    /** See {@link #findLoader}. */
    private final String methodAttr;

    /** Constructor. */
    public FinderFromMethod()
    {
        this( DFLT_METHOD_ATTR );
    }

    /**
     * See {@link #findLoader}.
     *
     * @param methodAttr The XML attribute that needs to be present on a plugin declaration in order to specify the
     *        method to load rules from.
     */
    public FinderFromMethod( String methodAttr )
    {
        this.methodAttr = methodAttr;
    }

    /**
     * If there exists a property with the name matching constructor param methodAttr, then locate the appropriate
     * Method on the plugin class and return an object encapsulating that info.
     * 
     * If there is no matching property provided, then just return null.
     * 
     * The returned object (when non-null) will invoke the target method on the plugin class whenever its addRules
     * method is invoked. The target method is expected to have the following prototype:
     *  public static void xxxxx(Digester d, String patternPrefix); 
     *
     * @param d The digester instance where locating plugin classes
     * @param pluginClass The plugin Java class
     * @param p The properties object that holds any xml attributes the user may have specified on the plugin
     *          declaration in order to indicate how to locate the plugin rules.
     * @return a source of digester rules for the specified plugin class.
     * @throws PluginException if the algorithm finds a source of rules, but there is something invalid
     *         about that source.
     */
    @Override
    public RuleLoader findLoader( Digester d, Class pluginClass, Properties p )
        throws PluginException
    {

        String methodName = p.getProperty( methodAttr );
        if ( methodName == null )
        {
            // nope, user hasn't requested dynamic rules to be loaded
            // from a specific class.
            return null;
        }

        return new LoaderFromClass( pluginClass, methodName );
    }

}