001/* $Id: FinderFromClass.java 471661 2006-11-06 08:09:25Z skitching $
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 
019package org.apache.commons.digester.plugins.strategies;
020
021import java.util.Properties;
022import org.apache.commons.digester.Digester;
023import org.apache.commons.digester.plugins.RuleFinder;
024import org.apache.commons.digester.plugins.RuleLoader;
025import org.apache.commons.digester.plugins.PluginException;
026
027/**
028 * A rule-finding algorithm which expects the caller to specify a classname and
029 * methodname as plugin properties.
030 *
031 * @since 1.6
032 */
033
034public class FinderFromClass extends RuleFinder {
035    public static String DFLT_RULECLASS_ATTR = "ruleclass";
036    public static String DFLT_METHOD_ATTR = "method";
037    public static String DFLT_METHOD_NAME = "addRules";
038    
039    private String ruleClassAttr;
040    private String methodAttr;
041    private String dfltMethodName;
042    
043    /**
044     * See {@link #findLoader}.
045     */
046    public FinderFromClass() {
047        this(DFLT_RULECLASS_ATTR, DFLT_METHOD_ATTR, DFLT_METHOD_NAME);
048    }
049
050    /**
051     * Create a rule-finder which invokes a user-specified method on a
052     * user-specified class whenever dynamic rules for a plugin need to be
053     * loaded. See the findRules method for more info.
054     *
055     * @param ruleClassAttr must be non-null.
056     * @param methodAttr may be null.
057     * @param dfltMethodName may be null.
058     */
059    public FinderFromClass(String ruleClassAttr, String methodAttr, 
060                String dfltMethodName) {
061        this.ruleClassAttr = ruleClassAttr;
062        this.methodAttr = methodAttr;
063        this.dfltMethodName = dfltMethodName;
064    }
065    
066    /**
067     * If there exists a property with the name matching constructor param
068     * ruleClassAttr, then load the specified class, locate the appropriate 
069     * rules-adding method on that class, and return an object encapsulating 
070     * that info.
071     * <p>
072     * If there is no matching property provided, then just return null.
073     * <p>
074     * The returned object (when non-null) will invoke the target method
075     * on the selected class whenever its addRules method is invoked. The
076     * target method is expected to have the following prototype:
077     * <code> public static void xxxxx(Digester d, String patternPrefix); </code>
078     * <p>
079     * The target method can be specified in several ways. If this object's
080     * constructor was passed a non-null methodAttr parameter, and the
081     * properties defines a value with that key, then that is taken as the
082     * target method name. If there is no matching property, or the constructor
083     * was passed null for methodAttr, then the dfltMethodName passed to the
084     * constructor is used as the name of the method on the target class. And
085     * if that was null, then DFLT_METHOD_NAME will be used.
086     * <p>
087     * When the user explicitly declares a plugin in the input xml, the
088     * xml attributes on the declaration tag are passed here as properties,
089     * so the user can select any class in the classpath (and any method on
090     * that class provided it has the correct prototype) as the source of
091     * dynamic rules for the plugged-in class.
092     */
093    public RuleLoader findLoader(Digester digester, Class pluginClass, 
094                        Properties p) throws PluginException {
095
096        String ruleClassName = p.getProperty(ruleClassAttr);
097        if (ruleClassName == null) {
098            // nope, user hasn't requested dynamic rules to be loaded
099            // from a specific class.
100            return null;
101        }
102        
103        // ok, we are in business
104        String methodName = null;
105        if (methodAttr != null) { 
106            methodName = p.getProperty(methodAttr);
107        }
108        if (methodName == null) {
109            methodName = dfltMethodName;
110        }
111        if (methodName == null) {
112            methodName = DFLT_METHOD_NAME;
113        }
114        
115        Class ruleClass;
116        try {
117            // load the plugin class object
118            ruleClass = 
119                digester.getClassLoader().loadClass(ruleClassName);
120        } catch(ClassNotFoundException cnfe) {
121            throw new PluginException(
122                "Unable to load class " + ruleClassName, cnfe);
123        }
124
125        return new LoaderFromClass(ruleClass, methodName);
126    }
127}
128