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
019 package org.apache.commons.digester.plugins.strategies;
020
021 import java.util.Properties;
022 import org.apache.commons.digester.Digester;
023 import org.apache.commons.digester.plugins.RuleFinder;
024 import org.apache.commons.digester.plugins.RuleLoader;
025 import 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
034 public 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