001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. You may obtain a copy of the License at
008 *
009 * http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017 package org.apache.commons.configuration.interpol;
018
019 import java.lang.reflect.Field;
020 import java.util.HashMap;
021 import java.util.Map;
022
023 import org.apache.commons.lang.ClassUtils;
024 import org.apache.commons.lang.text.StrLookup;
025 import org.apache.commons.logging.Log;
026 import org.apache.commons.logging.LogFactory;
027
028 /**
029 * <p>
030 * A specialized lookup implementation that allows access to constant fields of
031 * classes.
032 * </p>
033 * <p>
034 * Sometimes it is necessary in a configuration file to refer to a constant
035 * defined in a class. This can be done with this lookup implementation.
036 * Variable names passed in must be of the form
037 * <code>mypackage.MyClass.FIELD</code>. The <code>lookup()</code> method
038 * will split the passed in string at the last dot, separating the fully
039 * qualified class name and the name of the constant (i.e. <strong>static final</strong>)
040 * member field. Then the class is loaded and the field's value is obtained
041 * using reflection.
042 * </p>
043 * <p>
044 * Once retrieved values are cached for fast access. This class is thread-safe.
045 * It can be used as a standard (i.e. global) lookup object and serve multiple
046 * clients concurrently.
047 * </p>
048 *
049 * @version $Id: ConstantLookup.java 588329 2007-10-25 20:01:31Z oheger $
050 * @since 1.4
051 * @author <a
052 * href="http://commons.apache.org/configuration/team-list.html">Commons
053 * Configuration team</a>
054 */
055 public class ConstantLookup extends StrLookup
056 {
057 /** Constant for the field separator. */
058 private static final char FIELD_SEPRATOR = '.';
059
060 /** An internally used cache for already retrieved values. */
061 private static Map constantCache = new HashMap();
062
063 /** The logger. */
064 private Log log = LogFactory.getLog(getClass());
065
066 /**
067 * Tries to resolve the specified variable. The passed in variable name is
068 * interpreted as the name of a <b>static final</b> member field of a
069 * class. If the value has already been obtained, it can be retrieved from
070 * an internal cache. Otherwise this method will invoke the
071 * <code>resolveField()</code> method and pass in the name of the class
072 * and the field.
073 *
074 * @param var the name of the variable to be resolved
075 * @return the value of this variable or <b>null</b> if it cannot be
076 * resolved
077 */
078 public String lookup(String var)
079 {
080 if (var == null)
081 {
082 return null;
083 }
084
085 String result;
086 synchronized (constantCache)
087 {
088 result = (String) constantCache.get(var);
089 }
090 if (result != null)
091 {
092 return result;
093 }
094
095 int fieldPos = var.lastIndexOf(FIELD_SEPRATOR);
096 if (fieldPos < 0)
097 {
098 return null;
099 }
100 try
101 {
102 Object value = resolveField(var.substring(0, fieldPos), var
103 .substring(fieldPos + 1));
104 if (value != null)
105 {
106 synchronized (constantCache)
107 {
108 // In worst case, the value will be fetched multiple times
109 // because of this lax synchronisation, but for constant
110 // values this shouldn't be a problem.
111 constantCache.put(var, String.valueOf(value));
112 }
113 result = value.toString();
114 }
115 }
116 catch (Exception ex)
117 {
118 log.warn("Could not obtain value for variable " + var, ex);
119 }
120
121 return result;
122 }
123
124 /**
125 * Clears the shared cache with the so far resolved constants.
126 */
127 public static void clear()
128 {
129 synchronized (constantCache)
130 {
131 constantCache.clear();
132 }
133 }
134
135 /**
136 * Determines the value of the specified constant member field of a class.
137 * This implementation will call <code>fetchClass()</code> to obtain the
138 * <code>java.lang.Class</code> object for the target class. Then it will
139 * use reflection to obtain the field's value. For this to work the field
140 * must be accessable.
141 *
142 * @param className the name of the class
143 * @param fieldName the name of the member field of that class to read
144 * @return the field's value
145 * @throws Exception if an error occurs
146 */
147 protected Object resolveField(String className, String fieldName)
148 throws Exception
149 {
150 Class clazz = fetchClass(className);
151 Field field = clazz.getField(fieldName);
152 return field.get(null);
153 }
154
155 /**
156 * Loads the class with the specified name. If an application has special
157 * needs regarding the class loaders to be used, it can hook in here. This
158 * implementation delegates to the <code>getClass()</code> method of
159 * Commons Lang's
160 * <code><a href="http://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html">
161 * ClassUtils</a></code>.
162 *
163 * @param className the name of the class to be loaded
164 * @return the corresponding class object
165 * @throws ClassNotFoundException if the class cannot be loaded
166 */
167 protected Class fetchClass(String className) throws ClassNotFoundException
168 {
169 return ClassUtils.getClass(className);
170 }
171 }