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}. The {@code lookup()} 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 1210218 2011-12-04 20:57:48Z 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<String, String> constantCache = new HashMap<String, String>();
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()} 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 @Override
079 public String lookup(String var)
080 {
081 if (var == null)
082 {
083 return null;
084 }
085
086 String result;
087 synchronized (constantCache)
088 {
089 result = constantCache.get(var);
090 }
091 if (result != null)
092 {
093 return result;
094 }
095
096 int fieldPos = var.lastIndexOf(FIELD_SEPRATOR);
097 if (fieldPos < 0)
098 {
099 return null;
100 }
101 try
102 {
103 Object value = resolveField(var.substring(0, fieldPos), var
104 .substring(fieldPos + 1));
105 if (value != null)
106 {
107 synchronized (constantCache)
108 {
109 // In worst case, the value will be fetched multiple times
110 // because of this lax synchronisation, but for constant
111 // values this shouldn't be a problem.
112 constantCache.put(var, String.valueOf(value));
113 }
114 result = value.toString();
115 }
116 }
117 catch (Exception ex)
118 {
119 log.warn("Could not obtain value for variable " + var, ex);
120 }
121
122 return result;
123 }
124
125 /**
126 * Clears the shared cache with the so far resolved constants.
127 */
128 public static void clear()
129 {
130 synchronized (constantCache)
131 {
132 constantCache.clear();
133 }
134 }
135
136 /**
137 * Determines the value of the specified constant member field of a class.
138 * This implementation will call {@code fetchClass()} to obtain the
139 * {@code java.lang.Class} object for the target class. Then it will
140 * use reflection to obtain the field's value. For this to work the field
141 * must be accessable.
142 *
143 * @param className the name of the class
144 * @param fieldName the name of the member field of that class to read
145 * @return the field's value
146 * @throws Exception if an error occurs
147 */
148 protected Object resolveField(String className, String fieldName)
149 throws Exception
150 {
151 Class<?> clazz = fetchClass(className);
152 Field field = clazz.getField(fieldName);
153 return field.get(null);
154 }
155
156 /**
157 * Loads the class with the specified name. If an application has special
158 * needs regarding the class loaders to be used, it can hook in here. This
159 * implementation delegates to the {@code getClass()} method of
160 * Commons Lang's
161 * <code><a href="http://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html">
162 * ClassUtils</a></code>.
163 *
164 * @param className the name of the class to be loaded
165 * @return the corresponding class object
166 * @throws ClassNotFoundException if the class cannot be loaded
167 */
168 protected Class<?> fetchClass(String className) throws ClassNotFoundException
169 {
170 return ClassUtils.getClass(className);
171 }
172 }