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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *     http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.configuration.interpol;
18  
19  import java.lang.reflect.Field;
20  import java.util.HashMap;
21  import java.util.Map;
22  
23  import org.apache.commons.lang.ClassUtils;
24  import org.apache.commons.lang.text.StrLookup;
25  import org.apache.commons.logging.Log;
26  import org.apache.commons.logging.LogFactory;
27  
28  /**
29   * <p>
30   * A specialized lookup implementation that allows access to constant fields of
31   * classes.
32   * </p>
33   * <p>
34   * Sometimes it is necessary in a configuration file to refer to a constant
35   * defined in a class. This can be done with this lookup implementation.
36   * Variable names passed in must be of the form
37   * {@code mypackage.MyClass.FIELD}. The {@code lookup()} method
38   * will split the passed in string at the last dot, separating the fully
39   * qualified class name and the name of the constant (i.e. <strong>static final</strong>)
40   * member field. Then the class is loaded and the field's value is obtained
41   * using reflection.
42   * </p>
43   * <p>
44   * Once retrieved values are cached for fast access. This class is thread-safe.
45   * It can be used as a standard (i.e. global) lookup object and serve multiple
46   * clients concurrently.
47   * </p>
48   *
49   * @version $Id: ConstantLookup.java 1210218 2011-12-04 20:57:48Z oheger $
50   * @since 1.4
51   * @author <a
52   * href="http://commons.apache.org/configuration/team-list.html">Commons
53   * Configuration team</a>
54   */
55  public class ConstantLookup extends StrLookup
56  {
57      /** Constant for the field separator. */
58      private static final char FIELD_SEPRATOR = '.';
59  
60      /** An internally used cache for already retrieved values. */
61      private static Map<String, String> constantCache = new HashMap<String, String>();
62  
63      /** The logger. */
64      private Log log = LogFactory.getLog(getClass());
65  
66      /**
67       * Tries to resolve the specified variable. The passed in variable name is
68       * interpreted as the name of a <b>static final</b> member field of a
69       * class. If the value has already been obtained, it can be retrieved from
70       * an internal cache. Otherwise this method will invoke the
71       * {@code resolveField()} method and pass in the name of the class
72       * and the field.
73       *
74       * @param var the name of the variable to be resolved
75       * @return the value of this variable or <b>null</b> if it cannot be
76       * resolved
77       */
78      @Override
79      public String lookup(String var)
80      {
81          if (var == null)
82          {
83              return null;
84          }
85  
86          String result;
87          synchronized (constantCache)
88          {
89              result = constantCache.get(var);
90          }
91          if (result != null)
92          {
93              return result;
94          }
95  
96          int fieldPos = var.lastIndexOf(FIELD_SEPRATOR);
97          if (fieldPos < 0)
98          {
99              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 }