Classes in this File | Line Coverage | Branch Coverage | Complexity | ||||
ConstantLookup |
|
| 3.0;3 |
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</code>. The <code>lookup()</code> 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 490375 2006-12-26 21:28:04Z oheger $ | |
50 | * @since 1.4 | |
51 | * @author <a | |
52 | * href="http://jakarta.apache.org/commons/configuration/team-list.html">Commons | |
53 | * Configuration team</a> | |
54 | */ | |
55 | 110 | 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 | 48 | private static Map constantCache = new HashMap(); |
62 | ||
63 | /** The logger. */ | |
64 | 55 | 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()</code> 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 | public String lookup(String var) | |
79 | { | |
80 | 10 | if (var == null) |
81 | { | |
82 | 1 | return null; |
83 | } | |
84 | ||
85 | String result; | |
86 | 9 | synchronized (constantCache) |
87 | { | |
88 | 9 | result = (String) constantCache.get(var); |
89 | 9 | } |
90 | 9 | if (result != null) |
91 | { | |
92 | 1 | return result; |
93 | } | |
94 | ||
95 | 8 | int fieldPos = var.lastIndexOf(FIELD_SEPRATOR); |
96 | 8 | if (fieldPos < 0) |
97 | { | |
98 | 1 | return null; |
99 | } | |
100 | try | |
101 | { | |
102 | 7 | Object value = resolveField(var.substring(0, fieldPos), var |
103 | .substring(fieldPos + 1)); | |
104 | 4 | if (value != null) |
105 | { | |
106 | 4 | 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 | |
111 | // this shouldn't be a problem. | |
112 | 4 | constantCache.put(var, value); |
113 | 4 | } |
114 | 4 | result = value.toString(); |
115 | } | |
116 | 4 | } |
117 | catch (Exception ex) | |
118 | { | |
119 | 3 | log.warn("Could not obtain value for variable " + var, ex); |
120 | } | |
121 | ||
122 | 7 | return result; |
123 | } | |
124 | ||
125 | /** | |
126 | * Clears the shared cache with the so far resolved constants. | |
127 | */ | |
128 | public static void clear() | |
129 | { | |
130 | 7 | synchronized (constantCache) |
131 | { | |
132 | 7 | constantCache.clear(); |
133 | 7 | } |
134 | 7 | } |
135 | ||
136 | /** | |
137 | * Determines the value of the specified constant member field of a class. | |
138 | * This implementation will call <code>fetchClass()</code> to obtain the | |
139 | * <code>java.lang.Class</code> 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 | 7 | Class clazz = fetchClass(className); |
152 | 6 | Field field = clazz.getField(fieldName); |
153 | 4 | 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()</code> method of | |
160 | * Commons Lang's | |
161 | * <code><a href="http://jakarta.apache.org/commons/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 | 7 | return ClassUtils.getClass(className); |
171 | } | |
172 | } |