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.util.HashMap;
020 import java.util.Map;
021 import java.util.Set;
022
023 import org.apache.commons.lang.text.StrLookup;
024
025 /**
026 * <p>
027 * A class that handles interpolation (variable substitution) for configuration
028 * objects.
029 * </p>
030 * <p>
031 * Each instance of <code>AbstractConfiguration</code> is associated with an
032 * object of this class. All interpolation tasks are delegated to this object.
033 * </p>
034 * <p>
035 * <code>ConfigurationInterpolator</code> works together with the
036 * <code>StrSubstitutor</code> class from <a
037 * href="http://commons.apache.org/lang">Commons Lang</a>. By extending
038 * <code>StrLookup</code> it is able to provide values for variables that
039 * appear in expressions.
040 * </p>
041 * <p>
042 * The basic idea of this class is that it can maintain a set of primitive
043 * <code>StrLookup</code> objects, each of which is identified by a special
044 * prefix. The variables to be processed have the form
045 * <code>${prefix:name}</code>. <code>ConfigurationInterpolator</code> will
046 * extract the prefix and determine, which primitive lookup object is registered
047 * for it. Then the name of the variable is passed to this object to obtain the
048 * actual value. It is also possible to define a default lookup object, which
049 * will be used for variables that do not have a prefix or that cannot be
050 * resolved by their associated lookup object.
051 * </p>
052 * <p>
053 * When a new instance of this class is created it is initialized with a default
054 * set of primitive lookup objects. This set can be customized using the static
055 * methods <code>registerGlobalLookup()</code> and
056 * <code>deregisterGlobalLookup()</code>. Per default it contains the
057 * following standard lookup objects:
058 * </p>
059 * <p>
060 * <table border="1">
061 * <tr>
062 * <th>Prefix</th>
063 * <th>Lookup object</th>
064 * </tr>
065 * <tr>
066 * <td valign="top">sys</td>
067 * <td>With this prefix a lookup object is associated that is able to resolve
068 * system properties.</td>
069 * </tr>
070 * <tr>
071 * <td valign="top">const</td>
072 * <td>The <code>const</code> prefix indicates that a variable is to be
073 * interpreted as a constant member field of a class (i.e. a field with the
074 * <b>static final</b> modifiers). The name of the variable must be of the form
075 * <code><full qualified class name>.<field name></code>, e.g.
076 * <code>org.apache.commons.configuration.interpol.ConfigurationInterpolator.PREFIX_CONSTANTS
077 * </code>.</td>
078 * </tr>
079 * </table>
080 * </p>
081 * <p>
082 * After an instance has been created the current set of lookup objects can be
083 * modified using the <code>registerLookup()</code> and
084 * <code>deregisterLookup()</code> methods. The default lookup object (that is
085 * invoked for variables without a prefix) can be set with the
086 * <code>setDefaultLookup()</code> method. (If a
087 * <code>ConfigurationInterpolator</code> instance is created by a
088 * configuration object, this lookup points to the configuration itself, so that
089 * variables are resolved using the configuration's properties. This ensures
090 * backward compatibility to earlier version of Commons Configuration.)
091 * </p>
092 * <p>
093 * Implementation node: Instances of this class are not thread-safe related to
094 * modifications of their current set of registered lookup objects. It is
095 * intended that each instance is associated with a single
096 * <code>Configuration</code> object and used for its interpolation tasks.
097 * </p>
098 *
099 * @version $Id: ConfigurationInterpolator.java 561230 2007-07-31 04:17:09Z rahul $
100 * @since 1.4
101 * @author <a
102 * href="http://commons.apache.org/configuration/team-list.html">Commons
103 * Configuration team</a>
104 */
105 public class ConfigurationInterpolator extends StrLookup
106 {
107 /**
108 * Constant for the prefix of the standard lookup object for resolving
109 * system properties.
110 */
111 public static final String PREFIX_SYSPROPERTIES = "sys";
112
113 /**
114 * Constant for the prefix of the standard lookup object for resolving
115 * constant values.
116 */
117 public static final String PREFIX_CONSTANTS = "const";
118
119 /** Constant for the prefix separator. */
120 private static final char PREFIX_SEPARATOR = ':';
121
122 /** A map with the globally registered lookup objects. */
123 private static Map globalLookups;
124
125 /** A map with the locally registered lookup objects. */
126 private Map localLookups;
127
128 /** Stores the default lookup object. */
129 private StrLookup defaultLookup;
130
131 /**
132 * Creates a new instance of <code>ConfigurationInterpolator</code>.
133 */
134 public ConfigurationInterpolator()
135 {
136 synchronized (globalLookups)
137 {
138 localLookups = new HashMap(globalLookups);
139 }
140 }
141
142 /**
143 * Registers the given lookup object for the specified prefix globally. This
144 * means that all instances that are created later will use this lookup
145 * object for this prefix. If for this prefix a lookup object is already
146 * registered, the new lookup object will replace the old one. Note that the
147 * lookup objects registered here will be shared between multiple clients.
148 * So they should be thread-safe.
149 *
150 * @param prefix the variable prefix (must not be <b>null</b>)
151 * @param lookup the lookup object to be used for this prefix (must not be
152 * <b>null</b>)
153 */
154 public static void registerGlobalLookup(String prefix, StrLookup lookup)
155 {
156 if (prefix == null)
157 {
158 throw new IllegalArgumentException(
159 "Prefix for lookup object must not be null!");
160 }
161 if (lookup == null)
162 {
163 throw new IllegalArgumentException(
164 "Lookup object must not be null!");
165 }
166 synchronized (globalLookups)
167 {
168 globalLookups.put(prefix, lookup);
169 }
170 }
171
172 /**
173 * Deregisters the global lookup object for the specified prefix. This means
174 * that this lookup object won't be available for later created instances
175 * any more. For already existing instances this operation does not have any
176 * impact.
177 *
178 * @param prefix the variable prefix
179 * @return a flag whether for this prefix a lookup object had been
180 * registered
181 */
182 public static boolean deregisterGlobalLookup(String prefix)
183 {
184 synchronized (globalLookups)
185 {
186 return globalLookups.remove(prefix) != null;
187 }
188 }
189
190 /**
191 * Registers the given lookup object for the specified prefix at this
192 * instance. From now on this lookup object will be used for variables that
193 * have the specified prefix.
194 *
195 * @param prefix the variable prefix (must not be <b>null</b>)
196 * @param lookup the lookup object to be used for this prefix (must not be
197 * <b>null</b>)
198 */
199 public void registerLookup(String prefix, StrLookup lookup)
200 {
201 if (prefix == null)
202 {
203 throw new IllegalArgumentException(
204 "Prefix for lookup object must not be null!");
205 }
206 if (lookup == null)
207 {
208 throw new IllegalArgumentException(
209 "Lookup object must not be null!");
210 }
211 localLookups.put(prefix, lookup);
212 }
213
214 /**
215 * Deregisters the lookup object for the specified prefix at this instance.
216 * It will be removed from this instance.
217 *
218 * @param prefix the variable prefix
219 * @return a flag whether for this prefix a lookup object had been
220 * registered
221 */
222 public boolean deregisterLookup(String prefix)
223 {
224 return localLookups.remove(prefix) != null;
225 }
226
227 /**
228 * Returns a set with the prefixes, for which lookup objects are registered
229 * at this instance. This means that variables with these prefixes can be
230 * processed.
231 *
232 * @return a set with the registered variable prefixes
233 */
234 public Set prefixSet()
235 {
236 return localLookups.keySet();
237 }
238
239 /**
240 * Returns the default lookup object.
241 *
242 * @return the default lookup object
243 */
244 public StrLookup getDefaultLookup()
245 {
246 return defaultLookup;
247 }
248
249 /**
250 * Sets the default lookup object. This lookup object will be used for all
251 * variables without a special prefix. If it is set to <b>null</b>, such
252 * variables won't be processed.
253 *
254 * @param defaultLookup the new default lookup object
255 */
256 public void setDefaultLookup(StrLookup defaultLookup)
257 {
258 this.defaultLookup = defaultLookup;
259 }
260
261 /**
262 * Resolves the specified variable. This implementation will try to extract
263 * a variable prefix from the given variable name (the first colon (':') is
264 * used as prefix separator). It then passes the name of the variable with
265 * the prefix stripped to the lookup object registered for this prefix. If
266 * no prefix can be found or if the associated lookup object cannot resolve
267 * this variable, the default lookup object will be used.
268 *
269 * @param var the name of the variable whose value is to be looked up
270 * @return the value of this variable or <b>null</b> if it cannot be
271 * resolved
272 */
273 public String lookup(String var)
274 {
275 if (var == null)
276 {
277 return null;
278 }
279
280 int prefixPos = var.indexOf(PREFIX_SEPARATOR);
281 if (prefixPos >= 0)
282 {
283 String prefix = var.substring(0, prefixPos);
284 String name = var.substring(prefixPos + 1);
285 String value = fetchLookupForPrefix(prefix).lookup(name);
286 if (value != null)
287 {
288 return value;
289 }
290 }
291 return fetchNoPrefixLookup().lookup(var);
292 }
293
294 /**
295 * Returns the lookup object to be used for variables without a prefix. This
296 * implementation will check whether a default lookup object was set. If
297 * this is the case, it will be returned. Otherwise a <b>null</b> lookup
298 * object will be returned.
299 *
300 * @return the lookup object to be used for variables without a prefix
301 */
302 protected StrLookup fetchNoPrefixLookup()
303 {
304 return (getDefaultLookup() != null) ? getDefaultLookup() : StrLookup.noneLookup();
305 }
306
307 /**
308 * Obtains the lookup object for the specified prefix. This method is called
309 * by the <code>lookup()</code> method. This implementation will check
310 * whether a lookup object is registered for the given prefix. If not, a
311 * <b>null</b> lookup object will be returned.
312 *
313 * @param prefix the prefix
314 * @return the lookup object to be used for this prefix
315 */
316 protected StrLookup fetchLookupForPrefix(String prefix)
317 {
318 StrLookup lookup = (StrLookup) localLookups.get(prefix);
319 if (lookup == null)
320 {
321 lookup = StrLookup.noneLookup();
322 }
323 return lookup;
324 }
325
326 // static initializer, sets up the map with the standard lookups
327 static
328 {
329 globalLookups = new HashMap();
330 globalLookups.put(PREFIX_SYSPROPERTIES, StrLookup.systemPropertiesLookup());
331 globalLookups.put(PREFIX_CONSTANTS, new ConstantLookup());
332 }
333 }