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 833923 2009-11-08 20:53:52Z oheger $
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 /**
120 * Constant for the prefix of the standard lookup object for resolving
121 * environment properties.
122 * @since 1.7
123 */
124 public static final String PREFIX_ENVIRONMENT = "env";
125
126 /** Constant for the prefix separator. */
127 private static final char PREFIX_SEPARATOR = ':';
128
129 /** A map with the globally registered lookup objects. */
130 private static Map globalLookups;
131
132 /** A map with the locally registered lookup objects. */
133 private Map localLookups;
134
135 /** Stores the default lookup object. */
136 private StrLookup defaultLookup;
137
138 /** Stores a parent interpolator objects if the interpolator is nested hierarchically. */
139 private ConfigurationInterpolator parentInterpolator;
140
141 /**
142 * Creates a new instance of <code>ConfigurationInterpolator</code>.
143 */
144 public ConfigurationInterpolator()
145 {
146 synchronized (globalLookups)
147 {
148 localLookups = new HashMap(globalLookups);
149 }
150 }
151
152 /**
153 * Registers the given lookup object for the specified prefix globally. This
154 * means that all instances that are created later will use this lookup
155 * object for this prefix. If for this prefix a lookup object is already
156 * registered, the new lookup object will replace the old one. Note that the
157 * lookup objects registered here will be shared between multiple clients.
158 * So they should be thread-safe.
159 *
160 * @param prefix the variable prefix (must not be <b>null</b>)
161 * @param lookup the lookup object to be used for this prefix (must not be
162 * <b>null</b>)
163 */
164 public static void registerGlobalLookup(String prefix, StrLookup lookup)
165 {
166 if (prefix == null)
167 {
168 throw new IllegalArgumentException(
169 "Prefix for lookup object must not be null!");
170 }
171 if (lookup == null)
172 {
173 throw new IllegalArgumentException(
174 "Lookup object must not be null!");
175 }
176 synchronized (globalLookups)
177 {
178 globalLookups.put(prefix, lookup);
179 }
180 }
181
182 /**
183 * Deregisters the global lookup object for the specified prefix. This means
184 * that this lookup object won't be available for later created instances
185 * any more. For already existing instances this operation does not have any
186 * impact.
187 *
188 * @param prefix the variable prefix
189 * @return a flag whether for this prefix a lookup object had been
190 * registered
191 */
192 public static boolean deregisterGlobalLookup(String prefix)
193 {
194 synchronized (globalLookups)
195 {
196 return globalLookups.remove(prefix) != null;
197 }
198 }
199
200 /**
201 * Registers the given lookup object for the specified prefix at this
202 * instance. From now on this lookup object will be used for variables that
203 * have the specified prefix.
204 *
205 * @param prefix the variable prefix (must not be <b>null</b>)
206 * @param lookup the lookup object to be used for this prefix (must not be
207 * <b>null</b>)
208 */
209 public void registerLookup(String prefix, StrLookup lookup)
210 {
211 if (prefix == null)
212 {
213 throw new IllegalArgumentException(
214 "Prefix for lookup object must not be null!");
215 }
216 if (lookup == null)
217 {
218 throw new IllegalArgumentException(
219 "Lookup object must not be null!");
220 }
221 localLookups.put(prefix, lookup);
222 }
223
224 /**
225 * Deregisters the lookup object for the specified prefix at this instance.
226 * It will be removed from this instance.
227 *
228 * @param prefix the variable prefix
229 * @return a flag whether for this prefix a lookup object had been
230 * registered
231 */
232 public boolean deregisterLookup(String prefix)
233 {
234 return localLookups.remove(prefix) != null;
235 }
236
237 /**
238 * Returns a set with the prefixes, for which lookup objects are registered
239 * at this instance. This means that variables with these prefixes can be
240 * processed.
241 *
242 * @return a set with the registered variable prefixes
243 */
244 public Set prefixSet()
245 {
246 return localLookups.keySet();
247 }
248
249 /**
250 * Returns the default lookup object.
251 *
252 * @return the default lookup object
253 */
254 public StrLookup getDefaultLookup()
255 {
256 return defaultLookup;
257 }
258
259 /**
260 * Sets the default lookup object. This lookup object will be used for all
261 * variables without a special prefix. If it is set to <b>null</b>, such
262 * variables won't be processed.
263 *
264 * @param defaultLookup the new default lookup object
265 */
266 public void setDefaultLookup(StrLookup defaultLookup)
267 {
268 this.defaultLookup = defaultLookup;
269 }
270
271 /**
272 * Resolves the specified variable. This implementation will try to extract
273 * a variable prefix from the given variable name (the first colon (':') is
274 * used as prefix separator). It then passes the name of the variable with
275 * the prefix stripped to the lookup object registered for this prefix. If
276 * no prefix can be found or if the associated lookup object cannot resolve
277 * this variable, the default lookup object will be used.
278 *
279 * @param var the name of the variable whose value is to be looked up
280 * @return the value of this variable or <b>null</b> if it cannot be
281 * resolved
282 */
283 public String lookup(String var)
284 {
285 if (var == null)
286 {
287 return null;
288 }
289
290 int prefixPos = var.indexOf(PREFIX_SEPARATOR);
291 if (prefixPos >= 0)
292 {
293 String prefix = var.substring(0, prefixPos);
294 String name = var.substring(prefixPos + 1);
295 String value = fetchLookupForPrefix(prefix).lookup(name);
296 if (value == null && getParentInterpolator() != null)
297 {
298 value = getParentInterpolator().fetchLookupForPrefix(prefix).lookup(name);
299 }
300 if (value != null)
301 {
302 return value;
303 }
304 }
305 String value = fetchNoPrefixLookup().lookup(var);
306 if (value == null && getParentInterpolator() != null)
307 {
308 value = getParentInterpolator().fetchNoPrefixLookup().lookup(var);
309 }
310 return value;
311 }
312
313 /**
314 * Returns the lookup object to be used for variables without a prefix. This
315 * implementation will check whether a default lookup object was set. If
316 * this is the case, it will be returned. Otherwise a <b>null</b> lookup
317 * object will be returned (never <code>null</code>).
318 *
319 * @return the lookup object to be used for variables without a prefix
320 */
321 protected StrLookup fetchNoPrefixLookup()
322 {
323 return (getDefaultLookup() != null) ? getDefaultLookup() : StrLookup.noneLookup();
324 }
325
326 /**
327 * Obtains the lookup object for the specified prefix. This method is called
328 * by the <code>lookup()</code> method. This implementation will check
329 * whether a lookup object is registered for the given prefix. If not, a
330 * <b>null</b> lookup object will be returned (never <code>null</code>).
331 *
332 * @param prefix the prefix
333 * @return the lookup object to be used for this prefix
334 */
335 protected StrLookup fetchLookupForPrefix(String prefix)
336 {
337 StrLookup lookup = (StrLookup) localLookups.get(prefix);
338 if (lookup == null)
339 {
340 lookup = StrLookup.noneLookup();
341 }
342 return lookup;
343 }
344
345 /**
346 * Registers the local lookup instances for the given interpolator.
347 *
348 * @param interpolator the instance receiving the local lookups
349 * @since upcoming
350 */
351 public void registerLocalLookups(ConfigurationInterpolator interpolator)
352 {
353 interpolator.localLookups.putAll(localLookups);
354 }
355
356 /**
357 * Sets the parent interpolator. This object is used if the interpolation is nested
358 * hierarchically and the current interpolation object cannot resolve a variable.
359 *
360 * @param parentInterpolator the parent interpolator object or <code>null</code>
361 * @since upcoming
362 */
363 public void setParentInterpolator(ConfigurationInterpolator parentInterpolator)
364 {
365 this.parentInterpolator = parentInterpolator;
366 }
367
368 /**
369 * Requests the parent interpolator. This object is used if the interpolation is nested
370 * hierarchically and the current interpolation
371 *
372 * @return the parent interpolator or <code>null</code>
373 * @since upcoming
374 */
375 public ConfigurationInterpolator getParentInterpolator()
376 {
377 return this.parentInterpolator;
378 }
379
380 // static initializer, sets up the map with the standard lookups
381 static
382 {
383 globalLookups = new HashMap();
384 globalLookups.put(PREFIX_SYSPROPERTIES, StrLookup.systemPropertiesLookup());
385 globalLookups.put(PREFIX_CONSTANTS, new ConstantLookup());
386 globalLookups.put(PREFIX_ENVIRONMENT, new EnvironmentLookup());
387 }
388 }