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17 package org.apache.commons.configuration.interpol;
18
19 import java.util.HashMap;
20 import java.util.Map;
21 import java.util.Set;
22
23 import org.apache.commons.lang.text.StrLookup;
24
25 /***
26 * <p>
27 * A class that handles interpolation (variable substitution) for configuration
28 * objects.
29 * </p>
30 * <p>
31 * Each instance of <code>AbstractConfiguration</code> is associated with an
32 * object of this class. Each interpolation tasks are delegated to this object.
33 * </p>
34 * <p>
35 * <code>ConfigurationInterpolator</code> works together with the
36 * <code>StrSubstitutor</code> class from <a
37 * href="http://jakarta.apache.org/commons/lang">Commons Lang</a>. By extending
38 * <code>StrLookup</code> it is able to provide values for variables that
39 * appear in expressions.
40 * </p>
41 * <p>
42 * The basic idea of this class is that it can maintain a set of primitive
43 * <code>StrLookup</code> objects, each of which are identified by a special
44 * prefix. The variables to be processed have the form
45 * <code>${prefix:name}</code>. <code>ConfigurationInterpolator</code> will
46 * extract the prefix and determine, which primitive lookup object is registered
47 * for it. Then the name of the variable is passed to this object to obtain the
48 * actual value. It is also possible to define a default lookup object, which
49 * will be used for variables that do not have a prefix.
50 * </p>
51 * <p>
52 * When a new instance of this class is created it is initialized with a default
53 * set of primitive lookup objects. This set can be customized using the static
54 * methods <code>registerGlobalLookup()</code> and
55 * <code>deregisterGlobalLookup()</code>. Per default it contains the
56 * following standard lookup objects:
57 * </p>
58 * <p>
59 * <table border="1">
60 * <tr>
61 * <th>Prefix</th>
62 * <th>Lookup object</th>
63 * </tr>
64 * <tr>
65 * <td valign="top">sys</td>
66 * <td>With this prefix a lookup object is associated that is able to resolve
67 * system properties.</td>
68 * </tr>
69 * <tr>
70 * <td valign="top">const</td>
71 * <td>The <code>const</code> prefix indicates that a variable is to be
72 * interpreted as a constant member field of a class (i.e. a field with the
73 * <b>static final</b> modifiers). The name of the variable must be of the form
74 * <code><full qualified class name>.<field name></code>, e.g.
75 * <code>org.apache.commons.configuration.interpol.ConfigurationInterpolator.PREFIX_CONSTANTS
76 * </code>.</td>
77 * </tr>
78 * </table>
79 * </p>
80 * <p>
81 * After an instance has been created the current set of lookup objects can be
82 * modified using the <code>registerLookup()</code> and
83 * <code>deregisterLookup()</code> methods. The default lookup object (that is
84 * invoked for variables without a prefix) can be set with the
85 * <code>setDefaultLookup()</code> method. (If a
86 * <code>ConfigurationInterpolator</code> instance is created by a
87 * configuration object, this lookup points to the configuration itself, so that
88 * variables are resolved using the configuration's properties. This ensures
89 * backward compatibility to earlier version of Commons Configuration.)
90 * </p>
91 * <p>
92 * Implementation node: Instances of this class are not thread-safe related to
93 * modifications of their current set of registered lookup objects. It is
94 * intended that each instance is associated with a single
95 * <code>Configuration</conde>
96 * object and used for its interpolation tasks.</p>
97 *
98 * @version $Id: ConfigurationInterpolator.java 491243 2006-12-30 16:04:03Z oheger $
99 * @since 1.4
100 * @author <a
101 * href="http://jakarta.apache.org/commons/configuration/team-list.html">Commons
102 * Configuration team</a>
103 */
104 public class ConfigurationInterpolator extends StrLookup
105 {
106 /***
107 * Constant for the prefix of the standard lookup object for resolving
108 * system properties.
109 */
110 public static final String PREFIX_SYSPROPERTIES = "sys";
111
112 /***
113 * Constant for the prefix of the standard lookup object for resolving
114 * constant values.
115 */
116 public static final String PREFIX_CONSTANTS = "const";
117
118 /*** Constant for the prefix separator. */
119 private static final char PREFIX_SEPARATOR = ':';
120
121 /*** A map with the globally registered lookup objects. */
122 private static Map globalLookups;
123
124 /*** A map with the locally registered lookup objects. */
125 private Map localLookups;
126
127 /*** Stores the default lookup object. */
128 private StrLookup defaultLookup;
129
130 /***
131 * Creates a new instance of <code>ConfigurationInterpolator</code>.
132 */
133 public ConfigurationInterpolator()
134 {
135 synchronized (globalLookups)
136 {
137 localLookups = new HashMap(globalLookups);
138 }
139 }
140
141 /***
142 * Registers the given lookup object for the specified prefix globally. This
143 * means that all instances that are created later will use this lookup
144 * object for this prefix. If for this prefix a lookup object is already
145 * registered, the new lookup object will replace the old one. Note that the
146 * lookup objects registered here will be shared between multiple clients.
147 * So they should be thread-safe.
148 *
149 * @param prefix the variable prefix (must not be <b>null</b>)
150 * @param lookup the lookup object to be used for this prefix (must not be
151 * <b>null</b>)
152 */
153 public static void registerGlobalLookup(String prefix, StrLookup lookup)
154 {
155 if (prefix == null)
156 {
157 throw new IllegalArgumentException(
158 "Prefix for lookup object must not be null!");
159 }
160 if (lookup == null)
161 {
162 throw new IllegalArgumentException(
163 "Lookup object must not be null!");
164 }
165 synchronized (globalLookups)
166 {
167 globalLookups.put(prefix, lookup);
168 }
169 }
170
171 /***
172 * Deregisters the global lookup object for the specified prefix. This means
173 * that this lookup object won't be available for later created instances
174 * any more. For already existing instances this operation does not have any
175 * impact.
176 *
177 * @param prefix the variable prefix
178 * @return a flag whether for this prefix a lookup object had been
179 * registered
180 */
181 public static boolean deregisterGlobalLookup(String prefix)
182 {
183 synchronized (globalLookups)
184 {
185 return globalLookups.remove(prefix) != null;
186 }
187 }
188
189 /***
190 * Registers the given lookup object for the specified prefix at this
191 * instance. From now on this lookup object will be used for variables that
192 * have the specified prefix.
193 *
194 * @param prefix the variable prefix (must not be <b>null</b>)
195 * @param lookup the lookup object to be used for this prefix (must not be
196 * <b>null</b>)
197 */
198 public void registerLookup(String prefix, StrLookup lookup)
199 {
200 if (prefix == null)
201 {
202 throw new IllegalArgumentException(
203 "Prefix for lookup object must not be null!");
204 }
205 if (lookup == null)
206 {
207 throw new IllegalArgumentException(
208 "Lookup object must not be null!");
209 }
210 localLookups.put(prefix, lookup);
211 }
212
213 /***
214 * Deregisters the lookup object for the specified prefix at this instance.
215 * It will be removed from this instance.
216 *
217 * @param prefix the variable prefix
218 * @return a flag whether for this prefix a lookup object had been
219 * registered
220 */
221 public boolean deregisterLookup(String prefix)
222 {
223 return localLookups.remove(prefix) != null;
224 }
225
226 /***
227 * Returns a set with the prefixes, for which lookup objects are registered
228 * at this instance. This means that variables with these prefixes can be
229 * processed.
230 *
231 * @return a set with the registered variable prefixes
232 */
233 public Set prefixSet()
234 {
235 return localLookups.keySet();
236 }
237
238 /***
239 * Returns the default lookup object.
240 *
241 * @return the default lookup object
242 */
243 public StrLookup getDefaultLookup()
244 {
245 return defaultLookup;
246 }
247
248 /***
249 * Sets the default lookup object. This lookup object will be used for all
250 * variables without a special prefix. If it is set to <b>null</b>, such
251 * variables won't be processed.
252 *
253 * @param defaultLookup the new default lookup object
254 */
255 public void setDefaultLookup(StrLookup defaultLookup)
256 {
257 this.defaultLookup = defaultLookup;
258 }
259
260 /***
261 * Resolves the specified variable. This implementation will try to extract
262 * a variable prefix from the given variable name (the first colon (':') is
263 * used as prefix separator). It then passes the name of the variable with
264 * the prefix stripped to the lookup object registered for this prefix. If
265 * no prefix can be found, the default lookup object will be used.
266 *
267 * @param var the name of the variable whose value is to be looked up
268 * @return the value of this variable or <b>null</b> if it cannot be
269 * resolved
270 */
271 public String lookup(String var)
272 {
273 if (var == null)
274 {
275 return null;
276 }
277
278 int prefixPos = var.indexOf(PREFIX_SEPARATOR);
279 if (prefixPos < 0)
280 {
281 return fetchNoPrefixLookup().lookup(var);
282 }
283 else
284 {
285 String prefix = var.substring(0, prefixPos);
286 String name = var.substring(prefixPos + 1);
287 return fetchLookupForPrefix(prefix).lookup(name);
288 }
289 }
290
291 /***
292 * Returns the lookup object to be used for variables without a prefix. This
293 * implementation will check whether a default lookup object was set. If
294 * this is the case, it will be returned. Otherwise a <b>null</b> lookup
295 * object will be returned.
296 *
297 * @return the lookup object to be used for variables without a prefix
298 */
299 protected StrLookup fetchNoPrefixLookup()
300 {
301 return (getDefaultLookup() != null) ? getDefaultLookup() : StrLookup
302 .noneLookup();
303 }
304
305 /***
306 * Obtains the lookup object for the specified prefix. This method is called
307 * by the <code>lookup()</code> method. This implementation will check
308 * whether a lookup object is registered for the given prefix. If not, a
309 * <b>null</b> lookup object will be returned.
310 *
311 * @param prefix the prefix
312 * @return the lookup object to be used for this prefix
313 */
314 protected StrLookup fetchLookupForPrefix(String prefix)
315 {
316 StrLookup lookup = (StrLookup) localLookups.get(prefix);
317 if (lookup == null)
318 {
319 lookup = StrLookup.noneLookup();
320 }
321 return lookup;
322 }
323
324
325 static
326 {
327 globalLookups = new HashMap();
328 globalLookups.put(PREFIX_SYSPROPERTIES, StrLookup
329 .systemPropertiesLookup());
330 globalLookups.put(PREFIX_CONSTANTS, new ConstantLookup());
331 }
332 }