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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. All 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://commons.apache.org/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 is 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 or that cannot be
50 * resolved by their associated lookup object.
51 * </p>
52 * <p>
53 * When a new instance of this class is created it is initialized with a default
54 * set of primitive lookup objects. This set can be customized using the static
55 * methods <code>registerGlobalLookup()</code> and
56 * <code>deregisterGlobalLookup()</code>. Per default it contains the
57 * following standard lookup objects:
58 * </p>
59 * <p>
60 * <table border="1">
61 * <tr>
62 * <th>Prefix</th>
63 * <th>Lookup object</th>
64 * </tr>
65 * <tr>
66 * <td valign="top">sys</td>
67 * <td>With this prefix a lookup object is associated that is able to resolve
68 * system properties.</td>
69 * </tr>
70 * <tr>
71 * <td valign="top">const</td>
72 * <td>The <code>const</code> prefix indicates that a variable is to be
73 * interpreted as a constant member field of a class (i.e. a field with the
74 * <b>static final</b> modifiers). The name of the variable must be of the form
75 * <code><full qualified class name>.<field name></code>, e.g.
76 * <code>org.apache.commons.configuration.interpol.ConfigurationInterpolator.PREFIX_CONSTANTS
77 * </code>.</td>
78 * </tr>
79 * </table>
80 * </p>
81 * <p>
82 * After an instance has been created the current set of lookup objects can be
83 * modified using the <code>registerLookup()</code> and
84 * <code>deregisterLookup()</code> methods. The default lookup object (that is
85 * invoked for variables without a prefix) can be set with the
86 * <code>setDefaultLookup()</code> method. (If a
87 * <code>ConfigurationInterpolator</code> instance is created by a
88 * configuration object, this lookup points to the configuration itself, so that
89 * variables are resolved using the configuration's properties. This ensures
90 * backward compatibility to earlier version of Commons Configuration.)
91 * </p>
92 * <p>
93 * Implementation node: Instances of this class are not thread-safe related to
94 * modifications of their current set of registered lookup objects. It is
95 * intended that each instance is associated with a single
96 * <code>Configuration</code> object and used for its interpolation tasks.
97 * </p>
98 *
99 * @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
327 static
328 {
329 globalLookups = new HashMap();
330 globalLookups.put(PREFIX_SYSPROPERTIES, StrLookup.systemPropertiesLookup());
331 globalLookups.put(PREFIX_CONSTANTS, new ConstantLookup());
332 }
333 }