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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.tree.xpath;
18  
19  import java.util.ArrayList;
20  import java.util.Collections;
21  import java.util.List;
22  import java.util.StringTokenizer;
23  
24  import org.apache.commons.configuration.tree.ConfigurationNode;
25  import org.apache.commons.configuration.tree.ExpressionEngine;
26  import org.apache.commons.configuration.tree.NodeAddData;
27  import org.apache.commons.jxpath.JXPathContext;
28  import org.apache.commons.jxpath.ri.JXPathContextReferenceImpl;
29  import org.apache.commons.lang.StringUtils;
30  
31  /***
32   * <p>
33   * A specialized implementation of the <code>ExpressionEngine</code> interface
34   * that is able to evaluate XPATH expressions.
35   * </p>
36   * <p>
37   * This class makes use of <a href="http://jakarta.apache.org/commons/jxpath/">
38   * Commons JXPath</a> for handling XPath expressions and mapping them to the
39   * nodes of a hierarchical configuration. This makes the rich and powerfull
40   * XPATH syntax available for accessing properties from a configuration object.
41   * </p>
42   * <p>
43   * For selecting properties arbitrary XPATH expressions can be used, which
44   * select single or multiple configuration nodes. The associated
45   * <code>Configuration</code> instance will directly pass the specified
46   * property keys into this engine. If a key is not syntactically correct, an
47   * exception will be thrown.
48   * </p>
49   * <p>
50   * For adding new properties, this expression engine uses a specific syntax: the
51   * &quot;key&quot; of a new property must consist of two parts that are
52   * separated by whitespace:
53   * <ol>
54   * <li>An XPATH expression selecting a single node, to which the new element(s)
55   * are to be added. This can be an arbitrary complex expression, but it must
56   * select exactly one node, otherwise an exception will be thrown.</li>
57   * <li>The name of the new element(s) to be added below this parent node. Here
58   * either a single node name or a complete path of nodes (separated by the
59   * &quot;/&quot; character) can be specified.</li>
60   * </ol>
61   * Some examples for valid keys that can be passed into the configuration's
62   * <code>addProperty()</code> method follow:
63   * </p>
64   * <p>
65   *
66   * <pre>
67   * &quot;/tables/table[1] type&quot;
68   * </pre>
69   *
70   * </p>
71   * <p>
72   * This will add a new <code>type</code> node as a child of the first
73   * <code>table</code> element.
74   * </p>
75   * <p>
76   *
77   * <pre>
78   * &quot;/tables/table[1] @type&quot;
79   * </pre>
80   *
81   * </p>
82   * <p>
83   * Similar to the example above, but this time a new attribute named
84   * <code>type</code> will be added to the first <code>table</code> element.
85   * </p>
86   * <p>
87   *
88   * <pre>
89   * &quot;/tables table/fields/field/name&quot;
90   * </pre>
91   *
92   * </p>
93   * <p>
94   * This example shows how a complex path can be added. Parent node is the
95   * <code>tables</code> element. Here a new branch consisting of the nodes
96   * <code>table</code>, <code>fields</code>, <code>field</code>, and
97   * <code>name</code> will be added.
98   * </p>
99   *
100  * @since 1.3
101  * @author Oliver Heger
102  * @version $Id: XPathExpressionEngine.java 466413 2006-10-21 15:23:45Z oheger $
103  */
104 public class XPathExpressionEngine implements ExpressionEngine
105 {
106     /*** Constant for the path delimiter. */
107     static final String PATH_DELIMITER = "/";
108 
109     /*** Constant for the attribute delimiter. */
110     static final String ATTR_DELIMITER = "@";
111 
112     /*** Constant for the delimiters for splitting node paths. */
113     private static final String NODE_PATH_DELIMITERS = PATH_DELIMITER
114             + ATTR_DELIMITER;
115 
116     /***
117      * Executes a query. The passed in property key is directly passed to a
118      * JXPath context.
119      *
120      * @param root the configuration root node
121      * @param key the query to be executed
122      * @return a list with the nodes that are selected by the query
123      */
124     public List query(ConfigurationNode root, String key)
125     {
126         if (StringUtils.isEmpty(key))
127         {
128             List result = new ArrayList(1);
129             result.add(root);
130             return result;
131         }
132         else
133         {
134             JXPathContext context = createContext(root, key);
135             List result = context.selectNodes(key);
136             return (result != null) ? result : Collections.EMPTY_LIST;
137         }
138     }
139 
140     /***
141      * Returns a (canonic) key for the given node based on the parent's key.
142      * This implementation will create an XPATH expression that selects the
143      * given node (under the assumption that the passed in parent key is valid).
144      * As the <code>nodeKey()</code> implementation of
145      * <code>{@link org.apache.commons.configuration.tree.DefaultExpressionEngine DefaultExpressionEngine}</code>
146      * this method will not return indices for nodes. So all child nodes of a
147      * given parent whith the same name will have the same key.
148      *
149      * @param node the node for which a key is to be constructed
150      * @param parentKey the key of the parent node
151      * @return the key for the given node
152      */
153     public String nodeKey(ConfigurationNode node, String parentKey)
154     {
155         if (parentKey == null)
156         {
157             // name of the root node
158             return StringUtils.EMPTY;
159         }
160         else if (node.getName() == null)
161         {
162             // paranoia check for undefined node names
163             return parentKey;
164         }
165 
166         else
167         {
168             StringBuffer buf = new StringBuffer(parentKey.length()
169                     + node.getName().length() + PATH_DELIMITER.length());
170             if (parentKey.length() > 0)
171             {
172                 buf.append(parentKey);
173                 buf.append(PATH_DELIMITER);
174             }
175             if (node.isAttribute())
176             {
177                 buf.append(ATTR_DELIMITER);
178             }
179             buf.append(node.getName());
180             return buf.toString();
181         }
182     }
183 
184     /***
185      * Prepares an add operation for a configuration property. The expected
186      * format of the passed in key is explained in the class comment.
187      *
188      * @param root the configuration's root node
189      * @param key the key describing the target of the add operation and the
190      * path of the new node
191      * @return a data object to be evaluated by the calling configuration object
192      */
193     public NodeAddData prepareAdd(ConfigurationNode root, String key)
194     {
195         if (key == null)
196         {
197             throw new IllegalArgumentException(
198                     "prepareAdd: key must not be null!");
199         }
200 
201         int index = key.length() - 1;
202         while (index >= 0 && !Character.isWhitespace(key.charAt(index)))
203         {
204             index--;
205         }
206         if (index < 0)
207         {
208             throw new IllegalArgumentException(
209                     "prepareAdd: Passed in key must contain a whitespace!");
210         }
211 
212         List nodes = query(root, key.substring(0, index).trim());
213         if (nodes.size() != 1)
214         {
215             throw new IllegalArgumentException(
216                     "prepareAdd: key must select exactly one target node!");
217         }
218 
219         NodeAddData data = new NodeAddData();
220         data.setParent((ConfigurationNode) nodes.get(0));
221         initNodeAddData(data, key.substring(index).trim());
222         return data;
223     }
224 
225     /***
226      * Creates the <code>JXPathContext</code> used for executing a query. This
227      * method will create a new context and ensure that it is correctly
228      * initialized.
229      *
230      * @param root the configuration root node
231      * @param key the key to be queried
232      * @return the new context
233      */
234     protected JXPathContext createContext(ConfigurationNode root, String key)
235     {
236         JXPathContext context = JXPathContext.newContext(root);
237         context.setLenient(true);
238         return context;
239     }
240 
241     /***
242      * Initializes most properties of a <code>NodeAddData</code> object. This
243      * method is called by <code>prepareAdd()</code> after the parent node has
244      * been found. Its task is to interprete the passed in path of the new node.
245      *
246      * @param data the data object to initialize
247      * @param path the path of the new node
248      */
249     protected void initNodeAddData(NodeAddData data, String path)
250     {
251         String lastComponent = null;
252         boolean attr = false;
253         boolean first = true;
254 
255         StringTokenizer tok = new StringTokenizer(path, NODE_PATH_DELIMITERS,
256                 true);
257         while (tok.hasMoreTokens())
258         {
259             String token = tok.nextToken();
260             if (PATH_DELIMITER.equals(token))
261             {
262                 if (attr)
263                 {
264                     invalidPath(path, " contains an attribute"
265                             + " delimiter at an unallowed position.");
266                 }
267                 if (lastComponent == null)
268                 {
269                     invalidPath(path,
270                             " contains a '/' at an unallowed position.");
271                 }
272                 data.addPathNode(lastComponent);
273                 lastComponent = null;
274             }
275 
276             else if (ATTR_DELIMITER.equals(token))
277             {
278                 if (attr)
279                 {
280                     invalidPath(path,
281                             " contains multiple attribute delimiters.");
282                 }
283                 if (lastComponent == null && !first)
284                 {
285                     invalidPath(path,
286                             " contains an attribute delimiter at an unallowed position.");
287                 }
288                 if (lastComponent != null)
289                 {
290                     data.addPathNode(lastComponent);
291                 }
292                 attr = true;
293                 lastComponent = null;
294             }
295 
296             else
297             {
298                 lastComponent = token;
299             }
300             first = false;
301         }
302 
303         if (lastComponent == null)
304         {
305             invalidPath(path, "contains no components.");
306         }
307         data.setNewNodeName(lastComponent);
308         data.setAttribute(attr);
309     }
310 
311     /***
312      * Helper method for throwing an exception about an invalid path.
313      *
314      * @param path the invalid path
315      * @param msg the exception message
316      */
317     private void invalidPath(String path, String msg)
318     {
319         throw new IllegalArgumentException("Invalid node path: \"" + path
320                 + "\" " + msg);
321     }
322 
323     // static initializer: registers the configuration node pointer factory
324     static
325     {
326         JXPathContextReferenceImpl
327                 .addNodePointerFactory(new ConfigurationNodePointerFactory());
328     }
329 }