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30
31 This package provides an <em>object-model neutral</em> API for the
32 evaluation of XPath expressions and access to the evaluation
33 environment.
34
35 <p>
36 The XPath API supports <a href="http://www.w3.org/TR/xpath">
37 XML Path Language (XPath) Version 1.0</a>
38
39 <hr />
40
41 <ul>
42 <li><a href='#XPath.Overview'>1. XPath Overview</a></li>
43 <li><a href='#XPath.Expressions'>2. XPath Expressions</a></li>
44 <li><a href='#XPath.Datatypes'>3. XPath Data Types</a>
45 <ul>
46 <li><a href='#XPath.Datatypes.QName'>3.1 QName Types</a>
47 <li><a href='#XPath.Datatypes.Class'>3.2 Class Types</a>
48 <li><a href='#XPath.Datatypes.Enum'>3.3 Enum Types</a>
49 </ul>
50 </li>
51 <li><a href='#XPath.Context'>4. XPath Context</a></li>
52 <li><a href='#XPath.Use'>5. Using the XPath API</a></li>
53 </ul>
54 <p>
55 <a name="XPath.Overview"></a>
56 <h3>1. XPath Overview</h3>
57
58 <p>The XPath language provides a simple, concise syntax for selecting
59 nodes from an XML document. XPath also provides rules for converting a
60 node in an XML document object model (DOM) tree to a boolean, double,
61 or string value. XPath is a W3C-defined language and an official W3C
62 recommendation; the W3C hosts the XML Path Language (XPath) Version
63 1.0 specification.
64 </p>
65
66 <p>XPath started in life in 1999 as a supplement to the XSLT and
67 XPointer languages, but has more recently become popular as a
68 stand-alone language, as a single XPath expression can be used to
69 replace many lines of DOM API code.
70 </p>
71
72 <a name="XPath.Expressions"></a>
73 <h3>2. XPath Expressions</h3>
74
75 <p>An XPath <em>expression</em> is composed of a <em>location
76 path</em> and one or more optional <em>predicates</em>. Expressions
77 may also include XPath variables.
78 </p>
79
80 <p>The following is an example of a simple XPath expression:</p>
81
82 <blockquote>
83 <pre>
84 /foo/bar
85 </pre>
86 </blockquote>
87
88 <p>This example would select the <code><bar></code> element in
89 an XML document such as the following:</p>
90
91 <blockquote>
92 <pre>
93 <foo>
94 <bar/>
95 </foo>
96 </pre>
97 </blockquote>
98
99 <p>The expression <code>/foo/bar</code> is an example of a location
100 path. While XPath location paths resemble Unix-style file system
101 paths, an important distinction is that XPath expressions return
102 <em>all</em> nodes that match the expression. Thus, all three
103 <code><bar></code> elements in the following document would be
104 selected by the <code>/foo/bar</code> expression:</p>
105
106 <blockquote>
107 <pre>
108 <foo>
109 <bar/>
110 <bar/>
111 <bar/>
112 </foo>
113 </pre>
114 </blockquote>
115
116 <p>A special location path operator, <code>//</code>, selects nodes at
117 any depth in an XML document. The following example selects all
118 <code><bar></code> elements regardless of their location in a
119 document:</p>
120
121 <blockquote>
122 <pre>
123 //bar
124 </pre>
125 </blockquote>
126
127 <p>A wildcard operator, *, causes all element nodes to be selected.
128 The following example selects all children elements of a
129 <code><foo></code> element:
130
131 <blockquote>
132 <pre>
133 /foo/*
134 </pre>
135 </blockquote>
136
137 <p>In addition to element nodes, XPath location paths may also address
138 attribute nodes, text nodes, comment nodes, and processing instruction
139 nodes. The following table gives examples of location paths for each
140 of these node types:</p>
141
142 <table border="1">
143 <tr>
144 <td>Location Path</td>
145 <td>Description</td>
146 </tr>
147 <tr>
148 <td>
149 <code>/foo/bar/<strong>@id</strong></code>
150 </td>
151 <td>Selects the attribute <code>id</code> of the <code><bar></code> element
152 </td>
153 </tr>
154 <tr>
155 <td><code>/foo/bar/<strong>text()</strong></code>
156 </td>
157 <td>Selects the text nodes of the <code><bar></code> element. No
158 distinction is made between escaped and non-escaped character data.
159 </td>
160 </tr>
161 <tr>
162 <td><code>/foo/bar/<strong>comment()</strong></code>
163 </td>
164 <td>Selects all comment nodes contained in the <code><bar></code> element.
165 </td>
166 </tr>
167 <tr>
168 <td><code>/foo/bar/<strong>processing-instruction()</strong></code>
169 </td>
170 <td>Selects all processing-instruction nodes contained in the
171 <code><bar></code> element.
172 </td>
173 </tr>
174 </table>
175
176 <p>Predicates allow for refining the nodes selected by an XPath
177 location path. Predicates are of the form
178 <code>[<em>expression</em>]</code>. The following example selects all
179 <code><foo></code> elements that contain an <code>include</code>
180 attribute with the value of <code>true</code>:</p>
181
182 <blockquote>
183 <pre>
184 //foo[@include='true']
185 </pre>
186 </blockquote>
187
188 <p>Predicates may be appended to each other to further refine an
189 expression, such as:</p>
190
191 <blockquote>
192 <pre>
193 //foo[@include='true'][@mode='bar']
194 </pre>
195 </blockquote>
196
197 <a name="XPath.Datatypes"></a>
198 <h3>3. XPath Data Types</h3>
199
200 <p>While XPath expressions select nodes in the XML document, the XPath
201 API allows the selected nodes to be coalesced into one of the
202 following data types:</p>
203
204 <ul>
205 <li><code>Boolean</code></li>
206 <li><code>Number</code></li>
207 <li><code>String</code></li>
208 </ul>
209
210 <a name="XPath.Datatypes.QName"></a>
211 <h3>3.1 QName types</h3>
212 The XPath API defines the following {@link javax.xml.namespace.QName} types to
213 represent return types of an XPath evaluation:
214 <ul>
215 <li>{@link javax.xml.xpath.XPathConstants#NODESET}</li>
216 <li>{@link javax.xml.xpath.XPathConstants#NODE}</li>
217 <li>{@link javax.xml.xpath.XPathConstants#STRING}</li>
218 <li>{@link javax.xml.xpath.XPathConstants#BOOLEAN}</li>
219 <li>{@link javax.xml.xpath.XPathConstants#NUMBER}</li>
220 </ul>
221
222 <p>The return type is specified by a {@link javax.xml.namespace.QName} parameter
223 in method call used to evaluate the expression, which is either a call to
224 <code>XPathExpression.evalute(...)</code> or <code>XPath.evaluate(...)</code>
225 methods.
226
227 <p>When a <code>Boolean</code> return type is requested,
228 <code>Boolean.TRUE</code> is returned if one or more nodes were
229 selected; otherwise, <code>Boolean.FALSE</code> is returned.
230
231 <p>The <code>String</code> return type is a convenience for retrieving
232 the character data from a text node, attribute node, comment node, or
233 processing-instruction node. When used on an element node, the value
234 of the child text nodes is returned.
235
236 <p>The <code>Number</code> return type attempts to coalesce the text
237 of a node to a <code>double</code> data type.
238
239 <a name="XPath.Datatypes.Class"></a>
240 <h3>3.2 Class types</h3>
241 In addition to the QName types, the XPath API supports the use of Class types
242 through the <code>XPathExpression.evaluteExpression(...)</code> or
243 <code>XPath.evaluateExpression(...)</code> methods.
244
245 The XPath data types are mapped to Class types as follows:
246 <ul>
247 <li><code>Boolean</code> -- <code>Boolean.class</code></li>
248 <li><code>Number</code> -- <code>Number.class</code></li>
249 <li><code>String</code> -- <code>String.class</code></li>
250 <li><code>Nodeset</code> -- <code>XPathNodes.class</code></li>
251 <li><code>Node</code> -- <code>Node.class</code></li>
252 </ul>
253
254 <p>
255 Of the subtypes of Number, only Double, Integer and Long are supported.
256
257 <a name="XPath.Datatypes.Enum"></a>
258 <h3>3.3 Enum types</h3>
259 Enum types are defined in {@link javax.xml.xpath.XPathEvaluationResult.XPathResultType}
260 that provide mappings between the QName and Class types above. The result of
261 evaluating an expression using the <code>XPathExpression.evaluteExpression(...)</code>
262 or <code>XPath.evaluateExpression(...)</code> methods will be of one of these types.
263
264 <a name="XPath.Context"></a>
265 <h3>4. XPath Context</h3>
266
267 <p>XPath location paths may be relative to a particular node in the
268 document, known as the <code>context</code>. A context consists of:
269 <ul>
270 <li>a node (the context node)</li>
271 <li>a pair of non-zero positive integers (the context position and the context size)</li>
272 <li>a set of variable bindings</li>
273 <li>a function library</li>
274 <li>the set of namespace declarations in scope for the expression</li>
275 </ul>
276
277 <p>
278 It is an XML document tree represented as a hierarchy of nodes, a
279 {@link org.w3c.dom.Node} for example, in the JDK implementation.
280
281 <a name="XPath.Use"></a>
282 <h3>5. Using the XPath API</h3>
283
284 Consider the following XML document:
285 <blockquote>
286 <pre>
287 <widgets>
288 <widget>
289 <manufacturer/>
290 <dimensions/>
291 </widget>
292 </widgets>
293 </pre>
294 </blockquote>
295
296 <p>
297 The <code><widget></code> element can be selected with the following process:
298
299 <blockquote>
300 <pre>
301 // parse the XML as a W3C Document
302 DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
303 Document document = builder.parse(new File("/widgets.xml"));
304
305 //Get an XPath object and evaluate the expression
306 XPath xpath = XPathFactory.newInstance().newXPath();
307 String expression = "/widgets/widget";
308 Node widgetNode = (Node) xpath.evaluate(expression, document, XPathConstants.NODE);
309
310 //or using the evaluateExpression method
311 Node widgetNode = xpath.evaluateExpression(expression, document, Node.class);
312 </pre>
313 </blockquote>
314
315 <p>With a reference to the <code><widget></code> element, a
316 relative XPath expression can be written to select the
317 <code><manufacturer></code> child element:</p>
318
319 <blockquote>
320 <pre>
321 XPath xpath = XPathFactory.newInstance().newXPath();
322 <strong>String expression = "manufacturer";</strong>
323 Node manufacturerNode = (Node) xpath.evaluate(expression, <strong>widgetNode</strong>, XPathConstants.NODE);
324
325 //or using the evaluateExpression method
326 Node manufacturerNode = xpath.evaluateExpression(expression, <strong>widgetNode</strong>, Node.class);
327 </pre>
328 </blockquote>
329
330 <p>
331 In the above example, the XML file is read into a DOM Document before being passed
332 to the XPath API. The following code demonstrates the use of InputSource to
333 leave it to the XPath implementation to process it:
334
335 <blockquote>
336 <pre>
337 XPath xpath = XPathFactory.newInstance().newXPath();
338 String expression = "/widgets/widget";
339 InputSource inputSource = new InputSource("widgets.xml");
340 NodeList nodes = (NodeList) xpath.evaluate(expression, inputSource, XPathConstants.NODESET);
341
342 //or using the evaluateExpression method
343 XPathNodes nodes = xpath.evaluate(expression, inputSource, XPathNodes.class);
344 </pre>
345 </blockquote>
346
347 <p>
348 In the above cases, the type of the expected results are known. In case where
349 the result type is unknown or any type, the {@link javax.xml.xpath.XPathEvaluationResult}
350 may be used to determine the return type. The following code demonstrates the usage:
351 <blockquote>
352 <pre>
353 XPathEvaluationResult<?> result = xpath.evaluateExpression(expression, document);
354 switch (result.type()) {
355 case NODESET:
356 XPathNodes nodes = (XPathNodes)result.value();
357 ...
358 break;
359 }
360 </pre>
361 </blockquote>
362
363 <p>
364 The XPath 1.0 Number data type is defined as a double. However, the XPath
365 specification also provides functions that returns Integer type. To facilitate
366 such operations, the XPath API allows Integer and Long to be used in
367 {@code evaluateExpression} method such as the following code:
368 <blockquote>
369 <pre>
370 int count = xpath.evaluate("count(/widgets/widget)", document, Integer.class);
371 </pre>
372 </blockquote>
373
374 @since 1.5
375
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