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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>&lt;bar&gt;</code> element in
  89 an XML document such as the following:</p>
  90 
  91 <blockquote>
  92 <pre>
  93 &lt;foo&gt;
  94     &lt;bar/&gt;
  95 &lt;/foo&gt;
  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>&lt;bar&gt;</code> elements in the following document would be
 104 selected by the <code>/foo/bar</code> expression:</p>
 105 
 106 <blockquote>
 107 <pre>
 108 &lt;foo&gt;
 109     &lt;bar/&gt;
 110     &lt;bar/&gt;
 111     &lt;bar/&gt;
 112 &lt;/foo&gt;
 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>&lt;bar&gt;</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>&lt;foo&gt;</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>&lt;bar&gt;</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>&lt;bar&gt;</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>&lt;bar&gt;</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>&lt;bar&gt;</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>&lt;foo&gt;</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 <p>
 286 <blockquote>
 287 <pre>
 288 &lt;widgets&gt;
 289 &lt;widget&gt;
 290 &lt;manufacturer/&gt;
 291 &lt;dimensions/&gt;
 292 &lt;/widget&gt;
 293 &lt;/widgets&gt;
 294 </pre>
 295 </blockquote>
 296 
 297 <p>
 298 The <code>&lt;widget&gt;</code> element can be selected with the following process:
 299 
 300 <blockquote>
 301 <pre>
 302 // parse the XML as a W3C Document
 303 DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
 304 Document document = builder.parse(new File("/widgets.xml"));
 305 
 306 //Get an XPath object and evaluate the expression
 307 XPath xpath = XPathFactory.newInstance().newXPath();
 308 String expression = "/widgets/widget";
 309 Node widgetNode = (Node) xpath.evaluate(expression, document, XPathConstants.NODE);
 310 
 311 //or using the evaluateExpression method
 312 Node widgetNode = xpath.evaluateExpression(expression, document, Node.class);
 313 </pre>
 314 </blockquote>
 315 
 316 <p>With a reference to the <code>&lt;widget&gt;</code> element, a
 317 relative XPath expression can be written to select the
 318 <code>&lt;manufacturer&gt;</code> child element:</p>
 319 
 320 <blockquote>
 321 <pre>
 322 XPath xpath = XPathFactory.newInstance().newXPath();
 323 <strong>String expression = "manufacturer";</strong>
 324 Node manufacturerNode = (Node) xpath.evaluate(expression, <strong>widgetNode</strong>, XPathConstants.NODE);
 325 
 326 //or using the evaluateExpression method
 327 Node manufacturerNode = xpath.evaluateExpression(expression, <strong>widgetNode</strong>, Node.class);
 328 </pre>
 329 </blockquote>
 330 
 331 <p>
 332 In the above example, the XML file is read into a DOM Document before being passed
 333 to the XPath API. The following code demonstrates the use of InputSource to 
 334 leave it to the XPath implementation to process it:
 335 
 336 <blockquote>
 337 <pre>
 338 XPath xpath = XPathFactory.newInstance().newXPath();
 339 String expression = "/widgets/widget";
 340 InputSource inputSource = new InputSource("widgets.xml");
 341 NodeList nodes = (NodeList) xpath.evaluate(expression, inputSource, XPathConstants.NODESET);
 342 
 343 //or using the evaluateExpression method
 344 XPathNodes nodes = xpath.evaluate(expression, inputSource, XPathNodes.class);
 345 </pre>
 346 </blockquote>
 347 
 348 <p>
 349 In the above cases, the type of the expected results are known. In case where
 350 the result type is unknown or any type, the {@link javax.xml.xpath.XPathEvaluationResult}
 351 may be used to determine the return type. The following code demonstrates the usage:
 352 <blockquote>
 353 <pre>
 354 XPathEvaluationResult&lt;?&gt; result = xpath.evaluateExpression(expression, document);
 355 switch (result.type()) {
 356     case NODESET:
 357         XPathNodes nodes = (XPathNodes)result.value();
 358         ...
 359         break;
 360 }
 361 </pre>
 362 </blockquote>
 363 
 364 <p>
 365 The XPath 1.0 Number data type is defined as a double. However, the XPath 
 366 specification also provides functions that returns Integer type. To facilitate
 367 such operations, the XPath API allows Integer and Long to be used in 
 368 {@code evaluateExpression} method such as the following code: 
 369 <p>
 370 <blockquote>
 371 <pre>
 372 int count = xpath.evaluate("count(/widgets/widget)", document, Integer.class);
 373 </pre>
 374 </blockquote>
 375 
 376 @since 1.5
 377 
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