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  27 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  28                       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  29 <html xmlns="http://www.w3.org/1999/xhtml">
  30 <head>
  31 <title>javax.xml.xpath</title>
  32 <meta name="@author" content="mailto:Ben@galbraiths.org" />
  33 <meta name="@author" content="mailto:Norman.Walsh@Sun.com" />
  34 <meta name="@author" content="mailto:Jeff.Suttor@Sun.com" />
  35 <meta name="@see" content="http://www.w3.org/TR/xpath" />
  36 <meta name="@since" content="1.5" />
  37 </head>
  38 
  39 <body>
  40 
  41 <p>This package provides an <em>object-model neutral</em> API for the
  42 evaluation of XPath expressions and access to the evaluation
  43 environment.
  44 </p>
  45 
  46 <p>The following XML standards apply:</p>
  47 
  48 <ul>
  49 <li><a href="http://www.w3.org/TR/xpath">XML Path Language (XPath) Version 1.0</a></li>
  50 </ul>
  51 
  52 <hr />
  53 
  54 <h2>XPath Overview</h2>
  55 
  56 <p>The XPath language provides a simple, concise syntax for selecting
  57 nodes from an XML document. XPath also provides rules for converting a
  58 node in an XML document object model (DOM) tree to a boolean, double,
  59 or string value. XPath is a W3C-defined language and an official W3C
  60 recommendation; the W3C hosts the XML Path Language (XPath) Version
  61 1.0 specification.
  62 </p>
  63 
  64 <p>XPath started in life in 1999 as a supplement to the XSLT and
  65 XPointer languages, but has more recently become popular as a
  66 stand-alone language, as a single XPath expression can be used to
  67 replace many lines of DOM API code.
  68 </p>
  69 
  70 <h3>XPath Expressions</h3>
  71 
  72 <p>An XPath <em>expression</em> is composed of a <em>location
  73 path</em> and one or more optional <em>predicates</em>. Expressions
  74 may also include XPath variables.
  75 </p>
  76 
  77 <p>The following is an example of a simple XPath expression:</p>
  78 
  79 <pre>
  80 /foo/bar
  81 </pre>
  82 
  83 <p>This example would select the <code>&lt;bar&gt;</code> element in
  84 an XML document such as the following:</p>
  85 
  86 <pre>
  87 &lt;foo&gt;
  88 &lt;bar/&gt;
  89 &lt;/foo&gt;
  90 </pre>
  91 
  92 <p>The expression <code>/foo/bar</code> is an example of a location
  93 path. While XPath location paths resemble Unix-style file system
  94 paths, an important distinction is that XPath expressions return
  95 <em>all</em> nodes that match the expression. Thus, all three
  96 <code>&lt;bar&gt;</code> elements in the following document would be
  97 selected by the <code>/foo/bar</code> expression:</p>
  98 
  99 <pre>
 100 &lt;foo&gt;
 101 &lt;bar/&gt;
 102 &lt;bar/&gt;
 103 &lt;bar/&gt;
 104 &lt;/foo&gt;
 105 </pre>
 106 
 107 <p>A special location path operator, <code>//</code>, selects nodes at
 108 any depth in an XML document. The following example selects all
 109 <code>&lt;bar&gt;</code> elements regardless of their location in a
 110 document:</p>
 111 
 112 <pre>
 113 //bar
 114 </pre>
 115 
 116 <p>A wildcard operator, *, causes all element nodes to be selected.
 117 The following example selects all children elements of a
 118 <code>&lt;foo&gt;</code> element:</p>
 119 
 120 <pre>
 121 /foo/*
 122 </pre>
 123 
 124 <p>In addition to element nodes, XPath location paths may also address
 125 attribute nodes, text nodes, comment nodes, and processing instruction
 126 nodes. The following table gives examples of location paths for each
 127 of these node types:</p>
 128 
 129 <table border="1">
 130 <tr>
 131 <td>Location Path</td>
 132 <td>Description</td>
 133 </tr>
 134 <tr>
 135 <td>
 136 <code>/foo/bar/<strong>@id</strong></code>
 137 </td>
 138 <td>Selects the attribute <code>id</code> of the <code>&lt;bar&gt;</code> element
 139 </td>
 140 </tr>
 141 <tr>
 142 <td><code>/foo/bar/<strong>text()</strong></code>
 143 </td>
 144 <td>Selects the text nodes of the <code>&lt;bar&gt;</code> element. No
 145 distinction is made between escaped and non-escaped character data.
 146 </td>
 147 </tr>
 148 <tr>
 149 <td><code>/foo/bar/<strong>comment()</strong></code>
 150 </td>
 151 <td>Selects all comment nodes contained in the <code>&lt;bar&gt;</code> element.
 152 </td>
 153 </tr>
 154 <tr>
 155 <td><code>/foo/bar/<strong>processing-instruction()</strong></code>
 156 </td>
 157 <td>Selects all processing-instruction nodes contained in the
 158 <code>&lt;bar&gt;</code> element.
 159 </td>
 160 </tr>
 161 </table>
 162 
 163 <p>Predicates allow for refining the nodes selected by an XPath
 164 location path. Predicates are of the form
 165 <code>[<em>expression</em>]</code>. The following example selects all
 166 <code>&lt;foo&gt;</code> elements that contain an <code>include</code>
 167 attribute with the value of <code>true</code>:</p>
 168 
 169 <pre>
 170 //foo[@include='true']
 171 </pre>
 172 
 173 <p>Predicates may be appended to each other to further refine an
 174 expression, such as:</p>
 175 
 176 <pre>
 177 //foo[@include='true'][@mode='bar']
 178 </pre>
 179 
 180 <h3>Using the XPath API</h3>
 181 
 182 <p>
 183 The following example demonstrates using the XPath API to select one
 184 or more nodes from an XML document:</p>
 185 
 186 <pre>
 187 XPath xpath = XPathFactory.newInstance().newXPath();
 188 String expression = "/widgets/widget";
 189 InputSource inputSource = new InputSource("widgets.xml");
 190 NodeList nodes = (NodeList) xpath.evaluate(expression, inputSource, XPathConstants.NODESET);
 191 </pre>
 192 
 193 <h3>XPath Expressions and Types</h3>
 194 
 195 <p>While XPath expressions select nodes in the XML document, the XPath
 196 API allows the selected nodes to be coalesced into one of the
 197 following other data types:</p>
 198 
 199 <ul>
 200 <li><code>Boolean</code></li>
 201 <li><code>Number</code></li>
 202 <li><code>String</code></li>
 203 </ul>
 204 
 205 <p>The desired return type is specified by a {@link
 206 javax.xml.namespace.QName} parameter in method call used to evaluate
 207 the expression, which is either a call to
 208 <code>XPathExpression.evalute(...)</code> or to one of the
 209 <code>XPath.evaluate(...)</code> convenience methods. The allowed
 210 QName values are specified as constants in the {@link
 211 javax.xml.xpath.XPathConstants} class; they are:</p>
 212 
 213 <ul>
 214 <li>{@link javax.xml.xpath.XPathConstants#NODESET}</li>
 215 <li>{@link javax.xml.xpath.XPathConstants#NODE}</li>
 216 <li>{@link javax.xml.xpath.XPathConstants#STRING}</li>
 217 <li>{@link javax.xml.xpath.XPathConstants#BOOLEAN}</li>
 218 <li>{@link javax.xml.xpath.XPathConstants#NUMBER}</li>
 219 </ul>
 220 
 221 <p>When a <code>Boolean</code> return type is requested,
 222 <code>Boolean.TRUE</code> is returned if one or more nodes were
 223 selected; otherwise, <code>Boolean.FALSE</code> is returned.</p>
 224 
 225 <p>The <code>String</code> return type is a convenience for retrieving
 226 the character data from a text node, attribute node, comment node, or
 227 processing-instruction node. When used on an element node, the value
 228 of the child text nodes is returned.
 229 </p>
 230 
 231 <p>The <code>Number</code> return type attempts to coalesce the text
 232 of a node to a <code>double</code> data type.
 233 </p>
 234 
 235 <h3>XPath Context</h3>
 236 
 237 <p>XPath location paths may be relative to a particular node in the
 238 document, known as the <code>context</code>. Consider the following
 239 XML document:</p>
 240 
 241 <pre>
 242 &lt;widgets&gt;
 243 &lt;widget&gt;
 244 &lt;manufacturer/&gt;
 245 &lt;dimensions/&gt;
 246 &lt;/widget&gt;
 247 &lt;/widgets&gt;
 248 </pre>
 249 
 250 <p>The <code>&lt;widget&gt;</code> element can be selected with the
 251 following XPath API code:</p>
 252 
 253 <pre>
 254 // parse the XML as a W3C Document
 255 DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
 256 Document document = builder.parse(new File("/widgets.xml"));
 257 
 258 XPath xpath = XPathFactory.newInstance().newXPath();
 259 String expression = "/widgets/widget";
 260 Node widgetNode = (Node) xpath.evaluate(expression, document, XPathConstants.NODE);
 261 </pre>
 262 
 263 <p>With a reference to the <code>&lt;widget&gt;</code> element, a
 264 relative XPath expression can now written to select the
 265 <code>&lt;manufacturer&gt;</code> child element:</p>
 266 
 267 <pre>
 268 XPath xpath = XPathFactory.newInstance().newXPath();
 269 <strong>String expression = "manufacturer";</strong>
 270 Node manufacturerNode = (Node) xpath.evaluate(expression, <strong>widgetNode</strong>, XPathConstants.NODE);
 271 </pre>
 272 
 273 <ul>
 274 <li>Author <a href="mailto:Ben@galbraiths.org">Ben Galbraith</a></li>
 275 <li>Author <a href="mailto:Norman.Walsh@Sun.com">Norman Walsh</a></li>
 276 <li>Author <a href="mailto:Jeff.Suttor@Sun.com">Jeff Suttor</a></li>
 277 <li>See <a href="http://www.w3.org/TR/xpath">XML Path Language (XPath) Version 1.0</a></li>
 278 <li>Since 1.5</li>
 279 </ul>             
 280 </body>
 281 </html>