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evaluation of XPath expressions and access to the evaluation

environment.

<hr />

<p>The XPath language provides a simple, concise syntax for selecting

nodes from an XML document. XPath also provides rules for converting a

node in an XML document object model (DOM) tree to a boolean, double,

or string value. XPath is a W3C-defined language and an official W3C

recommendation; the W3C hosts the XML Path Language (XPath) Version

1.0 specification.

</p>

<p>XPath started in life in 1999 as a supplement to the XSLT and

XPointer languages, but has more recently become popular as a

stand-alone language, as a single XPath expression can be used to

replace many lines of DOM API code.

</p>

<p>An XPath <em>expression</em> is composed of a <em>location

path</em> and one or more optional <em>predicates</em>. Expressions

may also include XPath variables.

</p>

<p>The following is an example of a simple XPath expression:</p>

<pre>

/foo/bar

</pre>

<p>This example would select the <code>&lt;bar&gt;</code> element in

an XML document such as the following:</p>

<pre>

<foo>

</foo>

</pre>

<p>The expression <code>/foo/bar</code> is an example of a location

path. While XPath location paths resemble Unix-style file system

paths, an important distinction is that XPath expressions return

<em>all</em> nodes that match the expression. Thus, all three

<code>&lt;bar&gt;</code> elements in the following document would be

selected by the <code>/foo/bar</code> expression:</p>

<pre>

<foo>

</foo>

</pre>

<p>A special location path operator, <code>//</code>, selects nodes at

any depth in an XML document. The following example selects all

<code>&lt;bar&gt;</code> elements regardless of their location in a

document:</p>

<pre>

//bar

</pre>

<p>A wildcard operator, *, causes all element nodes to be selected.

The following example selects all children elements of a

<pre>

/foo/*

</pre>

<p>In addition to element nodes, XPath location paths may also address

attribute nodes, text nodes, comment nodes, and processing instruction

nodes. The following table gives examples of location paths for each

of these node types:</p>

<table border="1">

<tr>

<td>Location Path</td>

<td>Description</td>

</tr>

<tr>

<td>

<code>/foo/bar/<strong>@id</strong></code>

</td>

<td>Selects the attribute <code>id</code> of the <code>&lt;bar&gt;</code> element

</td>

</tr>

<tr>

<td><code>/foo/bar/<strong>text()</strong></code>

</td>

<td>Selects the text nodes of the <code>&lt;bar&gt;</code> element. No

distinction is made between escaped and non-escaped character data.

</td>

</tr>

<tr>

<td><code>/foo/bar/<strong>comment()</strong></code>

</td>

<td>Selects all comment nodes contained in the <code>&lt;bar&gt;</code> element.

</td>

</tr>

<tr>

<td><code>/foo/bar/<strong>processing-instruction()</strong></code>

</td>

<td>Selects all processing-instruction nodes contained in the

<code>&lt;bar&gt;</code> element.

</td>

</tr>

</table>

<p>Predicates allow for refining the nodes selected by an XPath

location path. Predicates are of the form

<code>[<em>expression</em>]</code>. The following example selects all

<code>&lt;foo&gt;</code> elements that contain an <code>include</code>

attribute with the value of <code>true</code>:</p>

<pre>

//foo[@include='true']

</pre>

<p>Predicates may be appended to each other to further refine an

expression, such as:</p>

<pre>

//foo[@include='true'][@mode='bar']

</pre>

<p>While XPath expressions select nodes in the XML document, the XPath

API allows the selected nodes to be coalesced into one of the

<ul>

<li><code>Boolean</code></li>

<li><code>Number</code></li>

<li><code>String</code></li>

</ul>

<ul>

<li>{@link javax.xml.xpath.XPathConstants#NODESET}</li>

<li>{@link javax.xml.xpath.XPathConstants#NODE}</li>

<li>{@link javax.xml.xpath.XPathConstants#STRING}</li>

<li>{@link javax.xml.xpath.XPathConstants#BOOLEAN}</li>

<li>{@link javax.xml.xpath.XPathConstants#NUMBER}</li>

</ul>

<p>When a <code>Boolean</code> return type is requested,

<code>Boolean.TRUE</code> is returned if one or more nodes were

<p>The <code>String</code> return type is a convenience for retrieving

the character data from a text node, attribute node, comment node, or

processing-instruction node. When used on an element node, the value

of the child text nodes is returned.

<p>The <code>Number</code> return type attempts to coalesce the text

of a node to a <code>double</code> data type.

<p>XPath location paths may be relative to a particular node in the

<pre>

<widgets>

<widget>

<manufacturer/>

<dimensions/>

</widget>

</widgets>

</pre>

<pre>

// parse the XML as a W3C Document

DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();

Document document = builder.parse(new File("/widgets.xml"));

XPath xpath = XPathFactory.newInstance().newXPath();

String expression = "/widgets/widget";

Node widgetNode = (Node) xpath.evaluate(expression, document, XPathConstants.NODE);

</pre>

<p>With a reference to the <code>&lt;widget&gt;</code> element, a

<code>&lt;manufacturer&gt;</code> child element:</p>

<pre>

XPath xpath = XPathFactory.newInstance().newXPath();

<strong>String expression = "manufacturer";</strong>

Node manufacturerNode = (Node) xpath.evaluate(expression, <strong>widgetNode</strong>, XPathConstants.NODE);

</pre>

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