Package javax.xml.xpath
The XPath API supports XML Path Language (XPath) Version 1.0
1. XPath Overview
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.
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.
2. XPath Expressions
An XPath expression is composed of a location path and one or more optional predicates. Expressions may also include XPath variables.
The following is an example of a simple XPath expression:
/foo/bar
This example would select the <bar>
element in
an XML document such as the following:
<foo> <bar/> </foo>
The expression /foo/bar
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
all nodes that match the expression. Thus, all three
<bar>
elements in the following document would be
selected by the /foo/bar
expression:
<foo> <bar/> <bar/> <bar/> </foo>
A special location path operator, //
, selects nodes at
any depth in an XML document. The following example selects all
<bar>
elements regardless of their location in a
document:
//bar
A wildcard operator, *, causes all element nodes to be selected.
The following example selects all children elements of a
<foo>
element:
/foo/*
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:
Location Path | Description |
---|---|
/foo/bar/@id
|
Selects the attribute id of the <bar> element
|
/foo/bar/text()
|
Selects the text nodes of the <bar> element. No
distinction is made between escaped and non-escaped character data.
|
/foo/bar/comment()
|
Selects all comment nodes contained in the <bar> element.
|
/foo/bar/processing-instruction()
|
Selects all processing-instruction nodes contained in the
<bar> element.
|
Predicates allow for refining the nodes selected by an XPath
location path. Predicates are of the form
[expression]
. The following example selects all
<foo>
elements that contain an include
attribute with the value of true
:
//foo[@include='true']
Predicates may be appended to each other to further refine an expression, such as:
//foo[@include='true'][@mode='bar']
3. XPath Data Types
While XPath expressions select nodes in the XML document, the XPath API allows the selected nodes to be coalesced into one of the following data types:
Boolean
Number
String
3.1 QName types
The XPath API defines the followingQName
types to
represent return types of an XPath evaluation:
XPathConstants.NODESET
XPathConstants.NODE
XPathConstants.STRING
XPathConstants.BOOLEAN
XPathConstants.NUMBER
The return type is specified by a QName
parameter
in method call used to evaluate the expression, which is either a call to
XPathExpression.evaluate(...)
or XPath.evaluate(...)
methods.
When a Boolean
return type is requested,
Boolean.TRUE
is returned if one or more nodes were
selected; otherwise, Boolean.FALSE
is returned.
The String
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.
The Number
return type attempts to coalesce the text
of a node to a double
data type.
3.2 Class types
In addition to the QName types, the XPath API supports the use of Class types through theXPathExpression.evaluateExpression(...)
or
XPath.evaluateExpression(...)
methods.
The XPath data types are mapped to Class types as follows:
Boolean
--Boolean.class
Number
--Number.class
String
--String.class
Nodeset
--XPathNodes.class
Node
--Node.class
Of the subtypes of Number
, only Double, Integer
and Long
are supported.
3.3 Enum types
Enum types are defined inXPathEvaluationResult.XPathResultType
that provide mappings between the QName and Class types above. The result of
evaluating an expression using the XPathExpression.evaluateExpression(...)
or XPath.evaluateExpression(...)
methods will be of one of these types.
Note the differences between the Enum and QName mappings:
NUMBER
The Enum mapping forNUMBER
supportsDouble, Integer
andLong
.
NODESET
The Enum mapping forNODESET
isXPathNodes
instead ofNodeList
in the QName mapping.
4. XPath Context
XPath location paths may be relative to a particular node in the
document, known as the context
. A context consists of:
- a node (the context node)
- a pair of non-zero positive integers (the context position and the context size)
- a set of variable bindings
- a function library
- the set of namespace declarations in scope for the expression
It is an XML document tree represented as a hierarchy of nodes, a
Node
for example, in the JDK implementation.
5. Using the XPath API
Consider the following XML document:<widgets> <widget> <manufacturer/> <dimensions/> </widget> </widgets>
The <widget>
element can be selected with the following process:
// parse the XML as a W3C Document DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder(); Document document = builder.parse(new File("/widgets.xml")); //Get an XPath object and evaluate the expression XPath xpath = XPathFactory.newInstance().newXPath(); String expression = "/widgets/widget"; Node widgetNode = (Node) xpath.evaluate(expression, document, XPathConstants.NODE); //or using the evaluateExpression method Node widgetNode = xpath.evaluateExpression(expression, document, Node.class);
With a reference to the <widget>
element, a
relative XPath expression can be written to select the
<manufacturer>
child element:
XPath xpath = XPathFactory.newInstance().newXPath(); String expression = "manufacturer"; Node manufacturerNode = (Node) xpath.evaluate(expression, widgetNode, XPathConstants.NODE); //or using the evaluateExpression method Node manufacturerNode = xpath.evaluateExpression(expression, widgetNode, Node.class);
In the above example, the XML file is read into a DOM Document before being passed to the XPath API. The following code demonstrates the use of InputSource to leave it to the XPath implementation to process it:
XPath xpath = XPathFactory.newInstance().newXPath(); String expression = "/widgets/widget"; InputSource inputSource = new InputSource("widgets.xml"); NodeList nodes = (NodeList) xpath.evaluate(expression, inputSource, XPathConstants.NODESET); //or using the evaluateExpression method XPathNodes nodes = xpath.evaluateExpression(expression, inputSource, XPathNodes.class);
In the above cases, the type of the expected results are known. In case where
the result type is unknown or any type, the XPathEvaluationResult
may be used to determine the return type. The following code demonstrates the usage:
XPathEvaluationResult<?> result = xpath.evaluateExpression(expression, document); switch (result.type()) { case NODESET: XPathNodes nodes = (XPathNodes)result.value(); ... break; }
The XPath 1.0 Number data type is defined as a double. However, the XPath
specification also provides functions that returns Integer type. To facilitate
such operations, the XPath API allows Integer and Long to be used in
evaluateExpression
method such as the following code:
int count = xpath.evaluateExpression("count(/widgets/widget)", document, Integer.class);
- Since:
- 1.5
-
ClassDescription
XPath
provides access to the XPath evaluation environment and expressions.XPath constants.TheXPathEvaluationResult
interface represents the result of the evaluation of an XPath expression within the context of a particular node.XPathResultType represents possible return types of an XPath evaluation.XPathException
represents a generic XPath exception.XPathExpression
provides access to compiled XPath expressions.XPathExpressionException
represents an error in an XPath expression.AnXPathFactory
instance can be used to createXPath
objects.XPathFactoryConfigurationException
represents a configuration error in aXPathFactory
environment.XPathFunction
provides access to XPath functions.XPathFunctionException
represents an error with an XPath function.XPathFunctionResolver
provides access to the set of user definedXPathFunction
s.XPathNodes represents a set of nodes selected by a location path as specified in XML Path Language (XPath) Version 1.0, 3.3 Node-sets.XPathVariableResolver
provides access to the set of user defined XPath variables.