XML XPath Evaluation
Remarks#
XPath expressions are used to navigate and select one or more nodes within an XML tree document, such as selecting a certain element or attribute node.
See this W3C recommendation for a reference on this language.
Evaluating a NodeList in an XML document
Given the following XML document:
<documentation>
<tags>
<tag name="Java">
<topic name="Regular expressions">
<example>Matching groups</example>
<example>Escaping metacharacters</example>
</topic>
<topic name="Arrays">
<example>Looping over arrays</example>
<example>Converting an array to a list</example>
</topic>
</tag>
<tag name="Android">
<topic name="Building Android projects">
<example>Building an Android application using Gradle</example>
<example>Building an Android application using Maven</example>
</topic>
<topic name="Layout resources">
<example>Including layout resources</example>
<example>Supporting multiple device screens</example>
</topic>
</tag>
</tags>
</documentation>
The following retrieves all example
nodes for the Java tag (Use this method if only evaluating XPath in the XML once. See other example for when multiple XPath calls are evaluated in the same XML file.):
XPathFactory xPathFactory = XPathFactory.newInstance();
XPath xPath = xPathFactory.newXPath(); //Make new XPath
InputSource inputSource = new InputSource("path/to/xml.xml"); //Specify XML file path
NodeList javaExampleNodes = (NodeList) xPath.evaluate("/documentation/tags/tag[@name='Java']//example", inputSource, XPathConstants.NODESET); //Evaluate the XPath
...
Parsing multiple XPath Expressions in a single XML
Using the same example as Evaluating a NodeList in an XML document, here is how you would make multiple XPath calls efficiently:
Given the following XML document:
<documentation>
<tags>
<tag name="Java">
<topic name="Regular expressions">
<example>Matching groups</example>
<example>Escaping metacharacters</example>
</topic>
<topic name="Arrays">
<example>Looping over arrays</example>
<example>Converting an array to a list</example>
</topic>
</tag>
<tag name="Android">
<topic name="Building Android projects">
<example>Building an Android application using Gradle</example>
<example>Building an Android application using Maven</example>
</topic>
<topic name="Layout resources">
<example>Including layout resources</example>
<example>Supporting multiple device screens</example>
</topic>
</tag>
</tags>
</documentation>
This is how you would use XPath to evaluate multiple expressions in one document:
XPath xPath = XPathFactory.newInstance().newXPath(); //Make new XPath
DocumentBuilder builder = DocumentBuilderFactory.newInstance();
Document doc = builder.parse(new File("path/to/xml.xml")); //Specify XML file path
NodeList javaExampleNodes = (NodeList) xPath.evaluate("/documentation/tags/tag[@name='Java']//example", doc, XPathConstants.NODESET); //Evaluate the XPath
xPath.reset(); //Resets the xPath so it can be used again
NodeList androidExampleNodes = (NodeList) xPath.evaluate("/documentation/tags/tag[@name='Android']//example", doc, XPathConstants.NODESET); //Evaluate the XPath
...
Parsing single XPath Expression multiple times in an XML
In this case, you want to have the expression compiled before the evaluations, so that each call to evaluate
does not compile
the same expression. The simple syntax would be:
XPath xPath = XPathFactory.newInstance().newXPath(); //Make new XPath
XPathExpression exp = xPath.compile("/documentation/tags/tag[@name='Java']//example");
DocumentBuilder builder = DocumentBuilderFactory.newInstance();
Document doc = builder.parse(new File("path/to/xml.xml")); //Specify XML file path
NodeList javaExampleNodes = (NodeList) exp.evaluate(doc, XPathConstants.NODESET); //Evaluate the XPath from the already-compiled expression
NodeList javaExampleNodes2 = (NodeList) exp.evaluate(doc, XPathConstants.NODESET); //Do it again
Overall, two calls to XPathExpression.evaluate()
will be much more efficient than two calls to XPath.evaluate()
.