Friday 15 February 2013

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Tutorial On How to Use Xpath on xml file or string in Java

Xpath is a W3C's XSLT standard to navigate through the Xml Document . for example consider this sample xml file for auction . each auction contains list of bids apart from other details
Now if you want to access first bid price in bids then the xpath would be
Java provides xpath support to navigate through the xml document .
classes which are required for applying xpath are following :
  First , get instance of XPath from XPathFactory
XPathFactory factory = XPathFactory.newInstance();
XPath xpath = factory.newXPath();
Create an InputSource instance from InputStream of xml file .
InputSource inputSource = new InputSource(stream);

now get the desired value by entering xpath expresion like this
You can specify required constants from XPathConstants class
String value = (String) xpath.evaluate("/auction/bids/bid[1]/price", inputSource,XPathConstants.STRING);

If you want complete list you can specify like this
NodeList list  = (NodeList) xpath.evaluate(expression, source,XPathConstants.NODESET);
for (int i = 0; i < nodes.getLength(); i++)
value = nodes.item(i).getTextContent();

 If the xml is present as string we can use StringReader to create InputSource
StringReader converts String to Stream of character

Common Error : when we use Xpath again with the same InputResource , it will throw Exception with message like this
javax.xml.xpath.XPathExpressionException at org.apache.xpath.jaxp.XPathImpl.evaluate( Stream Closed 

once the Xpath expression is evaluated on particular InputSource , it's stream is closed We have to use new InputSource with new InputStream for every Xpath evaluation .