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Use of "preceding" axis in XPath expression
Posted on: September 29, 2008 at 12:00 AM

In this part of tutorial we are going to describe you the use of "preceding" axis in XPath expression. "preceding" axis will select each and every element or node before the given context node.

Use of "preceding" axis in XPath expression

In this part of tutorial we are going to describe you the use of "preceding" axis in XPath expression. "preceding" axis will select each and every element or node before the given context node.

In this example we have created an XML file "persons.xml" as in our previous section, which is necessary to execute XPath query on to it. This "persons.xml" contains information related to name, age, gender of different persons.

Here is the full source code for persons.xml file as follows :

persons.xml

<?xml version="1.0" ?>

      <information>

        <person id="1">

        <name>Deep</name>

        <age>34</age>

        <gender>Male</gender>

        </person>

       

       <person id="2">

        <name>Kumar</name>

        <age>24</age>

        <gender>Male</gender>

        </person>

       

        <person id="3">

        <name>Deepali</name>

        <age>19</age>

        <gender>Female</gender>

        </person>

      

        <!-- more persons... -->

      </information>

We have put that persons.xml file in that current working directory. Now we have declared a class XPathPreceding and in this class we are parsing the XML file with JAXP. First of all we need to load the document into DOM Document object.

 
      DocumentBuilderFactory domFactory = 

        DocumentBuilderFactory.newInstance();

        domFactory.setNamespaceAware(true); 

        DocumentBuilder builder = domFactory.newDocumentBuilder();

        Document doc = builder.parse("persons.xml");

For downloading document we have created a new instance of DocumentBuilderFactory and then this instance is further going to be used in creating an object of DocumentBuilder. "setNamespaceAware()" tells that the parser produced by this code will provide full support for XML namespaces. Above lines of code parses "persons.xml" file and creates a Document object. Next we have created XPath object with the use of XPathFactory.

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

XPath expression "//person/preceding::*" will select all elements that are before the start of the context node.
Here is the example code for XPathPreceding.java as follows:

XPathPreceding.java


import org.w3c.dom.*;

import javax.xml.xpath.*;

import javax.xml.parsers.*;

import java.io.IOException;

import org.xml.sax.SAXException;



public class XPathPreceding {



  public static void main(String[] args) 

 throws ParserConfigurationException, SAXException, 

  IOException, XPathExpressionException {



  DocumentBuilderFactory domFactory = 

       DocumentBuilderFactory.newInstance();

  domFactory.setNamespaceAware(true); 

  DocumentBuilder builder = domFactory.newDocumentBuilder();

  Document doc = builder.parse("persons.xml");

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

  XPathExpression expr = xpath.compile("//person/preceding::*");

  // Selecting person element's preceding elements name 

  Object result = expr.evaluate(doc, XPathConstants.NODESET);

  NodeList nodes = (NodeList) result;

  for (int i = 0; i < nodes.getLength(); i++){

 System.out.println(nodes.item(i).getNodeName()); 

  }

    expr = xpath.compile("//person/preceding::*/name/text()");

  // Selecting all person element's precding elements values 

  result = expr.evaluate(doc, XPathConstants.NODESET);

  nodes = (NodeList) result;

  for (int i = 0; i < nodes.getLength(); i++) {

 System.out.print(nodes.item(i).getNodeValue()+",  "); 

  }  

  }

}

Output:

Download Source Code

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