Java SAX Parser - Query XML Document

Demo Example

Here is the input text file that we need to Query for rollno: 393

<?xml version = "1.0"?>
<class>
   <student rollno = "393">
      <firstname>dinkar</firstname>
      <lastname>kad</lastname>
      <nickname>dinkar</nickname>
      <marks>85</marks>
   </student>
   
   <student rollno = "493">
      <firstname>Vaneet</firstname>
      <lastname>Gupta</lastname>
      <nickname>vinni</nickname>
      <marks>95</marks>
   </student>
   
   <student rollno = "593">
      <firstname>jasvir</firstname>
      <lastname>singn</lastname>
      <nickname>jazz</nickname>
      <marks>90</marks>
   </student>
</class>

UserHandler.java

package com.howcodex.xml;

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;
 
public class UserHandler extends DefaultHandler {

   boolean bFirstName = false;
   boolean bLastName = false;
   boolean bNickName = false;
   boolean bMarks = false;
   String rollNo = null;

   @Override
   public void startElement(
      String uri, String localName, String qName, Attributes attributes)
      throws SAXException {

      if (qName.equalsIgnoreCase("student")) {
         rollNo = attributes.getValue("rollno");
      }
      if(("393").equals(rollNo) && 
         qName.equalsIgnoreCase("student")) {
         System.out.println("Start Element :" + qName);      
      }       
      if (qName.equalsIgnoreCase("firstname")) {
         bFirstName = true;
      } else if (qName.equalsIgnoreCase("lastname")) {
         bLastName = true;
      } else if (qName.equalsIgnoreCase("nickname")) {
         bNickName = true;
      }
      else if (qName.equalsIgnoreCase("marks")) {
         bMarks = true;
      }
   }

   @Override
   public void endElement(
      String uri, String localName, String qName) throws SAXException {
      
      if (qName.equalsIgnoreCase("student")) {
         if(("393").equals(rollNo) && qName.equalsIgnoreCase("student"))
         System.out.println("End Element :" + qName);
      }
   }

   @Override
   public void characters(char ch[], int start, int length) throws SAXException {

      if (bFirstName && ("393").equals(rollNo)) {
         //age element, set Employee age
         System.out.println("First Name: " + new String(ch, start, length));
         bFirstName = false;
      } else if (bLastName && ("393").equals(rollNo)) {
         System.out.println("Last Name: " + new String(ch, start, length));
         bLastName = false;
      } else if (bNickName && ("393").equals(rollNo)) {
         System.out.println("Nick Name: " + new String(ch, start, length));
         bNickName = false;
      } else if (bMarks && ("393").equals(rollNo)) {
         System.out.println("Marks: " + new String(ch, start, length));
         bMarks = false;
      }
   }
}

SAXQueryDemo.java

package com.howcodex.xml;

import java.io.File;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

public class SAXQueryDemo {

   public static void main(String[] args) {

      try {
         File inputFile = new File("input.txt");
         SAXParserFactory factory = SAXParserFactory.newInstance();
         SAXParser saxParser = factory.newSAXParser();
         UserHandler userhandler = new UserHandler();
         saxParser.parse(inputFile, userhandler);     
      } catch (Exception e) {
         e.printStackTrace();
      }
   }   
}

class UserHandler extends DefaultHandler {

   boolean bFirstName = false;
   boolean bLastName = false;
   boolean bNickName = false;
   boolean bMarks = false;
   String rollNo = null;

   @Override
   public void startElement(String uri, 
      String localName, String qName, Attributes attributes)
      throws SAXException {

      if (qName.equalsIgnoreCase("student")) {
         rollNo = attributes.getValue("rollno");
      }
      if(("393").equals(rollNo) && qName.equalsIgnoreCase("student")) {
         System.out.println("Start Element :" + qName);      
      }       
      if (qName.equalsIgnoreCase("firstname")) {
         bFirstName = true;
      } else if (qName.equalsIgnoreCase("lastname")) {
         bLastName = true;
      } else if (qName.equalsIgnoreCase("nickname")) {
         bNickName = true;
      }
      else if (qName.equalsIgnoreCase("marks")) {
         bMarks = true;
      }
   }

   @Override
   public void endElement(
      String uri, String localName, String qName) throws SAXException {
      
      if (qName.equalsIgnoreCase("student")) {

         if(("393").equals(rollNo) 
            && qName.equalsIgnoreCase("student"))
            System.out.println("End Element :" + qName);
      }
   }


   @Override
   public void characters(
      char ch[], int start, int length) throws SAXException {

      if (bFirstName && ("393").equals(rollNo)) {
         //age element, set Employee age
         System.out.println("First Name: " + new String(ch, start, length));
         bFirstName = false;
      } else if (bLastName && ("393").equals(rollNo)) {
         System.out.println("Last Name: " + new String(ch, start, length));
         bLastName = false;
      } else if (bNickName && ("393").equals(rollNo)) {
         System.out.println("Nick Name: " + new String(ch, start, length));
         bNickName = false;
      } else if (bMarks && ("393").equals(rollNo)) {
         System.out.println("Marks: " + new String(ch, start, length));
         bMarks = false;
      }
   }
}

This would produce the following result −

Start Element :student
First Name: dinkar
Last Name: kad
Nick Name: dinkar
Marks: 85
End Element :student