XML and Java - Parsing XML using

Parsing XML
If you are a beginner to XML using Java then this is the perfect sample to parse a XML file create Java Objects and manipulate them.

The idea here is to parse the employees.xml file with content as below

<?xml version="1.0" encoding="UTF-8"?>
<Personnel>
  <Employee type="permanent">
        <Name>Seagull</Name>
        <Id>3674</Id>
        <Age>34</Age>
   </Employee>
  <Employee type="contract">
        <Name>Robin</Name>
        <Id>3675</Id>
        <Age>25</Age>
    </Employee>
  <Employee type="permanent">
        <Name>Crow</Name>
        <Id>3676</Id>
        <Age>28</Age>
    </Employee>
</Personnel>

From the parsed content create a list of Employee objects and print it to the console. The output would be something like


Employee Details - Name:Seagull, Type:permanent, Id:3674, Age:34.
Employee Details - Name:Robin, Type:contract, Id:3675, Age:25.
Employee Details - Name:Crow, Type:permanent, Id:3676, Age:28.

We will start with a DOM parser to parse the xml file, create Employee value objects and add them to a list. To ensure we parsed the file correctly let's iterate through the list and print the employees data to the console. Later we will see how to implement the same using SAX parser.
In a real world situation you might get a xml file from a third party vendor which you need to parse and update your database.

Using DOM
    This program DomParserExample.java uses DOM API.

The steps are

    Get a document builder using document builder factory and parse the xml file to create a DOM object
    Get a list of employee elements from the DOM
    For each employee element get the id, name, age and type. Create an employee value object and add it to the list.
    At the end iterate through the list and print the employees to verify we parsed it right.

a) Getting a document builder

    private void parseXmlFile(){
        //get the factory
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();

        try {

            //Using factory get an instance of document builder
            DocumentBuilder db = dbf.newDocumentBuilder();

            //parse using builder to get DOM representation of the XML file
            dom = db.parse("employees.xml");


        }catch(ParserConfigurationException pce) {
            pce.printStackTrace();
        }catch(SAXException se) {
            se.printStackTrace();
        }catch(IOException ioe) {
            ioe.printStackTrace();
        }
    }


b) Get a list of employee elements
Get the rootElement from the DOM object.From the root element get all employee elements. Iterate through each employee element to load the data.


    private void parseDocument(){
        //get the root element
        Element docEle = dom.getDocumentElement();

        //get a nodelist of
 elements
        NodeList nl = docEle.getElementsByTagName("Employee");
        if(nl != null && nl.getLength() > 0) {
            for(int i = 0 ; i < nl.getLength();i++) {

                //get the employee element
                Element el = (Element)nl.item(i);

                //get the Employee object
                Employee e = getEmployee(el);

                //add it to list
                myEmpls.add(e);
            }
        }
    }

c) Reading in data from each employee.


    /**
     * I take an employee element and read the values in, create
     * an Employee object and return it
     */
    private Employee getEmployee(Element empEl) {

        //for each <employee> element get text or int values of
        //name ,id, age and name
        String name = getTextValue(empEl,"Name");
        int id = getIntValue(empEl,"Id");
        int age = getIntValue(empEl,"Age");

        String type = empEl.getAttribute("type");

        //Create a new Employee with the value read from the xml nodes
        Employee e = new Employee(name,id,age,type);

        return e;
    }


    /**
     * I take a xml element and the tag name, look for the tag and get
     * the text content
     * i.e for <employee><name>John</name></employee> xml snippet if
     * the Element points to employee node and tagName is 'name' I will return John
     */
    private String getTextValue(Element ele, String tagName) {
        String textVal = null;
        NodeList nl = ele.getElementsByTagName(tagName);
        if(nl != null && nl.getLength() > 0) {
            Element el = (Element)nl.item(0);
            textVal = el.getFirstChild().getNodeValue();
        }

        return textVal;
    }


    /**
     * Calls getTextValue and returns a int value
     */
    private int getIntValue(Element ele, String tagName) {
        //in production application you would catch the exception
        return Integer.parseInt(getTextValue(ele,tagName));
    }

d) Iterating and printing.


    private void printData(){

        System.out.println("No of Employees '" + myEmpls.size() + "'.");

        Iterator it = myEmpls.iterator();
        while(it.hasNext()) {
            System.out.println(it.next().toString());
        }
    }