Introduction to XML and XML With Java  

If you are looking for sample programs to parse a XML file using DOM/SAX parser or looking for a program to generate a XML file please proceed directly to programs.

This small tutorial introduces you to the basic concepts of XML and using Xerces parser for Java to generate and parse XML.
The intended audience are XML beginners with knowledge of Java.
Last updated:
03 Oct 2005.Modified programs runs in JDK 1.3, JDK 1.4 and JDK 1.5. Xerces parser is used with JDK 1.4 and 1.3.

Contents
Introduction:

    This is a gentle introduction to XML and using XML with Java. We see what makes XML tick and the advantages of XML . Also we have programs to show how to use Xerces parser for parsing and printing an XML document, to generate XML from arbitrary data.
These programs have been intentionally kept small to get the point across.

2 What is XML ?

    Extensible Markup Language
XML is text based markup language.
XML is more than html because html only deal with presentation and carries no information about the data itself.

In HTML
For example
<b>Seagull<b>
<i>3674<i>
<i> 34 </i>
could mean anything

In XML  we represent it as.

<Personnel>
    <Employee>
        <Name>Seagull</Name>
        < ID> 3674 </ID>
        <Age>34</Age>
    </Employee>
</Personnel>

And this clearly expresses Seagull is an employee name his ID is 3674 and his age is 34.

HTML has limited set of Tags
But XML can be extended (i.e we can create our own tags )

3 Advantages of XML

    Here are a few advantages.

3.1 Readability
    XML document is plain text and human readable, To edit/view XML documents any simple text editor will suffice .

3.2 Hierarchical
    XML document has a tree structure which is powerful enough to express complex data and simple enough to understand

3.3 Language Independent
    XML documents are language neutral.For e.g. a Java program can generate a XML which can be parsed by a program written in C++ or Perl.

3.4 OS Independent
    XML files are Operating System independent.

4 Uses of XML


4.1 MetaContent
    To describe the contents of a document.

4.2 Messaging
    Where applications or organizations exchanges data between them.

4.3 Database
    The data extracted from the database can be preserved with original information and can be used more than one application in different ways. One application might just display the data and the other application might perform some complex calculation on this data

5 Parsers

5.1 DOM (Document Object Model ) parser - Tree Structure based API:
    The Dom parser implements the dom api and it creates a DOM tree in memory for a XML document

5.2 SAX (Simple API For XML ) parser - Event Based API
    The SAX parser implements the SAX API and it is event driven interface. As it parses it invokes the callback methods

5.3 When to use DOM parser

  • Manipulate the document
  • Traverse the document back and forth
  • Small XML files
Drawbacks of DOM parser
    Consumes lot of memory

5.4 When to use SAX parser

  • No structural modification
  • Huge XML files
5.5 Validating And Non Validating
DOM and SAX can either be a validating or a non validating parser.
    A validating parser checks the XML file against the rules imposed by DTD or XML Schema.
    A non validating parser doesn't validate the XML file against a DTD or XML Schema.
Both Validating and non validating parser checks for the well formedness of the xml document 5.6 Well Formedness
    A well formed XML must satisfy these constraints
a) All tags must have a corresponding ending tag.
Well Formed Not Well formed
<Personnel>
    <Employee>
        <Name>Seagull</Name>
        < ID> 3674 </ID>
        <Age>34</Age>
    </Employee>
</Personnel> 		<Personnel>
    <Employee>
        <Name>Seagull</Name>
        < ID> 3674 </ID>
        <Age>34
     </Employee>
</Personnel>
(Note: Missing ending tag for Age)

b) No overlapping tags
Well Formed Not Well formed
<Personnel>
    <Employee>
        <Name>Seagull</Name>
        < ID> 3674 </ID>
        <Age>34</Age>
    </Employee>
</Personnel> 		<Personnel>
    <Employee>
        <Name>Seagull</Name>
        < ID> 3674 </ID>
        <Age>34</Age>
    </Personnel>
</Employee>
Note: </Employee> and </Personnel > tags are not in the right order

6 Programs

    All these Programs uses Xerces for Java Parser.

6.1 Parsing XML
    This is like "hello world"   to programming. One of the first encounters with XML for a beginner would be 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.

6.1.1 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());
		}
	}

6.1.2 Using SAX
This program SAXParserExample.java parses a XML document and prints it on the console.
Sax parsing is event based modelling.When a Sax parser parses a XML document and every time it encounters a tag it calls the corresponding tag handler methods

when it encounters a Start Tag it calls this method
    public void startElement(String uri,..

when it encounters a End Tag it calls this method
    public void endElement(String uri,...

Like the dom example this program also parses the xml file, creates a list of employees and prints it to the console. The steps involved are

  • Create a Sax parser and parse the xml
  • In the event handler create the employee object
  • Print out the data
Basically the class extends DefaultHandler to listen for call back events. And we register this handler with the Sax parser to notify us of call back events. We are only interested in start event, end event and character event.
In start event if the element is employee we create a new instant of employee object and if the element is Name/Id/Age we initialize the character buffer to get the text value.
In end event if the node is employee then we know we are at the end of the employee node and we add the Employee object to the list.If it is any other node like Name/Id/Age we call the corresponding methods like setName/SetId/setAge on the Employee object.
In character event we store the data in a temp string variable.

a) Create a Sax Parser and parse the xml 


	private void parseDocument() {

		//get a factory
		SAXParserFactory spf = SAXParserFactory.newInstance();
		try {

			//get a new instance of parser
			SAXParser sp = spf.newSAXParser();

			//parse the file and also register this class for call backs
			sp.parse("employees.xml", this);

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

b) In the event handlers create the Employee object and call the corresponding setter methods. 


	//Event Handlers
	public void startElement(String uri, String localName, String qName,
		Attributes attributes) throws SAXException {
		//reset
		tempVal = "";
		if(qName.equalsIgnoreCase("Employee")) {
			//create a new instance of employee
			tempEmp = new Employee();
			tempEmp.setType(attributes.getValue("type"));
		}
	}


	public void characters(char[] ch, int start, int length) throws SAXException {
		tempVal = new String(ch,start,length);
	}

	public void endElement(String uri, String localName,
		String qName) throws SAXException {

		if(qName.equalsIgnoreCase("Employee")) {
			//add it to the list
			myEmpls.add(tempEmp);

		}else if (qName.equalsIgnoreCase("Name")) {
			tempEmp.setName(tempVal);
		}else if (qName.equalsIgnoreCase("Id")) {
			tempEmp.setId(Integer.parseInt(tempVal));
		}else if (qName.equalsIgnoreCase("Age")) {
			tempEmp.setAge(Integer.parseInt(tempVal));
		}

	}

c) 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());
		}
	}

6.2 Generating XML
    The previous programs illustrated how to parse an existing XML file using both SAX and DOM Parsers.
But generating a XML file from scratch is a different story, for instance you might like to generate a xml file for the data extracted from a database.To keep the example simple this program XMLCreatorExample.java generates XML from a list preloaded with hard coded data. The output will be book.xml file with the following content. 


<?xml version="1.0" encoding="UTF-8"?>
<Books>
    <Book Subject="Java 1.5">
        <Author>Kathy Sierra .. etc</Author>
        <Title>Head First Java</Title>
    </Book>
    <Book Subject="Java Architect">
        <Author>Kathy Sierra .. etc</Author>
        <Title>Head First Design Patterns</Title>
    </Book>
</Books>

The steps involved are
  • Load Data
  • Get an instance of Document object using document builder factory
  • Create the root element Books
  • For each item in the list create a Book element and attach it to Books element
  • Serialize DOM to FileOutputStream to generate the xml file "book.xml".

a) Load Data. 



	/**
	 * Add a list of books to the list
	 * In a production system you might populate the list from a DB
	 */
	private void loadData(){

		myData.add(new Book("Head First Java",
			"Kathy Sierra .. etc","Java 1.5"));

		myData.add(new Book("Head First Design Patterns",
			"Kathy Sierra .. etc","Java Architect"));
	}

c) Getting an instance of DOM. 



	/**
	 * Using JAXP in implementation independent manner create a document object
	 * using which we create a xml tree in memory
	 */
	private void createDocument() {

		//get an instance of factory
		DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
		try {
		//get an instance of builder
		DocumentBuilder db = dbf.newDocumentBuilder();

		//create an instance of DOM
		dom = db.newDocument();

		}catch(ParserConfigurationException pce) {
			//dump it
			System.out.println("Error while trying to instantiate DocumentBuilder " + pce);
			System.exit(1);
		}

	}

c) Create the root element Books. 



	/**
	 * The real workhorse which creates the XML structure
	 */
	private void createDOMTree(){

		//create the root element 
		Element rootEle = dom.createElement("Books");
		dom.appendChild(rootEle);

		//No enhanced for
		Iterator it  = myData.iterator();
		while(it.hasNext()) {
			Book b = (Book)it.next();
			//For each Book object  create  element and attach it to root
			Element bookEle = createBookElement(b);
			rootEle.appendChild(bookEle);
		}

	}

d) Creating a book element. 



	/**
	 * Helper method which creates a XML element 
	 * @param b The book for which we need to create an xml representation
	 * @return XML element snippet representing a book
	 */
	private Element createBookElement(Book b){

		Element bookEle = dom.createElement("Book");
		bookEle.setAttribute("Subject", b.getSubject());

		//create author element and author text node and attach it to bookElement
		Element authEle = dom.createElement("Author");
		Text authText = dom.createTextNode(b.getAuthor());
		authEle.appendChild(authText);
		bookEle.appendChild(authEle);

		//create title element and title text node and attach it to bookElement
		Element titleEle = dom.createElement("Title");
		Text titleText = dom.createTextNode(b.getTitle());
		titleEle.appendChild(titleText);
		bookEle.appendChild(titleEle);

		return bookEle;

	}

e) Serialize DOM to FileOutputStream to generate the xml file "book.xml". 




	/**
	 * This method uses Xerces specific classes
	 * prints the XML document to file.
     */
	private void printToFile(){

		try
		{
			//print
			OutputFormat format = new OutputFormat(dom);
			format.setIndenting(true);

			//to generate output to console use this serializer
			//XMLSerializer serializer = new XMLSerializer(System.out, format);


			//to generate a file output use fileoutputstream instead of system.out
			XMLSerializer serializer = new XMLSerializer(
			new FileOutputStream(new File("book.xml")), format);

			serializer.serialize(dom);

		} catch(IOException ie) {
		    ie.printStackTrace();
		}
	}
Note:
The Xerces internal classes OutputFormat and XMLSerializer are in different packages.

In JDK 1.5 with built in Xerces parser they are under
com.sun.org.apache.xml.internal.serialize.OutputFormat
com.sun.org.apache.xml.internal.serialize.XMLSerializer

In Xerces 2.7.1 which we are using to run these examples they are under
org.apache.xml.serialize.XMLSerializer
org.apache.xml.serialize.OutputFormat
We are using Xerces 2.7.1 with JDK 1.4 and JDK 1.3 as the default parser with JDK 1.4 is Crimson and there is no built in parser with JDK 1.3.
Also please remember it is not advisable to use parser implementation specific classes like OutputFormat and XMLSerializer as they are only available in Xerces and if you switch to another parser in the future you may have to rewrite.
See below for steps to make your code JAXP compliant.


7 Instructions to run these programs

The instructions to compile and run these programs varies based on the JDK that you are using. This is due to the way the XML parser is bundled with various Java distributions.These instructions are for Windows OS.For Unix or Linux OS you just need to change the folder paths accordingly.
7.1 Using JDK 1.5

Xerces parser is bundled with the JDK 1.5 distribution.So you need not download the parser separately.

Running DOMParserExample

  1. Download DomParserExample.java, Employee.java, employees.xml to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath . DomParserExample.java
  4. To run, type
    java -classpath . DomParserExample
Running SAXParserExample
  1. Download SAXParserExample.java, Employee.java, employees.xml to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath . SAXParserExample.java
  4. To run,type
    java -classpath . SAXParserExample
Running XMLCreatorExample
  1. Download XMLCreatorExample.java, Book.java to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath . XMLCreatorExample.java
  4. To run, type
    java -classpath . XMLCreatorExample

7.2 Using JDK 1.4

Crimson parser is bundled by default with JDK 1.4.To override this parser we need to run our example with -Djava.endorsed.dirs option. Download Xerces 2.7.1 from here. Unzip and copy the jar files to c:\xerces\lib.If it is installed in any other folder modify the instructions below accordingly.
Running DOMParserExample

  1. Download DomParserExample.java, Employee.java, employees.xml to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath .;c:\xerces\lib\xercesImpl.jar DomParserExample.java
  4. To run, type
    java -classpath .; -Djava.endorsed.dirs=c:\xerces\lib DomParserExample
Running SAXParserExample
  1. Download SAXParserExample.java, Employee.java, employees.xml to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath .;c:\xerces\lib\xercesImpl.jar SAXParserExample.java
  4. To run, type
    java -classpath .; -Djava.endorsed.dirs=c:\xerces\lib SAXParserExample
Running XMLCreatorExample
  1. Download XMLCreatorExample.java, Book.java to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath .;c:\xerces\lib\xercesImpl.jar XMLCreatorExample.java
  4. To run, type
    java -classpath .; -Djava.endorsed.dirs=c:\xerces\lib XMLCreatorExample

7.3 Using JDK 1.3

No parser is bundled by default with JDK 1.3. All we need to do is set the xerces jar files in classpath for our compilation and execution. Download xerces 2.7.1 from here. Unzip and copy the jar files to c:\xerces\lib.If it is installed in any other folder modify the instructions below accordingly.
Running DOMParserExample

  1. Download DomParserExample.java, Employee.java, employees.xml to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath .;c:\xerces\lib\xercesImpl.jar;c:\xerces\lib\xml-apis.jar DomParserExample.java
  4. To run, type
    java -classpath .;c:\xerces\lib\xercesImpl.jar;c:\xerces\lib\xml-apis.jar DomParserExample
Running SAXParserExample
  1. Download SAXParserExample.java, Employee.java, employees.xml to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath .;c:\xerces\lib\xercesImpl.jar;c:\xerces\lib\xml-apis.jar SAXParserExample.java
  4. To run, type
    java -classpath .;c:\xerces\lib\xercesImpl.jar;c:\xerces\lib\xml-apis.jar SAXParserExample
Running XMLCreatorExample
  1. Download XMLCreatorExample.java, Book.java to c:\xercesTest
  2. Go to command prompt and type
    cd c:\xercesTest
  3. To compile, type
    javac -classpath .;c:\xerces\lib\xercesImpl.jar;c:\xerces\lib\xml-apis.jar XMLCreatorExample.java
  4. To run, type
    java -classpath .;c:\xerces\lib\xercesImpl.jar;c:\xerces\lib\xml-apis.jar XMLCreatorExample


Misc

Alternative to XMLSerializer
It is not advisable to use parser implementation specific classes like OutputFormat and XMLSerializer as they are only available in Xerces and if you switch to another parser in the future you may have to rewrite. To be JAXP compliant use the following to classes to generate the output. 


import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
The code to generate xml to a file would be something like 

Transformer tr = TransformerFactory.newInstance().newTransformer();
tr.setOutputProperty(OutputKeys.INDENT, "yes");
tr.setOutputProperty(OutputKeys.METHOD,"xml");
tr.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "3");

//to send the output to a file
tr.transform( new DOMSource(dom),new StreamResult(new FileOutputStream("test.xml")));


//to send the output to console
//tr.transform( new DOMSource(dom),new StreamResult(System.out));

What is endorsed standard?
An endorsed standard is a JavaTM API defined through a standards process other than the Java Community ProcessSM (JCPSM). Because endorsed standards are defined outside the JCP, it is anticipated that such standards may be revised between releases of the Java 2 Platform. In order to take advantage of new revisions to endorsed standards, developers and software vendors may use the Endorsed Standards Override Mechanism to provide newer versions of an endorsed standard than those included in the Java 2 Platform as released by Sun Microsystems.
By default java looks for endorsed jar files under 


<java-home>\lib\endorsed          [Microsoft Windows]
<java-home>/lib/endorsed          [Solaris or Linux]

This can be overridden using java system property java.endorsed.dirs as follows

java -Djava.endorsed.dirs=<folder_name>

For more information see Endorsed Standards Override Mechanism

Definitions
(From java.sun.com)

Attribute
A qualifier on an XML tag that provides additional information. For example, in the tag <slide title="My Slide">, title is an attribute, and My Slide is its value.
Tag
A piece of text that describes a unit of data, or element, in XML. A tag is surrounded by angle brackets
<Age>25</Age>
<Age> is the start tag
Element
A unit of XML data, delimited by tags. An XML element can enclose other elements.
<Personnel>
<Employee>..</Employee>
<Employee>..</Employee>
</Personnel>
In this Personnel has two Employee elements.
CDATA
A predefined XML tag for "Character DATA" that says "don't interpret these characters", as opposed to "Parsed Character Data" (PCDATA), in which the normal rules of XML syntax apply (for example, angle brackets demarcate XML tags, tags define XML elements, etc.).
CDATA sections are typically used to show examples of XML syntax. Like this:
<![CDATA[ <slide>..A sample slide..</slide> ]]>
which displays as:
<slide>..A sample slide.. </slide>
Declaration
The very first thing in an XML document, which declares it as XML. The minimal declaration is <?xml version="1.0"?>. The declaration is part of the document prolog.
Data
The contents of an element, generally used when the element does not contain any subelements.
<Age>25<Age>
Here 25 is the data
DTD
Document Type Definition. An optional part of the document prolog, as specified by the XML standard. The DTD specifies constraints on the valid tags and tag sequences that can be in the document.






The following segement contains the older version of the code.
Please use the latest programs found above.

6 Programs(Old Version)
    All these Programs uses Xerces for Java Parser.

6.1 Parsing and Printing (Old Version)
    This is like "hello world"  to programming.The first step we should do is to learn how to parse and print a simple XML document using both DOM and SAX.
This will help you to get the basic concepts in parsing and how does DOM API differ from SAX.

6.1.1 Using DOM (Old Version)
    This program PrintUsingDOM parses a XML file and prints it on the console . It uses DOM API.

This is a two stage process, first it parses the XML file and creates a tree structure in the memory.
The next stage is to traverse the tree and print the contents.

a) Construction of DOM tree

    //get an instance of DomParser
    DOMParserWrapper myDOMParser = (DOMParserWrapper) new dom.wrappers.DOMParser();
    //Parse the XML File which results in the creation of the Dom tree
    Document doc = myDOMParser.parse(xmlFile);

Once the Dom tree is constructed the Walk method is called which traverses the tree and prints the content.

    walk(doc);

b) Traversing the DOM tree
It does a Depth First Pre Order Traversal , for more on tree traversal techniques see here
As it goes through each node it prints it contents.
This snippet explains how to handle the element Node

//For an element node print the node name and also its attributes if any
            case Node.ELEMENT_NODE:
            {
                System.out.print('<' + node.getNodeName() );
                NamedNodeMap nnm = node.getAttributes();
                if(nnm != null )
                {
                    int len = nnm.getLength() ;
                    Attr attr;
                    for ( int i = 0; i < len; i++ )
                    {
                        attr = (Attr)nnm.item(i);
                        System.out.print(' '
                             + attr.getNodeName()
                             + "=\""
                             + attr.getNodeValue()
                             +  '"' );
                    }
                }
                System.out.print('>');

                break;

            }//end of element

This recurses through each node

        //recurse
        for(Node child = node.getFirstChild(); child != null; child = child.getNextSibling())
        {
            walk(child);
        }

Hence we print recursively all the contents to the console

6.1.2 Using SAX (Old Version)
    Sax is much more easier to understand and implement.
This program PrintUsingSAX parses a XML document and prints it on the console.
When a Sax parser parses a XML document and every time it encounters a tag it calls the corresponding tag handler methods

when it encounter a Start Tag it calls this method
    public void startElement(String name, AttributeList attrs)

when it encounter a End Tag it calls this method
    public void endElement(String name)

This program also parses a XML file and prints it on the console . It uses SAX API.
In this example,  the PrintUsingSax Class extends the HandlerBase class and implemets the call back  methods to handle the printing

The steps involved are

    Get a instance of SAX parser,
        Parser myParser = new SAXParser();

    Register the class which extends the HandlerBase so that it handle the call backs
        myParser.setDocumentHandler(this);

    Parse the XML file
        myParser.parse(fileName);

    Print the document as it is parsed

             //handle start element
             public void startElement(String name, AttributeList attrs)
             {

        System.out.print('<' + name );

        if (attrs != null)
        {

            int len = attrs.getLength();
            for (int i = 0; i < len; i++)
            {
                System.out.print(" "
                + attrs.getName(i)
                + "=\""
                + attrs.getValue(i)
                + "\"");
            }
        }//end of if
        System.out.print('>');

    } // startElement(String,AttributeList)


That’s how we print using a SAX parser

6.2 Generate XML (Old Version)
    The previous program illustrated how to Parse an existing XML file using both SAX and DOM Parsers.
But generating  a XML file from scratch is a different story , for instance you might like to generate a xml file for the  data extracted form the database.To keep the example simple this program GenerateXML generates a XML file from a Vector preloaded with hard coded data.

The steps involved are
   a)  Get an instance of DOM parser
   b)  Create a new Document
   c)  Load the Data
   d)  Create a DOM tree with this Data
   e)  Print the DOM tree which will be the XML file

In detail

a) Get an instance of DOM psrser
    DocumentBuilderFactory dbf =DocumentBuilderFactory.newInstance();
    DocumentBuilder db = dbf.newDocumentBuilder();

b) Create a new Document
    doc = db.newDocument();

c) Load the Data
In this case the vector is loaded with hard coded data for simplicty.
     private void loadData()
    {
        vData = new Vector();
        //trivial example
        //data is Name$Age$sex
        vData.addElement("John$35$M");
        vData.addElement("Jessie$30$F");
        vData.addElement("Jonathan$28$M");
        vData.addElement("Laila$20$F");
    }
 

d) Create a DOM tree with this Data
            root = doc.createElement("Employee");
            root.appendChild(doc.createTextNode("\n"));
 
            //add name Element
            item = doc.createElement("Name");
            item.appendChild(doc.createTextNode(name));
            root.appendChild(item);
            root.appendChild(doc.createTextNode("\n"));
 
            //add Age Element
            item = doc.createElement("Age");
            item.appendChild(doc.createTextNode(age));
            root.appendChild(item);
            root.appendChild(doc.createTextNode("\n"));
 
            //add Sex Element
            item = doc.createElement("Sex");
            item.appendChild(doc.createTextNode(sex));
            root.appendChild(item);
            root.appendChild(doc.createTextNode("\n"));
 
            //add to the Top Element
            main.appendChild(root);
            main.appendChild(doc.createTextNode("\n"));
 

This is nothing but a creating a tree structure,
Create a Root Node Employee
Append Child nodes Name, Age and Sex to the Root Node
Append Root Node to the Top Node Personal

This is similar to this

<Personnel>
    <Employee>
        <Name>John</Name>
        <Age>35</Age>
        <Sex> M </Sex>
    </Employee>
</Personnel>

e) Print the DOM tree
This is similar to the printing done in the previous program PrintUsingDOM.java

7 Instructions to run these programs (Old Version)
If you wish you can download Personnel.xml and Personnel.dtd Personnel.dtd to the same directory where you
have downloaded these programs.
Requirements
JDK of course
Xerces for Java Parser

Assumption
Xerces is installed in c:\xerces-1_3_0 if not change the path accordingly in these instructions

PrintUsingDOM.java
To compile
set CLASSPATH=c:\xerces-1_3_0\xerces.jar;c:\xerces-1_3_0\xercesSamples.jar;.;%classpath%;
javac PrintUsingDOM.java
 
To run
java PrintUsingDOM xmlFileName
  
or if you have downloaded the Personnel.xml and Personnel.DTD
java PrintUsingDOM Personnel.xml

PrintUsingSAX
To compile
set CLASSPATH=c:\xerces-1_3_0\xerces.jar;c:\xerces-1_3_0\xercesSamples.jar;.;%classpath%;
javac PrintUsingSAX.java
 
To run
java PrintUsingSAX xmlFileName
  
or if you have downloaded the sample xml file and DTD
java PrintUsingSAX Personnel.xml

GenerateXML.java
To compile
set CLASSPATH=c:\xerces-1_3_0\xerces.jar;c:\xerces-1_3_0\xercesSamples.jar;.;%classpath%;
javac GenerateXML.java
 
To run
java GenerateXML