XML DOM Tutorial

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XML DOM Tutorial

• CSC 309
• By Meng Lou

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DOM

• Introduction
• Overview
• Steps for DOM parsing
• Examples
• DOM or SAX?
• Summary

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Introduction

• DOM supports navigating and modifying XML
  documents.
• Hierarchical tree representation of documents
• Language Neutral, C, Java, CORBA
• www.w3c.org/DOM

4
Pros and Cons

• Advantages Robust API for the DOM TREE
  Relatively simple to modify the data structure
  and extract data
• Disadvantages Stores the entire document in
  memory As DOM was written for any language,
  method naming conventions dont follow standard
  Java conventions

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Overview of steps
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Steps for parsing

• Specify parser
• Create a document builder
• Invoke the parser to create a Document
  representing the XML document
• Normalize
• Obtain the root node
• Modify and examine the properties of nodes

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Specifying a Parser

• Use the command line java D option
• In the program, use System.setProperty, eg.
• System.setProperty( javax.xml.parsers.DocumentB
  uilderFactory, org.apache.xerces.jaxp.DocumentBu
  ilderFactoryImpl )

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Create a Document Handler

• Create an instance of builder factory, then use
  it to create a DocumentBuilder Object
• DocumentBuilderFactory builderFactory
  DocumentBuilderFactory.newInstance()
• DocumentBuilder builder builderFactory.newDocum
  entBuilder()

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Create a Dcoument

• Call the parse method
• Document doc builder.parse (someInputStream)
• The Document class represents the parsed result
  in a tree structure

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Normalize the Tree

• Normalization has two affects
• - Combines textual nodes that span multiple
  lines
• - Eliminates empty textual nodes
• doc.getDocumentElement().normalize()

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Obtain the root node

• Traversing begins at the root node
• Element rootElement doc.getDocumentElement()
• - Element is a subclass of the more general Node
  class represents an XML element
• - Node represents all the various components of
  an XML document
• eg. Document, Element, Attribute, Entity

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Examine and Modify Nodes

• Various properties
• - getNodeName
• - getNodeType
• - getAttributes
• - getChildNodes
• - setNodeValue
• - appendChild
• - removeChild
• - replaceChild

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Sample Code Bits

• //walk the DOM tree and print as u go
• public void walk(Node node)
• int type node.getNodeType()
• switch(type)
• case Node.DOCUMENT_NODE
• System.out.println("lt?xml
  version\"1.0\" encoding\""
• "UTF-8"
  "\"?gt")
• break
• //end of document
• case Node.ELEMENT_NODE
• System.out.print('lt'
  node.getNodeName() )
• NamedNodeMap nnm
  node.getAttributes()
• if(nnm ! null )
• int len nnm.getLength()

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DOM or SAX ?

• Dom
• - Suitable for small documents
• - Easily modify document
• - Memory intensive
• SAX (Simple API for XML)
• - Suitable for large documents
• - Only traverse document once
• - event Driven, saves memory

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Summary

• DOM is a tree representation of an XML document
  in memory
• JAXP provides a vendor-neutral interface to the
  underlying parser
• Every component of the XML document is a Node
• Use normalization to combine text elements that
  spans multiple lines