Java and XML

1
Java and XML

• Format of lecture
• What is XML?
• A sample XML file
• How to use XML with JSP.
• A demonstration of parsing an XML file in JSP.

2
What is XML?

• XML is principally concerned with the description
  and structure of data (content presentation are
  separate).
• Traditional methods of data storage and exchange
  employ a variety of schemes.
• Electronically, these usually are of the form of
  simple text files (Comma Separated Values etc.)
  or binary files.
• Both methods have their own advantages and
  disadvantages.
• CSV files contain data that can be easily read
  but lack a description of their own format.
• Binary files contain data and a description of
  its data format (as a Word document does) but are
  proprietary schemes that require specific
  applications to read them.

3
What is XML?

• XML stands for eXtensible Markup Language.
• Developed by World Wide Web Consortium (W3C).
• The aim of XML is to provide the best of both
  worlds a means to store data in an easy to read
  text format that also contains information that
  describes the format of the data itself.
• Open standard - looks similar to HTML (large user
  base) except the extensible nature of XML allows
  for the creation of user-defined tags.
• Is not to be thought of as purely a replacement
  for HTML (although it will eventually supplant
  it).

4
What is XML?

• XML offers a standard method for storing
  structured data.
• Language independence (English, Chinese etc.).
• Hierarchical structure allows for simple and
  efficient querying/parsing of the document.
• Simple data interchange between applications
  and/or distributed objects.
• Can improve upon and replace existing EDI
  (Electronic Document Interchange) solutions.
• Websites gain by having content and presentation
  separate.
• Cost benefits
• No need for private networks Internet used as
  exchange medium.
• Reduced time to implement/reduced maintenance.

5
Illustrative Example

• Paper-based or Simple Text File
• Yvan Cartwright
• Staffordshire University
• y.j.f.cartwright_at_staffs.ac.uk
• Comma Separated Values
• Yvan,Cartwright,Staffordshire University,y.j.f.ca
  rtwright_at_staffs.ac.uk
• Binary a string of 0s and 1s
• 0101 1001 0111 0110 0110 0001 0110 1110 0010
  0000 etc.

6
Simple Illustrative Example

• lt?xml version"1.0" encoding"UTF-8" ?gt
• ltcontactlist owner"Yvan"gt
• ltcontactgt
• ltnamegt
• ltfirstnamegtYvanlt/firstnamegt
• ltlastnamegtCartwrightlt/lastnamegt
• lt/namegt
• ltworkplacegtStaffordshire Universitylt/workplacegt
• ltemailgty.j.f.cartwright_at_staffs.ac.uklt/emailgt
• lt/contactgt
• lt/contactlistgt

7
How to use XML with JSP

• XML is a big topic!
• We are just going to look at one of the more
  fundamental aspects of XML.
• The XML Parser.
• An XML parser simply checks that your XML
  document is syntactically correct (well-formed)
  and contains correctly formatted data (valid).
• Once an XML document is parsed, the information
  it contains is accessible inside our web
  application.
• There are two ways of parsing an XML document
  SAX and DOM.

8
SAX Parser

• Event Driven An event is triggered each time
  the parser encounters a beginning, or ending tag.

9
DOM Parser

• A tree representing the document is built in
  memory.

10
Tree Structure

• contacts---contact
• ---name
• ---firstname
• ---Text
• ---lastname
• ---Text
• ---workplace
• ---Text
• ---email
• ---Text

11
Standard DOM Parsing

• Searching for a contact based on their email
  address
• Create a Document object.
• Load the XML file into the Document.
• Using iterations over the Document nodes, you can
  access any of the values or attributes that are
  stored in the XML file.
• When you find the record that contains the email
  address that you are looking for, do something
  with the information.

12
Standard DOM Parsing

• DocumentBuilderFactory docBuilderFactory
  DocumentBuilderFactory.newInstance()
• DocumentBuilder docBuilder docBuilderFactory.new
  DocumentBuilder()
• // load the XML file into a Document object
• Document doc docBuilder.parse(new
  File(myfile.xml))
• // what does this do?
• doc.normalize()
• // get the whole node structure
• NodeList nodelist getNodeList("/contacts",
  doc)

13
Standard DOM Parsing

• for(int index 0 index lt nodelist.getLength()
  index)
• Node node nodelist.item(index)
• if(node.getNodeType(TEXT_NODE))
• if(node.getNodeName().equals(firstname))
  firstname node.getNodeValue()
• if(node.getNodeName().equals(lastname))
  lastname node.getNodeValue()
• if(node.getNodeName().equals(workplace))
  workplace node.getNodeValue()
• if(node.getNodeName().equals(email)) email
  node.getNodeValue()
• if(email.equals(y.j.f.cartwright_at_staffs.ac.uk)
  )
• displayRecord(firstname, lastname,
  workplace, email)
• break

14
Standard DOM Parsing Problems

• The previous procedure is ok if you have a fairly
  flat XML structure that does not contain many
  different node types.
• If you have many contacts stored then it may be
  very slow to iterate through the nodes until you
  find the contact you are looking for.
• For more complex XML documents, you cant use a
  simple FOR loop to iterate.
• You end up with code that looks more like

15
Not Nice!

• Element root doc.getDocumentElement()
• Node configNode root.getFirstChild()
• NodeList childNodes configNode.getChildNodes()
  
• for (int childNum 0 childNum lt
  childNodes.getLength() childNum)
• if ( childNodes.item(childNum).getNodeType()
  Node.ELEMENT_NODE )
• Element child (Element) childNodes.item(
  childNum )
• if ( child.getTagName().equals( "header" ) )
• // Do something with the header
  System.out.print("Got a header!\n")

16
XPath

• XPath (XML Path Language) is a terse (non-XML)
  syntax for addressing portions of an XML
  document.
• A typical XPath expression is a Location Path
  consisting of a string of element or attribute
  qualifiers separated by forward slashes ("/"),
  similar in appearance to a file system path.
• E.g. this gets the firstname of the first contact
• //contact1/name/firstname

17
Using XPath

• Using the same code as per slide 12, we can then
  use the XPath syntax to retrieve any information
  we like from the Document.
• XPath is a new(ish) specification and as such it
  was only available via Java extensions.
• As of version 1.5 of the JDK, Java now natively
  supports XPath.

18
Search Using XPath

• // create the XPath and initialize it
• XPath xPath XPathFactory.newInstance().newXPath(
  )
• // now execute the XPath select statement to get
  the contact that matches the email address given
• NodeList nodes (NodeList) xPath.evaluate(//conta
  ctemaily.j.f.cartwright_at_staffs.ac.uk,
  nodelist, XPathConstants.NODESET)
• Node firstNameNode (NodeList)
  xPath.evaluate(//firstname, nodes,
  XPathConstants.NODESET)
• String firstname firstNameNode.getNodeValue()
• Node lastNameNode (NodeList) xPath.evaluate(//l
  astname, nodes, XPathConstants.NODESET)
• String lastname lastNameNode.getNodeValue()
• Node workPlaceNode (NodeList)
  xPath.evaluate(//workplace, nodes,
  XPathConstants.NODESET)
• String workplace workPlaceNode.getNodeValue()
• Node emailNode (NodeList) xPath.evaluate(//emai
  l, nodes, XPathConstants.NODESET)
• String email emailNode.getNodeValue()

19
XMLHelper2.java

• package helper
• import javax.xml.xpath.XPath
• import javax.xml.xpath.XPathConstants
• import javax.xml.xpath.XPathExpressionException
• import javax.xml.xpath.XPathFactory
• import org.w3c.dom.Document
• import org.w3c.dom.Node
• import org.w3c.dom.NodeList

20
Constructor

• public class XMLHelper2
• public XMLHelper2()
• // bit blank isnt it

21
getNodeListXPath()

• public NodeList getNodeListXPath(String
  expression, Document target) throws
  XPathExpressionException
• // create the XPath and initialize it
• XPath xPath XPathFactory.newInstance().newXPat
  h()
• // now execute the XPath select statement
• NodeList nodes (NodeList) xPath.evaluate(expre
  ssion, target, XPathConstants.NODESET)
• // return the resulting node
• return nodes

22
getNumberXPath()

• public double getNumberXPath(String expression,
  Document target) throws XPathExpressionException
• // create the XPath and initialize it
• XPath xPath XPathFactory.newInstance().newXPat
  h()
• // now execute the XPath select statement
• Double nodeNumber (Double)xPath.evaluate(expre
  ssion, target, XPathConstants.NUMBER)
• // return the resulting node
• return nodeNumber.doubleValue()

23
getStringXPath()

• public String getStringXPath(String expression,
  Document target) throws XPathExpressionException
• // create the XPath and initialize it
• XPath xPath XPathFactory.newInstance().newXPat
  h()
• // now execute the XPath select statement
• String nodeText (String)xPath.evaluate(express
  ion, target, XPathConstants.STRING)
• // return the resulting node
• return nodeText

24
getChildNodeText()

• private String getChildNodeText(Node node)
• String childNodeText node.getTextContent().tri
  m()
• if(childNodeText.equals("")
  childNodeText.equals("\r")) childNodeText
  null
• return childNodeText

25
getNodeValue()

• private String getNodeValue(String expression,
  Document doc) throws Exception
• String nodeValue null
• NodeList nodelist
• Node node
• nodelist getNodeList(expression, doc)

26
getNodeValue()

• for(int i 0 i lt nodelist.getLength() i)
• node nodelist.item(i)
• nodeValue getChildNodeText(node)
• if(nodeValue ! null) break
• return nodeValue

27
Using the XMLHelper class

• XMLHelper2 xmlHelper new XMLHelper2()
• DocumentBuilderFactory docBuilderFactory
  DocumentBuilderFactory.newInstance()
• DocumentBuilder docBuilder docBuilderFactory.new
  DocumentBuilder()
• // load the XML file into a Document object
• Document doc docBuilder.parse(new
  File(//trentdev/wwwdatajsp/JWWW/yvan/contacts.xml
  ))
• // what does this do?
• doc.normalize()
• // get the whole node structure
• NodeList nodelist xmlHelper.getNodeListXPath("/c
  ontacts", doc)