Simple API for XML

Common Interface implemented for many different XML Parsers
Standard interface for event-based XML parsing
SAX is not an XML Parser
SAX2 is a new Java-based release of SAX
SAX2 adds support for XML Namespaces
SAX 2.0/Java final release at :
http://www.megginson.com/SAX/Java/index.html

What is an Event-Based API?

Event-Based APIs report parsing events directly to the application through callbacks.

Start Element
Start Document
End Element

Application implements handlers to deal with events
Event-Based API provides a simpler, lower-level access to an XML document
Possible to parse large documents much larger than system memory
Application constructs data structure using callback event handlers

How SAX works?

XML Document

     <?xml version="1.0"?>
     <Name>
        <Last_Name>Punin</Last_Name>
        <First_Name>John</First_Name>
     </Name>

Events

     start document
     start element: Name
     start element: Last_Name
     characters: Punin
     end element: Last_Name
     start element: First_Name
     characters: John
     end element: First_Name
     end element: Name
     end document

SAX2/Java Parsers and Drivers

The Apache XML Project's Xerces Java Parser
http://xml.apache.org/xerces-j/index.html
Sun's JAXP
http://java.sun.com/xml/
SAXON XSLT processor
http://users.iclway.co.uk/mhkay/saxon/
Oracle XML Developer's Kit for Java
http://technet.oracle.com/tech/xml/xdk_java.html
ParserAdapter
http://www.megginson.com/SAX/Java/index.html

Working with SAX

Problem: Print a list of the Elements of this XML file:

   <?xml version="1.0"?> 
   <course>
      <name id="csci_2962">Programming XML in Java</name>
      <teacher id="jp">
         <name>John Punin</name>
      </teacher>
      <student id="js">
         <name>John Smith</name>
         <hw1>30</hw1>
         <hw2>70</hw2>
         <project>80</project>
         <final>85</final>
      </student>
      <student id="gl">
         <name>George Lucas</name>
         <hw1>80</hw1>
         <hw2>90</hw2>
         <project>100</project>
         <final>40</final>
      </student>
      <student id="lr">
         <name>Elizabeth Roberts</name>
         <hw1>60</hw1>
         <hw2>95</hw2>
         <project>50</project>
         <final>90</final>
      </student>
   </course>

Using Xerces XML Parser

import org.xml.sax.*;
import org.xml.sax.helpers.DefaultHandler;
import org.apache.xerces.parsers.SAXParser;

public class SaxNames extends DefaultHandler
{
    public static void main(String[] args)
    {
	  SaxNames SAXHandler = new SaxNames();
	  SAXParser parser = new SAXParser();
	  .
	  .
	  .
    }
}

DefaultHandler

Default implementations for all required callback methods
Required methods

void characters(char[] ch, int start, int length)
void endDocument()
void endElement(String uri, String localName, String rawName)
void startDocument()
void startElement(String uri, String localName, String rawName Attributes attributes)

SAXParser

SAXParser class is derived from the XMLParser class

     java.lang.Object
     |
     +--org.apache.xerces.framework.XMLParser
        |
        +--org.apache.xerces.parsers.SAXParser

Register the SAXHandler object to the SAXParser object
Four core SAX interfaces

Entity Resolver - handles external entities
DTDHandler - handles DTD events
ContentHandler - handles content of a document
ErrorHandler - handles error

Using Xerces XML Parser (2)

   import org.xml.sax.*;
   import org.xml.sax.helpers.DefaultHandler;
   import org.apache.xerces.parsers.SAXParser;

   public class SaxNames extends DefaultHandler
   {
      public void startElement(String uri, String localName, String rawName,
                               Attributes attributes)
      {
	 System.out.println("Element : " + localName);
      }

      public static void main(String[] args)
      {
         try{
            SaxNames SAXHandler = new SaxNames();
	    SAXParser parser = new SAXParser();
	    parser.setContentHandler(SAXHandler);
	    parser.setErrorHandler(SAXHandler);
	    parser.parse(args[0]);
	 }
	 catch(Exception e){
	    e.printStackTrace(System.err);
	 }
      }
   }

Compiling and Running SaxNames example

javac -classpath ":xerces.jar" SaxNames.java
java -classpath ":xerces.jar" SaxNames students.xml

     Element : course
     Element : name
     Element : teacher
     Element : student
     Element : name
     Element : hw1
     Element : hw2
     Element : project
     Element : final
     Element : student
     Element : name
     Element : hw1
     Element : hw2
     Element : project
     Element : final
     Element : student
     Element : name
     Element : hw1
     Element : hw2
     Element : project
     Element : final

xerces.jar, SaxNames.java, SaxNames.class and students.xml are in the same directory

Working with SAX (2)

Problem: Print a list of Students' Average and Course Average

   <?xml version="1.0"?> 
   <course>
      <name id="csci_2962">Programming XML in Java</name>
      <teacher id="jp">
         <name>John Punin</name>
      </teacher>
      <student id="js">
         <name>John Smith</name>
         <hw1>30</hw1>
         <hw2>70</hw2>
         <project>80</project>
         <final>85</final>
      </student>
      <student id="gl">
         <name>George Lucas</name>
         <hw1>80</hw1>
         <hw2>90</hw2>
         <project>100</project>
         <final>40</final>
      </student>
      <student id="lr">
         <name>Elizabeth Roberts</name>
         <hw1>60</hw1>
         <hw2>95</hw2>
         <project>50</project>
         <final>90</final>
      </student>
   </course>

Creating SAXHandler and Parser

   import org.xml.sax.*;
   import org.xml.sax.helpers.DefaultHandler;
   import org.apache.xerces.parsers.SAXParser;

   class SaxParserGrades extends DefaultHandler
   {
   ....
   }

   public class SaxGrades 
   {
      public static void main(String[] args)
      {
	 try{
	    SaxParserGrades SAXHandler = new SaxParserGrades();
	    SAXParser parser = new SAXParser();
	    parser.setContentHandler(SAXHandler);
	    parser.setErrorHandler(SAXHandler);
	    parser.parse(args[0]);
	 }
	 catch(Exception e){
	    e.printStackTrace(System.err);
	 }
      }
   }

Writing startElement and endElement

   static float grades[][] = new float[100][5];
   int gi = -1;
   int nstudent = 0;

   public void startElement(String uri, String localName, String rawName,
                            Attributes attributes)
   {
      if(localName.equals("hw1"))
	 gi = 0;
      else if(localName.equals("hw2"))
	 gi = 1;
      else if(localName.equals("project"))
	 gi = 2;
      else if(localName.equals("final"))
	 gi = 3;
      else if(localName.equals("student"))
	 nstudent++;
   }

   public void endElement(String uri, String localName, String rawName)
   {
      gi = -1;
   }

Writing characters

   public void characters(char characters[], int start, int length)
   {
      String chData = (new String(characters, start, length)).trim();
      if(chData.indexOf("\n") < 0 && chData.length() > 0) {
         if(gi >= 0){
            grades[nstudent-1][gi] = Integer.parseInt(chData);
         }
      }
   }

Writing endDocument

   public void endDocument()
   {
      float Ave = 0;
      int i = 0, j = 0;
      System.out.println("Grades");
      for(i = 0; i < nstudent ; i++){
         float total = 0;
         for(j = 0; j < 4; j++){
            total += grades[i][j];
         }		
         grades[i][4] = total/4;
         Ave += grades[i][4];
         System.out.println("Student " + i + "=" + grades[i][4]);
      }
      Ave /= nstudent;
      System.out.println("Class Average = " + Ave);
   }

Putting all together

   import org.xml.sax.*;
   import org.xml.sax.helpers.DefaultHandler;
   import org.apache.xerces.parsers.SAXParser;

   class SaxParserGrades extends DefaultHandler
   {
      static float grades[][] = new float[100][5];
      int gi = -1;
      int nstudent = 0;
    
      public void startElement(String uri, String localName, String rawName,
                               Attributes attributes)
      {
         if(localName.equals("hw1"))
            gi = 0;
         else if(localName.equals("hw2"))
            gi = 1;
         else if(localName.equals("project"))
            gi = 2;
         else if(localName.equals("final"))
            gi = 3;
         else if(localName.equals("student"))
         nstudent++;
      }

      public void endElement(String uri, String localName, String rawName)
      {
         gi = -1;
      }

      public void characters(char characters[], int start, int length)
      {
          String chData = (new String(characters, start,length)).trim();
          if(chData.indexOf("\n") < 0 && chData.length() > 0){
             if(gi >= 0){
                grades[nstudent-1][gi] = Integer.parseInt(chData);
             }
          }
      }

      public void endDocument()
      {
         float Ave = 0;
         int i = 0, j = 0;
         System.out.println("Grades");
         for(i = 0; i < nstudent ; i++){
            float total = 0;
            for(j = 0; j < 4; j++){
               total += grades[i][j];
            }		
            grades[i][4] = total/4;
            Ave += grades[i][4];
            System.out.println("Student " + i + "=" + grades[i][4]);
         }
         Ave /= nstudent;
         System.out.println("Class Average =" + Ave);
      }
   }

   public class SaxGrades 
   {
      public static void main(String[] args)
      {
         try{
            SaxParserGrades SAXHandler = new SaxParserGrades();
            SAXParser parser = new SAXParser();
            parser.setContentHandler(SAXHandler);
            parser.setErrorHandler(SAXHandler);
            parser.parse(args[0]);
      }
      catch(Exception e){
         e.printStackTrace(System.err);
      }
   }
}


Compiling and Running SaxGrades example

javac -classpath ":xerces.jar" SaxGrades.java
java -classpath ":xerces.jar" SaxGrades students.xml

     Grades
     Student 0= 66.25
     Student 1= 77.5
     Student 2= 73.75
     Class Average = 72.5


xerces.jar, SaxGrades.java, SaxGrades.class and students.xml are in
    the same directory



Working with SAX (Using Attributes)

Attributes Interface




  int getIndex(String rawName)
         Gets the index of an attribute given a raw name
  String getRawName(int index)
         Gets the attribute's raw name by index
  String getValue(int index)
         Gets an attribute's value by index
  String getValue(String rawName)
         Gets an attribute's value by raw name



Working with SAX (Using Attributes) (2)

Problem:   Compute the area of each of these figures:

     <?xml version="1.0"?> 
     <figures>
        <circle x="20" y="10" r="20"/>
        <rectangle x="-3" y="4" w="5" h="36"/>
        <ellipse x="-5" y="6" w="30" h="50"/>
        <rectangle x="7" y="23" w="58" h="45"/>
        <circle x="-2" y="5" r="35"/>
        <ellipse x="-10" y="-8" w="45" h="30"/>
     </figures>



Working with SAX (Using Attributes) (3)
   import org.xml.sax.*;
   import org.xml.sax.helpers.DefaultHandler;
   import org.apache.xerces.parsers.SAXParser;

   public class SaxFigures extends DefaultHandler
   {
       public void startElement(String uri, String localName, String rawName,
                                Attributes attributes)
       {
          if(localName.equals("circle"))
          {
             String sr = attributes.getValue("r");
             float radius = Float.valueOf(sr).floatValue();
             float area = (float)Math.PI*radius*radius;
             System.out.println("Circle : Radius = " + radius + 
                                " Area = " + area);
          }
          else if(localName.equals("ellipse"))
          {
             String sw = attributes.getValue("w");
             String sh = attributes.getValue("h");
             float width = Float.valueOf(sw).floatValue();
             float height = Float.valueOf(sh).floatValue();
             float area = (float)Math.PI*(width/2)*(height/2);
             System.out.println("Ellipse : Width = " + width + 
                                " Height = " + height + 
                                " Area = " + area);
          }
          else if(localName.equals("rectangle"))
          {
             String sh = attributes.getValue("h");
             String sw = attributes.getValue("w");
             float width = Float.valueOf(sw).floatValue();
             float height = Float.valueOf(sh).floatValue();
             float area = width * height;
             System.out.println("Rectangle : Width = " + width +
                                " Height = " + height + 
                                " Area = " + area);
          }
      }

      public static void main(String[] args)
      {
         try{
            SaxFigures SAXHandler = new SaxFigures();
            SAXParser parser = new SAXParser();
            parser.setContentHandler(SAXHandler);
            parser.setErrorHandler(SAXHandler);
            parser.parse(args[0]);
         }
         catch(Exception e){
            e.printStackTrace(System.err);
         }
      }
   }


Compiling and Running SaxFigures example

javac -classpath ":xerces.jar" SaxFigures.java
java -classpath ":xerces.jar" SaxFigures figures.xml

     Circle : Radius = 20.0 Area = 1256.6371
     Rectangle : Width = 5.0 Height = 36.0 Area = 180.0
     Ellipse : Width = 30.0 Height = 50.0 Area = 1178.0973
     Rectangle : Width = 58.0 Height = 45.0 Area = 2610.0
     Circle : Radius = 35.0 Area = 3848.4512
     Ellipse : Width = 45.0 Height = 30.0 Area = 1060.2876


xerces.jar, SaxFigures.java, SaxFigures.class and figures.xml are in 
    the same directory