3) Streaming API for XML
3.1) Introduction
Streaming API for XML, simply called StaX, is an API for reading and writing XML Documents. Why need another XML Parsing API when we already have SAX (Simple API for XML Parsing) and DOM (Document Object Model)? Both SAX and DOM parsers have their own advantages and disadvantages and StaX provides a solution for the disadvantages that are found in both SAX and DOM. It is not that StaX replaces SAX and DOM.
SAX, which provides an Event-Driven XML Processing, follows the Push-Parsing Model. What this model means is that in SAX, Applications will register Listeners in the form of Handlers to the Parser and will get notified through Call-back methods. Here the SAX Parser takes the control over Application thread by Pushing Events to the Application. So SAX is a Push-Parsing model. Whereas StaX is a Pull-Parsing model meaning that Application can take the control over parsing the XML Documents by pulling (taking) the Events from the Parser.
The disadvantage of DOM Parser is, it will keep the whole XML Document Tree in memory and certainly this would be problematic if the size of the Document is large. StaX doesn’t follow this type of model and it also has options for Skipping a Portion of a large Document during Reading.
The core StaX API falls into two categories and they are listed below. They are
- Cursor API
- Event Iterator API
Applications can any of these two API for parsing XML Documents. Let us see what these APIs’ are in detail in the following sections.
3.2) Cursor API
The Cursor API is used to traverse over the XML Document. Think of a Cursor as some kind of Pointer pointing at the start of the XML Document and then Forwarding the Document upon properly instructed. The working model of this Cursor API is very simple. When given a XML Document and asked to parse, the Parser will start reading the XML Document, and if any of the Nodes (like Start Element, Attribute, Text, End Element) are found it will stop and will give information about the Nodes to the Processing Application if requested. This cursor is a Forward only Cursor, it can never go backwards. Both Reading and Writing operations is possible in this cursor API.
3.3) Sample Application
Let us consider a sample Application which will read data from and to the XML Document with the help of the Cursor API. Following is the sample XML Document. The below XML File contains a list of Events for a person in his/her Calendar.
myCalendar.xml
<calendar>
<event type = "meeting">
<whom>With my Manager</whom>
<where>At the Conference Hall</where>
<date>June 09 2007</date>
<time>10.30AM</time>
</event>
<event type = "birthday">
<whom>For my Girl Friend</whom>
<date>01 December</date>
</event>
</calendar>
ReadingUsingCursorApi.java
package net.javabeat.articles.java6.newfeatures.stax;
import java.io.*;
import javax.xml.stream.*;
import javax.xml.stream.events.*;
public class ReadingUsingCurorApi {
private XMLInputFactory inputFactory = null;
private XMLStreamReader xmlReader = null;
public ReadingUsingCurorApi() {
inputFactory = XMLInputFactory.newInstance();
}
public void read() throws Exception{
xmlReader = inputFactory.createXMLStreamReader(
new FileReader(".\\src\\myCalendar.xml"));
while (xmlReader.hasNext()){
Integer eventType = xmlReader.next();
if (eventType.equals(XMLEvent.START_ELEMENT)){
System.out.print(" " + xmlReader.getName() + " ");
}else if (eventType.equals(XMLEvent.CHARACTERS)){
System.out.print(" " + xmlReader.getText() + " ");
}else if (eventType.equals(XMLEvent.ATTRIBUTE)){
System.out.print(" " + xmlReader.getName() + " ");
}else if (eventType.equals(XMLEvent.END_ELEMENT)){
System.out.print(" " + xmlReader.getName() + " ");
}
}
xmlReader.close();
}
public static void main(String args[]){
try{
ReadingUsingCurorApi obj = new ReadingUsingCurorApi();
obj.read();
}catch(Exception exception){
exception.printStackTrace();
}
}
}
XMLInputFactory is the Factory Class for creating Input Stream objects which is represented by XMLStreamReader. An instance of type XMLStreamReader is created by calling XMLInputFactory.createXMLStreamReader() by passing the XML File to be parsed. At this stage, the Parser is ready to read the XML Contents if a combination call to XMLStreamReader.hasNext() and XMLStreamReader.next() is made. The entire Document is traversed in the while loop and the appropriate node's value is taken by checking the various Element Types.
3.4) Event Iterator API
The Working Model of this Event Iterator API is no more different from the Cursor API. As the Parser starts traversing over the XML Document and if any of the Nodes are found, it will provide this information to the Application that is processing in the form of XML Events. Applications can loop over the entire Document, by requesting for the Next Event. This Event Iterator API is implemented on top of Cursor API.
3.5) Sample Application
Now let us take over a Sample Application using the Event Iterator API which is parsing on the XML Document myCalendar.xml.
ReadingUsingEventIterator.java
package net.javabeat.articles.java6.newfeatures.stax;
import java.io.*;
import javax.xml.stream.*;
import javax.xml.stream.events.*;
public class ReadingUsingEventIteratorApi {
private XMLInputFactory inputFactory = null;
private XMLEventReader xmlEventReader = null;
public ReadingUsingEventIteratorApi() {
inputFactory = XMLInputFactory.newInstance();
}
public void read() throws Exception{
xmlEventReader = inputFactory.createXMLEventReader(
new FileReader(".\\src\\myCalendar.xml"));
while (xmlEventReader.hasNext()){
XMLEvent xmlEvent = xmlEventReader.nextEvent();
if (xmlEvent.isStartElement()){
System.out.print(" " + xmlEvent.asStartElement().getName() + " ");
}else if (xmlEvent.isCharacters()){
System.out.print(" " + xmlEvent.asCharacters().getData() + " ");
}else if (xmlEvent.isEndElement()){
System.out.print(" " + xmlEvent.asEndElement().getName() + " ");
}
}
xmlEventReader.close();
}
public static void main(String args[]){
try{
ReadingUsingEventIteratorApi obj = new ReadingUsingEventIteratorApi();
obj.read();
}catch(Exception exception){
exception.printStackTrace();
}
}
}
If XMLStreamReader class represents the Reader for stream reading the XML Contents, then XMLEventReader represents the class for reading the XML Document as XML Events (represented by javax.xml.stream.events.XMLEvent). The rest of the reading logic is the same as that of the ReadingUsingCurorApi.java.
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