Clean up SAX Handler - xml-parsing

Question

I've made a SAX parser for parsing XML files with a number of different tags. For performance reasons, I chose SAX over DOM. And I'm glad I did because it works fast and good. The only issue I currently have is that the main class (which extends DefaultHandler) is a bit large and not very easy on the eyes. It contains a huge if/elseif block where I check the tag name, with some nested if's for reading specific attributes. This block is located in the StartElement method.
Is there any nice clean way to split this up? I would like to have a main class which reads the files, and then a Handler for every tag. In this Tag Handler, I'd like to read the attributes for that tag, do something with them, and then go back to the main handler to read the next tag which again gets redirected to the appropriate handler.
My main handler also has a few global Collection variables, which gather information regarding all the documents I parse with it. Ideally, I would be able to add something to those collections from the Tag Handlers.
A code example would be very helpful, if possible. I read something on this site about a Handler Stack, but without code example I was not able to reproduce it.
Thanks in advance :)

Answer 1

I suggest setting up a chain of SAX filters. A SAX filter is just like any other SAX Handler, except that it has another SAX handler to pass events into when it's done. They're frequently used to perform a sequence of transformations to an XML stream, but they can also be used to factor things the way you want.
You don't mention the language you're using, but you mention DefaultHandler so I'll assume Java. The first thing to do is to code up your filters. In Java, you do this by implementing XMLFilter (or, more simply, by subclassing XMLFilterImpl)
import java.util.Collection;
import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.XMLFilterImpl;
public class TagOneFilter extends XMLFilterImpl {
private Collection<Object> collectionOfStuff;
public TagOneFilter(Collection<Object> collectionOfStuff) {
this.collectionOfStuff = collectionOfStuff;
}
#Override
public void startElement(String uri, String localName, String qName,
Attributes atts) throws SAXException {
if ("tagOne".equals(qName)) {
// Interrogate the parameters and update collectionOfStuff
}
// Pass the event to downstream filters.
if (getContentHandler() != null)
getContentHandler().startElement(uri, localName, qName, atts);
}
}
Next, your main class, which instantiates all of the filters and chains them together.
import java.util.ArrayList;
import java.util.Collection;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.XMLReaderFactory;
public class Driver {
public static void main(String[] args) throws Exception {
Collection<Object> collectionOfStuff = new ArrayList<Object>();
XMLReader parser = XMLReaderFactory.createXMLReader();
TagOneFilter tagOneFilter = new TagOneFilter(collectionOfStuff);
tagOneFilter.setParent(parser);
TagTwoFilter tagTwoFilter = new TagTwoFilter(collectionOfStuff);
tagTwoFilter.setParent(tagOneFilter);
// Call parse() on the tail of the filter chain. This will finish
// tying the filters together before executing the parse at the
// XMLReader at the beginning.
tagTwoFilter.parse(args[0]);
// Now do something interesting with your collectionOfStuff.
}
}