Java and XML, Second Edition

More Handlers

In the last chapter, I showed you the ContentHandler and ErrorHandler interfaces and briefly mentioned the EntityResolver and DTDHandler interfaces as well. Now that you’ve got a good understanding of SAX basics, you’re ready to look at these two other handlers.^[5] You’ll find that you use EntityResolver every now and then (more if you’re writing applications to be resold), and that the DTDHandler is something rarely ever pulled out of your bag of tricks.

Using an EntityResolver

The first of these new handlers is org.xml.sax.EntityResolver. This interface does exactly what it says: resolves entities (or at least declares a method that resolves entities, but you get the idea). The interface defines only a single method, and it looks like this:

public InputSource resolveEntity(String publicID, String systemID)
    throws SAXException, IOException;

You can create an implementation of this interface, and register it with your XMLReader instance (through setEntityResolver( ), not surprisingly). Once that’s done, every time the reader comes across an entity reference, it passes the public ID and system ID for that entity to the resolveEntity( ) method of your implementation. Now you can change the normal process of entity resolution.

Typically, the XML reader resolves the entity through the specified public or system ID, whether it be a file, URL, or other resource. And if the return value from the resolveEntity( ) method is null, this process executes unchanged. As a result, ...