Java and XML

“Gotcha!”

To continue with the theme of trying to provide some cautionary advice on your path to XML mastery, some additional pitfalls associated with XML validation are included here. These are often problems run into by newer XML developers, as the solutions are not immediately apparent. Take heed of them, as they have caused many a developer long hours of tedious debugging, or simple confusion at unexpected application output.

Handling Validation and Handling DTDs

One of the most common misunderstandings about using SAX for validation is thinking that validating an XML document is contingent upon registering a SAX DTDHandler implementation with the XML parser. Often, time and effort are spent to implement the DTDHandler interface and register it with the parser, and time is not spent setting the validation feature of the parser. This mistake arises from a mistaken association between handling a DTD and actually using the DTD for validation. In this case, the DTD would be parsed, and all DTD callback events would occur (if any were needed). However, the XML document itself would not be validated, but simply parsed. Keep in mind that the output from parsing a valid XML document looks almost identical to output from a non-validated XML document; always be aware when validation is occurring to avoid application bugs:

try { // Instantiate a parser XMLReader parser = XMLReaderFactory.createXMLReader( "org.apache.xerces.parsers.SAXParser"); // Register the content handler parser.setContentHandler(contentHandler); ...