How can I display XSLT transformer errors on a web page ?

Question

Hi,
I have some JSP pages that access DB, create an XML based on DB data and then transform it into HTML through an XSLT stylesheet. Developing the XSL code it's easy to make mistakes and generate errors on trasformation, but what I receive on the web page is only a "Could not compile stylesheet" TransformerConfigurationException, while the real cause of the error is displayed only on tomcat logs. This is the code for transformation:
static public void applyXSLT(Document docXML, InputStream isXSL, PrintWriter pw) throws TransformerException, Exception {
        // instantiate the TransformerFactory.
        TransformerFactory tFactory = TransformerFactory.newInstance();
        // creates an error listener
        XslErrorListener xel = new XslErrorListener();
        // sets the error listener for the factory
        tFactory.setErrorListener(xel);
        // generate the transformer
        Transformer transformer = tFactory.newTransformer(new SAXSource(new InputSource(isXSL)));
        // transforms the XML Source and sends the output to the HTTP response
        transformer.transform(new DOMSource(docXML), new StreamResult(pw));
}If an exception is thrown during the execution of this code, its error message is displayed on the web page.
This is the listener class:
public class XslErrorListener implements ErrorListener {
    public XslErrorListener() {
    public void warning(TransformerException ex) {
        // logs on error log
        System.err.println("\n\nWarning on XEL: " + ex.getMessage());
    public void error(TransformerException ex) throws TransformerException {
        // logs on error log
        System.err.println("\n\nError on XEL: " + ex.getMessage());
        // and throws it
        throw ex;
    public void fatalError(TransformerException ex) throws TransformerException {
        // logs on error log
        System.err.println("\n\nFatal Error on XEL: " + ex.getMessage());
        // and throws it
        throw ex;
}When I have an error in the XSL stylesheet (for examples a missing closing tag), I can find on tomcat logs the real cause of the error:
[Fatal Error] :59:10: The element type "table" must be terminated by the matching end-tag "</table>".
Error on XEL: The element type "table" must be terminated by the matching end-tag "</table>".but on my web page is reported just the TransformerConfigurationException message that is:
"Could not compile stylesheet".
How can I display the real cause of the error directly on the web page?
Thanks,
Andrea

Answer 1

This code is part of a bigger project that let developers edit XSL stylesheets through a file upload on the system and we can't impose the use of any tool for checking the xsl. So, I need to display the transformer error on the web page.I see. This code is part of an editorial/developmental tool for developers to create and edit XSL stylesheets.
As part of the editorial process, XSL errors during editing can be considered a normal condition. In other words, it is normal to expect that the developers will generate XSL errors as they are developing stylesheets.
In this light, handling the XSL transformation errors is a business requirement that you need to handle. Using the Java Exceptions mechanisms, e.g. try / catch are inappropriate to handle business requirements, in my opinion.
I suggest that you look at how you handle the occurence of XSL errors differently than what you currently have. You need to:
(1) capture the Transformation exception on the server;
(2) extract the message from the exception and put it into a message that can be easily understood by the user;
The current error message that you have going to the web browser is not useful.
And you should not have the Transformation exception sent to the web browser either.
What you are attempting to do with the exception is not appropriate.
Handle the Transformation exception on the Business tier and use it to create a useful message that is then sent to the Presentation tier. In other words, do not send Java exceptions to web browser.
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