SOCKET

XSLT, Servlet Filter and a Squeak on Browser Reload

The SOCKET view layer uses XSLT in a servlet filter to transform XML content to HTML for the browser.
Everything worked fine - until, that is, one clicked the refresh button on the browser when the thing squeaked about a premature end to the file that was being transformed.
The problem went away with Cache-Control = no-store, Pragma = no-store, and Expires = a long time ago set on the response headers. (Firefox doesn't work with 'no-cache' instead of 'no-store', IE is indifferent.) Not quite, though. When the university proxy server was switched on through the browser LAN settings, again the refresh failed.
I've only just stumbled upon an explanation of what's going on in Eric Burke's book, Java & XSLT, out of O'Reilly. Even the expert said that it was a difficult one to track down.
The error arises from the fact that in certain circumstances Tomcat re-uses the same HttpServletResponse object. One of these circumstances is the browser reload. Burke states that the servlet specs are not definitive on this point so that implementation dependent behaviour will be observed. We haven't tested on anything other than Tomcat 5 and 5.5, while Burke referred to version 4.
And so, the problem is finally laid to rest. It put up a good fight. The solution is to test for a zero or null output byte array from the ByteArrayPrintWriter, and, if positive, send back the original unwrapped response with

chain.doFilter(request, response);
return;

instead of the

chain.doFilter(request, wrappedResponse);

which results in the squeak. Job's a good 'un.

Posted at 06:12AM Jul 30, 2006 by Brian Peter Clark in View Factory & XSLT  |