Question

I’m building a web application which is supposed to measure httprequests/response times as well as response size. for this task I used: http://angelborroy.wordpress.com/2009/03/04/dump-request-and-response-using-javaxservletfilter/ filter, which does everything I need only it dumps requests/response data instead of print the response size.
It works just fine with Servlets, but there just seems to be a problem with JSPs… when I’m trying to load up a JSP in the browser, it displays a blank page, and the filter echos that the response size is 0.

Again, it works flawlessly when the requested page is a servlet… any idea what can cause this issue?

UPDATE: I’ve been trying to find any filter which wraps a servlet response and then able to forward the response back to the client (even as is!) but without any luck, if anyone can point me out to a working example, it doesn’t matter what the wrapper does, just a simple wrapper which does something on the response content and forwards it back to the client after the change, i’ll more than grateful.

P.S

I’m using Apache Tomcat 7

Answer 1

Flush the writer and the stream after doFilter():

filterChain.doFilter(req, wrappedResp);
pw.getWriter().flush();
pw.getStream().flush();

Please note that the linked codes does not throw IllegalStateException when:

• you call getOutputStream() and the getWriter method has been called on this response

or

• you call getWriter() and the getOutputStream method has already been called for this response object.

Check the source code of javamelody too. Their codes looks more robust.

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The PrintWriter uses an inner BufferedReader and it needs a flush. It was just a quick solution so maybe the getStream().flush() isn’t really required. Please notice that I called pw.get*().flush(), not response.get*().flush(). But it’s still just a quick answer for the specific problem when JSPs are not showed. If you need a production grade solution check the source of javamelody. They probably handle a lot more corner cases (like the mentioned IllegalStateException) than the referred article.
I would start with the source of javamelody and I’d cut out the unnecessary features.