I hope you aren't expecting to redirect when a file upload has reached a certain size limit. HTTP doesn't work that way.
When an HttpServletRequest runs, the entire fire has already been transferred. It doesn't run concurrently with the upload.
You can set a limit on how much data will be accepted in web.xml, and that will cause the data to be rejected before your application code gets run. I'm not sure if it's smart enough to forcibly shut down the incoming network connection immediately or if it simply keeps going and discards the data. In either case, however, that won't allow you to redirect in application code, because the application code, like I said, only gets control after the webapp server has already accepted and cached the data.
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