This solution may have been considered adequate in a trivial solutions sandbox back in the days of Servlet 2.0 - but, now-a-days there are much more robust, and much more adequate solutions. Using the pre-existence of a session as a handler is completely useless in a modern application (especially a model 2 application). What if this form submission needs information from previous form submissions? (A multiple page process).
This solution is entirely inadequate. Use a synchronizer token or something along those lines if you want to succeed.
It felt like this article was written in 1999, not 2003 - most of the information, although useful to new programmers, needs to be revised with focus on new environments. First comment on that? JSPS!!! Embedded HTML in a servlet? Not worth the time!