If using threadlocal sessions (As in HibernateUtil.java)
then does this mean that one should never close the session because if one closes the session, there is no way to "reopen" the session.

As a result does it mean that one has to call sessionfactory.opensession repeatedly for each request ? is keeping the threadlocal session an optimization only or is there anything else to it ? is there any inherent problem with invoking opensession repeatedly other than performance?

thanks.

There are various strategies
http://www.hibernate.org/42.html
http://www.hibernate.org/43.html

I would say that session-per-request strategy is a good one.
Session is a lightweight object and it is OK to open it on request arrival and then close upon request completion.