I often see the log message "unclosed connection, forgot to call close() on your session?"
I think it may be a message that Hibernate closed connection whiche is lazy or forgotten to be closed.
I cannot discover a information about this at Web.

I want to know whether taht is true or not and When Hibernate starts this job ????.

You usually make a call to session.close() after a "unit of work" to close the connection in a non-managed environment.