A SessionFactory is an expensive-to-create, threadsafe object intended to be shared by all application threads. It is created once, usually on application startup, from a Configuration instance.

A Session is an inexpensive, non-threadsafe object that should be used once, for a single request, a conversation, single unit of work, and then discarded. A Session will not obtain a JDBC Connection (or a Datasource) unless it is needed, hence consume no resources until used.

To complete this picture you also have to think about database transactions. A database transaction has to be as short as possible, to reduce lock contention in the database. Long database transactions will prevent your application from scaling to highly concurrent load. Hence, it is almost never good design to hold a database transaction open during user think time, until the unit of work is complete.

What is the scope of a unit of work? Can a single Hibernate Session span several database transactions or is this a one-to-one relationship of scopes? When should you open and close a Session and how do you demarcate the database transaction boundaries?