getCurrentSession() :
The "current session" refers to a Hibernate Session bound by Hibernate behind the scenes, to the transaction scope. A Session is opened when getCurrentSession() is called for the first time and closed when the transaction ends. It is also flushed automatically before the transaction commits. You can call getCurrentSession() as often and anywhere you want as long as the transaction runs.

To enable this strategy in your Hibernate configuration:
set hibernate.transaction.manager_lookup_class to a lookup strategy for your JEE container
set hibernate.transaction.factory_class to org.hibernate.transaction.JTATransactionFactory

Only the Session that you obtained with sf.getCurrentSession() is flushed and closed automatically.

Example :

try {
UserTransaction tx = (UserTransaction)new InitialContext()
.lookup("java:comp/UserTransaction");

tx.begin();

// Do some work
sf.getCurrentSession().createQuery(...);
sf.getCurrentSession().persist(...);

tx.commit();
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}

openSession() : If you decide to use manage the Session yourself the go for sf.openSession() , you have to flush() and close() it.
It does not flush and close() automatically.

Example :
UserTransaction tx = (UserTransaction)new InitialContext()
.lookup("java:comp/UserTransaction");

Session session = factory.openSession();

try {
tx.begin();

// Do some work
session.createQuery(...);
session.persist(...);

session.flush(); // Extra work you need to do

tx.commit();
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
finally {
session.close(); // Extra work you need to do
}   _{Your Comment}