JMS connections are served from a connection pool. This means that calling getConnection() on a Queue connection factory is fast.
Previous to version 8.1, it was possible to reuse a connection with a servlet or EJB component. That is, the servlet could call getConnection() in its init() method and then continually call getSession() for each servlet invocation. If you use JMS within a global transaction, that no longer works: applications can only call getSession() once for each connection. After than, the connection must be closed (which doesn’t actually close the connection; it merely returns it to the pool). This is a general feature of portable Java EE 1.4 applications; the Sun Java System Application Server enforces that restriction where previous (Java EE 1.3-based) application servers did not.