Now Im really confused !!
Only the transactional methods the MessageDrivenContext inherits from EJBContext are available to message-driven beans. The home methods--getEJBHome() and getEJBLocalHome()--throw a RuntimeException if invoked, because MDBs do not have home interfaces or EJB home objects. The security methods--getCallerPrincipal() and isCallerInRole()--also throw a RuntimeException if invoked on a MessageDrivenContext. When an MDB services a JMS message there is no "caller," so there is no security context to be obtained from the caller. Remember that JMS is asynchronous and doesn't propagate the sender's security context to the receiver--that wouldn't make sense, since senders and receivers tend to operate in different environments.
extracted from Ejb 3 (Oreilly)