The move from JAAS to JASPI subtly changes how we interact with identities. In the world of JAAS we deal with Subjects who are the entities making a request, typically a user, whilst Java EE deals with Principals, the representation of that entity such as a username. The difference may not seem great, but a Subject may have several Principals and this has caused some headaches when using JAAS, leaving determination of the relevant Principal to the implementation.
The days of JAAS have long been numbered however, and JSR-196 (also known as JASPI or JASPIC) is emerging at last; inclusion in JEE6 has definitely helped to push JASPI beyond just Glassfish support.
One of the changes is using the CallerPrincipalCallback to present to the container which Principal is applicable; and which is then available in the ServletRequest using getUserPrincipal(…).
Some background music for mulling over Subjects and Principals: Subject’s theme from Aldo Nova