Flex applications integrates well with any server environment, including J2EE. To effectively implement secure web applications in a J2EE environment, you should understand the following concepts:

Authentication 

The process of gathering user credentials (user name and password) and validating them in the system. This requires checking the credentials against a user repository such as a database, flat file, or LDAP implementation, and authenticating that the user is who they say they are.

Authorization 

The process of making sure that the authenticated user is allowed to view or access a given resource. If a user is not authorized to view a resource, the container does not allow access.