title: spring security - one active role
body: <p>Imagine that users can have one or more roles in the system. after 
  login the user have to choose on of these roles. at this point only 
  this selected role should be checked by spring security. </p>
  
  <p>e.g. user has ROLE_A, ROLE_B </p>
  
  <p>requestmaps: </p>
  
  <p>/book/** -> ROLE_A, ROLE_B 
  /author/** -> ROLE A </p>
  
  <p>the normal behaviour would be that user can access all defined 
  actions. in my special case the user would select ROLE_A and then get 
  access only for /author/** actions. in other words, it is possible to 
  define one active role in the security context, so that spring 
  security would perform security checks based on this single role?</p>