¿Cómo configuro el prefijo de rol en"" con un servicio personalizado de detalles de usuario en Spring?

    <beans:bean id="authService" class="com.cisco.badges.business.services.AuthenticationService"/>

<authentication-manager>
        <authentication-provider user-service-ref="authService">
            <password-encoder ref="passwordEncoder">
                <salt-source ref="saltSource" />
            </password-encoder>
        </authentication-provider>
    </authentication-manager>

@Service("authService")
public class AuthenticationService extends BaseService implements UserDetailsService, IAuthenticationService {

    @Autowired
    IUserRepository userRepository;

    @Autowired
    IAuthorityRepository authorityRepository;

    public AuthenticationService() {

    }

    public UserDetails loadUserByUsername(String username)
            throws UsernameNotFoundException {

        User user = userRepository.findByUsername(username);

        if(user == null)
            throw new UsernameNotFoundException("No user with username '" + username + "' found!");

        List<GrantedAuthority> authList = new ArrayList<GrantedAuthority>();

        for (Role role : user.getRoles()) {
            authList.add(new GrantedAuthorityImpl(role.getName()));
        }

        UserAuthentication userAuthentication = new UserAuthentication(user.getUsername(), user.getPassword(), user.getEnabled() == 0 ? false : true, true, true, true, authList);

        userAuthentication.setSalt(user.getSalt());
        userAuthentication.setId(user.getId());

        return (UserDetails)userAuthentication;
    }
}