Q:

Principal stays string albeit using a custom user details class in Spring security

I have a custom UserDetailsService as follows:
@Service
public class AuthenticatedUserService implements UserDetailsService {

    private final UserRepository userRepository;

    @Autowired
    public AuthenticatedUserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    @Override
    public UserDetails loadUserByUsername(String username) {
        UserCredentials userCredentials = userRepository.findByUsername(username);
        if (userCredentials == null) {
            throw new UsernameNotFoundException("The user " + username + " does not exist");
        }
        return new AuthenticatedPrincipal(userCredentials);
    }
}

My AuthenticatedPrincipal looks like this:
public class AuthenticatedPrincipal implements UserDetails {
    private UserCredentials userCredentials;

    public AuthenticatedPrincipal() {

    }

    public AuthenticatedPrincipal(UserCredentials userCredentials) {
        this.userCredentials = userCredentials;
    }

    @Override
    public Collection<? extends GrantedAuthority> getAuthorities() {
        List<GrantedAuthority> authorities = new ArrayList<>();
        ...
        return authorities;
    }

    @Override
    public String getPassword() {
        return userCredentials.getPassword();
    }

    @Override
    public String getUsername() {
        return userCredentials.getUsername();
    }

    @Override
    public boolean isAccountNonExpired() {
        return true;
    }

    @Override
    public boolean isAccountNonLocked() {
        return true;
    }

    @Override
    public boolean isCredentialsNonExpired() {
        return true;
    }

    @Override
    public boolean isEnabled() {
        return true;
    }
}

I'm using my service this way:
@Configuration
@EnableWebSecurity
public class WebSecurityConfig extends WebSecurityConfigurerAdapter {

    @Qualifier("authenticatedUserService")
    @Autowired
    private UserDetailsService userDetailsService;

    @Autowired
    protected void configureGlobal(AuthenticationManagerBuilder auth) throws Exception {
        // auth.ldapAuthentication()

        auth.userDetailsService(userDetailsService)
            .passwordEncoder(passwordEncoder);
    }

    ...
}

And everything works. I can login just fine. BUT I never get my custom principal. In every place a just get a plain string containing the user name. For example, I want to implement a custom SecurityExpressionRoot. 
public class CustomMethodSecurityExpressionRoot
    extends SecurityExpressionRoot implements MethodSecurityExpressionOperations {

    public CustomMethodSecurityExpressionRoot(Authentication authentication) {
        super(authentication);
    }

    public boolean isMember(Long OrganizationId) {
        // Throws an exception, because principal is just a string
        AuthenticatedPrincipal user = (AuthenticatedPrincipal) this.getPrincipal();
        return false;
    }

The same happens when I try to insert it into one of my controllers.
public ResponseEntity listMessages(@AuthenticationPrincipal Principal principal) {

I don't know what to do and try any more. I don't see any difference from what I've done and this tutorial, for example. 

A:

Okay, I've managed to track down and solve my issue. I am using JWT and was using a custom filter, which sets the authentication. Instead of returning a proper Authentication object, I had been using the default UsernamePasswordAuthenticationToken. To solve this, I went ahead and created my own token class: 
public class TokenBasedAuthentication extends AbstractAuthenticationToken {
    private final UserDetails principal;

    public TokenBasedAuthentication(AuthenticatedPrincipal principal) {
        super(principal.getAuthorities());
        this.principal = principal;
    }

    @Override
    public boolean isAuthenticated() {
        return true;
    }

    @Override
    public UserDetails getPrincipal() {
        return principal;
    }
}

Then, in my filter, I went ahead and created a proper authentication object:
AuthenticatedPrincipal userDetails = (AuthenticatedPrincipal) userDetailsService.loadUserByUsername(username);
TokenBasedAuthentication authentication = new TokenBasedAuthentication(userDetails);
SecurityContextHolder.getContext().setAuthentication(authentication);

Now I'm getting the right principal as well. Actually should have figured that out sooner, but all well, I guess.

