SecurityContextHolder.getContext().getAuthentication().getCredentials() returns null after authentication

I have created a simple spring web mvc app and I have one problem. After authentication I try to get an authentication object and for some reason it's credentials is null.

In this project I have a custom AuthenticationProvider which looks like this:

@Component
public class CustomAuthenticationProvider implements AuthenticationProvider {

    @Autowired
    private UserService userService;

    @Autowired
    private RoleService roleService;

    @PostConstruct
    public void init() {
        roleService.AddStandardRolesIfNeeded();
        userService.AddUserWithAdminRoleIfNotExists("a"); 
    }


    public Authentication authenticate(Authentication authentication) throws AuthenticationException {


        Object credentials = authentication.getCredentials();
        if(!(credentials instanceof String)) {
            return null;
        }

        String username = authentication.getName();
        String password = credentials.toString(); //password isn't null here


        User user = userService.findByUsernameAndPassword(username, password);


        if(user == null) {
            throw new BadCredentialsException("Authentication failed for " + username);
        }


        List<GrantedAuthority> authorities = new ArrayList<>();
        for(Role role : user.getRoles()) {
            authorities.add(new SimpleGrantedAuthority(role.getName()));
        }

        Authentication auth = new 
                UsernamePasswordAuthenticationToken(username, password, authorities);


        return auth;
    }

    public boolean supports(Class<?> authentication) {
        return authentication.equals(UsernamePasswordAuthenticationToken.class);
    }

}

I am interested in if it is intentionally done in spring security or I am missing something.

I solved this by passing user object to UsernamePasswordAuthenticationToken constructor instead of username in place of principal.

I changed this:

Authentication auth = new 
                UsernamePasswordAuthenticationToken(username, password, authorities);

to this:

Authentication auth = new 
                    UsernamePasswordAuthenticationToken(user, password, authorities);

And in controller I get the user so:

User user = (User)SecurityContextHolder.getContext().getAuthentication().getPrincipal();