We will implement UserDetailsService inside the existing UserApplicationServiceImpl and let's change the UserApplicationService interface to extend UserDetailsService, as shown here:

public interface UserApplicationService extends UserDetailsService {
  ...
}

Here is the change to UserApplicationServiceImpl:

...
public class UserApplicationServiceImpl implements UserApplicationService {
  ...
  @Autowired
  private UserRepository userRepository;

  @Override
  public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
    if (StringUtils.isEmpty(username)) {
      throw new UsernameNotFoundException("No user found");
    }

    User user;
    if (username.contains("@")) {
      user = userRepository.findByEmailAddress(username);
    } else {
      user = userRepository.findByUsername(username);
    }
    if (user == null) {
      throw new UsernameNotFoundException("No user found by `" + 
      username + "`");
    }
    return new SimpleUser(user);
  }
  ...
}

As you can see, we add a reference to UserRepository here. Inside the loadUserByUsername() method, we make sure username is not empty; otherwise, we simply throw the exception with no need to go further. The way of finding the user depends on whether the username property contains an @ or not. When no user is found, we throw an exception. Otherwise, we return an instance of SimpleUser, which implements the UserDetails interface.