org.springframework.security.authentication.InternalAuthenticationServiceException: UserDetailsService returned null, which is an interface contract violation

You can encounter the following exception:

org.springframework.security.authentication.InternalAuthenticationServiceException: UserDetailsService returned null, which is an interface contract violation

You should change from:

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        return userRepository.findByUsername(username);
    }

to:

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
User user = userRepository.findByUsername(username);
if (user == null) {
throw new UsernameNotFoundException(username);
}
return user;
    }