What is the difference between a Custom UserDetailsService and using in-memory authentication? कस्टम UserDetailsService और इन-मेमोरी ऑथेंटिकेशन में क्या अंतर है?

In-memory authentication defines a fixed set of users, passwords, and roles directly in configuration using InMemoryUserDetailsManager, which is useful only for demos, prototypes, or tests since users can't be added or changed without redeploying the application.

A custom UserDetailsService implementation loads user data from a real source, typically a database via a repository, overriding loadUserByUsername() to return a UserDetails object built from the stored user's credentials and authorities. This is the standard approach for any real application, since it allows dynamic user management and integrates with the rest of the persistence layer.

@Service
public class CustomUserDetailsService implements UserDetailsService {
    public UserDetails loadUserByUsername(String username) {
        User user = userRepository.findByUsername(username)
            .orElseThrow(() -> new UsernameNotFoundException("Not found"));
        return new org.springframework.security.core.userdetails.User(
            user.getUsername(), user.getPassword(), user.getAuthorities());
    }
}