Spring Security - Authrozation on rest apis

I am trying to implement role based authorization on my rest apis.
I have been able to successfully implement and run the project but there are some things that I am confused about.

CustomeUserDetailsService.java:

@Override
public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
    Optional<UsersAuth> optionalUsers = usersAuthRepository.findByName(username);

    optionalUsers
            .orElseThrow(() -> new UsernameNotFoundException("Username not found"));
    return optionalUsers
            .map(CustomUserDetails::new).get();
}

CustomUserDeatils.java:

 @Override
public Collection<? extends GrantedAuthority> getAuthorities() {

    return getRoles()
            .stream()
            .map(role -> new SimpleGrantedAuthority("ROLE_" + role.getRole()))
            .collect(Collectors.toList());
}

I want to know what the above two code snippets are actually doing. can someone explain to me? I have searched alot but did not understand. Specially what is the use of Granted authority? I know for a fact that CustomUserDetailsService is taking a username and if it does not exist, it throws an exception. If anyone could elaborate in detail? It would help me alot.

Optional<UsersAuth> optionalUsers = usersAuthRepository.findByName(username);

It is looking for user by his name probably inside databse or other in-memory store.

optionalUsers
            .orElseThrow(() -> new UsernameNotFoundException("Username not found"));

As you already know this line will throw exception if Optional is empty in other words user do not exist.

return optionalUsers
            .map(CustomUserDetails::new).get();

This line creates new object of CustomUserDeatils class that implements UserDetails interface and returns it. CustomUserDeatils has constructor with UsersAuth parameter.

return getRoles()
            .stream()
            .map(role -> new SimpleGrantedAuthority("ROLE_" + role.getRole()))
            .collect(Collectors.toList());

This is simple mapping of probably Role collection to SimpleGrantedAuthority list. As I mentioned earlier CustomUserDeatils class implements UserDetails. One of its method is Collection<? extends GrantedAuthority> getAuthorities() which is used to get info about authorities granted to user. AuthenticationProvider will use this info from UserDetailsService to create Authentication object. Spring Security will be checking if authenticated user has required roles etc using this mapped roles. SimpleGrantedAuthority is just implementation of GrantedAuthority. ROLE_ is default prefix used by RoleVoter when you are checking for roles in your controller method e.g @Secured("ROLE_ADMIN")

In the simplest way to answer, when you try to login with username and password, spring security calls loadUserByUsername with that username you provide. This method looks in the database if any user exists with the username. If doesnt throws exception. Otherwise returns the UserDetails object. Then spring security checks if the password of UserDetails object matched with the password you have provided. If doesnt throws exception. Otherwise provides you authentication token.

User can have role and this role can be used to permit/block apis for that user. For example in the database that user has role CUSTOMER and some other users have role ADMIN. Now you want certain api /abc to be exposed only to the users with CUSTOMER role. For this you have to do this

@PreAuthorize("hasAnyAuthority('CUSTOMER')")
@PostMapping(value = "/abc")
  public ResponseEntity<Map<String, Integer>> create(@Valid @RequestBody DTO dto) {
  ...

The above api is accessible to only user with role CUSTOMER