Spring Security 4 with custom authentication and local database - spring-security

Question

Spring Security 4 enables developers to write less code, but sometimes this also causes thing easily to get out of control. For example, now I am writing a login function, once the user pressed a button (login/unionauth URI), an OAuth 2.0 like authentication offered by a 3rd party would be launched, and finally the result comes back and we compare the user with our local database. In order to do so, first I have adapter class like this:
#Override
protected void configure(HttpSecurity http) throws Exception {
http
.antMatcher("/**")
.authorizeRequests()
.antMatchers("/", "/login**", "/webjars/**").permitAll()
.anyRequest().authenticated()
.and()
.exceptionHandling()
.authenticationEntryPoint(new LoginUrlAuthenticationEntryPoint("/"))
.and()
.logout()
.logoutSuccessUrl("/").permitAll()
.and()
.csrf()
.csrfTokenRepository(CookieCsrfTokenRepository.withHttpOnlyFalse())
.and()
.addFilterBefore(ssoFilter(), BasicAuthenticationFilter.class);
}
#Override
public void configure(AuthenticationManagerBuilder auth) throws Exception {
auth.userDetailsService(myUserDetailsService);
}
Then I have ssoFilter() like this:
public class UnionAuthenticationProcessingFilter extends AbstractAuthenticationProcessingFilter {
#Override
public Authentication attemptAuthentication(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse) throws AuthenticationException, IOException, ServletException {
// authentication steps.
UsernamePasswordAuthenticationToken token = new UsernamePasswordAuthenticationToken("tom", "N/A", null);
AuthenticationManager manager = this.getAuthenticationManager();
return manager.authenticate(token);
}
Now, the problem is, manager is null. Why it is null? I think in case of password username mode, the auth.userDetailsService(myUserDetailsService); in adapter would enable a DAO manager. In official website, it has this:
How to Add a Local User Database
Many applications need to hold data about their users locally, even if
authentication is delegated to an external provider. We don’t show the
code here, but it is easy to do in two steps.
Choose a backend for your database, and set up some repositories (e.g.
using Spring Data) for a custom User object that suits your needs and
can be populated, fully or partially, from the external
authentication.
Provision a User object for each unique user that logs in by
inspecting the repository in your /user endpoint. If there is already
a user with the identity of the current Principal, it can be updated,
otherwise created.
Hint: add a field in the User object to link to a unique identifier in
the external provider (not the user’s name, but something that is
unique to the account in the external provider).
Any idea how to add database into Spring Security OAuth2 or why the manager in the first paragraph is null?