Custom Authentication Manager with Spring Security and Java Configuration

First you must configure Spring security to use your custom AuthenticationProvider. So, in your spring-security.xml (or equivalent config file) you must define wich class is implementing this feature. For example:

<authentication-manager alias="authenticationManager">
    <authentication-provider ref="myAuthenticationProvider" />
</authentication-manager>

<!-- Bean implementing AuthenticationProvider of Spring Security -->
<beans:bean id="myAuthenticationProvider" class="com.teimas.MyAutenticationProvider">
</beans:bean>

Secondly you must implement AuthenticationProvider as in your example. Specially the method authenticate(Authentication authentication) in which your rest call must be. For example:

public Authentication authenticate(Authentication authentication) throws AuthenticationException {
    User user = null;
    try {
        //use a rest service to find the user. 
        //Spring security provides user login name in authentication.getPrincipal()
            user = userRestService.loadUserByUsername(authentication.getPrincipal().toString());
    } catch (Exception e) {
        log.error("Error loading user, not found: " + e.getMessage(), e);
    }

    if (user == null) {
        throw new UsernameNotFoundException(String.format("Invalid credentials", authentication.getPrincipal()));
    } else if (!user.isEnabled()) {
        throw new UsernameNotFoundException(String.format("Not found enabled user for username ", user.getUsername()));
    }
    //check user password stored in authentication.getCredentials() against stored password hash
    if (StringUtils.isBlank(authentication.getCredentials().toString())
        || !passwordEncoder.isPasswordValid(user.getPasswordHash(), authentication.getCredentials().toString()) {
        throw new BadCredentialsException("Invalid credentials");
    }

    //doLogin makes whatever is necesary when login is made (put info in session, load other data etc..)
    return doLogin(user);
} 

Take a look at my sample below. You have to return an UsernamePasswordAuthenticationToken. It contains the principal and the GrantedAuthorities. Hope I could help :)

public Authentication authenticate(Authentication authentication) throws AuthenticationException {
    String username = authentication.getPrincipal() + "";
    String password = authentication.getCredentials() + "";

    User user = userRepo.findOne(username);
    if (user == null) {
        throw new BadCredentialsException("1000");
    }
    if (!encoder.matches(password, user.getPassword())) {
        throw new BadCredentialsException("1000");
    }
    if (user.isDisabled()) {
        throw new DisabledException("1001");
    }
    List<Right> userRights = rightRepo.getUserRights(username);
    return new UsernamePasswordAuthenticationToken(username, null, userRights.stream().map(x -> new SimpleGrantedAuthority(x.getName())).collect(Collectors.toList()));
}

PS: userRepo and rightRepo are Spring-Data-JPA Repositories which access my custom User-DB

SpringSecurity JavaConfig:

@Configuration
@EnableWebMvcSecurity
public class MySecurityConfiguration extends WebSecurityConfigurerAdapter {

public MySecurityConfiguration() {
    super(false);
}

@Override
protected AuthenticationManager authenticationManager() throws Exception {
    return new ProviderManager(Arrays.asList((AuthenticationProvider) new AuthProvider()));
}

}

In its most simplest:

@Override
    public Authentication authenticate(Authentication auth) throws AuthenticationException {
        String username = auth.getName();
        String password = auth.getCredentials().toString();
        // to add more logic
        List<GrantedAuthority> grantedAuths = new ArrayList<>();
        grantedAuths.add(new SimpleGrantedAuthority("ROLE_USER"));
        return new UsernamePasswordAuthenticationToken(username, password, grantedAuths);
    }

My solution is almost the same as the first answer:

1) You need a class which implements the Authentication Provider

@Service
@Configurable
public class CustomAuthenticationProvider implements AuthenticationProvider    {
      @Override
      public Authentication authenticate(Authentication authentication) throws AuthenticationException {
    // Your code of custom Authentication
}
}

2) Opposite to the first answer you don't need to have following code in your WebSecurityConfiguration if you have only this custom provider.

@Override
protected AuthenticationManager authenticationManager() throws Exception {
     return new ProviderManager(Arrays.asList((AuthenticationProvider) new  AuthProvider()));
}

The issue is that Spring looks for available providers and use the default if nothing else is found. But as you have the implementation of the AuthenticationProvider - your implementation will be used.