Custom validation before login action with spring security?

I'm new to spring so not fully aware of how the UserDetailsService works.

What I would like is for my login form to act the same way my signup form does. However, instead, when sending a post request using login details the UserDetailsService class seems to execute first. Can someone explain how the UserDetailsService works (i.e. when its called) and how I can change my security config to execute whats in my controller POST method for login (as it does for signup).

Controller:

@Controller
public class controller{

@RequestMapping(value = "/signup", method = RequestMethod.POST)
public String registration(@ModelAttribute("userForm") final User userForm, final BindingResult bindingResult) {
    LOG.debug("Signing up with user: {}", userForm);

    signUpValidationService.validate(userForm, bindingResult);

    if (bindingResult.hasErrors()) {
        return "login/Signup";
    }

    userService.saveUser(userForm);
    userService.autologin(userForm.getEmail(), userForm.getPassword());

    return "about/Home";
}

@RequestMapping(value = "/login", method = RequestMethod.POST)
public String login(@ModelAttribute("userForm") final User userForm, final BindingResult bindingResult) {
    LOG.debug("Logging in with user: {}", userForm);

    loginValidationService.validate(userForm, bindingResult);

    if (bindingResult.hasErrors()) {
        return "login/Login";
    }

    userService.autologin(userForm.getEmail(), userForm.getPassword());

    LOG.debug("Successfully logged in with user: {}", userForm.getFirstName());

    return "about/Home";
}

}

My security config:

@Configuration
@EnableWebSecurity
@EnableGlobalMethodSecurity(prePostEnabled = true)
public class WebSecurityConfig
    extends WebSecurityConfigurerAdapter {

/**
 * {@inheritDoc}
 */
@Override
protected void configure(HttpSecurity http) throws Exception {

    http
            .csrf().disable()
            .authorizeRequests()
            .antMatchers("/resources/**", "/signup", "/", "/home", "/about")
            .permitAll()
            .anyRequest().authenticated()
            .and()
            .formLogin()
            .loginPage("/login")
            .successHandler(successHandler())
            .permitAll()
            .and()
            .logout().deleteCookies("JSESSIONID")
            .logoutUrl("/logout")
            .logoutSuccessUrl("/home")
            .permitAll()
            .and()
            .rememberMe().key("uniqueAndSecret").tokenValiditySeconds(86400)
            .and()
            .sessionManagement()
            .sessionFixation().migrateSession()
            .sessionCreationPolicy(SessionCreationPolicy.IF_REQUIRED)
            .invalidSessionUrl("/invalid_session")
            .maximumSessions(2)
            .expiredUrl("/expired_session");
}

/**
 * User login validation configuration.
 *
 * @param auth
 *         {@link AuthenticationManagerBuilder}
 * @param userDetailsService
 *         {@link UserDetailsService}
 * @throws Exception
 *         {@link Exception}
 */
@Autowired
public void configureGlobal(final AuthenticationManagerBuilder auth, final UserDetailsService userDetailsService)
        throws
        Exception {
    auth
            .userDetailsService(userDetailsService)
            .passwordEncoder(new BCryptPasswordEncoder());
}

/**
 * Handles login success configuration policy.
 *
 * @return {@link AuthenticationSuccessHandler}
 */
private final AuthenticationSuccessHandler successHandler() {
    return new UrlAuthenticationSuccessHandler();
}

/**
 * Sets security evaluation context.
 *
 * @return {@link SecurityEvaluationContextExtension}
 */
@Bean
public SecurityEvaluationContextExtension securityEvaluationContextExtension() {
    return new SecurityEvaluationContextExtension();
}
}