How to redirect to the homepage if the user accesses the login page after being logged in?

I've checked the topic more deeply than last time and found that you have to determine if user is authenticated by yourself in controller. Row Winch (Spring Security dev) says here:

Spring Security is not aware of the internals of your application (i.e. if you want to make your login page flex based upon if the user is logged in or not). To show your home page when the login page is requested and the user is logged in use the SecurityContextHolder in the login page (or its controller) and redirect or forward the user to the home page.

So solution would be determining if user requesting /auth/login is anonymous or not, something like below.

applicationContext-security.xml:

<http auto-config="true" use-expressions="true"
        access-decision-manager-ref="accessDecisionManager">
    <intercept-url pattern="/auth/login" access="permitAll" />
    <intercept-url pattern="/auth/logout" access="permitAll" />
    <intercept-url pattern="/admin/**" access="ADMINISTRATIVE_ACCESS" />
    <intercept-url pattern="/**" access="XYZ_ACCESS" />

    <form-login login-page="/auth/login"
        authentication-failure-url="/auth/loginFailed"
        authentication-success-handler-ref="authenticationSuccessHandler" />
    <logout logout-url="/auth/logout" logout-success-url="/auth/login" />
</http>

<beans:bean id="defaultTargetUrl" class="java.lang.String">
    <beans:constructor-arg value="/content" />
</beans:bean>

<beans:bean id="authenticationTrustResolver"
        class="org.springframework.security.authentication.AuthenticationTrustResolverImpl" />

<beans:bean id="authenticationSuccessHandler"
        class="com.example.spring.security.MyAuthenticationSuccessHandler">
    <beans:property name="defaultTargetUrl" ref="defaultTargetUrl" />
</beans:bean>

Add to applicationContext.xml bean definition:

<bean id="securityContextAccessor"
    class="com.example.spring.security.SecurityContextAccessorImpl" />

which is class

public final class SecurityContextAccessorImpl
      implements SecurityContextAccessor {

  @Autowired
  private AuthenticationTrustResolver authenticationTrustResolver;

  @Override
  public boolean isCurrentAuthenticationAnonymous() {
    final Authentication authentication =
        SecurityContextHolder.getContext().getAuthentication();
    return authenticationTrustResolver.isAnonymous(authentication);
  }
}

implementing simple interface

public interface SecurityContextAccessor {
  boolean isCurrentAuthenticationAnonymous();
}

(SecurityContextHolder accessing code is decoupled from controller, I followed suggestion from this answer, hence SecurityContextAccessor interface.)

And last but not least redirect logic in controller:

@Controller
@RequestMapping("/auth")
public class AuthController {
  @Autowired
  SecurityContextAccessor securityContextAccessor;

  @Autowired
  @Qualifier("defaultTargetUrl")
  private String defaultTargetUrl;

  @RequestMapping(value = "/login", method = RequestMethod.GET)
  public String login() {
    if (securityContextAccessor.isCurrentAuthenticationAnonymous()) {
      return "login";
    } else {
      return "redirect:" + defaultTargetUrl;
    }
  }
}

Defining defaultTargetUrl String bean seems like a hack, but I don't have better way not to hardcode url... (Actually in our project we use <util:constant> with class containing static final String fields.) But it works after all.

You could also restrict your login page to ROLE_ANONYMOUS and set an <access-denied-handler />:

<access-denied-handler ref="accessDeniedHandler" />
<intercept-url pattern="/auth/login" access="ROLE_ANONYMOUS" />

And in your handler check if the user is already authenticated:

@Service
public class AccessDeniedHandler extends AccessDeniedHandlerImpl {
    private final String HOME_PAGE = "/index.html";

    @Override
    public void handle(HttpServletRequest request, HttpServletResponse response, AccessDeniedException e) throws IOException, ServletException {
        Authentication auth = SecurityContextHolder.getContext().getAuthentication();
        if (auth != null && !(auth instanceof AnonymousAuthenticationToken)) {
            response.sendRedirect(HOME_PAGE);
        }

        super.handle(request, response, e);
    }
}

Implement a Redirect Interceptor for this purpose:

The Interceptor (implementing HandlerInterceptor interface) check if someone try to access the login page, and if this person is already logged in, then the interceptor sends a redirect to the index page.

public class LoginPageRedirectInterceptor extends HandlerInterceptorAdapter {    

    private String[] loginPagePrefixes = new String[] { "/login" };

    private String redirectUrl = "/index.html";

    private UrlPathHelper urlPathHelper = new UrlPathHelper();

    @Override
    public boolean preHandle(HttpServletRequest request,
                             HttpServletResponse response,
                             Object handler) throws Exception {

        if (isInLoginPaths(this.urlPathHelper.getLookupPathForRequest(request))
                           && isAuthenticated()) {
            response.setContentType("text/plain");
            sendRedirect(request, response);
            return false;
        } else {
            return true;
        }
    }

    private boolean isAuthenticated() {
        Authentication authentication =
                        SecurityContextHolder.getContext().getAuthentication();
        if (authentication == null) {
            return false;
        }
        if (authentication instanceof AnonymousAuthenticationToken) {
            return false;
        }
        return authentication.isAuthenticated();
    }

    private void sendRedirect(HttpServletRequest request,
                              HttpServletResponse response) {

        String encodedRedirectURL = response.encodeRedirectURL(
                                 request.getContextPath() + this.redirectUrl);
        response.setStatus(HttpStatus.SC_TEMPORARY_REDIRECT);
        response.setHeader("Location", encodedRedirectURL);
    }

    private boolean isInLoginPaths(final String requestUrl) {   
        for (String login : this.loginPagePrefixes) {
            if (requestUrl.startsWith(login)) {
                return true;
            }
        }
        return false;
    }
}