code: @Execute
      public ActionResponse index() {
          if (fessLoginAssist.getSavedUserBean().isPresent()) {
              return redirect(RootAction.class);
          }
          final SsoManager ssoManager = ComponentUtil.getSsoManager();
          final LoginCredential loginCredential = ssoManager.getLoginCredential();
          if (loginCredential == null) {
              if (logger.isDebugEnabled()) {
                  logger.debug("No user in SSO request.");
              }
              if (ssoManager.available()) {
                  saveError(messages -> messages.addErrorsSsoLoginError(GLOBAL));
              }
              return redirect(LoginAction.class);
          } else if (loginCredential instanceof ActionResponseCredential) {
              return ((ActionResponseCredential) loginCredential).execute();
          }
          try {
              return fessLoginAssist.loginRedirect(loginCredential, op -> {}, () -> {
                  activityHelper.login(getUserBean());
                  userInfoHelper.deleteUserCodeFromCookie(request);
                  return getHtmlResponse();
              });
          } catch (final LoginFailureException lfe) {
              if (logger.isDebugEnabled()) {
                  logger.debug("SSO login failure.", lfe);
              }
              if (ssoManager.available()) {
                  saveError(messages -> messages.addErrorsSsoLoginError(GLOBAL));
              }
              activityHelper.loginFailure(OptionalThing.of(loginCredential));
              return redirect(LoginAction.class);
          }
      }
docs: ==============