code: public void doSecured( StaplerRequest req, StaplerResponse rsp ) throws IOException, ServletException {
          // TODO fire something in SecurityListener? (seems to be used only for REST calls when LegacySecurityRealm is active)
  
          if(req.getUserPrincipal()==null) {
              // authentication must have failed
              rsp.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
              return;
          }
  
          // the user is now authenticated, so send him back to the target
          String path = req.getContextPath()+req.getOriginalRestOfPath();
          String q = req.getQueryString();
          if(q!=null)
              path += '?'+q;
  
          rsp.sendRedirect2(path);
      }
docs: Checks if the user was successfully authenticated.
  
  @see BasicAuthenticationFilter