Why Spring MVC does not allow to expose Model or BindingResult to an @ExceptionHandler?

Question

Situation

I\'m trying to group the code that logs the exceptions and render a nice view in a few methods. At the moment the logic is sometime in the @RequestHand

Answer 1

I've wondered this too.

In order to handle bean validation in a way that allows for a non-global error view to display any ConstraintViolationExceptions that may be thrown, I opted for a solution along the lines of what @Stefan Haberl proposed:

Explicitly catch the exception to tell Spring MVC that you know what you're doing (you could use the Template pattern to refactor exception handling logic into one single place)

I created a simple Action interface:

public interface Action {
  String run();
}

And an ActionRunner class which does the work of ensuring ConstraintViolationExceptions are handled nicely (basically the messages from each ConstraintViolationException is simply added to a Set and added to the model):

public class ActionRunner {
  public String handleExceptions(Model model, String input, Action action) {
    try {
      return action.run();
    }
    catch (RuntimeException rEx) {
      Set errors = BeanValidationUtils.getErrorMessagesIfPresent(rEx);
      if (!errors.isEmpty()) {
        model.addAttribute("errors", errors);
        return input;
      }
      throw rEx;
    }
  }
}

Java 8 makes this pretty nice to run within the controller action method:

@RequestMapping(value = "/event/save", method = RequestMethod.POST)
public String saveEvent(Event event, Model model, RedirectAttributes redirectAttributes) {
  return new ActionRunner().handleExceptions(model, "event/form", () -> {
    eventRepository.save(event);
    redirectAttributes.addFlashAttribute("messages", "Event saved.");
    return "redirect:/events";
  });
}

This is to wrap up those action methods for which I'd like to explicitly handle exceptions that could be thrown due to Bean Validation. I still have a global @ExceptionHandler but this deals with only "oh crap" exceptions.