Question

Is there any reason not to map Controllers as interfaces?

In all the examples and questions I see surrounding controllers, all are concrete classes. Is there a reason for this? I would like to separate the request mappings from the implementation. I hit a wall though when I tried to get a @PathVariable as a parameter in my concrete class.

My Controller interface looks like this:

@Controller
@RequestMapping("/services/goal/")
public interface GoalService {

    @RequestMapping("options/")
    @ResponseBody
    Map<String, Long> getGoals();

    @RequestMapping(value = "{id}/", method = RequestMethod.DELETE)
    @ResponseBody
    void removeGoal(@PathVariable String id);

}

And the implementing class:

@Component
public class GoalServiceImpl implements GoalService {

    /* init code */

    public Map<String, Long> getGoals() {
        /* method code */
        return map;
    }

    public void removeGoal(String id) {
        Goal goal = goalDao.findByPrimaryKey(Long.parseLong(id));
        goalDao.remove(goal);
    }

}

The getGoals() method works great; the removeGoal(String id) throws an exception

ExceptionHandlerExceptionResolver - Resolving exception from handler [public void
    todo.webapp.controllers.services.GoalServiceImpl.removeGoal(java.lang.String)]: 
    org.springframework.web.bind.MissingServletRequestParameterException: Required 
    String parameter 'id' is not present

If I add the @PathVariable annotation to the concrete class everything works as expected, but why should i have to re-declare this in the concrete class? Shouldn’t it be handled by whatever has the @Controller annotation?

Answer 1

Apparently, when a request pattern is mapped to a method via the @RequestMapping annotation, it is mapped to to the concrete method implementation. So a request that matches the declaration will invoke GoalServiceImpl.removeGoal() directly rather than the method that originally declared the @RequestMapping ie GoalService.removeGoal().

Since an annotation on an interface, interface method, or interface method parameter does not carry over to the implementation there is no way for Spring MVC to recognize this as a @PathVariable unless the implementing class declares it explicitly. Without it, any AOP advice that targets @PathVariable parameters will not be executed.