How to get different JSON content in the same $http.get - angularjs

Question

I'm using AngularJS to retrieve a list of objects from a Server. The server side is being made with Spring MVC.
So, I'd like to return a list of these objects as JSON format, but, I'd like to return an informative message object if the server coudn't retrieve the data for any reason.
Can I do it without encapsulating the List<> and the Message objects in a DTO (Data Transfer Object)?
How can I check on AngularJS the received JSON if it can be from 2 different types?
Here is a fragment of my controller class. It currently returns a list of the MyType I'd like to return. But the idea is to convert it to a generic type (Object, maybe?) and return from this request the list or the message, depending on the case.
#RequestMapping(value = RestPaths.LIST_MYTYPE_BY_OWNER, method = RequestMethod.GET)
public #ResponseBody List<MyType> listByOwner(#RequestBody Owner o) {
List<MyType> myType = myService.list(o);
return myType;
}
This is the service that consults the controller. How could I treat the data, considering that the JSON could have different formats?
this.listMyType = function(ownerId){
var deferred = $q.defer();
$http.get('/rest/my/list_by_owner',{})
.then(function (response) {
if (response.status == 200) {
deferred.resolve(response.data);
}
else {
deferred.reject('Error retrieving list of myType');
}
});
return deferred.promise;
}

Answer 1

I wouldn't use an exception for flow control. If both cases (list is empty or it is not) are valid, handle both of them in your handler method. Use ResponseEntity to encapsulate your response. Return a String message in one case (empty) and a List<MyType> in the other (not empty).
#RequestMapping(value = RestPaths.LIST_MYTYPE_BY_OWNER, method = RequestMethod.GET)
public ResponseEntity<?> listByOwner(#RequestBody Owner o) {
List<MyType> myType = myService.list(o);
if (myType.isEmpty()) {
return new ResponseEntity<>("The message", HttpStatus.NOT_FOUND);
}
return new ResponseEntity<>(myType, HttpStatus.OK);
}
All handler method return values are handled by HandlerMethodReturnValueHandler objects. The corresponding one for ResponseEntity is HttpEntityMethodProcessor. It handles the ResponseEntity's body the same way #ReponseBody would.
So a String would be written directly to the response as text/plain (or html) and the List<MyType> would be serialized to application/json by default (or whatever you have configured).
You can add response headers to the ResponseEntity which can also help determine how the content will be serialized (content-type, etc.).

Answer 2

My suggestion is to throw an exception, then add in a spring exception handler which sets the appropriate status code and message:
#RequestMapping(value = RestPaths.LIST_MYTYPE_BY_OWNER, method = RequestMethod.GET)
public #ResponseBody List<MyType> listByOwner(#RequestBody Owner o) {
List<MyType> myType = myService.list(o);
if (myType.size() == 0) {
throw new ResourceNotFoundException("List was empty");
}
return myType;
}
and
#ControllerAdvice
public class ControllerExceptionHandler {
#ExceptionHandler(ResourceNotFoundException.class)
public void handleException(Exception e, HttpServletReponse response) {
response.setStatus(HttpStatus.NOT_FOUND);
response.getOutputStream().write(e.message);
}
}
Here is a pretty good write up on spring mvc exception handling: https://spring.io/blog/2013/11/01/exception-handling-in-spring-mvc