ResponseEntity Spring MVC

Question

What is the best way to return an error message with ReponseEntity?

Say I have the following method

@Transactional
@RequestMapping(value = "/{id}", method = RequestMethod.GET)
public ResponseEntity<User> getUser(@PathVariable("id") Long id) {

    User user = userRepository.findOne(id);

    if (user == null) {
        return new ResponseEntity<>(HttpStatus.NOT_FOUND);
    }
    else {
        return new ResponseEntity<>(user, HttpStatus.OK);
    }

Now what if I want to return an error message to the front end? I can't do the following because the method return type is

ResponseEntity<User> 

not

ResponseEntity<String>

so this will not work

    if (user == null) {
        return new ResponseEntity<>("User does not exist", HttpStatus.NOT_FOUND);
    }

I could make the method return type

 ResponseEntity<Object> 

but that just seems slopy and bad practice. I want to be able to return at least a brief error message that gives some indication of what went wrong to the front end. How is the best way to go about doing this?

Update:

After some digging around I came up with this and it seems to work but curious if it would have a performance impact.

@RequestMapping(value = "/{id}", method = RequestMethod.GET)
public ResponseEntity<?> getUser(@PathVariable("id") Long id) {

    User user = userRepository.findOne(id);

    if (user == null) {
        return new ResponseEntity<>("User not found", HttpStatus.NOT_FOUND);
    }
    else {
        return new ResponseEntity<>(user, HttpStatus.OK);
    }
}

Answer 1

I realise you asked specifically about returning the error message using ReponseEntity, but you could also consider using Spring MVCs exception handling to acheive the same outcome:

// Example from the linked Spring article:

@RequestMapping(value="/orders/{id}", method=GET)
public String showOrder(@PathVariable("id") long id, Model model) {
    Order order = orderRepository.findOrderById(id);
    if (order == null) throw new OrderNotFoundException(id);
    model.addAttribute(order);
    return "orderDetail";
}


@ResponseStatus(value=HttpStatus.NOT_FOUND, reason="No such Order")  // 404
public class OrderNotFoundException extends RuntimeException {
    // ...
}