How to respond with HTTP status code in a Spring MVC @RestController @ResponseBody class returning an object?

A @RestController is not appropriate for this. If you need to return different types of responses, use a ResponseEntity<?> where you can explicitly set the status code.

The body of the ResponseEntity will be handled the same way as the return value of any @ResponseBody annotated method.

@RequestMapping(value = "/wells/{apiValue}", method = RequestMethod.GET)
public ResponseEntity<?> fetchWellData(@PathVariable String apiValue){
    try{
        OngardWell ongardWell = new OngardWell();
        ongardWell = ongardWellService.fetchOneByApi(apiValue);

        return new ResponseEntity<>(ongardWell, HttpStatus.OK);
    }catch(Exception ex){
        String errorMessage;
        errorMessage = ex + " <== error";
        return new ResponseEntity<>(errorMessage, HttpStatus.BAD_REQUEST);
    }
}

Note that you don’t need @ResponseBody on a @RequestMapping method within a @RestController annotated class.