how to return a specific status code in Kotlin - spring-boot

Question

I've set up a route that when I get a name in my post body I will search the DB and return an ID value.
What I want to do is once there is no ID present in the DB return a 204 status code.
But should that be handled in the service or in my controller?
and
How do I return my specific status code?
#ResponseStatus(HttpStatus.OK)
#PostMapping("/ID_values/")
fun getID(
#RequestBody
name: String
): ResponseEntity<String> = ResponseEntity.ok(IDLookupService.lookupIDValue(name))
}
#Service
class EmailLookupService(
private val IDRepo: IDRepo
) : Logging {
fun lookupIDValue(name: String): String {
val IDLookupResult = IDRepo.findById(name)
return if (IDLookupResult.isPresent) {
IDLookupResult.get().ID.toString()
} else {
// return status code 204
}
}
}

Answer 1

First, you should omit the #ResponseStatus(HttpStatus.OK) annotation if you do not wish to always return a status code of 200. Using that annotation, it would suffice to only specify the response body as return value (i.e specify return type String and then return only result in your example), and Spring would automatically wrap that into a response entity with HTTP-status OK.
Second, you need some way to tell the caller of IDLookupService.lookupIDValue (which should probably be called on an instance of IDLookupService and not the class itself) that there was nothing found. This could be done for instance by changing the return type to String? and return null if nothing was found.
Then you can change getID to return
val result = idLookupService.lookupIDValue(name)
return if(result != null) ResponseEntity.ok(result)
else ResponseEntity("not found", HttpStatus.NO_CONTENT)
If you wish to return something different than a String in the case there was nothing found (like an error object with detailed information; in the example here it is simply the text "not found"), you can change the response type of getID to ResponseEntity<*>.