Spring boot add error message to rest response without throwing exception but keeping typed response - rest

Question

My REST endpoints return ResponseEntity<SomeDto> or only SomeDto. Out of this i generating swagger to be consumed by front end. Of course the type information SomeDto is crucial.
Within my endpoint methods i am doing some basic checks to prevent exception further down the road (fail as fast as possible).
Returning ResponseEntity.badRequest().build() in such cases works fine, but i want to add a error message to the body. But adding a String to the body is not possible in regards to ResponseEntity<SomeDto>.
Question
How i can add a error message while maintaining the signature/ contract without the need of raising an exception (see "Effective Java 3rd edition - "Item 69: Use exceptions only for exceptional conditions")?
one (bad) way i see is to add an error message field to all my dtos (could be specified by an interface).

Answer 1

You don't need to add error model to individual object instead extend Response model from Base class which has error definition defined, this is how i would implement it.
static abstract class ApiResponse {
enum Status {
SUCCESS,
FAILURE,
}
#Getter
#Setter
private Status status;
#Getter
#Setter
private String errorMessage;
}
class NamesDto extends ApiResponse {
#Setter
#Getter
String[] names;
}
#GetMapping(value = "namesdto")
public ResponseEntity<ApiResponse> getNames() {
final NamesDto namesDto = new NamesDto();
namesDto.setStatus(ApiResponse.Status.FAILURE);
namesDto.names = new String[]{"john", "doe"};
return ResponseEntity.ok(namesDto);
}
#GetMapping(value = "errordto")
public ResponseEntity<ApiResponse> erroDto() {
final NamesDto namesDto = new NamesDto();
namesDto.setErrorMessage("No names found");
namesDto.setStatus(ApiResponse.Status.FAILURE);
return ResponseEntity.ok(namesDto);
}
#Getter/#Setter are imports from import lombok.* package

Answer 2

Either using ResponseEntity.badRequest().body(xxx).build() or throwing an exception is fine, as soon as the contract is the same.
You are using a lib to generate a Swagger Doc (such as Springfox) ? Well, it does fine for some situations but it can't perform miracles. The documentation generation should not affect how you write your code. In such a case, #ApiResponse is what you need.