RestMemberController.java

@PostMapping("/api/members")
public void newMember(@RequestBody @Valid RegisterRequest regReq, Errors errors) throws IOException {
  if (errors.hasErrors()) {
    String errorCodes = errors
      .getAllErrors()
      .stream()
      .map(error -> error.getCodes()[0])
      .collect(Collectors.joining(","));
    return ResponseEntity
      .status(HttpStatus.BAD_REQUEST)
      .body(new ErrorResponse("errorCodes = " + errorCodes));
  }
}