ControllerAdvice.java

@ControllerAdvice
class ValidationHandler extends ResponseEntityExceptionHandler {

    @Override
    @ResponseBody
    protected ResponseEntity<Object> handleMethodArgumentNotValid(
        MethodArgumentNotValidException ex,
        HttpHeaders headers,
        HttpStatus status,
        WebRequest request
    ) {
        HashMap<String, List<String>> body = new HashMap<>();

        List<String> errors = ex
            .getBindingResult()
            .getFieldErrors()
            .stream()
            .map(DefaultMessageSourceResolvable::getDefaultMessage)
            .collect(Collectors.toList());

        body.put("errors", errors);


        return handleExceptionInternal(ex, body, headers, HttpStatus.BAD_REQUEST, request);
    }
}