Spring Rest ErrorHandling @ControllerAdvice / @Valid

You are on the right track, but you need to override the handleMethodArgumentNotValid() instead of the handleException() method, e.g.

@ControllerAdvice
public class RestErrorHandler extends ResponseEntityExceptionHandler {

    @Override
    protected ResponseEntity<Object> handleMethodArgumentNotValid(
            MethodArgumentNotValidException exception,
            HttpHeaders headers,
            HttpStatus status,
            WebRequest request) {

        LOG.error(exception);
        String bodyOfResponse = exception.getMessage();
        return new ResponseEntity(errorMessage, headers, status);
    }
}

From the JavaDoc of MethodArgumentNotValidException:

Exception to be thrown when validation on an argument annotated with @Valid fails.

In other words, a MethodArgumentNotValidException is thrown when the validation fails. It is handled by the handleMethodArgumentNotValid() method provided by the ResponseEntityExceptionHandler, that needs to be overridden if you would like a custom implementation.

Better format of return message:

@ControllerAdvice
public class MyControllerAdvice extends ResponseEntityExceptionHandler {

@Override
protected ResponseEntity<Object> handleMethodArgumentNotValid(
        MethodArgumentNotValidException ex,
        HttpHeaders headers,
        HttpStatus status,
        WebRequest request) {
    BindingResult result = ex.getBindingResult();
    List<FieldErrorVM> fieldErrors = result.getFieldErrors().stream()
            .map(f -> new FieldErrorVM(f.getObjectName(), f.getField(), f.getCode()))
            .collect(Collectors.toList());

    return handleExceptionInternal(ex, "Method argument not valid, fieldErrors:" + fieldErrors ,new HttpHeaders(), HttpStatus.BAD_REQUEST, request);
    }

}