public MultipartMessageMapper build() {
        boolean requireAtLeastOneHolder = holders.isEmpty();
        if ( requireAtLeastOneHolder ) {
            throw new MIMEConvertException("require at least one holder");
        }

        boolean requireInjector = injector == null;
        if ( requireInjector ) {
            throw new MIMEConvertException("require setInjector");
        }

        MultipartMessageMapper mapper = new DefaultMultipartMessageMapper(holders, injector);

        return mapper;
    }