code: public static <T extends Message> Marshaller<T> jsonMarshaller(final T defaultInstance) {
      final Parser parser = JsonFormat.parser();
      final Printer printer = JsonFormat.printer();
      return jsonMarshaller(defaultInstance, parser, printer);
    }
docs: Create a {@code Marshaller} for json protos of the same type as {@code defaultInstance}.
  
  <p>This is an unstable API and has not been optimized yet for performance.