The Meta interface contains several POJO classes that represent RPC requests or responses. Currently a few of those classes have Jackson annotations such as @JsonCreator, @JsonProperty to help Jackson serialize the POJO to JSON and de-serialize from JSON to the object.
As Nick Dimiduk pointed out in http://mail-archives.apache.org/mod_mbox/calcite-dev/201503.mbox/%3CCANZa=GvKGD+BKJ4+eJMuO6iVhS+OkGSkG1VwDAZCy-ZijyyyPw@mail.gmail.com%3E these annotations are a "code smell" and should be removed. It makes it look as if Jackson is the only possible transport, which is not the case. We can continue to use Jackson as a transport, just specify the mappings elsewhere, not as annotations.