Jackson is compatible with the JAXB annotations. Hence JAXB doesn't support default values for XmlAttributes as the default behavior is to leave them out if value is null when serializing to XML.
There are a few options to achieve this for JSON.
You can annotate your POJO with @JsonSerialize(include=JsonSerialize.Inclusion.NON_NULL)
You can set the default behavior of the ObjectMapper to exclude null-values from serialisation. You do so by calling:
setSerializationInclusion(Inclusion.NON_NULL);
...on the ObjectMapper instance.