`JsonTypeInfo.As.PROPERTY` type id, property with same name, result in duplicate JSON property

Origin: https://stackoverflow.com/q/50163882/131929

The Swagger Java codegen produces a class like this:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "errorType", visible = true)
@JsonSubTypes({
    @JsonSubTypes.Type(value = SpecificError.class, name = "SpecificError"),
})
public class GeneralError {
    @JsonProperty("errorType")
    private String errorType = null;
    // accessors, even for errorType!, follow here

The Jackson serializer, version 2.9.4, produces JSON that includes the errorType property twice:

{"errorType":"SpecificError","message":"message","errorType":null}

My test bed is as follows:

SpecificError specificError = (SpecificError) new SpecificError().message("message")
ObjectMapper objectMapper = new ObjectMapper();
ObjectWriter writer = objectMapper.writer();
System.out.println(writer.writeValueAsString(clientError));

I’m not 100% sure if Jackson is supposed to behave like that given the annotations you see above. Based on what I read in the JsonTypeInfo Javadoc it seems incorrect. However, I also raised this as https://github.com/swagger-api/swagger-codegen/issues/8137.

[cowtowncoder]

@nickshoe Yes, I think you are right: visible is meant to “expose” type id metadata as a property. It’s bit of a work-around really, over the original case where type id was never exposed to other processing; and this before addition of EXISTING_PROPERTY. So the use case that was not covered originally (where type id maps to/from regular property) is supported by a combination of slightly cumbersome additions, as opposed to having been designed to work from the beginning.

[nickshoe]

@cowtowncoder I was wrong in my last comment, that happens only if one uses visible = true in the @JsonTypeInfo annotation. Since the default value of the visible attribute is false, then PROPERTY works as expected.