Polymorphic deserialization with `EXTERNAL_PROPERTY` fails if type property not marked as property

[prtmD]

I have class as follows

class XYZ {
    private String type;

    private TypeSpecific typeSpecific;

    public TypeSpecific getTypeSpecific() {
        return typeSpecific;
    }

    @JsonTypeInfo(
		use = JsonTypeInfo.Id.NAME, 
		include = JsonTypeInfo.As.EXTERNAL_PROPERTY, 
		property = "type"			
		)
    @JsonSubTypes({		
		@JsonSubTypes.Type(value = ATypeSpecific.class, name = "a")
    })
    public void setTypeSpecific(TypeSpecific typeSpecific) {
        this.typeSpecific = typeSpecific;
    }
}

Class ATypeSpecific extends TypeSpecific.

I want to deserialize JSON
{"type":"b"}
where typeSpecific will be set as null in object. But I am getting following exception:

com.fasterxml.jackson.databind.JsonMappingException: Missing property 'typeSpecific' for external type id 'type'

How do I deserialize above mentioned JSON into the object? If @JsonTypeInfo requires 'typeSpecific' property during deserialization, then is there a way to add it into JSON before deserialization?

Dependency versions:
jackson-annotations: 2.7.0
jackson-core: 2.7.4
jackson-databind: 2.7.4

[cowtowncoder]

First things first, try with the latest patch from the batch in question: in this case, 2.7.9.
Does the process still occur?

[prtmD]

I tried with latest patch, that is 2.7.9 and also with latest version of jackson-databind 2.8.6. But it is not working still.

[oliversaggau]

Any updates on this issue? I just tested with 2.8.8 and 2.9.0-pr3, but this still throws an exception. I found an old post on Stackoverflow with the same issue.

[Edit]
Okay, I just discovered FAIL_ON_MISSING_EXTERNAL_TYPE_ID_PROPERTY DeserializationFeature. By disabled this feature, I don't get the exception.

[cowtowncoder]

The problem here is this:

    private String type;

as there is no indication that this field is supposed to infer a property: it is private and has no annotations. It needs to be marked as a property; just indicating there should be one from polymorphic type id annotation is not enough.

Whether this should work without annotation is an interesting question: I can see why it would be nice; the problem however is that the way properties are introspected there is likely not enough information available to add inclusion.

[oliversaggau]

Ah ok, hadn't noticed the missing @JsonProperty in the OP's code. In my code I have the following code:

@JsonProperty("type")
private String type;

And I even tried adding @JsonTypeId, but both won't work.

[cowtowncoder]

@oliversaggau Ok, then there seems to be some other problem.

[buremba]

Any updates on this issue? If TypeSpecific marked as not required, there is no requirement for Jackson to try to parse the value of it.

[cowtowncoder]

@buremba Well the problem is that there IS a type identifier in there, and from databinding perspective it seems like a problem -- what is that type id for? Nothing? Why would something or someone serialize type without data being typed?

So in a way I am not sure if that ought to be allowed.

[buremba]

@cowtowncoder The type id may be useful by itself so don't assume that type id is there just for deserializing another value. For example, here is our use-case:
We have reports and each report has a type. Depending on that type, each report has options and we use different classes for different report types. We also have two different endpoints in our API server; list and get. The list reports the report names and types but not options because we just need to list the reports in a table. When the user clicks a report, we fetch the options from the server and return the data to the user.

[cowtowncoder]

@buremba Ok. That makes some more sense. Although I am bit worried about potential for masking issues, I guess I can see this as valid usage esp as there is no way to indicate difference like with PROPERTY vs EXISTING_PROPERTY (former implying its pure metadata, latter that it is explicit data also used as metadata).

[cowtowncoder]

Oh. I had forgotten that as per #1341 (and mentioned above), DeserializationFeature.FAIL_ON_MISSING_EXTERNAL_TYPE_ID_PROPERTY can be disabled to allow such value to be missed (unless declared required).
So, @buremba, you want to disable that.