TypeFactory#_fromVariable returns unknownType() even though it has enough information to provide a more specific type

[jkochaniak]

I'm using Jackson 2.2.2 in a Dropwizard service. I'm attempting to create a resource method with a bounded type parameter:

@POST
public <P extends Period> P createPeriod(@PathParam(VERSION_ID) String versionId,
                               P period)

However, when I attempt to post JSON to this endpoint, com.yammer.dropwizard.jersey.JacksonMessageBodyProvider deserializes the request as a LinkedHashMap instead of a Period. I believe there is enough information provided in the method declaration that Jackson should be able to deserialize this correctly. I've traced the problem to the TypeFactory#_fromVariable method.

JacksonMessageBodyProvider extends com.fasterxml.jackson.jaxrs.base.ProviderBase and calls super.readFrom(Class<Object> type, Type genericType, Annotation[] annotations, MediaType mediaType, MultivaluedMap<String,String> httpHeaders, InputStream entityStream) with the following arguments:

type: class Period
genericType: sun.reflect.generics.reflectiveObjects.TypeVariableImpl with name "P"
annotations: []
mediaType: application/json
and the provided headers and stream

This ends up calling the last line in ProviderBase#readFrom:

return reader.withType(genericType).readValue(jp);

ObjectReader#withType eventually calls TypeFactory#constructType(Type type), which passes a null context to _constructType(Type type, TypeBindings context). Since type is an instance of TypeVariable, it ends up calling TypeFactory#_fromVariable(TypeVariable<?> type, TypeBindings context).

Since context is null, this method immediately returns _unknownType(), which means the object ends up deserialized as a LinkedHashMap. However, if I call getBounds() on the type object while in the _fromVariable method, it returns an array containing Period.class. It seems that if we didn't short-circuit due to the null context, _fromVariable would eventually return the right type on the last line: return _constructType(bounds[0], context);

I've searched and couldn't find this exact issue. I looked at the latest code and see the same line, so I don't think upgrading would help.

[cowtowncoder]

Thanks, it does sounds like a bug. There are many problems with type handling, and some relate to missing information passed by JAX-RS, but in this case it does seem like bounds ought to be availble for use, since it is a local declaration

[cowtowncoder]

Hmmh. This is interesting... type-erased type would actually resolve correctly, and the problem is just with generic type. Anyway, I can resolve this locally, hope to fix it soon.

[cowtowncoder]

Turned out to be relatively easy to fix. And also helped resolve #609, awesome (and somewhat) synergy at play. Thank you again for reporting this! Fix will be in 2.5.2.

[jkochaniak]

Awesome, thank you!