NoCtorDeser can not deserialize parameterized types

[luozhenyu]

Code

The following code throws java.lang.ClassCastException. The type of response.getPayload() is not User but LinkedHashMap

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectReader;
import com.fasterxml.jackson.databind.json.JsonMapper;
import com.fasterxml.jackson.module.noctordeser.NoCtorDeserModule;

public class Demo {

    public static class Response<T> {

        private final T payload;

        public Response(T payload) {
            this.payload = payload;
        }

        public T getPayload() {
            return payload;
        }
    }

    public static class User {

        private final String name;

        public User(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }
    }

    public static void main(String[] args) throws Exception {
        String json = "{\"payload\":{\"name\":\"jackson\"}}";
        JsonMapper mapper = JsonMapper.builder()
            .addModule(new NoCtorDeserModule())
            .build();

        ObjectReader objectReader = mapper.readerFor(new TypeReference<Response<User>>() {
        });
        Response<User> response = objectReader.readValue(json);
        System.out.println(response.getPayload().getClass());
    }
}

Cause

@cowtowncoder It seems the JsonDeserializer is fetched by Class<?> instClass rather than a ParameterizedType.

https://github.com/FasterXML/jackson-modules-base/blob/2.15/no-ctor-deser/src/main/java/com/fasterxml/jackson/module/noctordeser/MissingInstantiatorHandler.java#L34-L37

Solution

The overrided method DeserializationProblemHandler#handleMissingInstantiator(DeserializationContext ctxt, Class<?> instClass, ValueInstantiator valueInst, JsonParser jsonParser, String msg) does not suppy generic information. Maybe we can extend class AnnotatedWithParams and override its public abstract Object call() throws Exception;. Then we set it to the StdValueInstantiator's _defaultCreator field?

[cowtowncoder]

In theory we could consider adding new version of handleMissingInstantiator() which takes JavaType (to retain generic type info). That seems like the way it could be done.
But that is called by handleMissingInstantiator() from DeserializationContext which would need to have overrides and delegation from old method as well, for backwards compatibility.

Although TBH, this module also uses a bit hacky way to go about construction as well. So maybe there could be different way that would not rely on having to find deserializer.

Not sure if AnnotatedWithParams route would be feasible; I guess one wouldn't need to get full deserializer if we could get ValueInstantiator to work.