How can I get generics info inside custom deserializer?

[yvasyliev]

Lets say I have the following code:

public class GenericsDeserializationDemo {
    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        Wrapper<WrappableImpl> value = mapper.readValue("{\"wrappedList\":[{\"id\":123}]}", new TypeReference<Wrapper<WrappableImpl>>() {
        });
        System.out.println(value.getWrappedList().get(0).getId());
    }
}

public interface Wrappable {
    int getId();
}

public class WrappableImpl implements Wrappable {
    private int id;

    @Override
    public int getId() {
        return id;
    }
}

public class WrappedListDeserializer extends StdDeserializer<List<? extends Wrappable>> {
    protected WrappedListDeserializer() {
        super((Class<?>) null);
    }

    @Override
    public List<? extends Wrappable> deserialize(JsonParser p, DeserializationContext ctxt) {
        JavaType type = getValueType(); // returns null
        // How do I get the actual type?
        return null;
    }
}

public class Wrapper<T extends Wrappable> {
    @JsonDeserialize(using = WrappedListDeserializer.class)
    private List<T> wrappedList;

    public List<T> getWrappedList() {
        return wrappedList;
    }
}

Is there any chance to get TypeReference<Wrapper<WrappableImpl>> inside WrappedListDeserializer?

[JooHyukKim]

Try invoking this.handledType() instead of getValueType() inside deserialize method.

[yvasyliev]

Same result

[yvasyliev]

After some research I found a solution that works for me. Here's my updated WrappedListDeserializer:

public class WrappedListDeserializer<T extends Wrappable> extends JsonDeserializer<List<T>> implements ContextualDeserializer {
    private JavaType type;

    @Override
    public List<T> deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        return ctxt.readValue(p, type);
    }


    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) {
        WrappedListDeserializer<T> deserializer = new WrappedListDeserializer<>();
        deserializer.type = property.getType();
        return deserializer;
    }
}

All other classes remain the same.

[cowtowncoder]

That is one way for sure.

But in general, full generic type should be available to Module when deserializer is requested, via Deserializers. SimpleModule does not use that (or expose mechanism), but custom Deserializers can implement callback that is called when deserialized is needed, and it gets fully resolved JavaType.