How can I determine the unerased type of a field?

Question

I have a generic response wrapper class:

public class Response <T> {
    T response;
}

and unrelated classes to be wrapped:

public class ServiceResponse {
    String someField;
}

When I make a service request, I get a JSON response that looks something like:

{ "code":200, "response":{"someField":"some text"} }

Now, all my service responses have the same outer wrapper, ie, they all have:

{ "code":200, "timestamp":"....", "response":... }

But the actual format/type of the response field is different for each service request. When I deserialize the response, I need to know the type of the response field so I can create the appropriate instance, if the deserialization was done within Response , I could use:

response = new T(jsonParser);

However, I'm doing all of this from within a library that is driven by reflection, so I normally deserialize the whole tree with code like:

wrapper = deserializer.parseObject(Response<ServiceResponse>.class)

but, at this point my parseObject method can't correctly determine the type of T.

I can use something like:

Response<ServiceResponse> response = new Response<>();
Field field = response.getClass().getDeclaredField("response");
Type type = field.getGenericType();

which then tells me that response is of type T but what I actually need is ServiceResponse

Per this SO question I tried casting as ParameterizedType but that would actually seem to apply to a field of type Response<ServiceResponse> and not the actual field within (and it fails because type can't be cast as ParameterizedType )

Is there any way to determine (at run time) the raw type of response ?

Eventually, I may wind up having to create an annotation providing more details about how to deserialize the field, probably by providing a function to do it, but would prefer a more transparent approach.

Another possibility might be to actually assign a void instance of T to response at initialization time and then I could grab the actual type from that...

Answer 1

Check out this post: http://mydailyjava.blogspot.com/2013/06/advanced-java-generics-retreiving.html

It's actually exactly what you're looking for.

According to this, you'll just need to extend your Response class and then query the generic type of its super.