How to pass parameter to constructor when using .getClass()?

Question

I have that line of code and it was working at that version:

...
Wrapper<Model> wrapped = restTemplate.getForObject(BASE_URL, Wrapper.class, map);
...

However I want to send parameter to constructor:

...
Wrapper<Model> wrapped = restTemplate.getForObject(BASE_URL, new Wrapper(Model.class).getClass(), map);
...

It throws me an exception:

org.springframework.web.client.ResourceAccessException: I/O error: No suitable constructor found for type [simple type, class a.b.c.d.model.Wrapper]: can not instantiate from JSON object (need to add/enable type information?)
 at [Source: org.apache.commons.httpclient.AutoCloseInputStream@ef9e8eb; line: 1, column: 3]; nested exception is org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class a.b.c.d.model.Wrapper]: can not instantiate from JSON object (need to add/enable type information?)
 at [Source: org.apache.commons.httpclient.AutoCloseInputStream@ef9e8eb; line: 1, column: 3]

How can I send parameter to an object that I will get the class of value of it?

Answer 1

Wrapper.class and new Wrapper().getClass() and new Wrapper(theParam).getClass() return the same value: Wrapper.class. All this if you have sutable constructor, i.e., constructor that is able to get argument theParam. In your case class Wrapper does not have constructor that accepts argument of type Class, so it complains about this.

Answer 2

I assume what you need is to indicate generic type of Wrapper for Jackson to use. There are couple of ways to do this:

Wrapper<Model> value = objectMapper.readValue(source, new TypeReference<Wrapper<Model>>() { });
Wrapper<Model> value = objectMapper.readValue(source, objectMapper.getTypeFactory().constructParametricType(Wrapper.class, Model.class);

I am not sure how either TypeReference or JavaType (which are generics-enabled alternatives to passing Class instances (that are type-erased, i.e. no generics!)) through Spring framework, but I assume it should be possible.

Alternatively, if that can't be made to work, try sub-classing Wrapper -- concrete sub-class WILL actually have information necessary:

public class ModelWrapper extends Wrapper { } ModelWrapper wrapped = restTemplate.getForObject(BASE_URL, ModelWrapper.class);