Framework Design: Jersey client test framework with TestNG. How to layout classes?

I have scoured the Internet and unfortunately found no solution to my problem. I am trying to create a full test framework for my REST API service. What I want to do is:

• Provide a set of REST processing classes that the test cases can call for sending request and receiving response.
• Create all application POJO classes which the test case can initialize and pass the objects to the REST Processors

I cannot seem to get a good layout of classes to do this. This is what I have so far; In each of the POJO classes I have created a method like-

    public String objectToJSON(MyPOJO requestJSONObject) {
        ObjectMapper mapperInstance = new ObjectMapper();
        return mapperInstance.writeValueAsString(requestJSONObject);
    }

And my REST processor for say, handling POST requests looks like-

public ClientResponse execRequest(Authorization authInstance, Object requestPOJO) {
    return webResourceInstance.type(MediaType.APPLICATION_JSON).header("Authorization", 
            encode(authInstance)).post(ClientResponse.class, **requestPOJO**);
}

This requestPOJO object in the post() method is creating a problem, in that my REST Processor never knows what type of Object it is handling and as a consequence the objectToJSON() method is never really visible.

I could implement the REST API code in the POJO itself but that doesn't sound like a very good idea. I am looking for a full framework design example on how I might go around laying out my classes for this framework.