Using Swagger 2, how to generate the API doc in this format;

{
    "user": {
        "userId": 1,
        "username": "sample_username"
    }
}

For this User model.

@ApiModel
public class User {
    private int userId;
    private String username;
}

Because normally it would generate:

{
  "userId": 1,
  "username": "sample_username"
}