How to get adaptive form field value in post.POST.jsp for the field having bind reference ?

I have created a simple adaptive form and I have created custom submit action(post.POST.jsp) for this form and calling OSGi service inside post.POST.jsp.
Now when I retrieve form field value in post.POST.jsp for field having No bind reference , I get it eazly as below(in classic way)

 
String firstName = request.getParameter("firstName");

but if I do the same for the field having bind reference it gives null.

I tried with below to get form data having bind reference 
Document dataXml = null;
    if(slingRequest.getParameter("jcr:data")!=null) {
        dataXml = new Document(slingRequest.getRequestParameter("jcr:data").get());
     }

and it works fine because I need to get all data in an XML Document and them read all fields value.

My question is -
1. Is there any other way(other than XML document) to get the form data in post.POST.jsp for the field having bind reference ?
2. What is the reason that form data is not retrieved in post.POST.jsp with classic approach for the field having bind reference ?

Thanks in advance !
Talk.