How to get DDL form's field names?

Question

I implemented MVCActionCommand to override ddl form's add method. I want to retreive the values of the field from that form.

@Override
public boolean processAction(ActionRequest actionRequest, ActionResponse actionResponse) 
throws PortletException {      
 try {
        String fieldName = ParamUtil.getString(actionRequest, "field_name");      
        System.out.println(fieldName);    //returned nothing
    } catch (Exception e) {
        System.out.println(e.getMessage());
    }
    return false;
}

Tried this too but gave me a 'null' value

ServiceContext serviceContext = ServiceContextFactory.getInstance(DDLRecord.class.getName(), actionRequest);
serviceContext.getAttribute('field_name');