title: Spring SimpleFormController form submission
body: <p>I've a small doubt. I use Spring SimpleFormController with a form backing object.</p>
  
  <p>Let's say my formBackobject has the following member:
   - Id
   - Name
   - Sex</p>
  
  <p>on the jsp page, I only bind name and sex to input field. when i submit the form, the backend controller can remember the value of the id field and so can distinguish whether this is a "new" or "edit' mode.</p>
  
  <p>Could you tell me the trick behind this?</p>
  
  <p>Thanks,</p>