How to bind subclass object in spring form submit as modelAttribute

Question

I Have

Class Shape {
      //Implementation
}
Class Round extends Shape{
      //Implementation
}

Controller I Have

@Requestmapping(value="/view/form")
public ModelAndView getForm(){
ModelAndView mav=new ModelAndView();
mav.addObject("shape",new Round());
}


@RequestMapping(value="/submit", method = RequestMethod.POST)    
public ModelAndView submitForm(@ModelAttribute("shape") Shape shape){
         if(shape instanceof Round){ //**Not giving positive result**

         }
    }

in Jsp

<form:form action="/submit" commandName="shape" method="post">

//form tags

</form:form>

when I submit the form with Round object. At controller side ModelAttribute is not giving instance of Round . its giving instance of shape only. How to do this

Answer 1

this will never work

<form:form action="/submit" commandName="shape" method="post">

you are submitting a shape from the form and expecting a shape in the controller method

public ModelAndView submitForm(@ModelAttribute("shape") Shape shape)

which will never give you an Round object.

simply submit a Round object from the form and use that.

 <form:form action="/submit" commandName="round" method="post">
 public ModelAndView submitForm(@ModelAttribute("round") Round round)

edited :-

have a hiddenInput type in the form which will tell controller the type of Shape it is passing, you can change the value of hidden tag dynamically upon user request.

<input type="hidden" name="type" value="round">

get the value of the type inside contoller and use it to cast the Shape object

     public ModelAndView submitForm(
     @ModelAttribute("shape") Shape shape,
     @RequestParam("type") String type)
     {
     if(type.equals("round")){
      //if type is a round you can cast the shape to a round
      Round round = (Round)shape; 
       }
    }