How to add checkbox value to temporary storage and put it in obj session using Struts 2

Hear my flag:

<input type="checkbox" value="${subFill.fillingname}" 
name="fillings"> ${subFill.fillingname}</input>

      

        
        
        
      

    

bean class:

public class AddCartBean {

public String fillings;

    public String getFillings() {
        return fillings;
    }

    public void setFillings(String fillings) {
        this.fillings = fillings;
        System.out.println("hiii"+fillings);
    }
}

      

        
        
        
      

    

action class:

public class CartAction extends ActionSupport implements SessionAware,ModelDriven<AddCartBean>{

    AddCartBean acb = new AddCartBean();


    @Override
    public void setSession(Map<String, Object> map) {
        sessionObj=(SessionMap<String, Object>) map;
    }

    @Override
    public AddCartBean getModel() {
        return acb;
    }


 public String mycart(){


 sessionObj.put("fillings",acb.getFillings());
         System.out.println("fillings are....."+acb.getFillings());
         return "success";
    }

      

        
        
        
      

    

when i first check the box and click the submit button check the box the value is appropriate for my action i can save it in session and get it in my jsp but my question is when the second time i click another check the box and click the submit button at the time I first submit the checkbox value will disappear and the new checkbox value appears, how can I get the first value of the submit checkbox and the second value of the submit checkbox?