Spring mvc 返回JSON

public ModelAndView updateContractTitle (HttpServletRequest request,
    HttpServletResponse response) throws Exception {
        ModelAndView mav = new ModelAndView();
        long applyId = Long.valueOf(request.getParameter("applyId"));
        AbstractApply apply = applyDao.getApplyById(applyId);
        apply.setApplyTitle(request.getParameter("contractTitle"));
        for(Object t : apply.getApplyTarget()){
            AbstractApplyTarget c = (AbstractApplyTarget) t;
            c.setTitle(apply.getApplyTitle());
        }
        applyDao.merge(apply);//以上是业务逻辑

        //开始返回
        MappingJacksonJsonView view = new MappingJacksonJsonView();
        Map attributes = new HashMap();
        attributes.put("success", Boolean.TRUE);

 attributes.put("contractNo", "contractNo");

        view.setAttributesMap(attributes);
        mav.setView(view);
        return mav;
    }

 

 

var URL = "approve.do?action=approveC"

            $.ajax({

                type:"POST",

                url:URL,

                data:null,

                success:function(msg){

                    alert(msg.contractNo);//取到返回的“ contractNo ”值

                },

                error:function(msg){

                }

            });