如何js将数组转化为json数组，然后后台如何解析？

页面代码：
var  jsonArr =  new  Array();
            // 组装JSON数组
            for  (  var  j = 0; j < changes.length; j++) {
                var  jsonObj = {};
                jsonObj[ "gradeId" ]=changes[j].gradeId;
                jsonObj[ "gradeName" ]=changes[j].gradeName;
                jsonObj[ "level" ]=changes[j].level;
                jsonObj[ "boundary" ]=changes[j].boundary;
                jsonObj[ "status" ]=changes[j].status;
                jsonArr.push(jsonObj)
            }

$.post( "update_gradeAction.action" ,
                    {gradeJson:jsonArr.toString()},
                    function (message){
                        alert( "success!" );
                    },
                    'json'

                    );







后台解析代码为


HttpServletRequest request = ServletActionContext.getRequest();

String gradeString = request.getParameter( "gradeJson" );
System.out.println(gradeString); // 此处为null或者偶尔为[{objcet objcet},{objcet objcet}..]
//String[] names = request.getParameterValues("gradeName");

JSONArray jsonArr = JSONArray.fromObject(gradeString); //此处为null，或者直接报错