关于ajax解析json

  $.ajax({
 type:"post",
 url:"SecondServlet",
 data:"&pid=1",
 //dataType:"json",
 success: function(data){ 
 ;
 var d=data;
                             alert(d);
alert(d.mydata[0].usernum);
//$("p").append(d.mydate[1].teamnum);
 
},
 error:function(err){alert("err");}
 

其中alert(d)显示的是{"mydata":[{"usernum":16789},{"teamnum":1970},{"resourcenum":108828}]}
而错误提示为Uncaught TypeError: Cannot read property '0' of undefined
我尝试着这样写

<script>
$(document).ready(function(){
var user = 
{ 
"username":"andy", 
"age":20, 
"info": { "tel": "123456", "cellphone": "98765"}, 
"address": 
[ 
{"city":"beijing","postcode":222333}, 
{"city":"newyork","postcode":"555666"} 
] 
} 

 
//alert(user.address[0]); 
alert(user.address[0].postcode); 
})</script>

直接把json写在js中可行能成功，但像之前从servlet拿到的data却不行，拿到的data还是jaon格式的。
ps：我已经把json.js 和jq.js导入了
求大神帮助 JSON
[解决办法]
success: function(data){
var d=eval(data); //json字符串转换成js对象
alert(data);
alert(d.mydata[0].usernum);
}
[解决办法]

  $.ajax({
             type:"post",
             url:"SecondServlet",
             data:"&pid=1",
             dataType:"json", 
 
             success: function(data){ 
                             ;
                             var d=data;
                             alert(d);//[object Object]
                            alert(d.mydata[0].usernum);
                            //$("p").append(d.mydate[1].teamnum);
                          
                            },
             error:function(err){alert("err");}

  $.ajax({
             type:"post",
             url:"SecondServlet",
             data:"&pid=1",
             //dataType:"json",
             success: function(data){ 
                             ;
                             var d=data;
                             alert(d);//{"mydata":[{"usernum":16789},{"teamnum":1970},{"resourcenum":108828}]}
                            d = eval('(' + d + ')');
                            alert(d.mydata[0].usernum); //16789 
 
                            //$("p").append(d.mydate[1].teamnum);
                          
                            },
             error:function(err){alert("err");}