Ext ComboBox 从Servlet 加载不出来数据

[code="servlet"]public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

    response.setContentType("text/html;charset=gbk");
    PrintWriter out = response.getWriter();     
    String json="[{cid:1,cname:'武汉市'},{cid:2,cname:'荆门市'},{cid:3,cname:'宜昌市'},{cid:4,cname:'沙洋市'},{cid:5,cname:'潜江市'}]";
    out.println(json);
    out.flush();
    out.close();
}[/code]

[code="js"]
Ext.onReady(function() {

        var proxy = new Ext.data.HttpProxy({
                    url : "CityJsonServlet"
                });

        var City = Ext.data.Record.create([{
                    name : "cid",
                    type : "int",
                    mapping : "cid"
                }, {
                    name : "cname",
                    type : "string",
                    mapping : "cname"
                }]);
        var reader = new Ext.data.JsonReader({}, City);

        var store = new Ext.data.Store({
                    proxy : proxy,
                    reader : reader,
                    autoLoad:true
                });

        var combobox = new Ext.form.ComboBox({
                    triggerAction : "All",
                    store : store,
                    displayField : "cname",
                    valueField : "cid",
                    mode : "remote",
                    emptyText : "请选择市",
                    renderTo : Ext.getBody()
                })

    })

[/code]
[code="jsp"]
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">



Dnamic ComboBox Example
href="./scripts/ext/resources/css/ext-all.css" />
src="./scripts/ext/adapter/ext/ext-base.js">

<br><br>



[/code]

[code="java"]public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

    response.setContentType("text/html;charset=gbk");
    PrintWriter out = response.getWriter();     
    String json="[{cid:1,cname:'武汉市'},{cid:2,cname:'荆门市'},{cid:3,cname:'宜昌市'},{cid:4,cname:'沙洋市'},{cid:5,cname:'潜江市'}]";

            out.write(json);//out.println(json);

            out.flush();
    out.close();
}[/code]

var proxy = new Ext.data.HttpProxy({

url : "CityJsonServlet"

});

改成

ar proxy = new Ext.data.HttpProxy({

url : "/CityJsonServlet"

});

不是正确的Json格式，应该是
"[{'cid':1,'cname':'武汉市'},{'cid':2,'cname':'荆门市'},{'cid':3,'cname':'宜昌市'},{'cid':4,'cname':'沙洋市'},{'cid':5,'cname':'潜江市'}]";