Extjs4获取不到grid数据

前台：
Ext.Loader.setConfig({enabled: true}); 
Ext.Loader.setPath('Ext.ux', 'extjs4/examples/ux');
Ext.require([
'Ext.grid.*',
'Ext.data.*',
'Ext.selection.CheckboxModel',
'Ext.ux.form.SearchField'
]);
Ext.onReady(function(){
var store = null;
Ext.create('Ext.Button', { 
text: '导出数据',
x:30,
y:30,
renderTo: 'button',  
handler: function(grid) { 
Ext.Ajax.request({ 
url: 'GridServlet', 
params: { 
  id: 1 
  },
success: function(response){ 
var jsonObj = response.responseText; 
store.load(jsonObj);
} 
}); 

} 
}); 

Ext.define('Mydata',{
extend:'Ext.data.Model',
fields:[
//第一个字段需要指定mapping，其他字段，可以省略掉。
{name:'UserName',mapping:'UserName'},
'Sex',
'Age',
'XueHao',
'BanJi'
]
});

store = Ext.create('Ext.data.Store',{
model:'Mydata'
});

//创建多选 
  var selModel = Ext.create('Ext.selection.CheckboxModel'); 

var grid = Ext.create('Ext.grid.Panel',{
store:store,
selModel:selModel,
columns:[
{text: "姓名", width: 120, dataIndex: 'UserName', sortable: true},
{text: "性别", flex: 1, dataIndex: 'Sex', sortable: false},
{text: "年龄", width: 100, dataIndex: 'Age', sortable: true},
{text: "学号", width: 100, dataIndex: 'XueHao', sortable: true},
{text: "班级", width: 100, dataIndex: 'BanJi', sortable: true}
],
height:400,
width:480,
x:20,
y:40,
title:'Extjs4 Grid搜索',
renderTo:'grid',
disableSelection: true,//值为TRUE，表示禁止选择行
dockedItems: [{ 
  dock: 'top', 
  xtype: 'toolbar', 
  items: { 
  width: 300, 
  fieldLabel: '搜索', 
  labelWidth: 50, 
  xtype: 'searchfield', 
  store: store 
  } 
  },{ 
  dock: 'bottom', 
  xtype: 'pagingtoolbar', 
  store: store, 
  pageSize: 2, 
  displayInfo: true, 
  displayMsg: '显示 {0} - {1} 条，共计 {2} 条', 
  emptyMsg: '没有数据' 
  }]
});

});

后台servlet：
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("application/x-json;charset=UTF-8");
String id = (String)request.getParameter("id");
PrintWriter out = response.getWriter();
StringBuffer jsonStr = new StringBuffer();
jsonStr.append("[{'Username':'李彦宏','Sex':'男','Age':25,'XueHao':00001,'BanJi':'一班'},{'Username':'李彦宏','Sex':'男','Age':25,'XueHao':00001,'BanJi':'一班'}]");
out.write(jsonStr.toString());
}

response.setContentType("application/x-json;charset=UTF-8");删了试试