Extjs GridPanel 返回数据不能正常显示的问题

ExtJS代码如下：Ext.onReady(function(){
var cm = new Ext.grid.ColumnModel(
{header:'学号', dataIndex:'id'},
{header:'姓名', dataIndex:'name'}
);
var store = new Ext.data.Store({
      proxy: new Ext.data.HttpProxy({
       url: 'test.action',
             method: 'POST'
      }),
         reader:new Ext.data.JsonReader({
        totalProperty: 'totalCount',        
         root: 'responseJson',
         fields:[
             {name:'id',},
             {name:'name'},
         ]
     })   
});

var grid = new Ext.grid.GridPanel({
renderTo:'grid',
width:600,
height:600,
loadMask:true,
store:store,
cm: cm
});

store.load();
});返回的JSon数据如下：
{"responseJson":[{"id":"id01","name":"name01"},{"id":"id11","name":"name11"},{"id":"id21","name":"name21"},{"id":"id31","name":"name31"},{"id":"id41","name":"name41"},{"id":"id51","name":"name51"},{"id":"id61","name":"name61"},{"id":"id71","name":"name71"},{"id":"id81","name":"name81"},{"id":"id91","name":"name91"}],"totalCount":10}使用的是struts2的json插件,action配置如下：<package name="login" extends="json-default">
<action name="test" class="com.test.login.LoginAction">
<result type="json">
</result>
</action>
</package>
Action代码如下：public class LoginAction {
public Integer totalCount;
public List<Map> responseJson;

public String execute() {
responseJson = new ArrayList<Map>();  
for(int i = 0; i < 10; ++i) {
Map<String, Object> stu = new HashMap<String, Object>();
stu.put("id", "id" + i + 1);
stu.put("name", "name" + i + 1);
responseJson.add(stu);
}
totalCount = 10;
return Action.SUCCESS; }
public Integer getTotalCount() {
return totalCount;
}
public void setTotalCount(Integer totalCount) {
this.totalCount = totalCount;
}
public List<Map> getResponseJson() {
return responseJson;
}
public void setResponseJson(List<Map> responseJson) {
this.responseJson = responseJson;
}
}