ext结合Struts2返回json数据，无法在页面显示？

Ext.onReady(function(){
        
         var meta = [
         {header:'编号',dataIndex:'id'},
         {header:'密码',dataIndex:'password'},
         {header:'名称',dataIndex:'username'}
         ];
        
         var store = new Ext.data.Store({
         proxy:new Ext.data.HttpProxy({url:'json.action',method:'post'}),
         reader:new Ext.data.JsonReader({
         totalProperty:'totalProperty',
         root:'root'
         },[
         {name:'id'},
         {name:'password'},
         {name:'username'}
         ])
         });
        
         var grid = new Ext.grid.GridPanel({
         renderTo:'grid',
         cm:new Ext.grid.ColumnModel(meta),
         autoHeight:true,
         store:store,
         bbar:new Ext.PagingToolbar({
         pageSize:10,
         store:store,
         displayInfo:true,
         displayMag:'显示第 {0} 条到 {1} 条记录,一共 {2} 条',
         emptyMsg:'没有记录'
         })
         });
         store.load();
         });
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控制台打印的json数据:{totalProperty:100,root:[{id:3,name:'tom',password:'test'},{id:4,name:'rose',password:'test'}]}
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firebug控制台:
json怎么是空的？