ExtJs2 Combobox组件远道加载数据失败 _JavaScript

ExtJs2 Combobox组件远程加载数据失败
问题描述：
1.Combobox组件远程加载失败，load之后没有任何选项，貌似是proxy获取不了Action传来的数据。
2.使用HttpProxy构造方法创建一个代理对象时，其构造方法需要一个Connection类型的参数，为什么我在按照Connection的配置项配置完成之后，页面加载前后listeners中的两个事件触发不了？

代码如下：
前台ExtJs代码：

JScript code

Ext.onReady(function(){    
    
    var httpProxy  = new Ext.data.HttpProxy({
        url:'httpComboboxAction.action',
        listeners:{
            requestcomplete:function(conn,response,options){
                alert(response.responseText());
            },
            beforerequest:function(conn,options){
                alert(111);
            }
        }
    });    
    

    var httpRecord = new Ext.data.Record.create([
        {name:'id',type:'int',mapping:'id'},
        {name:'cname',type:'string',mapping:'cname'}
    ]);
    var jsonReader = new Ext.data.JsonReader({},httpRecord);

    var httpStore = new Ext.data.Store({
        proxy:httpProxy,
        reader:jsonReader
    });
    
    var httpCombobox  = new Ext.form.ComboBox({
        store:httpStore,
        mode:'remote',
        renderTo:'httpCombobox',
        valueField:'id',
        displayField:'cname',
        triggerAction:'all'
    });
});

后台Action代码：

Java code

package org.action.extjstest;

import java.io.IOException;

import javax.servlet.http.HttpServletResponse;

import org.apache.struts2.ServletActionContext;

import com.opensymphony.xwork2.ActionSupport;

public class ComboboxAction extends ActionSupport{

    public String httpDataToCombobox() throws IOException{
        System.out.println("11111111");
        HttpServletResponse response = ServletActionContext.getResponse();
        response.setContentType("text/html;charset=utf-8");
//        response.setCharacterEncoding("utf-8");
        response.getWriter().write(
                "[{id:1,cname:北京},{id:2,cname:青岛},{id:3,cname:济南}]"
        );
        return null;
    }
}

------解决方案--------------------
response.getWriter().write(
"[{id:1,cname:北京},{id:2,cname:青岛},{id:3,cname:济南}]"
); 这个必须转化成json字符串的形式输出的,你的json格式也不对