var req;
    
    function Change_Select(){//当第一个下拉框的选项发生改变时调用该函数
      var city = document.getElementById('city').value;
      var url = "/shownettagmessage.ha?citypk="+ encodeURIComponent(city);
      if(window.XMLHttpRequest){
        req = new XMLHttpRequest();
      }else if(window.ActiveXObject){
        req = new ActiveXObject("Microsoft.XMLHTTP");
      }
      if(req){
        req.open("GET",url,true);
         //指定回调函数为callback
        req.onreadystatechange = callback;
        req.send(null);
      }
    }
    //回调函数
    function callback(){
      if(req.readyState ==4){
        if(req.status ==200){
          parseMessage();//解析XML文档
        }else{
          alert("不能得到信息:" + req.statusText);
        }
      }
    }
    
	//解析返回xml的方法
    function parseMessage(){
    
      
      var xmlDoc = req.responseText;
      
      alert(xmlDoc);
      
      var xSel = xmlDoc.split(',');
      
      var select_root = document.getElementById('nettag');
      
      select_root.options.length=0;
      
      
      for(var i=0;i<xSel.length;i++){
     
        var xValue = xSel[i];
		
        var xText = xSel[i];
        
        var option = new Option(xText, xValue);
        
        try{
          select_root.add(option);
        }catch(e){
        }
        
      }
     }