ajax 联动下拉框

<script type="text/javascript">
var http_request = false;
function send_request(method,url,content,responseType,callback)//定义发送请求的函数
{
http_request=false;
if(window.XMLHttpRequest)
{
http_request=new XMLHttpRequest();
if(http_request.overrideMimeType)
{
http_request.overrideMimeType("text/xml");
}
}
else
{
try
{
http_request=new ActiveXObject("Msxml2.XMLHTTP");
}
catch(e)
{
try
{
http_request=new ActiveXObject("Microsoft.XMLHTTP");
}
catch(e)
{}
}
}
if(!http_request)
{
window.alert("创建XMLHttpRequest对象失败");
return false;
}
if(responseType.toLowerCase()=="text")
{
http_request.onreadystatechange=callback;
}
else
{

window.alert("ERR");
return false;
}
if(method.toLowerCase()=="get")
{
http_request.open(method,url,true);
}
else if(method.toLowerCase()=="post")
{
http_request.open(method,url,true);
http_request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
}
else
{
window.alert("Err");
return false;
}
http_request.send(content);
}

function changcollege(va)//当学院下拉列表发生改变时触发的脚本事件
{
if(va!='0')
{
var speciality = document.getElementById("specialty");
speciality.disabled=false;

var url="college.do?type=college&id="+va;
send_request("GET",url,null,"text",populateClass3);
}
}
function populateClass3()//Ajax执行成功的回调函数
{
var f=document.getElementById("specialty");
if(http_request.readyState==4)
{
if(http_request.status==200)
{
var list=http_request.responseText;
var classList=list.split("|");
f.options.length=1;
for(var i=0;i<classList.length;i++)
//将取得的结果添加到下级的列表框中
{
var tmp=classList[i].split(",");
f.add(new Option(tmp[1],tmp[0]));
}
}
else
{
alert("您所请求的页面有异常。");
}
}
}
</script>

response.getWriter().print("{1:'wen 01',2'wen 02'}");

response.getWriter().close();