ajax 发送数据并显示返回数据

<script language="javascript">

function getInfo(url)
{
 if (window.XMLHttpRequest)
 {
  req=new XMLHttpRequest();
 }
 else if (window.ActiveXObject)
 {
  req=new ActiveXObject("Microsoft.XMLHTTP");
 }
 if(req)
 {
  req.open("GET",url,true);
  var content=document.getElementById("display");
  content.innerHTML="<table><tr><td><img src=/common/images/loading.gif align=absmiddle>    正在读取数据.....</td></tr></table>";
  req.onreadystatechange=callback;
  req.send(null);
 }
}
function callback()
{
 if(req.readyState==4)
 {
  if(req.status==200)
  {
   parseMessage();
  }
  else {alert("操作失败");}
 }
 else
 {
 }
}
function parseMessage()
{
 var content=document.getElementById("display");
 content.innerHTML=req.responseText;
}
</script>

<div id=display></div>