var xhr=window.XMLHttpRequest?new XMLHttpRequest():new ActiveXObject("microsoft.xmlhttp");
xhr.open("get","xxxx.do?_dc="+new Date().getTime(),true);
xhr.onreadystatechange=function(){
  if(xhr.readyState==4){
    if(xhr.status==200)alert(xhr.responseText);
    else alert(xhr.status+'\n\n'+xhr.responseText);
  }
}
xhr.send(null);

------解决方案--------------------
http://download.csdn.net/source/2256659
这里有个demo

------解决方案--------------------
//本代码用的是prototype框架实现，在IE中测试通过
//首先要引入prototype.js
<script type="text/javascript" src="../js/prototype.js" ></script>
//在JS中执行如下代码
function test(){  
 var http = new ActiveXObject("Msxml2.XMLHTTP");//生成一个XMLHTTP对象
 var url = "/checkAction.do";//调用的action
 http.open("GET", url, false);
 http.send(); //发送请求
 var msg = http.responseText;
 alert(msg); //返回值
}
//action中的重要代码如下：
HttpServletResponse response = context.getResponse();
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
out.print("Test String!"); //将信息写入response