问一个利用ajax从服务器端获取excel文件的问题

我原来不是用ajax方式做，是用：
<html:link page="/excel.do?action=byteacher&teacherid=1">生成EXCEL报表</html:link>
方式做的，服务器端的代码如下：



public class ExcelAction extends BaseAction {

public ActionForward doExcuteAction(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response) {

String actionType = request.getParameter("action");
if ("byteacher".equals(actionType.trim())) {
String teacherId = request.getParameter("teacherid");
                            //利用业务组件从后端获取数据
List teacherSelectedAssignments = this.adminBussiness
.getTeacherAssignment(Integer.parseInt(teacherId));
try {
response.setContentType("application/vnd.ms-excel"); 
//利用ExcelReportUtil工具类生成excel文件
                               ExcelReportUtil.writeAssignmentExcelByTeacher(
teacherSelectedAssignments, response.getOutputStream());

} catch (WriteException e) {
// TODO Auto-generated catch block
System.out.println("WriteException 错误");
} catch (IOException e) {
System.out.println("IO 错误");
}
}
return null;
}

}

这样作是没有问题的，可以生长excel报表！


后来我想改为用ajax来作：关键代码如下：

<script language="JavaScript">
HTTPRequest=function() 
{
http_request = false;
if (window.XMLHttpRequest)
{ // Mozilla, Safari,...
http_request = new XMLHttpRequest();
if (http_request.overrideMimeType)
{
http_request.overrideMimeType('text/xml');
}
} else if (window.ActiveXObject)
{ // IE
try {
http_request = new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e)
{
try
{
http_request = new ActiveXObject("Microsoft.XMLHTTP");
}
catch (e) {}
}
}

if (http_request) {
return http_request;
}

}

function getreportbyteacher(oSel){
alert(oSel.options[oSel.selectedIndex].value);
var http = new HTTPRequest();
    http.open("GET","/ecjtuthesis/excel.do?action=byteacher&teacherid="+oSel.options[oSel.selectedIndex].value,true);
    http.send();
}

</script>


……

<select name="assignTeacherId" id="teacher">
<c:forEach var="i" items="${teachers}">
<option value="${i.teacherId}">
<c:out value="${i.teacherName}" />
</option>
</c:forEach>
</select>
老师
<input type="button" value="生成报表" onclick="getreportbyteacher(document.all('teacher'));" />


…………
调试发现：
  http.open("GET","/ecjtuthesis/excel.do?action=byteacher&teacherid="+oSel.options[oSel.selectedIndex].value,true);
http.send();
信息发送到了服务器端，没有问题，就是不见生成的excel文件！各位有没有碰到类似的问题，帮我分析一下，谢谢