java文件下载不能弹出下载框

我想实现当前消息记录的下载，在js中
//导出聊天记录
function fnExportMsg(){
var infos = $("#imMsgBox").html();
$.ajaxSetup({async:false});
$.ajax({
type:"post",
url:G_BASE_URL+"im/testServlet",
data:"info="+infos
});
在servlet中的代码
public void service(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException {
request.setCharacterEncoding("utf-8");
System.out.println("==========begin");
String infos = request.getParameter("info"); System.out.println(infos);
if(infos!=null){
byte[] buf = infos.getBytes();
ByteArrayInputStream bais = new ByteArrayInputStream(buf);
BufferedInputStream bis = new BufferedInputStream(bais);

Date date = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMddhhmmssSSS");
String fileName = sdf.format(date)+".html";

response.reset();
response.setContentType("application/x-msdownload;charset=utf-8");
response.setHeader("Content-disposition","attachment;filename="+ fileName);

OutputStream out = response.getOutputStream();
byte[] buff = new byte[1024];
int len;
if((len = bis.read(buff,0,buff.length))!=-1){
out.write(buff, 0, len);
System.out.println("========== is here!");
}
out.flush();
bais.close();
bis.close();
out.close();
}else{
System.out.println("info is null");
}