HQL查询后ajax显示,刷新几次后数据不见了

查询action如下:输出了xml文档:
PrintWriter out = ServletActionContext.getResponse().getWriter();
String CONTENT_TYPE = "text/xml; charset=GBK";
ServletActionContext.getResponse().setContentType(CONTENT_TYPE);
ServletActionContext.getResponse().addHeader("Expires", "0");// 清除缓存
ServletActionContext.getResponse().addHeader("Cache-Control","no-store,must-revalidate");
ServletActionContext.getResponse().addHeader("Cache-Control","post-check=0,pre-check=0");
ServletActionContext.getResponse().addHeader("Pragma", "no-cache");
out.println(" <?xml version=\"1.0\" encoding=\"GBK\"?>");//生成XML文档
out.println(" <Powertype>");
for (int i = 0; i < li.size(); i++) {
out.println(" <xmllist>");
Powertype mes = (Powertype) li.get(i);
//System.out.println(mes.getId());
out.println(" <powertypeid>" + mes.getId()+ " </powertypeid>");
out.println(" <powertypename>" + mes.getVPowerTypeName()+ " </powertypename>");
out.println(" <powertypeorderid>" + mes.getIPowerOrderId()+ " </powertypeorderid>");
out.println(" <powertypecontent> </power