Jfinal输出流的问题?报错

@JFinal 你好，想跟你请教个问题：我在后台代码如下

public void export() {
        String sns = getPara(Globalconstant.sns);
        //todo
        String data = sns.replace(",", "\r\n");
        String fileName = System.currentTimeMillis() + ".csv";
        HttpServletResponse response = getResponse();

        //设置响应
        response.setCharacterEncoding("gbk");
        response.setContentType("text/csv; charset=utf-8");
        response.setHeader("Content-Disposition", "attachment; filename=" + fileName);
        PrintWriter writer = null;

        try {
            writer = response.getWriter();
            writer.write(data);
            writer.flush();
        } catch (IOException e) {
            log.info("", e);
        } finally {
            if (writer != null) {
                writer.close();
            }
        }
        renderNull();

后台没有报错 但是页面返回的是这个

刚接触jfinal， 不知道问题从哪个找？