写了一个执行下载的函数，@OK里面该怎么写，使用SpringMVC的时候，执行此函数可以成功下载，使用这个框架后，下载的文件大小是0

@At("/fileDownload")
@Ok("raw:stream")
public void fileDownLoad(@Param("filePath") String filePath,@Param("fileName") String fileName ,
                         HttpServletRequest request, HttpServletResponse response){

    response.setContentType("text/html;charset=UTF-8");
    try {
        request.setCharacterEncoding("UTF-8");
        BufferedInputStream bis = null;
        BufferedOutputStream bos = null;
        response.setContentType("application/octet-stream");
        response.reset();//清除response中的缓存
        //根据网络文件地址创建URL
        URL url = new URL(filePath);
        //获取此路径的连接
        URLConnection conn = url.openConnection();
        conn.connect();
        Long fileLength = conn.getContentLengthLong();//获取文件大小
        //设置reponse响应头，真实文件名重命名，就是在这里设置，设置编码
        response.setHeader("Content-disposition",
                "attachment; filename=" + new String(fileName.getBytes("utf-8"), "ISO8859-1"));
        response.setHeader("Content-Length", String.valueOf(fileLength));

        bis = new BufferedInputStream(conn.getInputStream());//构造读取流
        bos = new BufferedOutputStream(response.getOutputStream());//构造输出流
        byte[] buff = new byte[1024];
        int bytesRead;
        //每次读取缓存大小的流，写到输出流
        while (-1 != (bytesRead = bis.read(buff, 0, buff.length))) {
            bos.write(buff, 0, bytesRead);
        }
        response.flushBuffer();//将所有的读取的流返回给客户端
        bis.close();
        bos.close();

    } catch (IOException e) {
        e.getStackTrace();
    }
}