FTPファイルのダウンロード

    public void downloadFile(HttpServletResponse response, String filename, String filenameToDisplay) {
        try (
                FileInputStream inputStream = new FileInputStream(filename);
                OutputStream outputStream = response.getOutputStream()
        ) {
            response.setContentType("application/octet-stream");
            response.setHeader("Content-Transfer-Encoding", "binary");
            response.setHeader("Content-Disposition", "attachment; filename=" + URLEncoder.encode(filenameToDisplay, "UTF-8"));

            int length;
            byte[] buffer = new byte[1024];
            while ((length = inputStream.read(buffer)) != -1)
                outputStream.write(buffer, 0, length);
        } catch (IOException e) {
            throw new FileNotDownloadedException();
        }
    }