在Controller的方法中调用下面的方法，实现文件下载。
有两种需求：

1. 要下载的文件与服务在同一个服务器上，可通过文件路径创建File对象，下载文件；
2. 要下载的文件与服务在不同一个服务器上，可在服务程序中从文件服务器下载文件，转为byte数组，再调用该方法；

/**
 * 在Controller的方法中调用此方法，实现文件下载
 * @param resp 
 * @param fileBytes 文件的byte数组
 * @param fileName
 * @throws IOException
 */
private void downloadFile(HttpServletResponse resp, byte[] fileBytes, String fileName) throws IOException {
    resp.setHeader("content-type", "application/octet-stream");
    resp.setContentType("application/octet-stream");
    resp.setHeader("Content-Disposition", "attachment;filename=" + fileName);
    byte[] buff = new byte[1024];
    BufferedInputStream bufferedInputStream = null;
    OutputStream outputStream = null;
    try {
        outputStream = resp.getOutputStream();
 // ————————————————————————————
 // 文件File对象转BufferedInputStream: 给方法传入文件路径和文件名
 // File file = new File(filePath, fileName);
 // FileInputStream fis = new FileInputStream(file);
 // bufferedInputStream = new BufferedInputStream(fis);
 // ————————————————————————————
        bufferedInputStream = new BufferedInputStream(new ByteArrayInputStream(fileBytes), fileBytes.length);
        int num = bufferedInputStream.read(buff);
        while (num != -1) {
            outputStream.write(buff, 0, num);
            outputStream.flush();
            num = bufferedInputStream.read(buff);
        }
    } catch (IOException e) {
        throw new RuntimeException(e.getMessage());
    } finally {
        if (bufferedInputStream != null) {
            bufferedInputStream.close();
        }
    }
}