@GetMapping("/download")
public void download(@RequestParam String fileId, HttpServletResponse response) throws IOException {
        // 查询文件信息
        SysFileEntity sysFileEntity = fileService.queryFileById(fileId);
        response.setContentType("application/octet-stream");
        response.addHeader("Content-Length", "" + sysFileEntity.getSize());  // 文件大小
        response.addHeader("Content-Disposition", "attachment;filename=" + URLEncoder.encode(sysFileEntity .getFileName(), "UTF-8"));
        int bufferSize = 8 * 1024;
        File file = new File(sysFileEntity.getFilePath());
        # 指定 读、写缓冲区大小。不填时默认 缓冲区大小8192byte
        BufferedInputStream bufferedInputStream = new BufferedInputStream(new FileInputStream(file), bufferSize);
        BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(response.getOutputStream(), bufferSize);
        try {
            
            // 设置缓冲区
            byte[] bytes = new byte[bufferSize];
            int len;
            while ((len = bufferedInputStream.read(bytes, 0, bytes.length)) > 0){
                bufferedOutputStream.write(bytes, 0, len);
            }
            bufferedOutputStream.flush();
        }catch (Exception e){
            logger.error("下载附件出错", e);
        }finally {
            IoUtil.close(bufferedOutputStream);
            IoUtil.close(bufferedInputStream);
        }
    }

注意!!!!!!

response.setContentType("application/octet-stream");
response.addHeader("Content-Length", "" + sysFileEntity.getSize());  // 文件大小
response.addHeader("Content-Disposition", "attachment;filename=" + URLEncoder.encode(sysFileEntity .getFileName(), "UTF-8"));

上述代码需写在response.getWriter或response.getOutputStream方法被调用之前。否则response.setContentType会不生效。

1. 确保在调用response.setContentType方法之前，response.getWriter或response.getOutputStream方法没有被调用。因为一旦获取了响应的输出流或写入器，设置内容类型将不起作用。

2. 确保在调用response.setContentType方法之前，没有向客户端发送任何数据。一旦开始向客户端发送数据，设置内容类型将不起作用。