下载文件转换成流，这里说两种流的方式:

1. 文件流

2. 字节流

一，字节流

String filePath=“/opt/peoject/file/123/pdf”; //这个是你服务上存放文件位置

方法体，代码如下：

public byte[] downLoadFile(String filePath) throws Exception {
        byte[] buffer = null;
        try {
            File file = new File(filePath);
            FileInputStream fis = new FileInputStream(filePath);
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            byte[] b = new byte[1024];
            int n;
            while ((n = fis.read(b)) != -1) {
                bos.write(b, 0, n);
            }
            fis.close();
            bos.close();
            buffer = bos.toByteArray();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return buffer;
    }

后端 controller层：

@PostMapping("/downLoadFile")
public byte[] downLoadFile(HttpServletRequest request,HttpServletResponse response){
    return  downLoadFile2(response,filePath);
}

前端接收处理、

参考地址：文件下载

二，文件流

public void downLoadFile2(HttpServletResponse response,String filePath) throws Exception {
        File file = new File(filePath);
        //获取文件名称 （例如：123.pdf）
        String fileName=filePath.substring(filePath.lastIndexOf("\\"));
        InputStream inputStream = new FileInputStream(file);
        response.setContentType("application/pdf;charset=utf-8");
        response.setHeader("Content-Disposition", "attachment;filename=\"" + fileName + "\"");
        IOUtils.copy(inputStream, response.getOutputStream());
        response.flushBuffer();
        inputStream.close();
    }

后端controller

public void downLoadFile(HttpServletRequest request,HttpServletResponse response){
  downLoadFile2(response,filePath）;
}

下载后，浏览器这里会有显示文件