下载文件

1. restController的工作
   1. 首先读取文件

   File file = new File(filePath, fileName)
   InputStream inputStream = new FileInputStream(file);
   

   2. 设置response

   response.setContentType("application/msword;charset=utf-8");
   response.setHeader("Content-Disposition", "attachment");
   

   3. 写到输出流

   @GetMapping(value = "/download/userguide")
   public StreamingResponseBody getDocument(HttpServletResponse response) {
   //read file
   ...
   //set response
   ...
   //write to outputStream
   return outputStream -> {
       int nRead;
       byte[] data = new byte[1024];
       while ((nRead = inputStream.read(data, 0, data.length)) != -1) {
       outputStream.write(data, 0, nRead);
       }
   };
   

2. response.setContentType()的作用及参数
   response.setContentType(MIME)的作用是使客户端浏览器，区分不同种类的数据，并根据不同的MIME调用浏览器内不同的程序嵌入模块来处理相应的数据。
   具体文件类型对应的参数，可以参考下面链接
   https://blog.csdn.net/luman1991/article/details/53423305

3. 前端下载文件
   将button的click响应汉书

 downloadUserguide(){
        console.log("download user guide");
        const link: HTMLElement = document.createElement('a');
        link.setAttribute('href', SERVER_API_URL + 'uaa/api/download/userguide');
        link.setAttribute('download', 'CPE-UserGuide.docx');
        link.style.visibility = 'hidden';
        document.body.appendChild(link);
        link.click();
        document.body.removeChild(link);
    }