目录

• multipart原理简介
• • multipart的流是服务器临时文件流
  • multipartfile生成临时文件到默认文件夹
  • 临时文件的目录可配
  • Mutipartfile的文件默认来自临时文件
  • Mutipartfile产生临时文件的好处坏处
  • • 好处
    • 坏处
    • 纯流式上传application/octet-stream

      multipart原理简介

      multipart的流是服务器临时文件流

      multipart的流不是HTTPRequest的流，而是服务器的临时文件流

      multipart/form-data是表单上传，接口可以从Mutipartfile对象获取输入流getInputStream，表面上看没产生临时文件

      org.springframework.web.multipart.support.StandardMultipartHttpServletRequest的getInputStream

      		@Override
      		public InputStream getInputStream() throws IOException {
      			return this.part.getInputStream();
      		}
      

      这里面的流是从DiskFileItem（一眼磁盘）拿的，还告诉了你临时目录位置location

      

      multipartfile生成临时文件到默认文件夹

      从原理上看，在tomcat源码的org.apache.catalina.connector.Request类中，parseParts方法就将multipart格式的请求中文件生成到临时文件夹

      			File location;
                  String locationStr = mce.getLocation();
                  if (locationStr == null || locationStr.length() == 0) {
                      location = ((File) context.getServletContext().getAttribute(
                              ServletContext.TEMPDIR));
                  } else {
                      // If relative, it is relative to TEMPDIR
                      location = new File(locationStr);
                      if (!location.isAbsolute()) {
                          location = new File(
                                  (File) context.getServletContext().getAttribute(ServletContext.TEMPDIR),
                                  locationStr).getAbsoluteFile();
                      }
                  }
      

      临时文件的目录可配

      这个location就是javax.servlet.MultipartConfigElement的配置，可以通过spring.servlet.multipart系列配置的location指定临时文件目录，这个临时文件用完，也会被自动清除

          private final String location;// = "";
          private final long maxFileSize;// = -1;
          private final long maxRequestSize;// = -1;
          private final int fileSizeThreshold;// = 0;
      

      Mutipartfile的文件默认来自临时文件

      Mutipartfile的注释中说明了，文件是stored in memory or temporarily on disk，存在内存或磁盘临时文件的，所以上传大文件时，会发现前端进度条走完了，代码才走进自己写得接口代码

      /**
       * A representation of an uploaded file received in a multipart request.
       *
       * 
      The file contents are either stored in memory or temporarily on disk.
       * In either case, the user is responsible for copying file contents to a
       * session-level or persistent store as and if desired. The temporary storage
       * will be cleared at the end of request processing.
       *
       * @author Juergen Hoeller
       * @author Trevor D. Cook
       * @since 29.09.2003
       * @see org.springframework.web.multipart.MultipartHttpServletRequest
       * @see org.springframework.web.multipart.MultipartResolver
       */
      public interface MultipartFile extends InputStreamSource {
      

      Mutipartfile产生临时文件的好处坏处

      好处

      帮开发封装了上传逻辑

      可以在此基础实现切片上传到本地最后合并

      坏处

      对我来说

      文件转储时间翻倍：我的文件存储服务器不是接收请求的服务所在服务器，我还得把文件转存到文件服务器，而服务到文件服务器还得再传一遍，时间基本double

      前端进度不真实：前端显示的上传进度仅是浏览器给到服务的进度

      占用资源：临时文件多少占点服务器磁盘空间

      纯流式上传application/octet-stream

      一段半伪代码，application/octet-stream参数体只能有文件，参数通过放headers里给就可以解决这个问题

          @PostMapping(value = "/upload", headers = "content-type=application/octet-stream;charset=utf-8")
          public Object upload(HttpServletRequest request,
                                               @RequestHeader("userId") String userId,
                                               @RequestHeader("fileName") String fileName,
                                               @RequestHeader("fileSize") String fileSize,
                                               @RequestHeader("dirId") String dirId) {
              try (InputStream in = request.getInputStream()) {
                  fileName = URLDecoder.decode(fileName,"utf-8");
                  uploadFileServer(in, fileName, fileSize, userId, dirId);
                  return "success";
              } catch (Exception e) {
                  log.error("上传报错", e);
                  return "error";
              }
          }