spring框架上传文件原理探究

    <bean id="multipartResolver"
           class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
         <!-- set the max upload size100MB -->
         <property name="maxUploadSize">
             <value>104857600</value>
         </property>
         <property name="maxInMemorySize">
             <value>4096</value>
         </property>
         <property name="defaultEncoding">
             <value>utf-8</value>
         </property>
     </bean>

    @PostMapping(value = "/upload")
     public String upload(HttpServletRequest request){
         logger.info("上传文件");
         try {
  MultipartHttpServletRequest multipartRequest = (MultipartHttpServletRequest) request;
             MultipartFile file = multipartRequest.getFile("file");
           //处理上传的文件
         } catch (Exception e) {
             logger.error(e.getMessage(),e);
         }
     }

 DispatcherServlet：：protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
         HttpServletRequest processedRequest = request;
         HandlerExecutionChain mappedHandler = null;
         boolean multipartRequestParsed = false;
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        try {
             ModelAndView mv = null;
             Exception dispatchException = null;
            try {
                 processedRequest = checkMultipart(request);
 DispatcherServlet：： protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
         if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
             if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                 logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +
                         "this typically results from an additional MultipartFilter in web.xml");
             }
             else if (hasMultipartException(request) ) {
                 logger.debug("Multipart resolution failed for current request before - " +
                         "skipping re-resolution for undisturbed error rendering");
             }
             else {
                 try {
                     return this.multipartResolver.resolveMultipart(request);
                 }
                 catch (MultipartException ex) {
                     if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                         logger.debug("Multipart resolution failed for error dispatch", ex);
                         // Keep processing error dispatch with regular request handle below
                     }
                     else {
                         throw ex;
                     }
                 }
             }
         }
         // If not returned before: return original request.
         return request;
     }

    protected MultipartParsingResult parseFileItems(List<FileItem> fileItems, String encoding) {
         MultiValueMap<String, MultipartFile> multipartFiles = new LinkedMultiValueMap<String, MultipartFile>();
         Map<String, String[]> multipartParameters = new HashMap<String, String[]>();
         Map<String, String> multipartParameterContentTypes = new HashMap<String, String>();
        // Extract multipart files and multipart parameters.
         for (FileItem fileItem : fileItems) {
             if (fileItem.isFormField()) {
                 String value;
                 String partEncoding = determineEncoding(fileItem.getContentType(), encoding);
                 if (partEncoding != null) {
                     try {
                         value = fileItem.getString(partEncoding);
                     }
                     catch (UnsupportedEncodingException ex) {
                         if (logger.isWarnEnabled()) {
                             logger.warn("Could not decode multipart item '" + fileItem.getFieldName() +
                                     "' with encoding '" + partEncoding + "': using platform default");
                         }
                         value = fileItem.getString();
                     }
                 }
                 else {
                     value = fileItem.getString();
                 }
                 String[] curParam = multipartParameters.get(fileItem.getFieldName());
                 if (curParam == null) {
                     // simple form field
                     multipartParameters.put(fileItem.getFieldName(), new String[] {value});
                 }
                 else {
                     // array of simple form fields
                     String[] newParam = StringUtils.addStringToArray(curParam, value);
                     multipartParameters.put(fileItem.getFieldName(), newParam);
                 }
                 multipartParameterContentTypes.put(fileItem.getFieldName(), fileItem.getContentType());
             }
             else {
                 // multipart file field
                 CommonsMultipartFile file = new CommonsMultipartFile(fileItem);
                 multipartFiles.add(file.getName(), file);
                 if (logger.isDebugEnabled()) {
                     logger.debug("Found multipart file [" + file.getName() + "] of size " + file.getSize() +
                             " bytes with original filename [" + file.getOriginalFilename() + "], stored " +
                             file.getStorageDescription());
                 }
             }
         }
         return new MultipartParsingResult(multipartFiles, multipartParameters, multipartParameterContentTypes);
     }