spring框架上传文件原理探究

上述配置是springmvc框架下上传文件常见的配置，有了上面的配置，下面controller就可以很容易完成文件上传工作：

@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);
}
}

注意上面标红部分代码，那么CommonsMultipartResolver工作的背后原理是什么呢，笔者带着这个问题将一探究竟：

   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;
   }

上面multipartResolver.resolveMultipart就会调用CommonsMultipartResolver的resolveMultipart方法，最后真正转换发生在类CommonsFileUploadSupport的parseFileItems方法

   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);
   }

该类将来自客户端上传的文件转换为FileItem对象，这样就完成将multipart/form-data到MultipartFile对象的转换，

因为spring默认不启用MultipartResolver的支持，所以使用前需要进行MultipartResolver
Bean的定义，但是springboot默认支持了MultipartResolver，直接就可以使用！

spring框架上传文件原理探究

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