叙述
Spring 文件上传使用的总结及MultipartFile的解析
解决方案
MultipartFile
单文件上传
@RequestMapping(value = "/UploadOneServlet.do", method = RequestMethod.POST)
@ResponseBody
public String upload(@RequestParam("multipartFile") MultipartFile multipartFile) {
//...文件处理代码
}后端中的对应方法中使用MultipartFile类型参数进行接收。必须添加@RequestParam("multipartFile")参数，否则报错。@RequestParam的value与前端imput标签中的name对应。
多个文件上传
@RequestMapping(value = "/UploadOneServlet.do", method = RequestMethod.POST)
@ResponseBody
public String upload(@RequestParam("multipartFile") List<MultipartFile> multipartFile) {
//...文件处理代码
}后台使用List,即可前台上传的多个文件。如果使用单个MultipartFile类型进行接收则只能接收一个文件。
混合使用
@RequestMapping(value = "/UploadOneServlet.do", method = RequestMethod.POST)
@ResponseBody
public String upload(@RequestParam("multipartFile") List<MultipartFile> multipartFiles, @RequestParam("multipartFiles") MultipartFile multipartFile) {
}后台使用一个List<MultiPartFile>类型和MultiPartFile类型进行接收。@RequestParam的value与前端imput标签中的name一一对应。
解析多个文件
private void upload(List<MultipartFile> multipartFiles) throws Exception{
for (MultipartFile multipartFile : multipartFiles) {
String fileName = multipartFile.getOriginalFilename();
String filePath = ContextLoader.getCurrentWebApplicationContext().getServletContext().getRealPath("/") + "fileUpload";
String fileTotalName = filePath + File.separator + fileName;
File f = new File(fileTotalName);
multipartFile.transferTo(f);
}
}HttpServletRequest
使用HttpServletRequest接收
@RequestMapping(value = "/UploadOneServlet.do", method = RequestMethod.POST)
@ResponseBody
public String upload(HttpServletRequest request){
List<MultipartFile> files = new ArrayList<>();
MultipartHttpServletRequest multipartHttpServletRequest = (MultipartHttpServletRequest) request;
Iterator<String> a = multipartHttpServletRequest.getFileNames();//返回的数量与前端input数量相同, 返回的字符串即为前端input标签的name
while (a.hasNext()) {
String name = a.next();
List<MultipartFile> multipartFiles = multipartHttpServletRequest.getFiles(name);//获取单个input标签上传的文件，可能为多个
files.addAll(multipartFiles);
}
upload(files);
return "success";
}MultiPartFile类解析
Spring MVC中要使用MultipartFile进行文件上传，需要先注入MultipartResolver类型的Bean
之前解析DispatcherServlet类时得知所有最终会调用这个类中的doDispatch方法对request进行处理
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);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
//...剩余代码
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", err));
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}可以看到processedRequest = checkMultipart(request)对reuqest进行判断
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);
}
else {
throw ex;
}
}
}
}
return request;
}通过以上方法可以看到以下逻辑：
- 判断用户是否注入了
MultipartResolver类型的Bean,如果注入了则使用它去检测当前请求是否为文件上传请求 - 如果为文件上传请求，就通过
MultipartResolver去处理当前请求。 - 返回处理后的请求或为处理的请求。
接下来看看Spring是如何判断当前请求是文件上传请求的, 这里主要看CommonsMultipartResolver
public boolean isMultipart(HttpServletRequest request) {
return (request != null && ServletFileUpload.isMultipartContent(request));
}这里直接使用了Apache中的ServletFileUpload
public static final boolean isMultipartContent(
HttpServletRequest request) {
if (!POST_METHOD.equalsIgnoreCase(request.getMethod())) {
return false;
}
return FileUploadBase.isMultipartContent(new ServletRequestContext(request));
}这里可以看到request请求必须是POST类型，如果是，则用FileUploadBase进行进一步判断
public static final boolean isMultipartContent(RequestContext ctx) {
String contentType = ctx.getContentType();
if (contentType == null) {
return false;
}
if (contentType.toLowerCase(Locale.ENGLISH).startsWith(MULTIPART)) {// MULTIPART = "multipart/"
return true;
}
return false;
}以上方法说明：
只有当当前请求的contentType是以”multipart/“开头的时候，才会将此请求当做文件上传的请求。
综上，Spring判断当前请求是否是文件上传请求主要有两个条件：
- 当前请求为POST请求
- 当前请求的
contextType必须以”multipart/“开头
这正符合了前端<form>标签中的设置。
如果是文件上传请求，则会调用multipartResolver.resolveMultipart(request)对请求进行解析
public MultipartHttpServletRequest resolveMultipart(final HttpServletRequest request) throws MultipartException {
MultipartParsingResult parsingResult = parseRequest(request);
return new DefaultMultipartHttpServletRequest(request, parsingResult.getMultipartFiles(),
parsingResult.getMultipartParameters(), parsingResult.getMultipartParameterContentTypes());
}可以看到任然是调用parseRequest方法对request进行解析，并将结果存入MultipartParsingResult
protected MultipartParsingResult parseRequest(HttpServletRequest request) throws MultipartException {
String encoding = determineEncoding(request);
FileUpload fileUpload = prepareFileUpload(encoding);
try {
// 获取到上传文件的信息 一个FileItem对应一个文件，FileItem的fieldName与input框的name对应
List<FileItem> fileItems = ((ServletFileUpload) fileUpload).parseRequest(request);
//将其存入MultipartParsingResult类中
return parseFileItems(fileItems, encoding);
}
catch (FileUploadBase.SizeLimitExceededException ex) {
throw new MaxUploadSizeExceededException(fileUpload.getSizeMax(), ex);
}
catch (FileUploadBase.FileSizeLimitExceededException ex) {
throw new MaxUploadSizeExceededException(fileUpload.getFileSizeMax(), ex);
}
catch (FileUploadException ex) {
throw new MultipartException("Failed to parse multipart servlet request", ex);
}
}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()) {
//...省略代码
}
else {
// multipart file field
CommonsMultipartFile file = createMultipartFile(fileItem);
// file.getName()得到fileItem.getfieldName map键位input框的name, 值为具体文件类
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);
}初始化了MultipartParsingResult对象之后就能根据该对象去初始化MultipartHttpServletRequest对象，供接口中调用
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