HTTP上傳文件解析

瀏覽器上傳完整報文如下

POST http://localhost:8080/fileUpload/ HTTP/1.1
Host: localhost:8080
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Referer: http://localhost:8080/
Content-Type: multipart/form-data; boundary=---------------------------114782935826962
Content-Length: 937402
Connection: keep-alive
Upgrade-Insecure-Requests: 1
DNT: 1

-----------------------------114782935826962
Content-Disposition: form-data; name="text1"

text default
-----------------------------114782935826962
Content-Disposition: form-data; name="text2"

aωb
-----------------------------114782935826962
Content-Disposition: form-data; name="file1"; filename="timg.jpg"
Content-Type: image/jpeg
... contents of file goes here ...
----------------114782935826962
Content-Disposition: form-data; name="file2"; filename="timg.jpg"
Content-Type: image/jpeg
... contents of file goes here ...
----------------114782935826962--

 

從這里可以看出

1. 文件上傳時使用的Content-Type為 multipart/form-data

根據rfc1867：

The media-type multipart/form-data follows the rules of all multipart

   MIME data streams as outlined in RFC 1521. It is intended for use in

   returning the data that comes about from filling out a form.

這個類型允許傳遞所有MIME(Multipurpose Internet Mail Extensions多用途互聯網郵件擴展類型)數據流。

 

2. 其通過Content-Type中的boundary=---------------------------114782935826962來分割表單中的不同參數。

boundary是一個不會在文件流中出現的字符串，用以分割不同參數。

 

瀏覽器發送后，那么后台是怎么處理的，這里以springMVC為例子。

后台的SpringMvc接到后的處理方式如下：

1.  SpringMvc所有的請求都由DispatcherServlet進行分發,其具體代碼截取如下：

org.springframework.web.servlet.DispatcherServlet.doDispatch(HttpServletRequest, HttpServletResponse)

 

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    try{
            try{
                ...
                processedRequest = checkMultipart(request);//檢查並處理multipart的data，若存在file，則轉換為StandardMultipartHttpServletRequest返回請求
                multipartRequestParsed = (processedRequest != request);//標記是否有multipart處理過，若有，在最后要進行清理文件
                ...
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            
        }
        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()) {
                ...
            }
            else {
                // Clean up any resources used by a multipart request. 清理消息
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

接下來看看checkMultipart

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {//通過contentType中是否含有multipart/來判斷是否有文件
            if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                if (request.getDispatcherType().equals(DispatcherType.REQUEST)) {
                    logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
                }
            }
            else if (hasMultipartException(request) ) {
                logger.debug("Multipart resolution previously failed for current request - " +
                        "skipping re-resolution for undisturbed error rendering");
            }
            else {
                try {
                    return this.multipartResolver.resolveMultipart(request);//將request中的數據流轉成StandardMultipartHttpServletRequest
                }
                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;
}

org.springframework.web.multipart.support.StandardServletMultipartResolver.resolveMultipart(HttpServletRequest)

    @Override
    public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {
        return new StandardMultipartHttpServletRequest(request, this.resolveLazily);//里面執行了parseRequest()
    }

org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.parseRequest(HttpServletRequest)具體邏輯如下，其中從每部分中獲取Content-Disposition，包含文件名，和元素名稱，如果發現對象是文件的話（文件名不為空）則將文件放到request對象中去並返回。

 

private void parseRequest(HttpServletRequest request) {
        try {
            Collection<Part> parts = request.getParts();
            this.multipartParameterNames = new LinkedHashSet<>(parts.size());
            MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
            for (Part part : parts) {
                String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);//從每部分中獲取Content-Disposition，包含文件名，和元素名稱
                ContentDisposition disposition = ContentDisposition.parse(headerValue);
                String filename = disposition.getFilename();
                if (filename != null) {
                    if (filename.startsWith("=?") && filename.endsWith("?=")) {
                        filename = MimeDelegate.decode(filename);
                    }
                    files.add(part.getName(), new StandardMultipartFile(part, filename));
                }
                else {
                    this.multipartParameterNames.add(part.getName());
                }
            }
            setMultipartFiles(files);
        }
        catch (Throwable ex) {
            handleParseFailure(ex);
        }
    }

接下來就是正常的處理邏輯，去執行springMvc對應的方法，從頭中已經能拿到對應的file。

在請求完成后，會進行文件資源的清理。

org.springframework.web.multipart.support.StandardServletMultipartResolver.cleanupMultipart(MultipartHttpServletRequest)

 

@Override
    public void cleanupMultipart(MultipartHttpServletRequest request) {
        if (!(request instanceof AbstractMultipartHttpServletRequest) ||
                ((AbstractMultipartHttpServletRequest) request).isResolved()) {
            // To be on the safe side: explicitly delete the parts,
            // but only actual file parts (for Resin compatibility)
            try {
                for (Part part : request.getParts()) {
                    if (request.getFile(part.getName()) != null) {
                        part.delete();
                    }
                }
            }
            catch (Throwable ex) {
                LogFactory.getLog(getClass()).warn("Failed to perform cleanup of multipart items", ex);
            }
        }
    }

參考：

1.  https://tools.ietf.org/html/rfc1867 文件上傳的rfc規范