HTTP上传文件解析

浏览器上传完整报文如下

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--

从这里可以看出

1. 文件上传时使用的Content-Type为 multipart/form-data

根据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.

这个类型允许传递所有MIME(Multipurpose Internet Mail Extensions多用途互联网邮件扩展类型)数据流。

 

2. 其通过Content-Type中的boundary=---------------------------114782935826962来分割表单中的不同参数。

boundary是一个不会在文件流中出现的字符串，用以分割不同参数。

 

浏览器发送后，那么后台是怎么处理的，这里以springMVC为例子。

后台的SpringMvc接到后的处理方式如下：

1. SpringMvc所有的请求都由DispatcherServlet进行分发,其具体代码截取如下：

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

 

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    try{
            try{
                ...
                processedRequest = checkMultipart(request);//检查并处理multipart的data，若存在file，则转换为StandardMultipartHttpServletRequest返回请求
                multipartRequestParsed = (processedRequest != request);//标记是否有multipart处理过，若有，在最后要进行清理文件
                ...
            }
            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);
                }
            }
        }
    }

接下来看看checkMultipart

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {//通过contentType中是否含有multipart/来判断是否有文件
            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);//将request中的数据流转成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);//里面执行了parseRequest()
    }

org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.parseRequest(HttpServletRequest)具体逻辑如下，其中从每部分中获取Content-Disposition，包含文件名，和元素名称，如果发现对象是文件的话（文件名不为空）则将文件放到request对象中去并返回。

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);//从每部分中获取Content-Disposition，包含文件名，和元素名称
                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);
        }
    }

接下来就是正常的处理逻辑，去执行springMvc对应的方法，从头中已经能拿到对应的file。

在请求完成后，会进行文件资源的清理。

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);
            }
        }
    }

参考：

1. https://tools.ietf.org/html/rfc1867 文件上传的rfc规范