Spring Framework源码（十）：SpringMVC之文件上传

    我们这一章讲SpringMVC中文件上传的应用，首先我们还是从DispatcherServlet这个核心分发器开始讲起：

processedRequest = checkMultipart(request);

    还记得上一章讲doDispatch这个方法时见过的方法吧？现在我们来分析下这个方法的具体解析过程：

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {//检测multipartResolver是否已经初始化（在initStrategies这个方法中已经初始化了这个处理器），解析request判断是否有文件上传if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {//如果已经在web.xml配置了MultipartFilter过滤器并且已完成了解析则不需要再次处理requestif (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");}//如果在此前解析request的过程中发生了异常则不需要再次处理requestelse if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) instanceof MultipartException) {logger.debug("Multipart resolution failed for current request before - " +"skipping re-resolution for undisturbed error rendering");}//非以上情况则解析requestelse {return this.multipartResolver.resolveMultipart(request);}}return request;}

    以上完成了对request中是否有文件上传的判断，并返回了经过处理的request。但是到底springmvc是怎么解析request的呢？下面我们来看下MultipartResolver接口：

public interface MultipartResolver {//判断是否有文件上传boolean isMultipart(HttpServletRequest request);//解析requestMultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;//清除文件上传内容void cleanupMultipart(MultipartHttpServletRequest request);}

      我们以MultipartResolver的默认实现类StandardServletMultipartResolver为例进行讲解Multipart的解析过程，下面我们来看看isMultipart(HttpServletRequest)和resolveMultipart(HttpServletRequest)两个方法。

public boolean isMultipart(HttpServletRequest request) {//检测http的请求方式是否是postif (!"post".equals(request.getMethod().toLowerCase())) {return false;}//检查Content-Type,如果是以“multipart”开头则返回真String contentType = request.getContentType();return (contentType != null && contentType.toLowerCase().startsWith("multipart/"));}

     不难看出如果http请求是post方式并且contentType（form标签的enctype="multipart/form-data"）是“multipart/form-data”则说明该请求中附带文件需要处理。我们再看看resolveMultipart：

public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {return new StandardMultipartHttpServletRequest(request, this.resolveLazily);}

    它将一个request包装成了一个StandardMultipartHttpServletRequest，这个类会使用parseRequest方法解析http报文，将上传文件封装成StandardMultipartFile挨个存储到MultiValueMap<String, MultipartFile>类型的map中并关联到处理后的request。至于这个map中的file怎么被绑定到controller的参数中我们将在下一章中再做介绍。