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");
}
//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request
else 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request
else {
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request
MultipartHttpServletRequest 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post
if (!"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。