在spring mvc中，为了随时都能取到当前请求的request对象，可以通过RequestContextHolder的静态方法getRequestAttributes()获取Request相关的变量，如request, response等。
在jshop中，对RequestContextHolder的使用进一步封装，简化为RequestHolder类，如下：
public class RequestHolder {
public static HttpServletRequest getRequest(){
HttpServletRequest req = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest();
return req;
}
public static HttpServletResponse getResponse(){
HttpServletResponse resp = ((ServletWebRequest)RequestContextHolder.getRequestAttributes()).getResponse();
return resp;
}
}
在大部分的情况下，它都能很好地工作，但在商品管理编辑中，新增商品时，却出现了意外的问题：通过RequestHolder.getRequest().getParameter()得不到参数值，通过debug发现，通过spring mvc的method注入的request对象实际为MultipartHttpServletRequest,而通过RequestHolder.getRequest()获取到的request对象却是RequestFacfade的实例。
原来在商品新增时，由于使用了文件上传，form表单的enctype类型为”multipart/form-data”，spring mvc对文件上传的处理类实际却为spring-mvc.xml文件中配置的CommonsMultipartResolver， 该类先判断当前请求是否为multipart类型，如果是的话，将request对象转为MultipartHttpServletRequet,相关的源码见DisptcherServlet
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
……
processedRequest = checkMultipart(request);
multipartRequestParsed = processedRequest != request;
……
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
……
}
protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
if (request instanceof MultipartHttpServletRequest) {
logger.debug(“Request is already a MultipartHttpServletRequest – if not in a forward, ” +
“this typically results from an additional MultipartFilter in web.xml”);
}
else {
return this.multipartResolver.resolveMultipart(request);
}
}
// If not returned before: return original request.
return request;
}
那么，RequestContextHolder中的request又是从哪来的呢？
继续翻看DispatcherServlet的源码，从其父类FrameworkServlet中找到的processRequest()以相关方法源码：
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
……
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
initContextHolders(request, localeContext, requestAttributes);
try {
doService(request, response);
}
……
}
protected ServletRequestAttributes buildRequestAttributes(
HttpServletRequest request, HttpServletResponse response, RequestAttributes previousAttributes) {
if (previousAttributes == null || previousAttributes instanceof ServletRequestAttributes) {
return new ServletRequestAttributes(request);
}
else {
return null; // preserve the pre-bound RequestAttributes instance
}
}
private void initContextHolders(
HttpServletRequest request, LocaleContext localeContext, RequestAttributes requestAttributes) {
if (localeContext != null) {
LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
}
if (requestAttributes != null) {
RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
}
if (logger.isTraceEnabled()) {
logger.trace(“Bound request context to thread: ” + request);
}
}
从这里可以看到，initContextHolder()方法中完成了RequestContextHolder的requestAttributes设置，而doService()在这之后调用，DispatcherServlet中的processRequest()方法即在doService()之中，所以从RequestContextHolder中获取到的就是原来的RequestFacade对象，而不是经过spring mvc处理之后的MultipartHttpServletRequest对象，其后果就是，从RequestContextHolder获取request后，无法直接通过getParameter()获取参数值。
最便捷的解决办法：
直接将HttpServletRequest作为spring mvc的方法入参，即可以正确获取参数值。
