Spring MVC的RequestContextHolder使用误区

JShop简介：jshop是一套使用Java语言开发的B2C网店系统，致力于为个人和中小企业提供免费、好用的网店系统。

项目主页：http://git.oschina.net/dinguangx/jshop

在线演示：

• 前台: http://jshop.ofmall.org:81/jshop
• 后台: http://jshop.ofmall.org:81/jshop/manage/user/login (admin/123456)

    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org.apache.catalina.connector.RequestFacade的实例。 

public class RequestFacade implements HttpServletRequest 

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。

Jshop简介：http://git.oschina.net/dinguangx/jshop