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：

1.  
   public class RequestHolder {
2.  
   public static HttpServletRequest getRequest(){
3.  
   HttpServletRequest req = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest();
4.  
   return req;
5.  
   }
6.  
    
7.  
   public static HttpServletResponse getResponse(){
8.  
   HttpServletResponse resp = ((ServletWebRequest)RequestContextHolder.getRequestAttributes()).getResponse();
9.  
   return resp;
10.  
    }
11.  
    }

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

1.  
    
2.  
   protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
3.  
   HttpServletRequest processedRequest = request;
4.  
   ......
5.  
   processedRequest = checkMultipart(request);
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   multipartRequestParsed = processedRequest != request;
7.  
   ......
8.  
   // Actually invoke the handler.
9.  
   mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
10.  
    ......
11.  
    }
12.  
     
13.  
    protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
14.  
    if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
15.  
    if (request instanceof MultipartHttpServletRequest) {
16.  
    logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +
17.  
    "this typically results from an additional MultipartFilter in web.xml");
18.  
    }
19.  
    else {
20.  
    return this.multipartResolver.resolveMultipart(request);
21.  
    }
22.  
    }
23.  
    // If not returned before: return original request.
24.  
    return request;
25.  
    }

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：

1.  
    
2.  
   protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
3.  
   throws ServletException, IOException {
4.  
   ......
5.  
    
6.  
   RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
7.  
   ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
8.  
    
9.  
   WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
10.  
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
11.  
     
12.  
    initContextHolders(request, localeContext, requestAttributes);
13.  
     
14.  
    try {
15.  
    doService(request, response);
16.  
    }
17.  
    ......
18.  
    }
19.  
     
20.  
     
21.  
    protected ServletRequestAttributes buildRequestAttributes(
22.  
    HttpServletRequest request, HttpServletResponse response, RequestAttributes previousAttributes) {
23.  
     
24.  
    if (previousAttributes == null || previousAttributes instanceof ServletRequestAttributes) {
25.  
    return new ServletRequestAttributes(request);
26.  
    }
27.  
    else {
28.  
    return null; // preserve the pre-bound RequestAttributes instance
29.  
    }
30.  
    }
31.  
     
32.  
     
33.  
    private void initContextHolders(
34.  
    HttpServletRequest request, LocaleContext localeContext, RequestAttributes requestAttributes) {
35.  
     
36.  
    if (localeContext != null) {
37.  
    LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
38.  
    }
39.  
    if (requestAttributes != null) {
40.  
    RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
41.  
    }
42.  
    if (logger.isTraceEnabled()) {
43.  
    logger.trace("Bound request context to thread: " + request);
44.  
    }
45.  
    }
46.  
     

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

http://dinguangx.iteye.com/blog/2227049