Spring MVC的RequestContextHolder使用誤區

JShop簡介：jshop是一套使用Java語言開發的B2C網店系統，致力於為個人和中小企業提供免費、好用的網店系統。

項目主頁：http://git.oschina.net/dinguangx/jshop

在線演示：

• 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，為了隨時都能取到當前請求的request對象，可以通過RequestContextHolder的靜態方法getRequestAttributes()獲取Request相關的變量，如request, response等。 
        在jshop中，對RequestContextHolder的使用進一步封裝，簡化為RequestHolder類，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.  
    }

        在大部分的情況下，它都能很好地工作，但在商品管理編輯中，新增商品時，卻出現了意外的問題：通過RequestHolder.getRequest().getParameter()得不到參數值，通過debug發現，
通過spring mvc的method注入的request對象實際為MultipartHttpServletRequest,而通過RequestHolder.getRequest()獲取到的request對象卻是org.apache.catalina.connector.RequestFacade的實例。 

public class RequestFacade implements HttpServletRequest 

 

原來在商品新增時，由於使用了文件上傳，form表單的enctype類型為”multipart/form-data”，
spring mvc對文件上傳的處理類實際卻為spring-mvc.xml文件中配置的CommonsMultipartResolver， 
該類先判斷當前請求是否為multipart類型，如果是的話，將request對象轉為MultipartHttpServletRequet,相關的源碼見DisptcherServlet

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

那么，RequestContextHolder中的request又是從哪來的呢？ 
繼續翻看DispatcherServlet的源碼，從其父類FrameworkServlet中找到的processRequest()以相關方法源碼：

1.  
    
2.  
   protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
3.  
   throws ServletException, IOException {
4.  
   ......
5.  
    
6.  
   RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
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   ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
8.  
    
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   WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
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    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
11.  
     
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    initContextHolders(request, localeContext, requestAttributes);
13.  
     
14.  
    try {
15.  
    doService(request, response);
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    }
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 {
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    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.  
     

從這里可以看到，initContextHolder()方法中完成了RequestContextHolder的requestAttributes設置，
而doService()在這之后調用，DispatcherServlet中的processRequest()方法即在doService()之中，
所以從RequestContextHolder中獲取到的就是原來的RequestFacade對象，而不是經過spring mvc處理之后的MultipartHttpServletRequest對象，
其后果就是，從RequestContextHolder獲取request后，無法直接通過getParameter()獲取參數值。

最便捷的解決辦法： 
直接將HttpServletRequest作為spring mvc的方法入參，即可以正確獲取參數值。

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

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