AOP 获取 RequestContextHolder

转载： http://blog.csdn.net/lexang1/article/details/52619215

在使用spring时,经常需要在普通类中获取session,request等对像.
比如一些AOP拦截器类,在有使用struts2时,因为struts2有一个接口使用org.apache.struts2.ServletActionContext即可很方便的取到session对像.
用法:

//获取请求对像
    public static HttpServletRequest getRequest() {
        return  ServletActionContext.getRequest();
    }
    //获取输出对象
    public static HttpServletResponse getResponse(){
        return ServletActionContext.getResponse();

    }

但在单独使用spring时如何在普通类中获取session,reuqest呢?
首先要在web.xml增加如下代码:

<listener>
        <listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>
 </listener>

使用方法：

　　在普通bean中

@Autowired
private HttpSession session;

@Autowired
private HttpServletRequest request;

在普通类中

    public static HttpServletRequest getRequest(){
        HttpServletRequest req = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest();
        return req;
    }

    public static HttpServletResponse getResponse(){
        HttpServletResponse resp = ((ServletRequestAttributes)RequestContextHolder.getRequestAttributes()).getResponse();
        return resp;
    }

你必须要有一个request的引用，否则是取不到的。request可以通过控制器传入，有了request自然就可以取到session了，或者可以通过spring的WebUtils取Session数据，如下：
拦截器举例：

public boolean preHandle(HttpServletRequest request,
HttpServletResponse response, Object handler) throws
Exception {
String context = (String) WebUtils.getSessionAttribute(request,
"context_key");
return context != null ;
}

RequestContextHolder使用误区

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。