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。