转自：https://zhuanlan.zhihu.com/p/24293062?refer=dreawer
1.RequestContextHolder的使用
RequestContextHolder顾名思义,持有上下文的Request容器.使用是很简单的,具体使用如下:
//两个方法在没有使用JSF的项目中是没有区别的RequestAttributesrequestAttributes=RequestContextHolder.currentRequestAttributes();// RequestContextHolder.getRequestAttributes();//从session里面获取对应的值Stringstr=(String)requestAttributes.getAttribute("name",RequestAttributes.SCOPE_SESSION);HttpServletRequestrequest=((ServletRequestAttributes)requestAttributes).getRequest();HttpServletResponseresponse=((ServletRequestAttributes)requestAttributes).getResponse();
看到这一般都会想到几个问题:
request和response怎么和当前请求挂钩?
request和response等是什么时候设置进去的?
2.解决疑问
2.1 request和response怎么和当前请求挂钩?
首先分析RequestContextHolder这个类,里面有两个ThreadLocal保存当前线程下的request,关于ThreadLocal可以参考我的另一篇博文[Java学习记录--ThreadLocal使用案例]
//得到存储进去的requestprivatestaticfinalThreadLocal<RequestAttributes>requestAttributesHolder=newNamedThreadLocal<RequestAttributes>("Request attributes");//可被子线程继承的requestprivatestaticfinalThreadLocal<RequestAttributes>inheritableRequestAttributesHolder=newNamedInheritableThreadLocal<RequestAttributes>("Request context");
再看`getRequestAttributes()`方法,相当于直接获取ThreadLocal里面的值,这样就保证了每一次获取到的Request是该请求的request.
publicstaticRequestAttributesgetRequestAttributes(){RequestAttributesattributes=requestAttributesHolder.get();if(attributes==null){attributes=inheritableRequestAttributesHolder.get();}returnattributes;}
2.2request和response等是什么时候设置进去的?
找这个的话需要对springMVC结构的`DispatcherServlet`的结构有一定了解才能准确的定位该去哪里找相关代码.
在IDEA中会显示如下的继承关系.
左边1这里是Servlet的接口和实现类.
右边2这里是使得SpringMVC具有Spring的一些环境变量和Spring容器.类似的XXXAware接口就是对该类提供Spring感知,简单来说就是如果想使用Spring的XXXX就要实现XXXAware,spring会把需要的东西传送过来.
那么剩下要分析的的就是三个类,简单看下源码
1. HttpServletBean 进行初始化工作
2. FrameworkServlet 初始化 WebApplicationContext,并提供service方法预处理请
3. DispatcherServlet 具体分发处理.
那么就可以在FrameworkServlet查看到该类重写了service(),doGet(),doPost()...等方法,这些实现里面都有一个预处理方法`processRequest(request, response);`,所以定位到了我们要找的位置
查看`processRequest(request, response);`的实现,具体可以分为三步:
获取上一个请求的参数
重新建立新的参数
设置到XXContextHolder
父类的service()处理请求
恢复request
发布事件
protectedfinalvoidprocessRequest(HttpServletRequestrequest,HttpServletResponseresponse)throwsServletException,IOException{longstartTime=System.currentTimeMillis();ThrowablefailureCause=null;//获取上一个请求保存的LocaleContextLocaleContextpreviousLocaleContext=LocaleContextHolder.getLocaleContext();//建立新的LocaleContextLocaleContextlocaleContext=buildLocaleContext(request);//获取上一个请求保存的RequestAttributesRequestAttributespreviousAttributes=RequestContextHolder.getRequestAttributes();//建立新的RequestAttributesServletRequestAttributesrequestAttributes=buildRequestAttributes(request,response,previousAttributes);WebAsyncManagerasyncManager=WebAsyncUtils.getAsyncManager(request);asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(),newRequestBindingInterceptor());//具体设置的方法initContextHolders(request,localeContext,requestAttributes);try{doService(request,response);}catch(ServletExceptionex){failureCause=ex;throwex;}catch(IOExceptionex){failureCause=ex;throwex;}catch(Throwableex){failureCause=ex;thrownewNestedServletException("Request processing failed",ex);}finally{//恢复resetContextHolders(request,previousLocaleContext,previousAttributes);if(requestAttributes!=null){requestAttributes.requestCompleted();}if(logger.isDebugEnabled()){if(failureCause!=null){this.logger.debug("Could not complete request",failureCause);}else{if(asyncManager.isConcurrentHandlingStarted()){logger.debug("Leaving response open for concurrent processing");}else{this.logger.debug("Successfully completed request");}}}//发布事件publishRequestHandledEvent(request,response,startTime,failureCause);}}
再看initContextHolders(request, localeContext, requestAttributes)方法,把新的RequestAttributes设置进LocalThread,实际上保存的类型为ServletRequestAttributes,这也是为什么在使用的时候可以把RequestAttributes强转为ServletRequestAttributes.
privatevoidinitContextHolders(HttpServletRequestrequest,LocaleContextlocaleContext,RequestAttributesrequestAttributes){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);}}
因此RequestContextHolder里面最终保存的为ServletRequestAttributes,这个类相比`RequestAttributes`方法是多了很多.
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