springweb篇6讲16常见错误今天我们来学习Spring异常处理机制提供了一套

publicvoiddestroy()}NotAllowException就是一个简单的RuntimeExceptionpublicpublicclassNotAllowExceptionextendsRuntimeExceptionpublicNotAllowException() 5同时，新增了一个RestControllerAdvice来处理这个异常，处理方式也很简单，就是返回一个403的resultCode：publicclassNotAllowExceptionHandlerpublicStringhandle()return"{\"resultCode\": 9为了验证一下失败的情况，我们模拟了一个请求，在HTTP请求头里加上一个Token，值为111，这样就会错误了，我们会不会被NotAllowExceptionHandler处理11throw想下问题出在哪呢？我们不妨对Spring我们先来回顾一下 从这张图中可以看出，当所有的过滤器被执行完毕以后，Spring才会进入Servlet相关的处理，而DispatcherServlet才是整个Servlet处理的，它是前端控制器设计模式的实现，提供SpringWebMVC的集中点并负责职责的分派。正是在这里，Spring处理其实说到这里，我们已经了解到过滤器内异常无法被统一处理的大致原因，就是因为异常处理发生在上图的红域，即DispatcherSerlet中的doDisach)，而此时，过滤器已经全部执行完毕了。下面深入分析SpringWeb对异常统一处理的逻辑，深刻理解其内部原理首先我们来了解下ControllerAdvice是如何被Spring加载并对外的。在SpringWeb的配置类WebMvcConfigurationSupport中，被@Bean修饰的handlerExceptionResolveraddDefaultHandlerExceptionResolvers1123456789publicHandlerExceptionResolverhandlerExceptionResolver(@Qualifier("mvcContentNegotiationManager")ContentNegotiationManagerconList<HandlerExceptionResolver>exceptionResolvers=newArrayList<>();if(exceptionResolvers.isEmpty())}positecomposite=newreturn}最终按照下图的调用栈，Spring实例化了ExceptionHandlerExceptionResolver类。publicpublicvoidafterPropertiesSet()//Dothisfirst,itmayaddResponseBodyAdvice5并在initExceptionHandlerAdviceCache()中完成了所有ControllerAdvice中的ExceptionHandler@ControllerAdviceBean，把它们放到成员变量exceptionHandlerAdviceCache中。在我们这个案例里，就是指NotAllowExceptionHandler这个异常处理器。1123456789privatevoidinitExceptionHandlerAdviceCache()List<ControllerAdviceBean>adviceBeans=ControllerAdviceBean.findAnnotatedfor(ControllerAdviceBeanadviceBean:adviceBeans){Class<?>beanType=adviceBean.getBeanType();if(beanType==null){thrownewIllegalStateException("Unresolvabletypefor}ExceptionHandlerMethodResolverresolver=newifs())this.exceptionHandlerAdviceCache.put(adviceBean,}}到这，我们可以总结一下，WebMvcConfigurationSupport中的handlerExceptionResolver()实例化并了一个ExceptionHandlerExceptionResolver的实例，而所有被@ControllerAdvice注解修饰的异常处理器，都会在ExceptionHandlerExceptionResolver实例化的时候自动扫描并装载在其类成员变量exceptionHandlerAdviceCache中。privatevoidinitHandlerExceptionResolvers(ApplicationContextcontext)this.handlerExceptionResolversprivatevoidinitHandlerExceptionResolvers(ApplicationContextcontext)this.handlerExceptionResolvers=34(this.detectAllHandlerExceptionResolvers)5//FindallHandlerExceptionResolversintheApplicationContext,6Map<String,HandlerExceptionResolver>matchingBeans=7.beansOfTypeIncludingAncestors(context,8if(!matchingBeans.isEmpty())9this.handlerExceptionResolvers=new//WekeepHandlerExceptionResolversinsorted }14ControllerAdviceSpring下文贴出的是类DispatcherServlet中的方法doDispatch()的部分代码：1protectedvoiddoDispatch(HttpServletRequestrequest,234try5ModelAndViewmv=6ExceptiondispatchException=7try89//查找当前请求对应的handler}catch(Exceptionex)dispatchException=}catch(Throwableerr)dispatchException=newNestedServletException("Handler}processDispatchResult(processedRequest,processDispatchResult(processedRequest,response,mappedHandler,mv,}dispatchException，再交给processDispatchResultprivateprivatevoidprocessDispatchResult(HttpServletRequestrequest,@NullableHandlerExecutionChainmappedHandler,@NullableModelAndView@NullableExceptionexception)throwsExceptionbooleanerrorView=if(exception!=null)if(exceptioninstanceofModelAndViewDefiningException)mv=((ModelAndViewDefiningException) elseObjecthandler=(mappedHandler!=null?mv=processHandlerException(request,response,handler,errorView=(mv!= }ExceptionnullprocessHandlerExceptionprotectedprotectedModelAndViewprocessHandlerException(HttpServletRequestrequest,@NullableObjecthandler,Exceptionex)throwsExceptionModelAndViewexMv=if(this.handlerExceptionResolvers!=null)for(HandlerExceptionResolverresolver:exMv=resolver.resolveException(request,response,handler,if(exMv!=null) 14然后，processHandlerExceptionhandlerExceptionResolvers显然，这里的handlerExceptionResolvers一定包含我们的NotAllowExceptionHandler#NotAllowExceptionExceptionHandlerExceptionResolver包装类。SpringMVCFilter并将异常HandlerExceptionResolver进行解析处理。我们可以这样修改PermissionFilter，注入privateHandlerExceptionResolver12345612345678publicvoiddoFilter(ServletRequestrequest,ServletResponseresponse,FilHttpServletRequesthttpServletRequest=(HttpServletRequest)request;HttpServletResponsehttpServletResponse=(HttpServletResponse)responStringtoken=(!"111111".equals(token)){System.out.println("throwNotAllowException");resolver.resolveException(httpServletRequest,9}chain.doFilter(request,}当我们尝试用错误的Tokenthrowthrow返回的JSON11{"resultCode":成正确的Token案例2404继续沿用学生的案例，为了防止一些异常的，我们需要记录所有404状态的一般使用RESTfulJSON11{"resultCode":Spring404ExceptionHandlerpublicclassMyExceptionHandlerpublicStringhandle404()return"{\"resultCode\": 10URLregStudent成/regStudent1，得到了以下结果：11{"timestamp":"2021-05- ","status":404,"error":"NotSpring导致Spring没有使用我们定义的异常处理器呢？我们可以从异常处理的处理代码开始分析，DispatcherServlet中的doDispatch()核protectedprotectedvoiddoDispatch(HttpServletRequestrequest,HttpServletResponsemappedHandler=if(mappedHandler==null)noHandlerFound(processedRequest, 91123456789protectedvoidnoHandlerFound(HttpServletRequestrequest,(this.throwExceptionIfNoHandlerFound)thrownewNoHandlerFoundException(request.getMethod(),getRequestUri(reqnewServletServerHttpRequest(request).getHeaders());}else}}noHandlerFound()的逻辑非常简单，如果throwExceptionIfNoHandlerFound属性为true，则直接抛出NoHandlerFoundException异常，反之则会进一步获取到对应的请求到这，离我们非常近了，我们只需要将throwExceptionIfNoHandlerFound默认设trueNoHandlerFoundExceptiondoDispatch()内的catch俘获。进而就像案例1介绍的一样，最终能够执行我们自定义的异常处理器于是，我们开始尝试，因为throwExceptionIfNoHandlerFound对应的Spring配置项为throw-exception-if-no-handler-found，其加入到perties配置文件中，设置其值为true。实际上这里还存在另一个坑，在SpringWeb的WebMvcAutoConfiguration类中，其默认添加的两个ResourceHandler，一个是用来处理请求路径/webjars/**，而另一个是即前请求没有定义任何对应的请求处理器，getHandler()也一定会获取到一个Handler来处理当前请求，因为第二个匹配/**路径的ResourceHandler决定了任何请求路径都会被其处理。mappedHandler==null判断条件不会成立，显然就不可能走noHandlerFound()，那么就不会抛出NoHandlerFoundException异常，也无法被后续的ResourceHandler的详细逻辑。首先我们来了解下ControllerAdvice是如何被Spring加载并对外的同样是在WebMvcConfigurationSupport类中，被@Bean修饰的resourceHandlerMap()，它新建了ResourceHandlerRegistry类实例，并通过addResourceHandlers()将ResourceHandler到ResourceHandlerRegistry类实1123456789public(@Qualifier("mvcUrlPathHelper")UrlPathHelperurlPathHelper,@Qualifier("mvcPathMatcher")PathMatcherpathMatcher,@Qualifier("mvcContentNegotiationManager")ContentNegotiationManagercon@Qualifier("mvcConversionService")FormattingConversionServiceconversio@Qualifier("mvcResourceUrlProvider")ResourceUrlProviderresourceUrlProvAssert.state(this.applicationContext!=null,"NoApplicationContextset");Assert.state(this.servletContext!=null,"NoServletContextset");ResourceHandlerRegistryregistry=newResourceHandlerRegistry(this.applicathis.servletContext,contentNegotiationManager,urlPathHelper);=if(handlerMapreturnnull;}==null)return;}最终通过ResourceHandlerRegistry类实例中的getHandlerMap()返回了SimpleUrlHandlerMap实例，它装载了所有ResourceHandler的集合并到Spring容器中：1123456789()Map<String,HttpRequestHandler>urlMap=newfor(ResourceHandlerRegistrationregistration:this.registrations){for(StringpathPattern:registration.getPathPatterns()){ResourceHttpRequestHandlerhandler=urlMap.put(pathPattern,handler);}}returnnew(urlMap,}可以了解到，当前方法中的addResourceHandlers()最终执行到了WebMvcAutoConfigurationaddResourceHandlers()，通过这个方法，我们可以知道当前有哪些ResourceHandler的集合被到了Spring容器中：11publicvoidaddResourceHandlers(ResourceHandlerRegistryregistry)223456789ifs())logger.debug("Defaultresourcehandling}DurationcachePeriod=this.resourceProperties.getCache().getPeriod();CacheControlcacheControl=this.resourceProperties.getCache().getCachecontForPattern("/webjars/**"))}StringstaticPathPattern=ForPattern(staticPathPattern))}}ResourceHandler，一个是用来处理请求路径/webjars/**，而另一个是/**。this.resourceProperties.isAddMaps()为false，那么会直接返回，后续的两ResourceHandler也不会被添加。11234ifs())logger.debug("Defaultresourcehandling}至此，有两个ResourceHandler被实例化且到了Spirng容器中，一个处理路径/webjars/**的请求，另一个处理路径为/**同样，当第一次请求发生时，DispatcherServlet中的initHandlerMaps()将会获取所有到Spring的HandlerMap类型的实例，而SimpleUrlHandlerMap恰好实现了HandlerMap接口，这些SimpleUrlHandlerMap类型的实例则会被入到类成员变量handlerMaps中。7BeanFactoryUtils.beansOfTypeIncludingAncestors(context,8(!matchingBeans.isEmpty())9this.handlerMaps=new//WekeepHandlerMapsinsorted }}}123456privatevoids(ApplicationContextcontext)123456privatevoids(ApplicationContextcontext)s=//FindallMap<String,s)sintheApplicationContext,including>matchingBeans我们来回顾一下DispatcherServlet中的doDispatch()代码protectedprotectedvoiddoDispatch(HttpServletRequestrequest,HttpServletResponsemappedHandler=if(mappedHandler==null)noHandlerFound(processedRequest, 9这里的getHandler()将会遍历成员变量handlerMap112345678protectedHandlerExecutionChaingetHandler(HttpServletReq