這一次搞懂SpringMVC原理說明
前言
前面幾篇文章,學習了Spring IOC、Bean例項化過程、AOP、事務的原始碼和設計思想,瞭解了Spring的整體執行流程,但如果是web開發,那麼必不可少的還有Spring MVC,本篇主要分析在請求呼叫過程中SpringMVC的實現原理,通過本篇要搞懂它是怎麼解決請求、引數、返回值對映等問題的。
正文
請求入口
我們都知道前端呼叫後端介面時,都會通過Servlet進行轉發,而Servlet的宣告週期包含下面四個階段:
例項化(new)
初始化(init)
執行(service呼叫doGet/doPost)
銷燬(destroy)
前兩個階段在Spring啟動階段就做好了(init根據配置可能是第一次請求時才會呼叫),銷燬是服務關閉的時候進行,本文主要分析的就是請求執行階段。我們知道SpringMVC的核心就是DispatcherServlet,該類是對Servlet的擴充套件,所以直接從該類的service方法開始,但在此類中沒有service方法,那肯定是在其父類中,我們先來看看其繼承體系:
逐個往上找,在FrameworkServlet方法中就有一個service方法:
protected void service(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException { HttpMethod httpMethod = HttpMethod.resolve(request.getMethod()); if (httpMethod == HttpMethod.PATCH || httpMethod == null) { processRequest(request,response); } else { super.service(request,response); } } protected void service(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) { long lastModified = getLastModified(req); if (lastModified == -1) { doGet(req,resp); } else { long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); if (ifModifiedSince < lastModified) { maybeSetLastModified(resp,lastModified); doGet(req,resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } } else if (method.equals(METHOD_HEAD)) { long lastModified = getLastModified(req); maybeSetLastModified(resp,lastModified); doHead(req,resp); } else if (method.equals(METHOD_POST)) { doPost(req,resp); } else if (method.equals(METHOD_PUT)) { doPut(req,resp); } else if (method.equals(METHOD_DELETE)) { doDelete(req,resp); } else if (method.equals(METHOD_OPTIONS)) { doOptions(req,resp); } else if (method.equals(METHOD_TRACE)) { doTrace(req,resp); } else { String errMsg = lStrings.getString("http.method_not_implemented"); Object[] errArgs = new Object[1]; errArgs[0] = method; errMsg = MessageFormat.format(errMsg,errArgs); resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED,errMsg); } }
但其主要還是呼叫父類HttpServlet中的方法,而該類又會根據不同的請求方式會調到子類中,最後的核心方法就是DispatcherServlet中的doDispatch方法:
protected void doDispatch(HttpServletRequest request,HttpServletResponse response) throws Exception { HttpServletRequest processedRequest = request; HandlerExecutionChain mappedHandler = null; boolean multipartRequestParsed = false; //非同步管理 WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); try { ModelAndView mv = null; Exception dispatchException = null; try { //檔案上傳 processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); //這個方法很重要,重點看 // Determine handler for the current request. mappedHandler = getHandler(processedRequest); if (mappedHandler == null) { noHandlerFound(processedRequest,response); return; } //獲取跟HandlerMethod匹配的HandlerAdapter物件 // Determine handler adapter for the current request. HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // Process last-modified header,if supported by the handler. String method = request.getMethod(); boolean isGet = "GET".equals(method); if (isGet || "HEAD".equals(method)) { long lastModified = ha.getLastModified(request,mappedHandler.getHandler()); if (new ServletWebRequest(request,response).checkNotModified(lastModified) && isGet) { return; } } //前置過濾器,如果為false則直接返回 if (!mappedHandler.applyPreHandle(processedRequest,response)) { return; } //呼叫到Controller具體方法,核心方法呼叫,重點看看 // Actually invoke the handler. mv = ha.handle(processedRequest,response,mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(processedRequest,mv); //中置過濾器 mappedHandler.applyPostHandle(processedRequest,mv); } catch (Exception ex) { dispatchException = ex; } catch (Throwable err) { // As of 4.3,we're processing Errors thrown from handler methods as well,// making them available for @ExceptionHandler methods and other scenarios. dispatchException = new NestedServletException("Handler dispatch failed",err); } //檢視渲染及後置過濾器執行 processDispatchResult(processedRequest,mappedHandler,mv,dispatchException); } catch (Exception ex) { triggerAfterCompletion(processedRequest,ex); } catch (Throwable err) { triggerAfterCompletion(processedRequest,new NestedServletException("Handler processing failed",err)); } finally { if (asyncManager.isConcurrentHandlingStarted()) { // Instead of postHandle and afterCompletion if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest,response); } } else { // Clean up any resources used by a multipart request. if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } }
MVC的所有處理邏輯都在這個方法中,先總結一下這個方法的實現邏輯,首先根據請求的url拿到快取中的HandlerMethod物件和執行鏈物件,HandlerMethod中封裝了controller物件、方法物件和方法引數等資訊,執行鏈則是包含了一個個HandlerInterceptor攔截器;然後再通過HandlerMethod拿到對應的HandlerAdapter,這個物件的作用就是去適配我們的controller;準備工作做完後,首先會執行前置過濾,如果被攔截則直接返回,否則就去呼叫controller中的方法執行我們的業務邏輯並返回一個ModelView物件;接著執行中置過濾器,以及處理全域性異常捕獲器捕獲到異常;最後進行檢視渲染返回並執行後置過濾器進行資源釋放等工作。
以上就是MVC的整體執行流程,下面就逐個來分析,首先進入getHandler方法:
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { //handlerMappering例項 if (this.handlerMappings != null) { for (HandlerMapping mapping : this.handlerMappings) { //獲取HandlerMethod和過濾器鏈的包裝類 HandlerExecutionChain handler = mapping.getHandler(request); if (handler != null) { return handler; } } } return null; }
是委託給HandlerMapping物件的,這是一個介面,主要的實現類是RequestMappingHandlerMapping,同樣先來看看其繼承體系:
這個類是管理請求和處理類之間的對映關係的,你是否疑惑它是在哪裡例項化的呢?下面先來看看MVC元件的初始化。
元件初始化
這裡我以自動化配置的註解方式說明,Spring提供了一個@EnableWebMvc,通過前面的學習我們知道在這個註解中必定匯入了一個配置類,點進去可以看到是DelegatingWebMvcConfiguration,這個類就是負責MVC的元件和擴充套件實現的初始化,其本身我們先不看,先看其父類WebMvcConfigurationSupport,這個類我們應該不陌生,要做一些自定義擴充套件時就需要繼承該類(如攔截器Interceptor),同樣作用的類還有WebMvcConfigurerAdapter,這個類是對前者相對安全的擴充套件,為什麼是相對安全呢?因為繼承前者會導致自動配置失效,而使用後者則不必擔心此問題,只需要在類上加上@EnableWebMvc註解。
在WebMvcConfigurationSupport中我們可以看到很多@Bean標註的方法,也就是mvc元件的例項化,這裡主要看看requestMappingHandlerMapping,其餘的可自行閱讀理解,也就是一些Bean的註冊:
public RequestMappingHandlerMapping requestMappingHandlerMapping() { RequestMappingHandlerMapping mapping = createRequestMappingHandlerMapping(); mapping.setOrder(0); mapping.setInterceptors(getInterceptors()); mapping.setContentNegotiationManager(mvcContentNegotiationManager()); mapping.setCorsConfigurations(getCorsConfigurations()); ......省略 return mapping; }
這裡主要看getInterceptors方法如何獲取攔截器的:
protected final Object[] getInterceptors() { if (this.interceptors == null) { InterceptorRegistry registry = new InterceptorRegistry(); //鉤子方法,需要自己定義 addInterceptors(registry); registry.addInterceptor(new ConversionServiceExposingInterceptor(mvcConversionService())); registry.addInterceptor(new ResourceUrlProviderExposingInterceptor(mvcResourceUrlProvider())); this.interceptors = registry.getInterceptors(); } return this.interceptors.toArray(); }
第一次進來會呼叫addInterceptors新增攔截器,這是一個模板方法,在子類DelegatingWebMvcConfiguration中實現:
private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite(); protected void addInterceptors(InterceptorRegistry registry) { this.configurers.addInterceptors(registry); } public void addInterceptors(InterceptorRegistry registry) { for (WebMvcConfigurer delegate : this.delegates) { delegate.addInterceptors(registry); } }
可以看到最終是呼叫WebMvcConfigurer的addInterceptors方法,也就是我們對WebMvcConfigurerAdapter的自定義擴充套件。看到這裡我們應該明白了MVC的元件是如何新增到IOC容器中的,但是DispatcherServlet又是怎麼獲取到它們的呢?回到之前的程式碼中,在DispatcherServlet這個類中有一個onRefresh方法,這個方法又呼叫了initStrategies方法完成了MVC九大元件的註冊:
protected void onRefresh(ApplicationContext context) { initStrategies(context); } protected void initStrategies(ApplicationContext context) { initMultipartResolver(context); initLocaleResolver(context); initThemeResolver(context); initHandlerMappings(context); initHandlerAdapters(context); initHandlerExceptionResolvers(context); initRequestToViewNameTranslator(context); initViewResolvers(context); initFlashMapManager(context); } private void initHandlerMappings(ApplicationContext context) { this.handlerMappings = null; if (this.detectAllHandlerMappings) { // Find all HandlerMappings in the ApplicationContext,including ancestor contexts. Map<String,HandlerMapping> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context,HandlerMapping.class,true,false); if (!matchingBeans.isEmpty()) { this.handlerMappings = new ArrayList<>(matchingBeans.values()); // We keep HandlerMappings in sorted order. AnnotationAwareOrderComparator.sort(this.handlerMappings); } } else { try { HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME,HandlerMapping.class); this.handlerMappings = Collections.singletonList(hm); } catch (NoSuchBeanDefinitionException ex) { // Ignore,we'll add a default HandlerMapping later. } } if (this.handlerMappings == null) { this.handlerMappings = getDefaultStrategies(context,HandlerMapping.class); } }
以initHandlerMappings為例,其它元件實現邏輯基本一樣。首先從IOC容器中拿到handlerMappings的所有實現類(WebMvcConfigurationSupport中注入的物件就在這裡被獲取到),若沒有,則從DispatcherServlet.properties配置檔案中(這個配置在spring-webmvc工程下org/springframework/web/servlet/DispatcherServlet.properties)獲取預設的配置:
org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\ org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\ org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\ org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,\ org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\ org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
但是onRefresh又是在什麼時候呼叫的呢?有兩個地方,一個是Servlet初始化時會呼叫到initWebApplicationContext進行容器的初始化,這個方法中就會觸發onRefresh;另外還有一個,在FrameworkServlet中有一個onApplicationEvent方法,而這個方法又會被內部類ContextRefreshListener呼叫,這個類實現了ApplicationListener介面,表示會接收容器重新整理事件。
以上就就是MVC HandlerMapping元件的初始化邏輯,其它元件實現邏輯相同,下面不再分析。
呼叫Controller
回到getHandler方法,其呼叫的是AbstractHandlerMapping類的方法:
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { //根據請求的uri拿到對應的HandlerMethod物件 Object handler = getHandlerInternal(request); if (handler == null) { handler = getDefaultHandler(); } if (handler == null) { return null; } // Bean name or resolved handler? if (handler instanceof String) { String handlerName = (String) handler; handler = obtainApplicationContext().getBean(handlerName); } //獲取HandlerMethod和過濾器鏈的包裝類 HandlerExecutionChain executionChain = getHandlerExecutionChain(handler,request); if (logger.isTraceEnabled()) { logger.trace("Mapped to " + handler); } else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) { logger.debug("Mapped to " + executionChain.getHandler()); } //是否是跨域請求,就是檢視request請求頭中是否有Origin屬性 if (CorsUtils.isCorsRequest(request)) { //自定義的鉤子方法獲取跨域配置 CorsConfiguration globalConfig = this.corsConfigurationSource.getCorsConfiguration(request); //註解獲取跨域配置 CorsConfiguration handlerConfig = getCorsConfiguration(handler,request); CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig); //這裡設定了跨域的過濾器CorsInterceptor executionChain = getCorsHandlerExecutionChain(request,executionChain,config); } return executionChain; }
先看AbstractHandlerMethodMapping.getHandlerInternal:
protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception { //從request物件中獲取uri,/common/query2 String lookupPath = getUrlPathHelper().getLookupPathForRequest(request); this.mappingRegistry.acquireReadLock(); try { //根據uri從對映關係中找到對應的HandlerMethod物件 HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath,request); //把Controller類例項化 return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null); } finally { this.mappingRegistry.releaseReadLock(); } } protected HandlerMethod lookupHandlerMethod(String lookupPath,HttpServletRequest request) throws Exception { List<Match> matches = new ArrayList<>(); // 根據url拿到對應的RequestMappingInfo List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath); if (directPathMatches != null) { addMatchingMappings(directPathMatches,matches,request); } if (matches.isEmpty()) { // No choice but to go through all mappings... addMatchingMappings(this.mappingRegistry.getMappings().keySet(),request); } if (!matches.isEmpty()) { Comparator<Match> comparator = new MatchComparator(getMappingComparator(request)); matches.sort(comparator); Match bestMatch = matches.get(0); if (matches.size() > 1) { if (logger.isTraceEnabled()) { logger.trace(matches.size() + " matching mappings: " + matches); } if (CorsUtils.isPreFlightRequest(request)) { return PREFLIGHT_AMBIGUOUS_MATCH; } Match secondBestMatch = matches.get(1); //如果兩個RequestMappinginfo什麼都相同,報錯 if (comparator.compare(bestMatch,secondBestMatch) == 0) { Method m1 = bestMatch.handlerMethod.getMethod(); Method m2 = secondBestMatch.handlerMethod.getMethod(); String uri = request.getRequestURI(); throw new IllegalStateException( "Ambiguous handler methods mapped for '" + uri + "': {" + m1 + "," + m2 + "}"); } } request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE,bestMatch.handlerMethod); handleMatch(bestMatch.mapping,lookupPath,request); return bestMatch.handlerMethod; } else { return handleNoMatch(this.mappingRegistry.getMappings().keySet(),request); } } private void addMatchingMappings(Collection<T> mappings,List<Match> matches,HttpServletRequest request) { for (T mapping : mappings) { // 拿到匹配的RequestMappingInfo物件,有可能url相同,@RequestMapping的屬性(請求方式、引數等)匹配不上 T match = getMatchingMapping(mapping,request); if (match != null) { //RequestMappingInfo物件和HandlerMethod物件封裝到Match物件中,其實就是註解屬性和Method物件的對映 matches.add(new Match(match,this.mappingRegistry.getMappings().get(mapping))); } } }
這裡邏輯很簡單,就是通過請求url從urlLookup中拿到對應的RequestMappingInfo(每一個 @RequestMapping對應一個RequestMappingInfo物件)物件,再根據RequestMappingInfo物件從mappingLookup拿到對應的HandlerMethod並返回。
但這裡你可能會比較好奇urlLookup和mappingLookup從哪裡來的,仔細觀察你會發現當前這個類實現了一個介面InitializingBean,實現了這個介面的類會在該類的Bean例項化完成後呼叫afterPropertiesSet方法,上面的對映關係就是在這個方法中做的。實際上這個方法不止完成了上面兩個對映關係,還有下面兩個:
corsLookup:handlerMethod -> corsConfig
registry:RequestMappingInfo -> MappingRegistration(包含url、handlerMethod、RequestMappingInfo、name等資訊)
這裡就不展開分析了,奉上一張時序圖,讀者可根據下面的時序圖自行分析:
拿到HandlerMethod物件後,又會通過getHandlerExecutionChain方法去獲取到所有的HandlerInterceptor攔截器物件,並連同HandlerMethod物件一起封裝為HandlerExecutionChain。之後是獲取跨域配置,這裡不詳細分析。
拿到HandlerExecutionChain物件後返回到doDispatch方法,又呼叫了getHandlerAdapter
方法拿到HandlerAdapter:
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException { //根據handlerMethod物件,找到合適的HandlerAdapter物件,這裡用到了策略模式 if (this.handlerAdapters != null) { for (HandlerAdapter adapter : this.handlerAdapters) { if (adapter.supports(handler)) { return adapter; } } } }
這裡的handlerAdapters變數值從哪裡來?相信不用我再分析,主要看這裡的設計思想,典型的策略模式。
之後呼叫完前置過濾器後,才是真正呼叫我們controller方法的邏輯,通過HandlerAdapter.handle去呼叫,最終會呼叫到ServletInvocableHandlerMethod.invokeAndHandle:
public void invokeAndHandle(ServletWebRequest webRequest,ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception { //具體呼叫邏輯,重點看 Object returnValue = invokeForRequest(webRequest,mavContainer,providedArgs); setResponseStatus(webRequest); if (returnValue == null) { if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) { mavContainer.setRequestHandled(true); return; } } else if (StringUtils.hasText(getResponseStatusReason())) { mavContainer.setRequestHandled(true); return; } mavContainer.setRequestHandled(false); Assert.state(this.returnValueHandlers != null,"No return value handlers"); try { //返回值處理 this.returnValueHandlers.handleReturnValue( returnValue,getReturnValueType(returnValue),webRequest); } catch (Exception ex) { if (logger.isTraceEnabled()) { logger.trace(formatErrorForReturnValue(returnValue),ex); } throw ex; } }
這個方法裡面主要看invokeForRequest和handleReturnValue的呼叫,前者是完成引數繫結並呼叫controller,後者則是對返回值進行處理並封裝到ModelAndViewContainer中。先來看invokeForRequest:
public Object invokeForRequest(NativeWebRequest request,@Nullable ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception { //獲取引數陣列 Object[] args = getMethodArgumentValues(request,providedArgs); if (logger.isTraceEnabled()) { logger.trace("Arguments: " + Arrays.toString(args)); } return doInvoke(args); }
doInvoke就是完成反射呼叫,主要還是看引數繫結的實現邏輯,在getMethodArgumentValues方法中:
protected Object[] getMethodArgumentValues(NativeWebRequest request,Object... providedArgs) throws Exception { if (ObjectUtils.isEmpty(getMethodParameters())) { return EMPTY_ARGS; } //入參的包裝類,裡面包裝了引數型別,引數名稱,引數註解等等資訊 MethodParameter[] parameters = getMethodParameters(); Object[] args = new Object[parameters.length]; for (int i = 0; i < parameters.length; i++) { MethodParameter parameter = parameters[i]; //設定引數名稱解析器 parameter.initParameterNameDiscovery(this.parameterNameDiscoverer); args[i] = findProvidedArgument(parameter,providedArgs); if (args[i] != null) { continue; } //典型的策略模式,根據parameter能否找到對應引數的處理類,能找到就返回true if (!this.resolvers.supportsParameter(parameter)) { throw new IllegalStateException(formatArgumentError(parameter,"No suitable resolver")); } try { //具體引數值解析過程,重點看看 args[i] = this.resolvers.resolveArgument(parameter,request,this.dataBinderFactory); } catch (Exception ex) { // Leave stack trace for later,exception may actually be resolved and handled.. if (logger.isDebugEnabled()) { String error = ex.getMessage(); if (error != null && !error.contains(parameter.getExecutable().toGenericString())) { logger.debug(formatArgumentError(parameter,error)); } } throw ex; } } return args; }
引數、返回值解析
因為引數型別非常多,同時還會伴隨各種註解,如:@RequestBody、@RequestParam、@PathVariable等,所以引數解析的工作是非常繁雜的,同時還要考慮到擴充套件性,所以SpringMVC依然採用了策略模式來完成對各種引數型別的解析繫結,其頂層介面就是HandlerMethodArgumentResolver,而預設SpringMVC提供的解析方式就高達20多種:
上面是類圖,讀者可根據自己熟悉的引數型別找到對應的類進行分析,最核心的還是要掌握這裡的設計思想。
接著方法呼叫完成後就是對返回值的處理,同樣的,返回值型別也是非常多,也可以使用各種註解標註,所以也是使用策略模式實現,其頂層介面是HandlerMethodReturnValueHandler,實現類如下:
呼叫完成之後就是執行後續操作了:執行中置過濾器、處理全域性異常、檢視渲染以及執行後置過濾器,這些與主流程沒有太大關係,本篇不展開分析了,最後是MVC的執行時序圖:
總結
本篇是Spring核心原理系列的最後一篇,前前後後花了一個月時間,終於從巨集觀上大致上理解了Spring的實現原理和執行機制,明白了之前專案中一些坑是如何產生的,最主要的是學到設計模式的運用以及如何利用Spring的一些常用的擴充套件點進行自定義擴充套件。但對於Spring這個龐大的體系來說,還有很多是要去理解學習的,尤其是設計思想,只有長期琢磨才能深刻的理解掌握。在我之前的文章中包括本篇還有很多沒分析到的細節,在後面我會不定期分享出來。希望能給大家一個參考,也希望大家多多支援我們。