Spring MVC源码(二) ----- DispatcherServlet 请求处理流程 面试必问
前端控制器
前端控制器,即所谓的Front Controller,体现的是设计模式中的前端控制器模式。前端控制器处理所有从用户过来的请求。所有用户的请求都要通过前端控制器。SpringMVC框架和其他请求驱动的表示层框架一样,也是围绕一个将请求分发到相应控制器的核心Servlet来设计的。DispatcherServlet和其他框架中的Servlet不一样的地方在于,它和Spring容器无缝整合在了一起,因此你可以在SpringMVC中使用Spring容器所有的特性。
DispatcherServlet这个前端控制器,在SpringMVC中的作用,以官方文档中的配图来说明:
整个流程可以被大致描述为:一个http请求到达服务器,被DispatcherServlet接收。DispatcherServlet将请求委派给合适的处理器Controller,此时处理控制权到达Controller对象。Controller内部完成请求的数据模型的创建和业务逻辑的处理,然后再将填充了数据后的模型即model和控制权一并交还给DispatcherServlet,委派DispatcherServlet来渲染响应。DispatcherServlet再将这些数据和适当的数据模版视图结合,向Response输出响应。
DispatcherServlet
SpringMVC完成初始化流程之后,就进入Servlet标准生命周期的第二个阶段,即“service”阶段。在“service”阶段中,每一次Http请求到来,容器都会启动一个请求线程,通过service()方法,委派到doGet()或者doPost()这些方法,完成Http请求的处理。
在初始化流程中,SpringMVC巧妙的运用依赖注入读取参数,并最终建立一个与容器上下文相关联的Spring子上下文。这个子上下文,就像Struts2中xwork容器一样,为接下来的Http处理流程中各种编程元素提供了容身之所。如果说将Spring上下文关联到Servlet容器中,是SpringMVC框架的第一个亮点,那么在请求转发流程中,SpringMVC对各种处理环节编程元素的抽象,就是另外一个独具匠心的亮点。
Struts2采取的是一种完全和Web容器隔离和解耦的事件机制。诸如Action对象、Result对象、Interceptor对象,这些都是完全脱离Servlet容器的编程元素。Struts2将数据流和事件处理完全剥离开来,从Http请求中读取数据后,下面的事件处理流程就只依赖于这些数据,而完全不知道有Web环境的存在。
反观SpringMVC,无论HandlerMapping对象、HandlerAdapter对象还是View对象,这些核心的接口所定义的方法中,HttpServletRequest和HttpServletResponse对象都是直接作为方法的参数出现的。这也就意味着,框架的设计者,直接将SpringMVC框架和容器绑定到了一起。或者说,整个SpringMVC框架,都是依托着Servlet容器元素来设计的。下面就来看一下,源码中是如何体现这一点的。
请求转发的入口
就像任何一个注册在容器中的Servlet一样,DispatcherServlet也是通过自己的service()方法来接收和转发Http请求到具体的doGet()或doPost()这些方法的。以一次典型的GET请求为例,经过HttpServlet基类中service()方法的委派,请求会被转发到doGet()方法或者doPost()方法中。
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); long lastModified; if (method.equals("GET")) { lastModified = this.getLastModified(req); if (lastModified == -1L) { this.doGet(req, resp); } else { long ifModifiedSince = req.getDateHeader("If-Modified-Since"); if (ifModifiedSince < lastModified) { this.maybeSetLastModified(resp, lastModified); this.doGet(req, resp); } else { resp.setStatus(304); } } } else if (method.equals("HEAD")) { lastModified = this.getLastModified(req); this.maybeSetLastModified(resp, lastModified); this.doHead(req, resp); } else if (method.equals("POST")) { this.doPost(req, resp); } else if (method.equals("PUT")) { this.doPut(req, resp); } else if (method.equals("DELETE")) { this.doDelete(req, resp); } else if (method.equals("OPTIONS")) { this.doOptions(req, resp); } else if (method.equals("TRACE")) { this.doTrace(req, resp); } else { String errMsg = lStrings.getString("http.method_not_implemented"); Object[] errArgs = new Object[]{method}; errMsg = MessageFormat.format(errMsg, errArgs); resp.sendError(501, errMsg); } }
doGet() 和 doPost() 方法,在DispatcherServlet的父类FrameworkServlet类中被覆写。
protected final void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.processRequest(request, response); } protected final void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.processRequest(request, response); }
可以看到,这里只是简单的转发到processRequest()这个方法。
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); Throwable failureCause = null; // Expose current LocaleResolver and request as LocaleContext. LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext(); LocaleContextHolder.setLocaleContext(buildLocaleContext(request), this.threadContextInheritable); // Expose current RequestAttributes to current thread. RequestAttributes previousRequestAttributes = RequestContextHolder.getRequestAttributes(); ServletRequestAttributes requestAttributes = null; if (previousRequestAttributes == null || previousRequestAttributes.getClass().equals(ServletRequestAttributes.class)) { requestAttributes = new ServletRequestAttributes(request); RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable); } if (logger.isTraceEnabled()) { logger.trace("Bound request context to thread: " + request); } try { doService(request, response); } finally { // Clear request attributes and reset thread-bound context. LocaleContextHolder.setLocaleContext(previousLocaleContext, this.threadContextInheritable); if (requestAttributes != null) { RequestContextHolder.setRequestAttributes(previousRequestAttributes, this.threadContextInheritable); requestAttributes.requestCompleted(); } if (this.publishEvents) { // Whether or not we succeeded, publish an event. long processingTime = System.currentTimeMillis() - startTime; this.webApplicationContext.publishEvent( new ServletRequestHandledEvent(this, request.getRequestURI(), request.getRemoteAddr(), request.getMethod(), getServletConfig().getServletName(), WebUtils.getSessionId(request), getUsernameForRequest(request), processingTime, failureCause)); } } }
可以看到,processRequest()方法只是做了一些线程安全的隔离,真正的请求处理,发生在doService()方法中。点开FrameworkServlet类中的doService()方法。
protected abstract void doService(HttpServletRequest request, HttpServletResponse response) throws Exception;
又是一个抽象方法,这也是SpringMVC类设计中的惯用伎俩:父类抽象处理流程,子类给予具体的实现。真正的实现是在DispatcherServlet类中。
让我们接着看DispatcherServlet类中实现的doService()方法。
@Override protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { if (logger.isDebugEnabled()) { String requestUri = urlPathHelper.getRequestUri(request); logger.debug("DispatcherServlet with name '" + getServletName() + "' processing " + request.getMethod() + " request for [" + requestUri + "]"); } // Keep a snapshot of the request attributes in case of an include, // to be able to restore the original attributes after the include. Map<String, Object> attributesSnapshot = null; if (WebUtils.isIncludeRequest(request)) { logger.debug("Taking snapshot of request attributes before include"); attributesSnapshot = new HashMap<String, Object>(); Enumeration<?> attrNames = request.getAttributeNames(); while (attrNames.hasMoreElements()) { String attrName = (String) attrNames.nextElement(); if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) { attributesSnapshot.put(attrName, request.getAttribute(attrName)); } } } // Make framework objects available to handlers and view objects. request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext()); request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver); request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver); request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource()); FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response); if (inputFlashMap != null) { request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap)); } request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap()); request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager); try { doDispatch(request, response); } finally { // Restore the original attribute snapshot, in case of an include. if (attributesSnapshot != null) { restoreAttributesAfterInclude(request, attributesSnapshot); } } }
几个requet.setAttribute()方法的调用,将前面在初始化流程中实例化的对象设置到http请求的属性中,供下一步处理使用,其中有容器的上下文对象、本地化解析器等SpringMVC特有的编程元素。不同于Struts2中的ValueStack,SpringMVC的数据并没有从HttpServletRequest对象中抽离出来再存进另外一个编程元素,这也跟SpringMVC的设计思想有关。因为从一开始,SpringMVC的设计者就认为,不应该将请求处理过程和Web容器完全隔离。
所以,你可以看到,真正发生请求转发的方法doDispatch()中,它的参数是HttpServletRequest和HttpServletResponse对象。这给我们传递的意思也很明确,从request中能获取到一切请求的数据,从response中,我们又可以往服务器端输出任何响应,Http请求的处理,就应该围绕这两个对象来设计。我们不妨可以将SpringMVC这种设计方案,是从Struts2的过度设计中吸取教训,而向Servlet编程的一种回归和简化。
而对请求的处理交给doDispatcher方法
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { HttpServletRequest processedRequest = request; HandlerExecutionChain mappedHandler = null; boolean multipartRequestParsed = false; WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); try { ModelAndView mv = null; Exception dispatchException = null; try { // 处理文件上传 processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // 决定当前请求的Handler mappedHandler = getHandler(processedRequest); if (mappedHandler == null || mappedHandler.getHandler() == null) { noHandlerFound(processedRequest, response); return; } // 决定当前请求的HandlerAdapter HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // 处理last-modified请求头 String method = request.getMethod(); boolean isGet = "GET".equals(method); if (isGet || "HEAD".equals(method)) { long lastModified = ha.getLastModified(request, mappedHandler.getHandler()); if (logger.isDebugEnabled()) { logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified); } if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { return; } } // 拦截器的前置处理 if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Handler实际执行请求 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } // 设置默认视图名 applyDefaultViewName(processedRequest, mv); // 拦截器后置处理 mappedHandler.applyPostHandle(processedRequest, response, 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); } // 选择视图并渲染视图 processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } catch (Exception ex) { triggerAfterCompletion(processedRequest, response, mappedHandler, ex); } catch (Throwable err) { triggerAfterCompletion(processedRequest, response, mappedHandler, 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); } } } }
先看doDispatcher方法执行的主要操作时序图
请求路由
getHandler方法就是从HandlerMapping中查询匹配当前request的Handler。我们看到只要一匹配上 handler 就不再循环,直接返回
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { for (HandlerMapping hm : this.handlerMappings) { HandlerExecutionChain handler = hm.getHandler(request); if (handler != null) { return handler; } } return null; }
HandlerMapping的getHandler方法在抽象基类AbstractHandlerMapping
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { // 由子类根据request获取Handler Object handler = getHandlerInternal(request); // 如果没匹配到,则获取默认Handler if (handler == null) { handler = getDefaultHandler(); } if (handler == null) { return null; } // 如果返回的Handler为String,则使用Spring容器实例化 if (handler instanceof String) { String handlerName = (String) handler; handler = getApplicationContext().getBean(handlerName); } // 查询匹配的拦截器,组装Handler生成HandlerExecutionChain HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request); if (CorsUtils.isCorsRequest(request)) { CorsConfiguration globalConfig = this.corsConfigSource.getCorsConfiguration(request); CorsConfiguration handlerConfig = getCorsConfiguration(handler, request); CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig); executionChain = getCorsHandlerExecutionChain(request, executionChain, config); } return executionChain; }
最终返回的Handler是由拦截器链和Handler共同组成的,而具体匹配Handler的方法是交给子类来完成的。上一章组件初始化中提到生产环境下使用的是RequestMappingHandlerMapping,getHandlerInternal方法的实现在它的基类AbstractHandlerMethodMapping。
protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception { // 从request获取匹配url String lookupPath = getUrlPathHelper().getLookupPathForRequest(request); if (logger.isDebugEnabled()) { logger.debug("Looking up handler method for path " + lookupPath); } this.mappingRegistry.acquireReadLock(); try { // 查询匹配的HandlerMethod HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request); if (logger.isDebugEnabled()) { if (handlerMethod != null) { logger.debug("Returning handler method [" + handlerMethod + "]"); } else { logger.debug("Did not find handler method for [" + lookupPath + "]"); } } return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null); } finally { this.mappingRegistry.releaseReadLock(); } }
可以看到返回的Handler的类型为HandlerMethod,它对应于Controller中的方法。上一章也提过,在AbstractHandlerMethodMapping中有一个MappingRegistry,统一管理URL和Controller方法的映射关系,lookupHandlerMethod就是对MappingRegistry的操作。
protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception { List<Match> matches = new ArrayList<Match>(); // 从mappingRegistry获取匹配到的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(), matches, request); } // 对匹配项进行排序 if (!matches.isEmpty()) { Comparator<Match> comparator = new MatchComparator(getMappingComparator(request)); Collections.sort(matches, comparator); if (logger.isTraceEnabled()) { logger.trace("Found " + matches.size() + " matching mapping(s) for [" + lookupPath + "] : " + matches); } Match bestMatch = matches.get(0); if (matches.size() > 1) { if (CorsUtils.isPreFlightRequest(request)) { return PREFLIGHT_AMBIGUOUS_MATCH; } Match secondBestMatch = matches.get(1); if (comparator.compare(bestMatch, secondBestMatch) == 0) { Method m1 = bestMatch.handlerMethod.getMethod(); Method m2 = secondBestMatch.handlerMethod.getMethod(); throw new IllegalStateException("Ambiguous handler methods mapped for HTTP path '" + request.getRequestURL() + "': {" + m1 + ", " + m2 + "}"); } } handleMatch(bestMatch.mapping, lookupPath, request); return bestMatch.handlerMethod; } else { // 无匹配项处理 return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request); } }
通过mappingRegistry匹配返回RequestMappingInfo,对应于每个有@RequestMapping注解解析后的Method。
我们来看看,HandlerExecutionChain类的代码。
package org.springframework.web.servlet; import java.util.ArrayList; import java.util.Arrays; import java.util.List; import org.springframework.util.CollectionUtils; public class HandlerExecutionChain { private final Object handler; private HandlerInterceptor[] interceptors; private List<HandlerInterceptor> interceptorList; public HandlerExecutionChain(Object handler) { this(handler, null); } public HandlerExecutionChain(Object handler, HandlerInterceptor[] interceptors) { if (handler instanceof HandlerExecutionChain) { HandlerExecutionChain originalChain = (HandlerExecutionChain) handler; this.handler = originalChain.getHandler(); this.interceptorList = new ArrayList<HandlerInterceptor>(); CollectionUtils.mergeArrayIntoCollection(originalChain.getInterceptors(), this.interceptorList); CollectionUtils.mergeArrayIntoCollection(interceptors, this.interceptorList); } else { this.handler = handler; this.interceptors = interceptors; } } public Object getHandler() { return this.handler; } public void addInterceptor(HandlerInterceptor interceptor) { initInterceptorList(); this.interceptorList.add(interceptor); } public void addInterceptors(HandlerInterceptor[] interceptors) { if (interceptors != null) { initInterceptorList(); this.interceptorList.addAll(Arrays.asList(interceptors)); } } private void initInterceptorList() { if (this.interceptorList == null) { this.interceptorList = new ArrayList<HandlerInterceptor>(); } if (this.interceptors != null) { this.interceptorList.addAll(Arrays.asList(this.interceptors)); this.interceptors = null; } } public HandlerInterceptor[] getInterceptors() { if (this.interceptors == null && this.interceptorList != null) { this.interceptors = this.interceptorList.toArray(new HandlerInterceptor[this.interceptorList.size()]); } return this.interceptors; } }
一个拦截器列表,一个执行对象,这个类的内容十分的简单,它蕴含的设计思想,却十分的丰富。
1.拦截器组成的列表,在执行对象被调用的前后,会依次执行。这里可以看成是一个的AOP环绕通知,拦截器可以对处理对象随心所欲的进行处理和增强。这里明显是吸收了Struts2中拦截器的设计思想。这种AOP环绕式的扩展点设计,也几乎成为所有框架必备的内容。
2.实际的处理对象,即handler对象,是由Object对象来引用的。
private final Object handler;
当我们拿到HandlerExecutionChain,就完成了request到Controller的路由操作。
适配器匹配
有了Handler后,需要合适的HandlerAdapter对其进行操作,因而就要根据Handler进行匹配。
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException { for (HandlerAdapter ha : this.handlerAdapters) { if (logger.isTraceEnabled()) { logger.trace("Testing handler adapter [" + ha + "]"); } if (ha.supports(handler)) { return ha; } } throw new ServletException("No adapter for handler [" + handler + "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler"); }
HandlerAdapter接口中定义了supports方法,用于检测是否支持Handler。生产环境使用的RequestMappingHandlerAdapter在其基类AbstractHandlerMethodAdapter中实现了supports方法。
public final boolean supports(Object handler) { return (handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler)); }
supportsInternal方法在RequestMappingHandlerAdapter的实现里默认返回true。因而RequestMappingHandlerAdapter就是用来支持类型为HandlerMethod的Handler的处理的。
拦截器处理
在SpringMVC中的拦截器接口HandlerInterceptor中定义了三个方法
public interface HandlerInterceptor { // 在Handler找到后,执行前拦截 boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception; // 在Handler执行后,视图渲染前拦截 void postHandle( HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception; // 请求处理完成,视图渲染后执行资源清理等 void afterCompletion( HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception; }
可以很清晰地对应到doDispatcher方法中。需要注意的有几点
- 前置处理preHandle,返回值为boolean。如果返回true,则执行下一个,如果返回false,则认为当前拦截器完成了请求,DispatcherServlet会直接返回,在返回前会调用所有拦截器的afterCompletion方法,完成清理工作。
- afterCompletion方法在遇到任何情况时都需要被执行,无论是成功返回还是抛出异常。
执行请求
HandlerAdapter的handle方法完成请求的真正执行。在AbstractHandlerMethodAdapter中由handleInternal执行。
protected ModelAndView handleInternal(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception { ModelAndView mav; checkRequest(request); // 执行HandlerMethod mav = invokeHandlerMethod(request, response, handlerMethod); // 处理缓存 if (!response.containsHeader(HEADER_CACHE_CONTROL)) { if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) { applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers); } else { prepareResponse(response); } } return mav; }
在invokeHandlerMethod中,HandlerMethod被封装ServletInvocableHandlerMethod,包裹上方法执行需要的信息。
protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception { ServletWebRequest webRequest = new ServletWebRequest(request, response); try { WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod); ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory); // 封装HandlerMethod ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod); invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers); invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers); invocableMethod.setDataBinderFactory(binderFactory); invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer); ModelAndViewContainer mavContainer = new ModelAndViewContainer(); mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request)); modelFactory.initModel(webRequest, mavContainer, invocableMethod); mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect); // 异步请求处理 AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response); asyncWebRequest.setTimeout(this.asyncRequestTimeout); WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); asyncManager.setTaskExecutor(this.taskExecutor); asyncManager.setAsyncWebRequest(asyncWebRequest); asyncManager.registerCallableInterceptors(this.callableInterceptors); asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors); if (asyncManager.hasConcurrentResult()) { Object result = asyncManager.getConcurrentResult(); mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0]; asyncManager.clearConcurrentResult(); if (logger.isDebugEnabled()) { logger.debug("Found concurrent result value [" + result + "]"); } invocableMethod = invocableMethod.wrapConcurrentResult(result); } // 执行处理 invocableMethod.invokeAndHandle(webRequest, mavContainer); if (asyncManager.isConcurrentHandlingStarted()) { return null; } // 封装数据和视图 return getModelAndView(mavContainer, modelFactory, webRequest); } finally { webRequest.requestCompleted(); } }
再到ServletInvocableHandlerMethod的invokeAndHandle方法
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception { // 执行request 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); try { // 对返回值进行处理 this.returnValueHandlers.handleReturnValue( returnValue, getReturnValueType(returnValue), mavContainer, webRequest); } catch (Exception ex) { if (logger.isTraceEnabled()) { logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex); } throw ex; } } public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception { // 执行方法参数 Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs); if (logger.isTraceEnabled()) { logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) + "' with arguments " + Arrays.toString(args)); } Object returnValue = doInvoke(args); if (logger.isTraceEnabled()) { logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) + "] returned [" + returnValue + "]"); } return returnValue; } protected Object doInvoke(Object... args) throws Exception { ReflectionUtils.makeAccessible(getBridgedMethod()); return getBridgedMethod().invoke(getBean(), args); }
需要说明的一点是方法执行完成的返回值通过返回值处理器HandlerMethodReturnValueHandler进行处理。在RequestMappingHandlerAdapter的初始化中,内置了众多的HandlerMethodReturnValueHandler来处理多种类型的返回值。
在完成请求执行后,doDispatcher方法中做了一个默认View的设置。
applyDefaultViewName(processedRequest, mv); private void applyDefaultViewName(HttpServletRequest request, ModelAndView mv) throws Exception { if (mv != null && !mv.hasView()) { mv.setViewName(getDefaultViewName(request)); } }
而这个getDefaultViewName是通过RequestToViewNameTranslator的实现类来解析的
protected String getDefaultViewName(HttpServletRequest request) throws Exception { return this.viewNameTranslator.getViewName(request); }
默认实现DefaultRequestToViewNameTranslator,根据配置的一些通用url进行匹配
public String getViewName(HttpServletRequest request) { String lookupPath = this.urlPathHelper.getLookupPathForRequest(request); return (this.prefix + transformPath(lookupPath) + this.suffix); }
视图渲染
当请求完成后,返回的ModelAndView需要渲染到浏览器进行显示。doDispatcher方法中processDispatchResult用来处理视图。
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception { boolean errorView = false; // 异常处理 if (exception != null) { if (exception instanceof ModelAndViewDefiningException) { logger.debug("ModelAndViewDefiningException encountered", exception); mv = ((ModelAndViewDefiningException) exception).getModelAndView(); } else { Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null); mv = processHandlerException(request, response, handler, exception); errorView = (mv != null); } } // Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { // 渲染执行 render(mv, request, response); if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else { if (logger.isDebugEnabled()) { logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() + "': assuming HandlerAdapter completed request handling"); } } if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Concurrent handling started during a forward return; } // 完成后执行拦截器的afterCompletion if (mappedHandler != null) { mappedHandler.triggerAfterCompletion(request, response, null); } }
render方法执行渲染,最终由View实现类执行
view.render(mv.getModelInternal(), request, response);
抽象类AbstractView执行对数据进行组装,输出操作交由子类完成
public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception { if (logger.isTraceEnabled()) { logger.trace("Rendering view with name '" + this.beanName + "' with model " + model + " and static attributes " + this.staticAttributes); } // 组装数据 Map<String, Object> mergedModel = createMergedOutputModel(model, request, response); prepareResponse(request, response); // 渲染输出 renderMergedOutputModel(mergedModel, getRequestToExpose(request), response); }
以通用的InternalResourceView举例
protected void renderMergedOutputModel( Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception { // Expose the model object as request attributes. exposeModelAsRequestAttributes(model, request); // Expose helpers as request attributes, if any. exposeHelpers(request); // Determine the path for the request dispatcher. String dispatcherPath = prepareForRendering(request, response); // Obtain a RequestDispatcher for the target resource (typically a JSP). RequestDispatcher rd = getRequestDispatcher(request, dispatcherPath); if (rd == null) { throw new ServletException("Could not get RequestDispatcher for [" + getUrl() + "]: Check that the corresponding file exists within your web application archive!"); } // If already included or response already committed, perform include, else forward. if (useInclude(request, response)) { response.setContentType(getContentType()); if (logger.isDebugEnabled()) { logger.debug("Including resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'"); } rd.include(request, response); } else { // Note: The forwarded resource is supposed to determine the content type itself. if (logger.isDebugEnabled()) { logger.debug("Forwarding to resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'"); } rd.forward(request, response); } }
最终由java Servlet的RequestDispatcher完成输出。其实就是做了一个跳转
本章中以请求的正向主流程解析了DispatcherServlet及相关类完成此过程的源码,其主要过程则是HandlerExecutionChain,HandlerMapping,HandlerAdapter,View等组件的交互过程,贴两张网上的核心原理图,希望对大家理解SpringMVC的原理有帮助。
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