spring mvc中DispatcherServlet如何得到ModelAndView的

首先看下面这种张图，这张图说明了spring mvc整体的流程。

本文讲的就是如何从DispatcherServlet中得到ModerAndView的过程。

首先看DispatherServlet这个类的doService方法，学过servlet的人都知道，它是web容器处理请求的入口。

 protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
         if (logger.isDebugEnabled()) {
             String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
             logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
                     " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
         }

         // 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)) {
             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 {
             if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
                 return;
             }
             // Restore the original attribute snapshot, in case of an include.
             if (attributesSnapshot != null) {
                 restoreAttributesAfterInclude(request, attributesSnapshot);
             }
         }
     }

可以看到，doService做的事情一共就两件，一是保存request的Attribute并在框架处理完之后恢复回去，以防止attribute在框架处理的过程中被迫坏；第二件事就是在第36行调用doDispatch，进入真正的spring mvc流程中。我们来看doDispatch函数，这个函数是DispatchServlet控制整个spring mvc流程的核心，文章开始的那张图所描述的流程都是在这个函数中完成的。我们来看一下这个函数。

 /**
      * Process the actual dispatching to the handler.
      * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
      * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
      * to find the first that supports the handler class.
      * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
      * themselves to decide which methods are acceptable.
      * @param request current HTTP request
      * @param response current HTTP response
      * @throws Exception in case of any kind of processing failure
      */
     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 {
                 processedRequest = checkMultipart(request);
                 multipartRequestParsed = (processedRequest != request);

                 // Determine handler for the current request.
                 mappedHandler = getHandler(processedRequest);
                 if (mappedHandler == null || mappedHandler.getHandler() == null) {
                     noHandlerFound(processedRequest, response);
                     return;
                 }

                 // 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 (logger.isDebugEnabled()) {
                         logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                     }
                     if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                         return;
                     }
                 }

                 if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                     return;
                 }

                 try {
                     // Actually invoke the handler.
                     mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                 }
                 finally {
                     if (asyncManager.isConcurrentHandlingStarted()) {
                         return;
                     }
                 }

                 applyDefaultViewName(request, mv);
                 mappedHandler.applyPostHandle(processedRequest, response, mv);
             }
             catch (Exception ex) {
                 dispatchException = ex;
             }
             processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
         }
         catch (Exception ex) {
             triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
         }
         catch (Error err) {
             triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
         }
         finally {
             if (asyncManager.isConcurrentHandlingStarted()) {
                 // Instead of postHandle and afterCompletion
                 mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                 return;
             }
             // Clean up any resources used by a multipart request.
             if (multipartRequestParsed) {
                 cleanupMultipart(processedRequest);
             }
         }
     }

这个方法的核心工作内容包括这么几件：处理http请求得到模型和视图（存在一个ModelAndView对象中）、处理拦截器、渲染视图、卸载multipart内容，以及获取用于处理模型所需的处理器HanderExecutionChain和适配器HandlerAdapter。

http的请求具体会被怎么处理将取决于处理器执行器HanderExecutionChain和适配器HandlerAdapter，所以我们先看这两个对象是怎么得到的。方法注释的已经大致说明了这两个对象是如何得到的：HanderExecutionChain是通过应用servlet所拥有的HanderMapping生成的，HandlerAdapter是通过依次查询servlet所拥有的适配器中能够支持执行器的适配器得到的。

那么我们就来看一下HanderExecutionChain和HandlerAdapter到底是怎么得到的。

看28行的getHandler，HanderExecutionChain是从这里获取的，跟进去看：

 protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
         for (HandlerMapping hm : this.handlerMappings) {
             if (logger.isTraceEnabled()) {
                 logger.trace(
                         "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
             }
             HandlerExecutionChain handler = hm.getHandler(request);
             if (handler != null) {
                 return handler;
             }
         }
         return null;
     }

看第7行，真的是通过DistpacherServlet的HandlerMappings得到的,跟进去看怎么得到的：

发现这个servlet拥有的一个HM是AbstractHanderMethondMapping对象，跟进他的方法，发现它在第3行获取了一个HandlerMethod（名为handler的object对象）。

 @Override
     public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
         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 = getApplicationContext().getBean(handlerName);
         }
         return getHandlerExecutionChain(handler, request);
     }

这个HandlerMethod对象存的是什么呢？我们看看：

原来是它封装了我们映射到的控制器，包括bean，以及map到的方法。有了这个封装对象，剩下的事情就好办了。

继续往下看便到了第15行，跟进去getHandlerExecutionChain方法：

 protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
         HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
                 (HandlerExecutionChain) handler : new HandlerExecutionChain(handler));
         chain.addInterceptors(getAdaptedInterceptors());

         String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
         for (MappedInterceptor mappedInterceptor : this.mappedInterceptors) {
             if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
                 chain.addInterceptor(mappedInterceptor.getInterceptor());
             }
         }

         return chain;
     }

可以看到，这个方法new了HandlerExecutionChain对象，并且把HM的拦截器和用户的拦截去都加进去了。至此HandlerExecutionChain的获取过程就讲完了。

接下来看适配器HandlerAdapter是怎么来的。

     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");
     }

真的就是看servlet有那些适配器，然后一个个查询是否支持，最后返回。

HanderExecutionChain和HandlerAdapter都有了，那么接下来就要看它们是怎么获取到模型和视图了。

跟进去dispatch方法的56行，一路下去，来到了 ModelAndView invokeHandleMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod)  方法，这个方法是HandlerAdapter适配器的方法啊， 来看看这个方法：

 private ModelAndView invokeHandleMethod(HttpServletRequest request,
             HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

         ServletWebRequest webRequest = new ServletWebRequest(request, response);

         WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
         ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
         ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

         ModelAndViewContainer mavContainer = new ModelAndViewContainer();
         mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
         modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
         mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

         AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
         asyncWebRequest.setTimeout(this.asyncRequestTimeout);

         final 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 + "]");
             }
             requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
         }

         requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

         if (asyncManager.isConcurrentHandlingStarted()) {
             return null;
         }

         return getModelAndView(mavContainer, modelFactory, webRequest);
     }

终于知道为什么需要适配器了，原来这个适配器的方法中，就适配了spring mvc和servlet，还有几个适配对象，其中最重要的是ServletWebRequest webRequest ，它适配了request。另外还有ModelAndViewContainer mavContainer，通过它持有模型和视图。

另外看这三行，这三行采用了工厂模式。

第一行获取了数据绑定的工厂，最重要的是它有handlerMethod。然后它会传给第三行，得到ServletInvocableHandlerMethod requestMappingMethod 。这个 requestMappingMethod 就是我们进入控制器调用的关键。

第二行获取了模型工厂，有了它，就可以创建模型了。

值得注意的是，这三个对象，每个里面都有handlerMethod的信息。

WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

看第35行，requestMappingMethod.invokeAndHandle(webRequest, mavContainer)，这里开始真正的填充模型了，怎么填充呢？其实猜都能猜到，首先它自己有handlermethod，又传入了webRequest和mavContainer，很容易想到，它肯定是根据请求，通过反射获取控制器的requestMap方法，将参数传入并调用方法，最后得到控制器处理后的模型，并得到控制器指定的视图。至于传入的参数是如何得到的，我在另一篇文章李有比较详细的描述，请看spring mvc中的控制器方法中的参数从哪里传进来这篇文章。

看起来貌似已经把整个流程讲完了，但是等等，有个很重要的问题，生成处理器执行器HandlerExecutionChain的时候需要从HM里get到一个真正的处理器传入给HandlerExecutionChain的构造器。这个处理器怎么来的？这个带着这个疑问，我们继续来看他们是怎么得到的。

前面已经说了，这个handler是在HandlerExecutionChain getHandler(HttpServletRequest request) 的第三行调用HM的Object handler = getHandlerInternal(request)得到的，我们看一看getHandlerInternal这个函数：

 protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
         String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
         if (logger.isDebugEnabled()) {
             logger.debug("Looking up handler method for path " + lookupPath);
         }
         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);
     }

可以看到，getHandlerInternal是通过请求的路径来查询得到那个关键的处理器（在这里是一个HandlerMethod，它含有控制器的requestMap方法签名），再看一下第6行，它是怎么通过路径查询到处理器的，跟进去lookupHandlerMethod函数：

 protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
         List<Match> matches = new ArrayList<Match>();
         List<T> directPathMatches = this.urlMap.get(lookupPath);
         if (directPathMatches != null) {
             addMatchingMappings(directPathMatches, matches, request);
         }
         if (matches.isEmpty()) {
             // No choice but to go through all mappings...
             addMatchingMappings(this.handlerMethods.keySet(), matches, request);
         }

         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) {
                 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 {
             return handleNoMatch(handlerMethods.keySet(), lookupPath, request);
         }
     }

lookupHandlerMethod函数把获取的处理器包装成一个个Match，并对Match进行了排序（因为一个path可能不仅仅对应一个处理器，比如存在两个requestMap路径相同的函数，那么一个path就会对应两个处理器）之后取最匹配的那个。在这个过程中甚至还做了重复性检测，如果有两个一模一样的处理器存在，那说明我们的控制器写的有歧义了，直接抛出异常。我们看到第5行 addMatchingMappings(directPathMatches, matches, request); ，这里是真正获取HandlerMethod的地方，跟进去看：

     private void addMatchingMappings(Collection<T> mappings, List<Match> matches, HttpServletRequest request) {
         for (T mapping : mappings) {
             T match = getMatchingMapping(mapping, request);
             if (match != null) {
                 matches.add(new Match(match, this.handlerMethods.get(mapping)));
             }
         }
     }

直接看到第5行 matches.add(new Match(match, this.handlerMethods.get(mapping))) ，这里传入了一个重要的this.handlerMethods.get(mapping)，this.handlerMethods 是一个LinkedHashMap，存放了此HM所有的处理器，根据mapping对象进行索引（mapping对象标识了唯一的请求映射），看一看这个handlerMethods 里有啥：

果然我们写的控制器方法对应的handler，都在这里头保存着。

接下来我们看看，这些handler是怎么保存到HM中的。我们在HM中找到了一个initHandlerMethods方法，从名字中就可以看出来这个方法是初始化methods用的，我们看看这个方法的内容：

 protected void initHandlerMethods() {
     if (logger.isDebugEnabled()) {
         logger.debug("Looking for request mappings in application context: " + getApplicationContext());
     }

     String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
             BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
             getApplicationContext().getBeanNamesForType(Object.class));

     for (String beanName : beanNames) {
         if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX) &&
                 isHandler(getApplicationContext().getType(beanName))){
             detectHandlerMethods(beanName);
         }
     }
     handlerMethodsInitialized(getHandlerMethods());
 }

它从ApplicationContext中获取了bean的名字，然后根据名字，在第13行中通过 detectHandlerMethods(beanName) 得到真正的bean：

 protected void detectHandlerMethods(final Object handler) {
         Class<?> handlerType =
                 (handler instanceof String ? getApplicationContext().getType((String) handler) : handler.getClass());

         // Avoid repeated calls to getMappingForMethod which would rebuild RequestMappingInfo instances
         final Map<Method, T> mappings = new IdentityHashMap<Method, T>();
         final Class<?> userType = ClassUtils.getUserClass(handlerType);

         Set<Method> methods = HandlerMethodSelector.selectMethods(userType, new MethodFilter() {
             @Override
             public boolean matches(Method method) {
                 T mapping = getMappingForMethod(method, userType);
                 if (mapping != null) {
                     mappings.put(method, mapping);
                     return true;
                 }
                 else {
                     return false;
                 }
             }
         });

         for (Method method : methods) {
             registerHandlerMethod(handler, method, mappings.get(method));
         }
     }

看第24行，在这里注册HM所持的HandlerMethod，跟进去：

 protected void registerHandlerMethod(Object handler, Method method, T mapping) {
         HandlerMethod newHandlerMethod = createHandlerMethod(handler, method);
         HandlerMethod oldHandlerMethod = this.handlerMethods.get(mapping);
         if (oldHandlerMethod != null && !oldHandlerMethod.equals(newHandlerMethod)) {
             throw new IllegalStateException("Ambiguous mapping found. Cannot map '" + newHandlerMethod.getBean() +
                     "' bean method \n" + newHandlerMethod + "\nto " + mapping + ": There is already '" +
                     oldHandlerMethod.getBean() + "' bean method\n" + oldHandlerMethod + " mapped.");
         }

         this.handlerMethods.put(mapping, newHandlerMethod);
         if (logger.isInfoEnabled()) {
             logger.info("Mapped \"" + mapping + "\" onto " + newHandlerMethod);
         }

         Set<String> patterns = getMappingPathPatterns(mapping);
         for (String pattern : patterns) {
             if (!getPathMatcher().isPattern(pattern)) {
                 this.urlMap.add(pattern, mapping);
             }
         }
     }

看第10行 this.handlerMethods.put(mapping, newHandlerMethod) ，一个个Put 进去了，这样，就完成了HandlerMethod的注册。在第18行 this.urlMap.add(pattern, mapping) ，完成了路径到mapping的映射。

最后只要框架在启动是调用initMethod方法，就可以完成处理器的注册了。

自此，DispatcherServlet如何得到ModelAndView的过程就讲完了。