spring mvc DispatcherServlet详解之一---处理请求深入解析

要深入理解spring mvc的工作流程，就需要先了解spring mvc的架构：

从上图可以看到 前端控制器DispatcherServlet在其中起着主导作用，理解了DispatcherServlet 就完全可以说弄清楚了spring mvc。

为了加深对spring mvc的整个工作流程的理解，本文从分析DispatcherServlet的工作过程来一窥spring mvc的整个面貌。

1. 初始化

protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context); //文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；
initLocaleResolver(context); //本地化解析
initThemeResolver(context);　　　//主题解析
initHandlerMappings(context); //通过HandlerMapping，将请求映射到处理器
initHandlerAdapters(context); //通过HandlerAdapter支持多种类型的处理器
initHandlerExceptionResolvers(context); //如果执行过程中遇到异常将交给HandlerExceptionResolver来解析
initRequestToViewNameTranslator(context); //直接解析请求到视图名
initViewResolvers(context); //通过ViewResolver解析逻辑视图名到具体视图实现
initFlashMapManager(context); //flash映射管理器
}

单个resolver
initMultipartResolver，initLocaleResolver，initThemeResolver，initRequestToViewNameTranslator，initFlashMapManager 这五个初始化方法流程相同，都是使用
context.getBean(String name, Class<FlashMapManager> requiredType)的方式获取到相应的Resolver。以initMultipartResolver为例，见如下： /*** Initialize the MultipartResolver used by this class.* <p>If no bean is defined with the given name in the BeanFactory for this namespace,* no multipart handling is provided.*/private void initMultipartResolver(ApplicationContext context) {try {this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);if (logger.isDebugEnabled()) {logger.debug("Using MultipartResolver [" + this.multipartResolver + "]");}}catch (NoSuchBeanDefinitionException ex) {// Default is no multipart resolver.this.multipartResolver = null;if (logger.isDebugEnabled()) {logger.debug("Unable to locate MultipartResolver with name '" + MULTIPART_RESOLVER_BEAN_NAME +"': no multipart request handling provided");}}}

多个resolver

initHandlerMappings，initHandlerAdapters，initHandlerExceptionResolvers，initViewResolvers 获取方式相同，使用：
BeanFactoryUtils.beansOfTypeIncludingAncestors(ListableBeanFactory lbf, Class<HandlerMapping> type, boolean includeNonSingletons, boolean allowEagerInit) 的方式获取到相应的Resolver。以initHandlerMappings为例，见如下： /*** Initialize the HandlerMappings used by this class.* <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,* we default to BeanNameUrlHandlerMapping.*/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<HandlerMapping>(matchingBeans.values());// We keep HandlerMappings in sorted order.OrderComparator.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. }}// Ensure we have at least one HandlerMapping, by registering// a default HandlerMapping if no other mappings are found.if (this.handlerMappings == null) {this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);if (logger.isDebugEnabled()) {logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");}}}

那么深入看一下BeanFactoryUtils.beansOfTypeIncludingAncestors 究竟做了什么？返回指定类型和子类型的所有bean，若该bean factory 是一个继承类型的beanFactory，这个方法也会获取祖宗factory中定义的指定类型的bean。

/*** Return all beans of the given type or subtypes, also picking up beans defined in* ancestor bean factories if the current bean factory is a HierarchicalBeanFactory.* The returned Map will only contain beans of this type.* <p>Does consider objects created by FactoryBeans if the "allowEagerInit" flag is set,* which means that FactoryBeans will get initialized. If the object created by the* FactoryBean doesn't match, the raw FactoryBean itself will be matched against the* type. If "allowEagerInit" is not set, only raw FactoryBeans will be checked* (which doesn't require initialization of each FactoryBean).* <p><b>Note: Beans of the same name will take precedence at the 'lowest' factory level,* i.e. such beans will be returned from the lowest factory that they are being found in,* hiding corresponding beans in ancestor factories.</b> This feature allows for* 'replacing' beans by explicitly choosing the same bean name in a child factory;* the bean in the ancestor factory won't be visible then, not even for by-type lookups.* @param lbf the bean factory* @param type type of bean to match* @param includeNonSingletons whether to include prototype or scoped beans too* or just singletons (also applies to FactoryBeans)* @param allowEagerInit whether to initialize <i>lazy-init singletons</i> and* <i>objects created by FactoryBeans</i> (or by factory methods with a* "factory-bean" reference) for the type check. Note that FactoryBeans need to be* eagerly initialized to determine their type: So be aware that passing in "true"* for this flag will initialize FactoryBeans and "factory-bean" references.* @return the Map of matching bean instances, or an empty Map if none* @throws BeansException if a bean could not be created*/public static <T> Map<String, T> beansOfTypeIncludingAncestors(ListableBeanFactory lbf, Class<T> type, boolean includeNonSingletons, boolean allowEagerInit)throws BeansException {Assert.notNull(lbf, "ListableBeanFactory must not be null");Map<String, T> result = new LinkedHashMap<String, T>(4);result.putAll(lbf.getBeansOfType(type, includeNonSingletons, allowEagerInit));if (lbf instanceof HierarchicalBeanFactory) {HierarchicalBeanFactory hbf = (HierarchicalBeanFactory) lbf;if (hbf.getParentBeanFactory() instanceof ListableBeanFactory) {Map<String, T> parentResult = beansOfTypeIncludingAncestors((ListableBeanFactory) hbf.getParentBeanFactory(), type, includeNonSingletons, allowEagerInit);for (Map.Entry<String, T> entry : parentResult.entrySet()) {String beanName = entry.getKey();if (!result.containsKey(beanName) && !hbf.containsLocalBean(beanName)) {result.put(beanName, entry.getValue());}}}}return result;}

 2. 提供服务

     我们来看看这个servlet是如何提供服务的？

@Overrideprotected 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);}}}

   从上面我们可以看到，提供服务只要分4步：

  1. 保存现场。保存request 熟悉的快照，以便能在必要时恢复。

  2. 将框架需要的对象放入request中，以便view和handler使用。

  3. 请求分发服务.

  4. 恢复现场。

其中最重要的是请求分发服务：

/*** 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);}}}

分发过程如下：

1. 判断是否设置了multipart resolver，设置的话转换为multipart request，没有的话则继续下面的步骤。

2. 根据当前request，获取hangdler。

3. 根据当前request，获取HandlerAdapter。

4. 如果支持http请求头，处理 last-modified header请求头。

5. 应用已注册interceptor的preHandle方法

6. HandlerAdapter处理请求。

7. 设置默认视图。

8. 应用已注册interceptor的postHandle方法。

9. 处理异常或者视图渲染。

小结：

   DispatherServlet整个过程的细节一章之内很难描述的面面俱到，只能分析部分流程，想了解更具体的实现需要从源代码中去寻找。

 

转载于:https://www.cnblogs.com/davidwang456/p/4096530.html

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