在web.xml中，可以找到这么一段配置文件，它的作用是将DispatchServlet作为一个servlet加载到servlet容器中 其中，是将某一类请求转交给这个servlet进行处理

<servlet><servlet-name>SpringMVC</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value>classpath:spring-mvc.xml</param-value></init-param><load-on-startup>1</load-on-startup>//是否开启异步支持<!--<async-supported>true</async-supported>-->
</servlet>
<servlet-mapping><servlet-name>SpringMVC</servlet-name><url-pattern>/</url-pattern>
</servlet-mapping>
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然后我们开始解析一下DispatchServlet具体做了些什么事情

1）加载DispatcherServlet.properties中的属性

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";...static {// Load default strategy implementations from properties file.// This is currently strictly internal and not meant to be customized// by application developers.try {ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);}catch (IOException ex) {throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());}
}
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找到DispatcherServlet.properties，其中内容如下

# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolverorg.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolverorg.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMappingorg.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapterorg.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolverorg.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslatororg.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolverorg.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
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所以我们可以知道，这个DispatcherServlet.properties的作用是将默认实现类的路径以键值对的形式加载进来 LocaleResolver(本地化解析器，AcceptHeaderLocaleResolver) ThemeResolver(主题解析器，FixedThemeResolver) HandlerMapping(映射处理器，BeanNameUrlHandlerMapping) HandlerAdapter(处理适配器，多个) ViewResolver(视图解析器，InternalResourceViewResolver) RequestToViewNameTranslator(将请求名称转化为视图名称，DefaultRequestToViewNameTranslator)

2）初始化加载SpringMVC框架下需要配置的基本的bean

protected void initStrategies(ApplicationContext context) {initMultipartResolver(context);initLocaleResolver(context);initThemeResolver(context);initHandlerMappings(context);initHandlerAdapters(context);initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);
}
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这里的作用是将SpringMVC框架所需的各个基本的bean初始化，并加载到容器中

3）重写doService()方法

DispatchServlet实际上继承于FrameworkServlet，并且在其具体实现中重写了doService()方法。 doService()主要是将一些全局属性set到request中，具体的请求处理则调用并在doDispatch()中处理

4）doDipatch()将请求处理后转发到Controller

/**
* 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 {//获取当前要处理的requestHttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {ModelAndView mv = null;Exception dispatchException = null;try {//判断是否有文件上传processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.//获取HandlerExecutionChain，其中包含了private HandlerInterceptor[] interceptors，即拦截器的一个数组mappedHandler = getHandler(processedRequest);if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.//获取HandlerAdapterHandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.//从request中获取HTTP请求方法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;}}//如果有拦截器，则处理其前置方法preHandleif (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.//返回处理后的modelViewmv = ha.handle(processedRequest, response, mappedHandler.getHandler());//判断是否是异步请求if (asyncManager.isConcurrentHandlingStarted()) {return;}//如果view为空，返回一个默认的viewapplyDefaultViewName(processedRequest, mv);//处理拦截器的后置方法postHandlemappedHandler.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 {//判断是否是异步请求if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.//删除上传资源if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}
}
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