第二章 SpringMVC之 DispatcherServlet解析
一. DispatcherServlet上下文的形成
web.xml
- <?xml version="1.0" encoding="UTF-8"?>
- <web-app version="2.5"
- xmlns="http://java.sun.com/xml/ns/javaee"
- xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
- xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
- http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
- <display-name></display-name>
- <welcome-file-list>
- <welcome-file>index.jsp</welcome-file>
- </welcome-file-list>
- <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*:springController.xml</param-value>
- </init-param>
- </servlet>
- <servlet-mapping>
- <servlet-name>springMVC</servlet-name>
- <url-pattern>/</url-pattern>
- </servlet-mapping>
- <span style="color:#ff6666;"> <context-param>
- <param-name>contextConfigLocation</param-name>
- <param-value>classpath*:springController.xml</param-value>
- </context-param>
- <listener>
- <listener-class>
- org.springframework.web.context.ContextLoaderListener
- </listener-class>
- </listener>
- </web-app></span>
从图中可以看出DispatcherServlet初始的上下文加载的bean是对SpringMVC有效的bean,例如HandlerMapping,HandlerAdapter,ViewResoler,ContextLoaderListener初始的上下文是对整个应用程序共享的bean,例如DAO,serivce等。最后DispatcherServlet初始的上下文会继承ContextLoaderListener的上下文。
二. DispatcherServlet的初始化
我们启动服务器的时候,程序只是初始化了DispatcherServlet的上下文,还没有对DispatcherServlet进行初始化,就像我们去买台式机,买回来的放在一个箱子里的有显示器,机箱,主板,显卡等,要想用电脑我们还得将电脑组装起来,初始化就好比要将我们配置文件中配置了的HandlerMapping,HandlerAdapter,ViewResoler组装起来,然后我们才可以真正的使用。下面看一下DispatcherServlet初始化的源码
- protected void initStrategies(ApplicationContext context) {
- initMultipartResolver(context);
- initLocaleResolver(context);
- initThemeResolver(context);
- initHandlerMappings(context);
- initHandlerAdapters(context);
- initHandlerExceptionResolvers(context);
- initRequestToViewNameTranslator(context);
- initViewResolvers(context);
- }
- /**
- * 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.
- }
- }
- <bean class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping"/>
- <bean id="urlMapping" class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
- <property name="mappings">
- <props>
- <prop key="/springMVC.d">HelloWorld</prop>
- </props>
- </property>
- </bean>
三. DispatcherServlet的工作流程
用过python Django框架的都知道Django对于访问方式的配置就是,一个url路径和一个函数配对,你访问这个url,就会直接调用这个函数,简单明了。对于java的面向对象来说,就要分两步走。第一步首先要找到是哪个对象,即handler,即我们写的action或是controller。第二步要找到访问的函数,即action中的方法。所以就出现了两个源码接口 HandlerMapping和HandlerAdapter,前者负责第一步,后者负责第二步。借用网上的SpringMVC架构图。
看一下Dispatcher中的doDispatcher源码
- //前端控制器分派方法
- protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
- HttpServletRequest processedRequest = request;
- HandlerExecutionChain mappedHandler = null;
- int interceptorIndex = -1;
- try {
- ModelAndView mv;
- boolean errorView = false;
- try {
- //检查是否是请求是否是multipart(如文件上传),如果是将通过MultipartResolver解析
- processedRequest = checkMultipart(request);
- //步骤2、请求到处理器(页面控制器)的映射,通过HandlerMapping进行映射
- mappedHandler = getHandler(processedRequest, false);
- if (mappedHandler == null || mappedHandler.getHandler() == null) {
- noHandlerFound(processedRequest, response);
- return;
- }
- //步骤3、处理器适配,即将我们的处理器包装成相应的适配器(从而支持多种类型的处理器)
- HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
- // 304 Not Modified缓存支持
- //此处省略具体代码
- // 执行处理器相关的拦截器的预处理(HandlerInterceptor.preHandle)
- //此处省略具体代码
- // 步骤4、由适配器执行处理器(调用处理器相应功能处理方法)
- mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
- // Do we need view name translation?
- if (mv != null && !mv.hasView()) {
- mv.setViewName(getDefaultViewName(request));
- }
- // 执行处理器相关的拦截器的后处理(HandlerInterceptor.postHandle)
- //此处省略具体代码
- }
- catch (ModelAndViewDefiningException ex) {
- logger.debug("ModelAndViewDefiningException encountered", ex);
- mv = ex.getModelAndView();
- }
- catch (Exception ex) {
- Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
- mv = processHandlerException(processedRequest, response, handler, ex);
- errorView = (mv != null);
- }
- //步骤5 步骤6、解析视图并进行视图的渲染
- //步骤5 由ViewResolver解析View(viewResolver.resolveViewName(viewName, locale))
- //步骤6 视图在渲染时会把Model传入(view.render(mv.getModelInternal(), request, response);)
- if (mv != null && !mv.wasCleared()) {
- render(mv, processedRequest, 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");
- }
- }
- // 执行处理器相关的拦截器的完成后处理(HandlerInterceptor.afterCompletion)
- //此处省略具体代码
- catch (Exception ex) {
- // Trigger after-completion for thrown exception.
- triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
- throw ex;
- }
- catch (Error err) {
- ServletException ex = new NestedServletException("Handler processing failed", err);
- // Trigger after-completion for thrown exception.
- triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
- throw ex;
- }
- finally {
- // Clean up any resources used by a multipart request.
- if (processedRequest != request) {
- cleanupMultipart(processedRequest);
- }
- }
- }
核心架构的具体流程步骤如下:
1、 首先用户发送请求——>DispatcherServlet,前端控制器收到请求后自己不进行处理,而是委托给其他的解析器进行处理,作为统一访问点,进行全局的流程控制;
2、 DispatcherServlet——>HandlerMapping, HandlerMapping将会把请求映射为HandlerExecutionChain对象(包含一个Handler处理器(页面控制器)对象、多个HandlerInterceptor拦截器)对象,通过这种策略模式,很容易添加新的映射策略;
3、 DispatcherServlet——>HandlerAdapter,HandlerAdapter将会把处理器包装为适配器,从而支持多种类型的处理器,即适配器设计模式的应用,从而很容易支持很多类型的处理器;
4、 HandlerAdapter——>处理器功能处理方法的调用,HandlerAdapter将会根据适配的结果调用真正的处理器的功能处理方法,完成功能处理;并返回一个ModelAndView对象(包含模型数据、逻辑视图名);
5、 ModelAndView的逻辑视图名——> ViewResolver, ViewResolver将把逻辑视图名解析为具体的View,通过这种策略模式,很容易更换其他视图技术;
6、 View——>渲染,View会根据传进来的Model模型数据进行渲染,此处的Model实际是一个Map数据结构,因此很容易支持其他视图技术;
7、返回控制权给DispatcherServlet,由DispatcherServlet返回响应给用户,到此一个流程结束。
此处我们只是讲了核心流程,没有考虑拦截器、本地解析、文件上传解析等。