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ContextLoaderListener的作用就是启动web容器时，自动装配ApplicationContext的配置信息。因为它实现了ServletContextListener接口，当web容器启动时，会触发ServletContextListener的contextInitialized方法。
public interface ServletContextListener extends EventListener { public void contextInitialized ( ServletContextEvent sce ); public void contextDestroyed ( ServletContextEvent sce );} 一般在web.xml中都会有如下配置，ContextLoaderListener是一个上下文载入器，同时它实现了ServletContextListener接口，也是一个最常用的Servlet监听器。
org.springframework.web.context.ContextLoaderListener
ServletContext启动之后会调用ServletContextListener的contextInitialized方法，我们进入到ContextLoaderListener的contextInitialized方法体。
public class ContextLoaderListener implements ServletContextListener { private ContextLoader contextLoader; public void contextInitialized(ServletContextEvent event) { this.contextLoader = createContextLoader(); this.contextLoader.initWebApplicationContext(event.getServletContext()); } protected ContextLoader createContextLoader() { return new ContextLoader(); } ……}public WebApplicationContext initWebApplicationContext(ServletContext servletContext) throws IllegalStateException, BeansException { if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) { throw new IllegalStateException( "Cannot initialize context because there is already a root application context present - " + "check whether you have multiple ContextLoader* definitions in your web.xml!"); } try { //载入根上下文的双亲上下文 ApplicationContext parent = loadParentContext(servletContext); this.context = createWebApplicationContext(servletContext, parent); servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context); currentContextPerThread.put(Thread.currentThread().getContextClassLoader(), this.context); …… return this.context; } …… } 上述代码首先判断在ServletContext中是否已经有根上下文存在了，如果存在，则抛出异常提示创建失败，如果不存在。
protected WebApplicationContext createWebApplicationContext( ServletContext servletContext, ApplicationContext parent) throws BeansException { Class contextClass = determineContextClass(servletContext); if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) { throw new ApplicationContextException("Custom context class [" + contextClass.getName() + "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]"); } ConfigurableWebApplicationContext wac = (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass); wac.setParent(parent); wac.setServletContext(servletContext); wac.setConfigLocation(servletContext.getInitParameter(CONFIG_LOCATION_PARAM)); customizeContext(servletContext, wac); wac.refresh(); return wac; } protected Class determineContextClass(ServletContext servletContext) throws ApplicationContextException { String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM); if (contextClassName != null) { try { return ClassUtils.forName(contextClassName); } catch (ClassNotFoundException ex) { throw new ApplicationContextException( "Failed to load custom context class [" + contextClassName + "]", ex); } } else { contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName()); try { return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader()); } catch (ClassNotFoundException ex) { throw new ApplicationContextException( "Failed to load default context class [" + contextClassName + "]", ex); } } } static { try { ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class); defaultStrategies = PropertiesLoaderUtils.loadProperties(resource); } catch (IOException ex) { throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage()); } } private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties"; 根据上述静态代码块的内容，我们查看当前类ContextLoader同样目录下的ContextLoader.properties文件，内容如下：
org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
综上分析，在初始化的过程中，首先读取ContextLoader.properties内容，根据其中的配置提取将要实现WebApplicationContext接口的实现类，并根据这个实现类通过反射方式创建实例对象。
再回到ContextLoader的initWebApplicationContext方法中，createWebApplicationContext方法执行结束后，需要将实例对象记录到servletContext中，并映射当前的类加载器与创建的实例对象到全局变量currentContextPerThread中。
在完成对ContextLoaderListener的初始化之后，Web容器开始初始化DispatcherServlet。在Servlet的初始化阶段会调用其init方法，DispatcherServlet继承自FrameworkServlet，而FrameworkServlet继承自HttpServletBean，在HttpServletBean中找到了init方法。
public final void init() throws ServletException { …… PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties); if (!pvs.isEmpty()) { try { BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this); ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext()); bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment())); initBeanWrapper(bw); bw.setPropertyValues(pvs, true); } catch (BeansException ex) { if (logger.isErrorEnabled()) { logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex); } throw ex; } } initServletBean(); if (logger.isDebugEnabled()) { logger.debug("Servlet '" + getServletName() + "' configured successfully"); } } DispatcherServlet的初始化过程主要是先将web.xml中的init节点内容解析成Servlet类型实例，再将Servlet类型实例转换成BeanWrapper类型实例。最后进入initServletBean方法进行具体的初始化，FrameworkServlet扩展了此方法。
protected final void initServletBean() throws ServletException { …… try { this.webApplicationContext = initWebApplicationContext(); initFrameworkServlet(); } …… } protected WebApplicationContext initWebApplicationContext() { WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext()); WebApplicationContext wac = null; if (this.webApplicationContext != null) { // A context instance was injected at construction time -> use it wac = this.webApplicationContext; if (wac instanceof ConfigurableWebApplicationContext) { ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac; if (!cwac.isActive()) { // The context has not yet been refreshed -> provide services such as // setting the parent context, setting the application context id, etc if (cwac.getParent() == null) { // The context instance was injected without an explicit parent -> set // the root application context (if any; may be null) as the parent cwac.setParent(rootContext); } configureAndRefreshWebApplicationContext(cwac); } } } if (wac == null) { // No context instance was injected at construction time -> see if one // has been registered in the servlet context. If one exists, it is assumed // that the parent context (if any) has already been set and that the // user has performed any initialization such as setting the context id wac = findWebApplicationContext(); } if (wac == null) { // No context instance is defined for this servlet -> create a local one wac = createWebApplicationContext(rootContext); } if (!this.refreshEventReceived) { // Either the context is not a ConfigurableApplicationContext with refresh // support or the context injected at construction time had already been // refreshed -> trigger initial onRefresh manually here. onRefresh(wac); } if (this.publishContext) { // Publish the context as a servlet context attribute. String attrName = getServletContextAttributeName(); getServletContext().setAttribute(attrName, wac); if (this.logger.isDebugEnabled()) { this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() + "' as ServletContext attribute with name [" + attrName + "]"); } } return wac; } protected void onRefresh(ApplicationContext context) { initStrategies(context); } protected void initStrategies(ApplicationContext context) { initMultipartResolver(context); initLocaleResolver(context); initThemeResolver(context); initHandlerMappings(context); initHandlerAdapters(context); initHandlerExceptionResolvers(context); initRequestToViewNameTranslator(context); initViewResolvers(context); initFlashMapManager(context); } onRefresh是FrameworkServlet类中提供的模板方法，在其子类DispatcherServlet中进行了重写：
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"); } } } private void initLocaleResolver(ApplicationContext context) { try { this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class); if (logger.isDebugEnabled()) { logger.debug("Using LocaleResolver [" + this.localeResolver + "]"); } } catch (NoSuchBeanDefinitionException ex) { // We need to use the default. this.localeResolver = getDefaultStrategy(context, LocaleResolver.class); if (logger.isDebugEnabled()) { logger.debug("Unable to locate LocaleResolver with name '" + LOCALE_RESOLVER_BEAN_NAME + "': using default [" + this.localeResolver + "]"); } } } private void initThemeResolver(ApplicationContext context) { try { this.themeResolver = context.getBean(THEME_RESOLVER_BEAN_NAME, ThemeResolver.class); if (logger.isDebugEnabled()) { logger.debug("Using ThemeResolver [" + this.themeResolver + "]"); } } catch (NoSuchBeanDefinitionException ex) { // We need to use the default. this.themeResolver = getDefaultStrategy(context, ThemeResolver.class); if (logger.isDebugEnabled()) { logger.debug("Unable to locate ThemeResolver with name '" + THEME_RESOLVER_BEAN_NAME + "': using default [" + this.themeResolver + "]"); } } } private void initHandlerMappings(ApplicationContext context) { this.handlerMappings = null; if (this.detectAllHandlerMappings) { // Find all HandlerMappings in the ApplicationContext, including ancestor contexts. Map matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerMappings = new ArrayList<>(matchingBeans.values()); // We keep HandlerMappings in sorted order. AnnotationAwareOrderComparator.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"); } } } private void initHandlerAdapters(ApplicationContext context) { this.handlerAdapters = null; if (this.detectAllHandlerAdapters) { // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts. Map matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerAdapters = new ArrayList<>(matchingBeans.values()); // We keep HandlerAdapters in sorted order. AnnotationAwareOrderComparator.sort(this.handlerAdapters); } } else { try { HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class); this.handlerAdapters = Collections.singletonList(ha); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we'll add a default HandlerAdapter later. } } // Ensure we have at least some HandlerAdapters, by registering // default HandlerAdapters if no other adapters are found. if (this.handlerAdapters == null) { this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class); if (logger.isDebugEnabled()) { logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default"); } } } private void initHandlerExceptionResolvers(ApplicationContext context) { this.handlerExceptionResolvers = null; if (this.detectAllHandlerExceptionResolvers) { // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts. Map matchingBeans = BeanFactoryUtils .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values()); // We keep HandlerExceptionResolvers in sorted order. AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers); } } else { try { HandlerExceptionResolver her = context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class); this.handlerExceptionResolvers = Collections.singletonList(her); } catch (NoSuchBeanDefinitionException ex) { // Ignore, no HandlerExceptionResolver is fine too. } } // Ensure we have at least some HandlerExceptionResolvers, by registering // default HandlerExceptionResolvers if no other resolvers are found. if (this.handlerExceptionResolvers == null) { this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class); if (logger.isDebugEnabled()) { logger.debug("No HandlerExceptionResolvers found in servlet '" + getServletName() + "': using default"); } } } private void initRequestToViewNameTranslator(ApplicationContext context) { try { this.viewNameTranslator = context.getBean(REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class); if (logger.isDebugEnabled()) { logger.debug("Using RequestToViewNameTranslator [" + this.viewNameTranslator + "]"); } } catch (NoSuchBeanDefinitionException ex) { // We need to use the default. this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class); if (logger.isDebugEnabled()) { logger.debug("Unable to locate RequestToViewNameTranslator with name '" + REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME + "': using default [" + this.viewNameTranslator + "]"); } } } private void initViewResolvers(ApplicationContext context) { this.viewResolvers = null; if (this.detectAllViewResolvers) { // Find all ViewResolvers in the ApplicationContext, including ancestor contexts. Map matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false); if (!matchingBeans.isEmpty()) { this.viewResolvers = new ArrayList<>(matchingBeans.values()); // We keep ViewResolvers in sorted order. AnnotationAwareOrderComparator.sort(this.viewResolvers); } } else { try { ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class); this.viewResolvers = Collections.singletonList(vr); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we'll add a default ViewResolver later. } } // Ensure we have at least one ViewResolver, by registering // a default ViewResolver if no other resolvers are found. if (this.viewResolvers == null) { this.viewResolvers = getDefaultStrategies(context, ViewResolver.class); if (logger.isDebugEnabled()) { logger.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default"); } } } private void initFlashMapManager(ApplicationContext context) { try { this.flashMapManager = context.getBean(FLASH_MAP_MANAGER_BEAN_NAME, FlashMapManager.class); if (logger.isDebugEnabled()) { logger.debug("Using FlashMapManager [" + this.flashMapManager + "]"); } } catch (NoSuchBeanDefinitionException ex) { // We need to use the default. this.flashMapManager = getDefaultStrategy(context, FlashMapManager.class); if (logger.isDebugEnabled()) { logger.debug("Unable to locate FlashMapManager with name '" + FLASH_MAP_MANAGER_BEAN_NAME + "': using default [" + this.flashMapManager + "]"); } } } 转载地址：http://jpgmi.baihongyu.com/