ContextLoaderListener和DispatcherServlet都会在Web容器启动的时候加载一下bean配置. 区别在于:
DispatcherServlet一般会加载MVC相关的bean配置管理(如: ViewResolver, Controller, MultipartResolver, ExceptionHandler, etc.)
ContextLoaderListener一般会加载整个Spring容器相关的bean配置管理(如: Log, Service, Dao, PropertiesLoader, DataSource Bean, etc.)
ContextLoaderListener继承ContextLoader类，实现ServletContextListener接口（该接口监听servlet容器的启动和销毁），ContextLoaderListener重写了监听容器启动和销毁的方法，在启动时（contextInitialized），会调用initWebApplicationContext方法，进行web容器初始化。
/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
} /**
* Close the root web application context.
*/
@Override
public void contextDestroyed(ServletContextEvent event) {
closeWebApplicationContext(event.getServletContext());
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}
在initWebApplicationContext方法时父类contextloader内的方法（精简）：
在方法内回调用createWebApplicationContext（）创建容器，该容器回根据spring内部的配置文件创建，最后强制为ConfigurableWebApplicationContext对象
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
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 {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);//ConfigurableWebApplicationContext类型对象
}
if (this.context instanceof ConfigurableWebApplicationContext) {//进行初始化操作
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
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 ->
// determine parent for root web application context, if any.
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
configureAndRefreshWebApplicationContext(cwac, servletContext);//真正初始化，在该方法内部有refresh()方法，在方法回调用ConfigurableWebApplicationContext父类的refresh()方法进行容器初始化
//也就是AbstractApplicationContext类中的refresh()方法
}
}
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
return this.context;
}
}
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
Class<?> contextClass = determineContextClass(sc);
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}
protected Class<?> determineContextClass(ServletContext servletContext) {
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
if (contextClassName != null) {
try {
return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class [" + contextClassName + "]", ex);
}
}
else {
contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
try {//通过反射得到一个class对象
return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load default context class [" + contextClassName + "]", ex);
}
}
}
DispatcherServlet本质是一个servlet，能够拦截请求
servlet会有初始化init，同dispatcherservlet，会进行一些初始化操作：
protected void initStrategies(ApplicationContext context) {
//初始化多部请求解析器，没有默认的实现
initMultipartResolver(context); //文件上传
//初始化地域解析器，默认实现是AcceptHeaderLocaleResolver
initLocaleResolver(context);
//初始化主题解析器，默认实现是FixedThemeResolver
initThemeResolver(context);
//初始化处理器映射，这是个集合， 默认实现是BeanNameUrlHandlerMapping和DefaultAnnotationHandlerMapping
initHandlerMappings(context);
//初始化处理器适配器，这是个集合，默认实现是HttpRequestHandlerAdapter，SimpleControllerHandlerAdapter和AnnotationMethodHandlerAdapter
initHandlerAdapters(context);
//初始化处理器异常解析器，这是个集合，默认实现是AnnotationMethodHandlerExceptionResolver,ResponseStatusExceptionResolver和DefaultHandlerExceptionResolver
initHandlerExceptionResolvers(context);
//初始化请求到视图名解析器，默认实现是DefaultRequestToViewNameTranslator
initRequestToViewNameTranslator(context);
//初始化视图解析器，这是个集合，默认实现是InternalResourceViewResolver
initViewResolvers(context);
}