public abstract class GenericServlet implements Servlet {
      /**
       * Called by the servlet container to indicate to a servlet that the
       * servlet is being placed into service. 
       */
      public void init(ServletConfig config) throws ServletException {
            this.config = config;
            this.init();
      }
}
  

public abstract class HttpServletBean extends HttpServlet {
        /**
         * 覆写了父类 GenericServlet#init() 方法
         */
	@Override
	public final void init() throws ServletException {
		// Set bean properties from init parameters.
		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);
                                /**
                                 * 如果你有设置 init-param 'contextConfigLocation'，那么就会调用 DispatcherServlet#setContextConfigLocation 方法
                                 * 如果你有设置 init-param 'contextClass'，就会调用 DispatcherServlet#setContextClass 方法
                                 * 如果你有设置 init-param 'contextInitializerClasses'，就会调用 DispatcherServlet#setContextInitializerClasses 方法
                                 */
				bw.setPropertyValues(pvs, true);
			}
			catch (BeansException ex) {
				if (logger.isErrorEnabled()) {
					logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
				}
				throw ex;
			}
		}
		// Let subclasses do whatever initialization they like.
		initServletBean();
	}
}
  

在 ContextLoader # initWebApplicationContext(ServletContext servletContext) 方法中，

把创建好的 WebApplicationContext 实例，通过调用 servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context) 设置到 Servlet 容器上下文中。

键值为 org.springframework.web.context.WebApplicationContext.ROOT

在 FrameworkServlet # initWebApplicationContext() 方法中， 调用 WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext()) 获取 Web 应用“根”上下文

// 传入空字符串"",得到的是长度为0的String数组
public void setConfigLocations(@Nullable String... locations) {
    if (locations != null) {
        Assert.noNullElements(locations, "Config locations must not be null");
        this.configLocations = new String[locations.length];
        for(int i = 0; i < locations.length; ++i) {
            this.configLocations[i] = this.resolvePath(locations[i]).trim();
        }
    } else {
        this.configLocations = null;
    }
}

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
	String[] configLocations = getConfigLocations();
	if (configLocations != null) {
                // 配置数组长度为0时，不会进入函数体内部，就不会执行 reader.loadBeanDefinitions
		for (String configLocation : configLocations) {
			reader.loadBeanDefinitions(configLocation);
		}
	}
}

public void refresh() throws BeansException, IllegalStateException {
	synchronized (this.startupShutdownMonitor) {
		// Prepare this context for refreshing.
		prepareRefresh();
		// Tell the subclass to refresh the internal bean factory.
		ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
		// Prepare the bean factory for use in this context.
		prepareBeanFactory(beanFactory);
		try {
			// Allows post-processing of the bean factory in context subclasses.
			postProcessBeanFactory(beanFactory);
			// Invoke factory processors registered as beans in the context.
			invokeBeanFactoryPostProcessors(beanFactory);
			// Register bean processors that intercept bean creation.
			registerBeanPostProcessors(beanFactory);
			// Initialize message source for this context.
			initMessageSource();
			// Initialize event multicaster for this context.
			initApplicationEventMulticaster();
			// Initialize other special beans in specific context subclasses.
			onRefresh();
			// Check for listener beans and register them.
			registerListeners();
			// Instantiate all remaining (non-lazy-init) singletons.
			finishBeanFactoryInitialization(beanFactory);
			// Last step: publish corresponding event.
			finishRefresh();
		}
		catch (BeansException ex) {
			if (logger.isWarnEnabled()) {
				logger.warn("Exception encountered during context initialization - " +
						"cancelling refresh attempt: " + ex);
			}
			// Destroy already created singletons to avoid dangling resources.
			destroyBeans();
			// Reset 'active' flag.
			cancelRefresh(ex);
			// Propagate exception to caller.
			throw ex;
		}
		finally {
			// Reset common introspection caches in Spring's core, since we
			// might not ever need metadata for singleton beans anymore...
			resetCommonCaches();
		}
	}
}

protected void finishRefresh() {
	// Clear context-level resource caches (such as ASM metadata from scanni
	clearResourceCaches();
	// Initialize lifecycle processor for this context.
	initLifecycleProcessor();
	// Propagate refresh to lifecycle processor first.
	getLifecycleProcessor().onRefresh();
        // 这个事件会被 FrameworkServlet 接收和处理
	// Publish the final event.
	publishEvent(new ContextRefreshedEvent(this));
	// Participate in LiveBeansView MBean, if active.
	LiveBeansView.registerApplicationContext(this);
}

wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

private class ContextRefreshListener implements ApplicationListener {
	@Override
	public void onApplicationEvent(ContextRefreshedEvent event) {
		FrameworkServlet.this.onApplicationEvent(event);
	}
}

该配置文件和 DispatcherServlet.class 在同一文件夹下，key 为接口类名称，value 为逗号分隔的实现类名称