Spring管理bean：

1.從web.xml裡面配置的ContextLoaderListener開始。

2.ContextLoaderListener繼承ContextLoader

3.執行ContextLoaderListener的contextInitialized方法，獲得servletContext.

4.把servletContext傳入ContextLoader的initWebApplicationContext(ServletContext servletContext)方法並執行。

5.通過ContextLoader.createWebApplicationContext(servletContext)獲得this.context（

預設情況下這個context就是XmlWebApplicationContext。

這個XmlWebApplicationContext繼承一個AbstractApplicationContext類。

再執行ContextLoader.configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc)方法，

執行AbstractApplicationContext的.refresh();方法

	@Override
	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) {
				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;
			}
		}
 

換言之，已經把該準備的都準備好了，只需要有請求來獲取bean，就根據情況或返回已經初始化的bean或進行bean的Instantiation 和 Initialization。