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Spring IoC作用是管理Bean的生命周期并提供用户使用bean，所以我们有必要来看看Spring如何管理这些Bean的，前面我们提到IoC基本功能由BeanFactory系列类型实现，BeanFactory系列中就是DefaultListableBeanFactory这个类型完成的IoC基本功能。先来看下继承体系：

我们需要理解的是，Spring在启动时，整个流程是怎样的。从大体流程上，我们应该可以预料到，肯定是ApplicationContext方法先被调用，然后才是BeanFactory方法被调用，但是主要操作上肯定是先BeanFactory操作，再是ApplicationContext。也就是说ApplicationContext是入口，他肯定会调用BeanFactory进行操作的。下面我们来看看具体主要流程。

我们知道ApplicationContext在context包中的唯一子接口是ConfigurableApplicationContext，这个接口中有一个方法

/** * Load or refresh the persistent representation of the configuration, * which might an XML file, properties file, or relational database schema. * 
As this is a startup method, it should destroy already created singletons * if it fails, to avoid dangling resources. In other words, after invocation * of that method, either all or no singletons at all should be instantiated. * @throws BeansException if the bean factory could not be initialized * @throws IllegalStateException if already initialized and multiple refresh * attempts are not supported */void refresh() throws BeansException, IllegalStateException;

上面的注释翻一下来的意思是，加载或刷新持久化的配置描述，这个描述可以是一个xml文件，properties文件或者相关的数据库模式。他作为一个启动方法，在失败时他必须销毁已经创建的单例beans，以防资源被挂起。换句话说，这个方法被调用后，要么全部的单例被创建要么一个也不被创建。这个注释应该是比较片面的，我们只能获取到一部分信息。只要知道他是启动方法就行了。

AbstractApplicationContext是实现ConfigurableApplicationContext的抽象类，里面实现了refresh()方法，所有其他实现了ApplicationContext接口的实现类型都会调用这个方法来启动一个IoC容器，并且我们知道，AbstractApplicationContext没有持有DefaultListableBeanFactory成员变量，我们来看看这个方法，AbstractApplicationContext#refresh()

@Overridepublic 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();		}	}}

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