l 主流程入口：
ApplicationContext context = new ClassPathXmlApplicationContext（“spring.xml”）
l ClassPathXmlApplicationContext类：重载的构造方法依次调用，进入下面代码
l AbstractApplicationContext的refresh方法：初始化spring容器的核心代码
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
//1、 Prepare this context for refreshing.
prepareRefresh();
//创建DefaultListableBeanFactory（真正生产和管理bean的容器）
//BeanDefinition处理
// 1 定位XML文件(Resource、ResourceLoader)
// 2 加载XML（Document、Dom4j解析）
// 3 从Document对象中解析BeanDefinition然后注册到BeanDefinitionRegistry（Map）
//通过NamespaceHandler解析自定义标签的功能（比如:context标签、aop标签、tx标签）
//2、 Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
//3、 Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
//4、 Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
//实例化并调用实现了BeanFactoryPostProcessor接口的Bean
//比如：PropertyPlaceHolderConfigurer（context:property-placeholer）
//就是此处被调用的，作用是替换掉BeanDefinition中的占位符（${}）中的内容
//5、 Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
//创建并注册BeanPostProcessor到BeanFactory中（Bean的后置处理器）
//比如：AutowiredAnnotationBeanPostProcessor（实现@Autowired注解功能）
//RequiredAnnotationBeanPostProcessor（实现@Required注解功能）
//这些注册的BeanPostProcessor
//6、 Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
//7、 Initialize message source for this context.
initMessageSource();
//8、 Initialize event multicaster for this context.
initApplicationEventMulticaster();
//9、 Initialize other special beans in specific context subclasses.
onRefresh();
//10、 Check for listener beans and register them.
registerListeners();
//1 通过构造方法，创建Bean的实例。此处的Bean是非懒加载方式的单例Bean（未设置属性）
//2 填充属性（DI，依赖注入）---循环依赖（A中有B的依赖、B中有A的依赖）
//3 初始化实例（比如调用init-method方法）
// 调用BeanPostProcessor（后置处理器）对实例bean进行后置处理（AOP功能）
//11、 Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
//12、 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();
}
}
}