Spring容器初始话原理图

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();
            }
    }
}