Spring的执行流程

一.Spring IOC

IOC容器是一个Map结构，里面包含了控制反转及依赖注入。

1. 控制反转：控制反转就是对对象控制权的转移，从程序代码本身反转到了外部容器。把对象的创建、初始化、销毁等工作交给spring容器来做。由spring容器控制对象的生命周期。
2. DI依赖注入：DI—Dependency Injection，即“依赖注入”：组件之间依赖关系由容器在运行期决定，形象的说，即由容器动态的将某个依赖关系注入到组件之中。依赖注入的目的并非为软件系统带来更多功能，而是为了提升组件重用的频率，并为系统搭建一个灵活、可扩展的平台。通过依赖注入机制，我们只需要通过简单的配置，而无需任何代码就可指定目标需要的资源，完成自身的业务逻辑，而不需要关心具体的资源来自何处，由谁实现。

二.IOC容器的创建流程

创建Spring容器我们用的最多的是AnnotationConfigApplicationContext及ClassPathXmlApplicationContext等等，通过源码发现他们的根接口都是BeanFactory。

//创建基于注解的springIOC容器
ApplicationContext applicationContext = new AnnotationConfigApplicationContext(BeanConfig.class);
//创建基于配置文件的springIOC容器
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("/spring-beans.xml");

无论是那种创建方式，通过源码发现他们都调用的了他们的父类AbstractApplicationContext#refresh()方法，refresh()方法如下：

@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
			// 初始化spring上下文
			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);
				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);
				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);
				beanPostProcess.end();
				// 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();
				contextRefresh.end();
			}
		}
	} 

在refresh方法中主要做了如下事情：

1. 初始化Spring上下文 prepareRefresh()
   主要是设置启动时间，并初始化earlyApplicationListeners及earlyApplicationEvents
2. 创建BeanFactory obtainFreshBeanFactory()
   refreshBeanFactory()：如果BeanFactory存在则关闭工程，并重新创建BeanFactory，加载BeanDefinitions
3. 设置BeanFactory上下文 prepareBeanFactory(beanFactory);
   设置BeanFactory属性
4. 后置处理BeanFactory
5. 调用已注册的BeanFactoryPostProcessors invokeBeanFactoryPostProcessors(beanFactory);
6. 注册BeanPostProcessors registerBeanPostProcessors(beanFactory);
7. 初始化国际化 initMessageSource();
8. 初始化多播器 initApplicationEventMulticaster();
9. 注册监听器 registerListeners();
10. 非懒加载Bean对象实例化 finishBeanFactoryInitialization(beanFactory);
    解决循环依赖及添加动态代理
11. 发布相应事件，完成刷新 finishRefresh();