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本文源码基于Spring 5.2.7
Spring中，容器的填充全靠org.springframework.context.support.AbstractApplicationContext#refresh()方法，这个方法的每一步都是值得分析的，这每一步都会分为一篇或多篇文章来解析，这里对这些步骤作一个整体说明。
Spring的核心能力就是作为控制反转容器使用，通过名称可以看出，这个方法是用来刷新容器的，可想而知这个方法包含了Spring核心能力，包括AOP的处理过程都在这里。
作为一个服务框架，Spring提供了强大的扩展能力，Spring的设计是定义很多post processor接口，启动时，会按照次序执行这些post processor，这样，用户可以自行在启动的不同时间节点，扩展自己的功能，这种设计方式就是模板方法。
org.springframework.context.support.AbstractApplicationContext#refresh()
@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) {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();}}
}一、postProcessBeanFactory()
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);先看这个方法，这句话的意思就是调用AbstractApplicationContext的子类的postProcessBeanFactory()方法，这个描述容易和调用BeanFactoryPostProcessor类这个过程混淆，这个模板方法是留给Spring内部扩展使用的，开发者无法介入。
org.springframework.context.support.AbstractApplicationContext#postProcessBeanFactory()
在application context标准初始化完成后修改它内部的bean factory。此时所有的bean definitions已经被加载，但是bean都还没有初始化。这样的话就能够在某个ApplicationContext的实现类中注册特定的BeanPostProcessors。
AbstractApplicationContext中的这个方法是个空方法，留给子类去实现。
/*** Modify the application context's internal bean factory after its standard* initialization. All bean definitions will have been loaded, but no beans* will have been instantiated yet. This allows for registering special* BeanPostProcessors etc in certain ApplicationContext implementations.* @param beanFactory the bean factory used by the application context*/
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
}二、invokeBeanFactoryPostProcessors()
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);然后是invokeBeanFactoryPostProcessors()，这个方法的作用是调用在容器注册的BeanFactoryPostProcessor类方法，BeanFactoryPostProcessor是留给Spring内部以及开发者扩展的接口，前提是要注册到容器中来。
这里定义了2种类型的post processor，分别是
org.springframework.beans.factory.config.BeanFactoryPostProcessor
org.springframework.beans.factory.config.BeanPostProcessor
开发者可以定义自己的post processor，且执行顺序是先执行BeanFactoryPostProcessor后执行BeanPostProcessor，同一种类型的post processor可以定义优先级来控制执行顺序，这样开发者就能在启动时扩展自己的功能。
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