Spring后置处理器BeanPostProcessor

1. 概述
   BeanPostProcessor[interface]是spring的后置处理器,在bean初始化前后进行一些初始化工作。

   postProcessBeforeInitialization在初始化之前工作
   postProcessAfterInitialization在初始化之后工作

2. Spring底层对BeanPostProcessor的使用
   ApplicationContextAwareProcessor中使用
   
   在spring类中需要使用到spring的上下文（ApplicationContext），只需要实现ApplicationContextAware，后置处理器会自动赋值

   public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
   		if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
   				bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
   				bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)){
   			return bean;
   		}
   
   		AccessControlContext acc = null;
   
   		if (System.getSecurityManager() != null) {
   			acc = this.applicationContext.getBeanFactory().getAccessControlContext();
   		}
   
   		if (acc != null) {
   			AccessController.doPrivileged((PrivilegedAction