什么是后置处理器
这里引入spring 的一个 重要部件 PostProcessor, 我习惯把它叫做：后置处理器；
通俗来说，就是：定义一个 XxxPostProcessor 接口，定义一组行为；而后在具体的bean加载过程中，我们可以在 BeanFactory 初始化时，根据自己的实际需要，向BeanFactory 中注入相关的 PostProcessor；
而后在某些特殊节点(时机)，获取某一类的 "后置处理器" 并全部执行之即可。
这里需要注意的是：后置处理器，一般需要被理解成，某某行为-后置处理器；它可能是对某某行为返回结果的校验，也可能是对其的包装，(希望你了解设计模式之：包装器模式)。
spring 源码中已知的，顶级PostProcessor
全局一共6个，那么我们通过，其中跟我们当前分析的代码关联度最高的， BeanPostProssor 来做个介绍：
package org.springframework.beans.factory.config;
import org.springframework.beans.BeansException;
import org.springframework.lang.Nullable;
/**
* Factory hook that allows for custom modification of new bean instances,
* e.g. checking for marker interfaces or wrapping them with proxies.
*
* <p>ApplicationContexts can autodetect BeanPostProcessor beans in their
* bean definitions and apply them to any beans subsequently created.
* Plain bean factories allow for programmatic registration of post-processors,
* applying to all beans created through this factory.
*
* <p>Typically, post-processors that populate beans via marker interfaces
* or the like will implement {@link #postProcessBeforeInitialization},
* while post-processors that wrap beans with proxies will normally
* implement {@link #postProcessAfterInitialization}.
*/
public interface BeanPostProcessor {
/**
* Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
* The returned bean instance may be a wrapper around the original.
* <p>The default implementation returns the given {@code bean} as-is.
* @param bean the new bean instance
* @param beanName the name of the bean
* @return the bean instance to use, either the original or a wrapped one;
*/
@Nullable
default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
/**
* Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
* The returned bean instance may be a wrapper around the original.
* <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean
* instance and the objects created by the FactoryBean (as of Spring 2.0). The
* post-processor can decide whether to apply to either the FactoryBean or created
* objects or both through corresponding {@code bean instanceof FactoryBean} checks.
* <p>This callback will also be invoked after a short-circuiting triggered by a
* in contrast to all other BeanPostProcessor callbacks.
* <p>The default implementation returns the given {@code bean} as-is.
* @param bean the new bean instance
* @param beanName the name of the bean
* @return the bean instance to use, either the original or a wrapped one;
*/
@Nullable
default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
}
可以看接口中的这俩方法名：
-
postProcessBeforeInitialization: 后置处理器 of 初始化前
所有bean在"初始化前"都会执行，实现了接口BeanPostProcessor 的后置处理器，所实现的 postProcessBeforeInitialization 方法。
该方法返回的可能是该bean的实例，也可能是被代理包装后的该bean -
postProcessAfterInitialization: 后置处理器 of 初始化后
所有bean在 "初始化后" 都会执行，实现了接口BeanPostProcessor 的后置处理器，所实现的 postProcessBeforeInitialization 方法。
该方法返回的可能是该bean的实例，也可能是被代理包装后的该bean
我这里的翻译较为缩略，英文尚可的伙计可以自取源码中的注释。
其它 "后置处理器"
其实大同小异，其它的就不再展开介绍。