BeanFactoryPostProcessor与BeanPostProcessor

从名称上不难看出一个是对BeanFactory的后置处理另外一个是对Bean的后置处理。在通过ApplicationContext的BeanFactoryPostProcessor支持两种方式的配置，即通过硬编码方式和xml配置文件配置;BeanPostProcessor一般使用xml配置方式注册

BeanFactoryPostProcessor是在ApplicationContext引入的用于处理BeanFactory，定义如下

/**
 * 允许自定义修改应用程序上下文的bean定义，调整上下文的基础bean工厂的bean属性值。
 *
 * <p>应用程序上下文可以在其bean定义中自动检测BeanFactoryPostProcessor bean
 * ，并在创建任何其他bean之前应用它们。
 *
 * BeanFactoryPostProcessor可以与bean定义交互并修改bean定义，但绝不能与bean实例交互。
 * 这样做可能会导致bean过早实例化，违反容器并导致意外的副作用。 如果需要bean实例交互，
 * 请考虑实现{@link BeanPostProcessor}
 *
 * @author Juergen Hoeller
 * @since 06.07.2003
 * @see BeanPostProcessor
 * @see PropertyResourceConfigurer
 */
public interface BeanFactoryPostProcessor {

	/**
	 * 在标准初始化之后修改应用程序上下文的内部bean工厂。
	 * 将加载所有bean定义，但尚未实例化任何bean。 这允许覆盖或添加属性，甚至是初始化bean。
	 * @param beanFactory the bean factory used by the application context
	 * @throws org.springframework.beans.BeansException in case of errors
	 */
	void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

BeanPostProcessor是BeanFactory引入的用于处理bean

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.beans.factory.config;

import org.springframework.beans.BeansException;

/**
 * 工厂钩子，允许自定义修改新的bean实例，例如 检查标记
 *
 * <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}.
 *
 * @author Juergen Hoeller
 * @since 10.10.2003
 * @see InstantiationAwareBeanPostProcessor
 * @see DestructionAwareBeanPostProcessor
 * @see ConfigurableBeanFactory#addBeanPostProcessor
 * @see BeanFactoryPostProcessor
 */
public interface BeanPostProcessor {

	/**
	 * 在任何bean初始化回调之前（例如InitializingBean的{@code afterPropertiesSet}或自定义init方法），
	 * 将此BeanPostProcessor应用于给定的新bean实例。 bean已经填充了属性值。返回的bean实例可能是原始实例的包装器
	 * @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;
	 * if {@code null}, no subsequent BeanPostProcessors will be invoked
	 * @throws org.springframework.beans.BeansException in case of errors
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 */
	Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;

	/**
	 * 在</ i>任何bean初始化回调之后（如InitializingBean的{@code afterPropertiesSet}或自定义init方法），
	 * 将此BeanPostProcessor应用于给定的新bean实例<i>。 bean已经填充了属性值
	 * 返回的bean实例可能是原始实例的包装器。
	 * <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.
	 *
	 * 对于factorBean，这个回调方法同时会被应用于FactoryBean实例和FactroyBean创建的实例对象
	 * 后处理器可以通过相应的{@code bean instanceof FactoryBean}检查来决定是应用于FactoryBean还是应用于创建的对象
	 * 与所有其他BeanPostProcessor回调相反方法，在由{@ linkInstantiationAwareBeanPostProcessor＃postProcessBeforeInstantiation}
	 * 触发的短路之后也将调用此回调
	 * @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;
	 * if {@code null}, no subsequent BeanPostProcessors will be invoked
	 * @throws org.springframework.beans.BeansException in case of errors
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 * @see org.springframework.beans.factory.FactoryBean
	 */
	Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

BeanFactorPostProcessor用于处理beanFactory，我们可以通过它改变beanFactory的相关属性，并让其作用于beanFactory中的bean，有如下使用场景

CustomerEditorConfigure:帮助我们注册PropertyEditorSupport，用于最后给bean实例赋值时使用

PropertyPlaceHolderConfigurer:使用PropertyPlaceholderConfigurer可以在XML配置文件中加入外部属性文件，当然也可以指定外部文件的编码。PropertyPlaceholderConfigurer可以将上下文（配置文 件）中的属性值放在另一个单独的标准java Properties文件中去。在XML文件中用${key}替换指定的properties文件中的值。这样的话，只需要对properties文件进 行修改，而不用对xml配置文件进行修改

BeanPostProcessor作用于bean,Spring中对它的应用点如： ApplicationContextAwareProcessor,用于帮我们实现Aware接口的功能

当然还有其他的一些比如ApplicationListenerDetector:用于检测ApplicationContext的ApplicationListener，监听ApplicationContext发布的事件；我们还可以通过它修改bean的属性等等。