PostProcessor

为什么要有这个后置处理器这种接口，了解Spring容器的朋友应该知道，所有的ApplicationContext都需要调用AbstractApplicationContext#refresh方法，这是一个模板方法，既然使用了模板方法设计模式，那么就应该提供hook，也就是钩子函数。而PostProcessor就是为了实现这种定制化的扩展需求而制定的。

UML

• BeanPostProcessor

• BeanFactoryPostProcessor

PostProcessor概览

BeanFactoryPostProcessor

容器级别的后置处理器，其内部仅声明了一个方法。Spring会根据声明的顺序对后置处理器进行调用，而BeanFactoryPostProcessor会在容器初始化期间将容器本身交由接口实现类去处理。获取到了容器，就可以做很多定制化的操作了。

@FunctionalInterface
public interface BeanFactoryPostProcessor {

	/**
	 * 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 overriding or adding
	 * properties even to eager-initializing beans.
	 * @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;

}
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BeanDefinitionRegistryPostProcessor

从注解容器解析BeanDefinition的篇章中，你会了解到BeanDefinitionRegistryPostProcessor是一种特殊的BeanFactoryPostProcessor,但是Spring会优先处理这类后置处理器，再处理常规的一些BeanFactoryPostProcessor.可以理解成VIP级别的BeanFactoryPostProcessor，优先于常规BeanFactoryPostProcessor执行。
其内部也声明了一个专用的注册后置处理方法。postProcessBeanDefinitionRegistry

public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {

	/**
	 * Modify the application context's internal bean definition registry after its
	 * standard initialization. All regular bean definitions will have been loaded,
	 * but no beans will have been instantiated yet. This allows for adding further
	 * bean definitions before the next post-processing phase kicks in.
	 * @param registry the bean definition registry used by the application context
	 * @throws org.springframework.beans.BeansException in case of errors
	 */
	void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;

}
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BeanPostProcessor

容器创建Bean时的hook实例，你可以通过实现BeanPostProcessor来影响所有Bean的生命周期，包括Initialization的前置与后置处理.在后面学习Spring Bean的生命周期中，会接触BeanPostProcessor

public interface BeanPostProcessor {

	@Nullable
	default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}

	@Nullable
	default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}

}
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使用postProcessor

MyBeanFactoryPostProcessor

• 基础对象

@Data
public class Root {

	private String name;

	private String description;

	private boolean isRoot;
}
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• 通过后置处理器往容器注入BeanDefinition

package com.xjm.bean.postprocessor;

import com.xjm.model.Root;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.support.*;
import org.springframework.context.annotation.Configuration;

/**
 * @author jaymin
 * 2021/1/7 23:10
 */
@Configuration
public class MyBeanFactoryPostProcessor implements BeanDefinitionRegistryPostProcessor {

	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

	}

	/**
	 * 标准初始化后，修改应用程序上下文的内部BeanDefiniton注册表。<br>
	 * 所有常规bean定义都将被加载，但尚未实例化任何bean。 <br>
	 * 这允许在下一个后处理阶段开始之前添加更多的BeanDeinition。 <br>
	 * Spring工厂级别的BeanFactoryPostProcessor,优先级别高于常规的BeanFactoryPostProcessor.<br>
	 * 第三方框架整合时，可以实现这个注册接口进行BeanDefinition注册.如Mybatis.<br>
	 * 设计模式:责任链模式<br>
	 * @param registry the bean definition registry used by the application context
	 * @throws BeansException
	 */
	@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
		BeanDefinition beanDefinition = getBeanDefinition(Root.class);
		registry.registerBeanDefinition("root",beanDefinition);
		System.out.println("Customer BeanPostProcessors execute.");
	}

	private BeanDefinition getBeanDefinition(Class<?> clazz){
		AbstractBeanDefinition beanDefinition = BeanDefinitionBuilder.genericBeanDefinition(clazz).getBeanDefinition();
		return beanDefinition;
	}
}
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• test

@ComponentScan根据自己的包路径进行修改,此处程序正常运行，说明Root已经注册到容器中了.

@Configuration
@ComponentScan(value = "com.xjm")
public class BeanDefinitionDemoByAnnotation {
	public static void main(String[] args) {
		ApplicationContext applicationContext = new AnnotationConfigApplicationContext(AnnotationContextDemo.class);
		Root root = applicationContext.getBean("root", Root.class);
	}
}
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MyBeanPostProcessor

简单实现BeanPostProcessor,主要是观察其行为的作用域.

@Configuration
public class MyBeanPostProcessor implements BeanPostProcessor {

	@Override
	@Nullable
	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
		System.out.println(beanName + " invoking MyBeanPostProcessor#postProcessBeforeInitialization");
		return bean;
	}

	/**
	 * 可以对所有的Bean做统一操作
	 * @param bean the new bean instance
	 * @param beanName the name of the bean
	 * @return
	 * @throws BeansException
	 */
	@Override
	@Nullable
	public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
		System.out.println(beanName + " invoking MyBeanPostProcessor#postProcessAfterInitialization");
		return bean;
	}

}
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• result

可以看到，实现的MyBeanPostProcessor影响了所有Bean的初始化过程.

总结

• 后置处理器是Spring留给开发者的扩展性钩子接口。
• 从后置处理器上可以简略分为:BeanFactoryPostProcessor和BeanPostProcessor.其中,BeanFactoryPostProcessor可以操作容器，BeanPostProcessor会影响所有Bean的生命周期.
• BeanDefinitionRegistryPostProcessor是一种特殊的BeanFactoryPostProcessor，优于其他BeanFactoryPostProcessor执行.

扩展阅读

Spring系列六：Spring BeanPostProcessor
谈谈Spring中的BeanPostProcessor接口
Spring 执行顺序：PostProcessor 接口
Spring BeanPostProcessor