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
BeanPostProcessor -
BeanFactoryPostProcessor
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;
}
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;
}
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;
}
}
使用postProcessor
MyBeanFactoryPostProcessor
- 基础对象
@Data
public class Root {
private String name;
private String description;
private boolean isRoot;
}
- 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。
* 所有常规bean定义都将被加载，但尚未实例化任何bean。
* 这允许在下一个后处理阶段开始之前添加更多的BeanDeinition。
* Spring工厂级别的BeanFactoryPostProcessor,优先级别高于常规的BeanFactoryPostProcessor.
* 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.
* 设计模式:责任链模式
* @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;
}
}
- 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);
}
}
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;
}
}
- result
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