spring源码分析-BeanPostProcessor

spring-系列前言接口描述

BeanPostProcessorInstantiationAwareBeanPostProcessorSmartInstantiationAwareBeanPostProcessorDestructionAwareBeanPostProcessorMergedBeanDefinitionPostProcessor 总结

BeanPostProcessor接口是spring重要的接口之一，Bean的增强功能大部分都是基于这个接口来实现。如通过 @Autowired、@Value、@Resource、@PostConstruct 和 @PreDestroy等注解都是基于BeanPostProcessor的子接口InstantiationAwareBeanPostProcessor接口实现逻辑处理。本文跟着博主一起来揭开BeanPostProcessor的神秘面纱。

BeanPostProcessor是允许自定义修改新建bean实例的工厂钩子，spring为了增强该接口的功能定义了4个子接口InstantiationAwareBeanPostProcessor，SmartInstantiationAwareBeanPostProcessor，DestructionAwareBeanPostProcessor和MergedBeanDefinitionPostProcessor下面讨论他们。

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;
	}

}

根据源码会发现postProcessBeforeInitialization(Object bean, String beanName) 是在初始化（InitializingBean的afterPropertiesSet或自定义init方法）之前执行，可以修改或包装Bean的实例。 postProcessAfterInitialization(Object bean, String beanName) 方法则相反在初始化之后执行，可以修改或包装Bean的实例。

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {

	
	@Nullable
	default Object postProcessBeforeInstantiation(Class beanClass, String beanName) throws BeansException {
		return null;
	}

	
	default boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
		return true;
	}

	
	@Nullable
	default PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName)
			throws BeansException {

		return null;
	}

	
	@Deprecated
	@Nullable
	default PropertyValues postProcessPropertyValues(
			PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {

		return pvs;
	}

}

根据源码很显然InstantiationAwareBeanPostProcessor是为Bean的实例化和属性赋值而生的，当存在多个InstantiationAwareBeanPostProcessor接口时只要有一个实例化成功，将直接返回。postProcessProperties(PropertyValues pvs, Object bean, String beanName)是属性值注入的入口方法， @Resource注入方式使用了CommonAnnotationBeanPostProcessor实现，@Autowired和 @Value由AutowiredAnnotationBeanPostProcessor类实现。

public interface SmartInstantiationAwareBeanPostProcessor extends InstantiationAwareBeanPostProcessor {

	
	@Nullable
	default Class predictBeanType(Class beanClass, String beanName) throws BeansException {
		return null;
	}

	
	@Nullable
	default Constructor[] determineCandidateConstructors(Class beanClass, String beanName)
			throws BeansException {

		return null;
	}

	
	default Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
		return bean;
	}

}

很显然SmartInstantiationAwareBeanPostProcessor是InstantiationAwareBeanPostProcessor的扩展，添加了一下默认预测方法，目标都是为了Bean的实例化和注入，例如AutowiredAnnotationBeanPostProcessor就是继承该接口。

public interface DestructionAwareBeanPostProcessor extends BeanPostProcessor {

	
	void postProcessBeforeDestruction(Object bean, String beanName) throws BeansException;

	
	default boolean requiresDestruction(Object bean) {
		return true;
	}

}

用于添加Bean销毁前回调。

public interface MergedBeanDefinitionPostProcessor extends BeanPostProcessor {

	
	void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class beanType, String beanName);

	
	default void resetBeanDefinition(String beanName) {
	}

}

MergedBeanDefinitionPostProcessor是spring内部实现合并bean定义的后处理器回调接口。一般情况下不用研究。

BeanPostProcessor是spring对Bean功能增强的实现，其中InstantiationAwareBeanPostProcessor，SmartInstantiationAwareBeanPostProcessor，DestructionAwareBeanPostProcessor和MergedBeanDefinitionPostProcessor就是spring在内部增强Bean的体现，一般情况下不推荐直接使用这些内部接口。当我们自己开发一个spring插件或者是个性化处理功能时可以考虑使用BeanPostProcessor接口来实现。