转自： https://www.jianshu.com/p/d26e8ec9c077?

BeanPostProcessor也称为Bean后置处理器，它是Spring中定义的接口，在Spring容器的创建过程中（具体为Bean初始化前后）会回调BeanPostProcessor中定义的两个方法。BeanPostProcessor的源码如下

public interface BeanPostProcessor {


    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException; Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException; }

其中postProcessBeforeInitialization方法会在每一个bean对象的初始化方法调用之前回调；postProcessAfterInitialization方法会在每个bean对象的初始化方法调用之后被回调。具体执行时期可以参考Spring中Bean的生命周期源码探究。

自定义BeanPostProcessor

查看BeanPostProcessor源码，可以看到它两个方法的参数都相同，其中第一个参数Object bean表示当前正在初始化的bean对象。此外两个方法都返回Object类型的实例，返回值既可以是将入参Object bean原封不动的返回出去，也可以对当前bean进行包装再返回。来看看下面的自定义BeanPostProcessor

/**
 * 后置处理器：初始化前后进行处理工作
 * 需要将后置处理器加入到容器中
 */
@Component
public class MyBeanPostProcessor implements BeanPostProcessor { @Override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { System.out.println("postProcessBeforeInitialization..."+beanName+"..."+bean); return bean; } @Override public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { System.out.println("postProcessAfterInitialization..."+beanName+"..."+bean); return bean; } }

Spring容器中加入MyBeanPostProcessor之后，针对容器中每个创建的Bean对象（Spring自身创建的Bean和应用程序创建的Bean），都会回调postProcessBeforeInitialization和postProcessAfterInitialization方法。

执行原理

BeanPostProcessor的执行是定义在容器的刷新过程中，容器刷新对象具体的方法为：AbstractApplicationContext.refresh()。
在refresh方法执行的调用栈中会去调用AbstractAutowireCapableBeanFactory.doCreateBean()方法，该方法节选源码如下

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) { if (mbd == null || !mbd.isSynthetic()) { wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName); } invokeInitMethods(beanName, wrappedBean, mbd); if (mbd == null || !mbd.isSynthetic()) { wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName); } return wrappedBean; }

看到在调用初始化方法前后会分别调用applyBeanPostProcessorsBeforeInitialization()和applyBeanPostProcessorsAfterInitialization()。applyBeanPostProcessorsBeforeInitialization()方法的源码如下

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException { Object result = existingBean; //获取所有的BeanPostProcessor进行遍历 for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) { result = beanProcessor.postProcessBeforeInitialization(result, beanName); if (result == null) { return result; } } return result; }

可以看到其逻辑为遍历得到容器中所有的BeanPostProcessor，然后一次执行postProcessBeforeInitialization，一但返回null，就跳出for循环不执行后面的BeanPostProcessor.postProcessorsBeforeInitialization()。也就是说如果返回的是null那么我们通过getBean方法将得不到目标Bean。
applyBeanPostProcessorsBeforeInitialization()方法的逻辑和上面一致，就是将循环执行的beanProcessor.postProcessBeforeInitialization()替换成beanProcessor.postProcessAfterInitialization()
Spring底层的很多功能特性都是借助BeanPostProcessor的子类来实现。

常见BeanPostProcessor分析

下图是debug过程中，ApplicationContext对象中的包含的BeanPostProcessor。具体包含哪些BeanPostProcessor和具体应用程序相关，除了下标3中的MyBeanPostProcessor为自定义的BeanPostProcessor，其余均为Spring自带的BeanPostProcessor。

下面来分析一下ApplicationContextAwareProcessor和AutowiredAnnotationProcessor的执行原理。

ApplicationContextAwareProcessor

ApplicationContextAwareProcessor后置处理器的作用是，当应用程序定义的Bean实现ApplicationContextAware接口时注入ApplicationContext对象。

@Component
public class Car implements ApplicationContextAware { private ApplicationContext applicationContext; public Car(){ System.out.println("car instance..."); } @Override public void setApplicationContext(ApplicationContext applicationContext) throws BeansException { System.out.println("ApplicationContextAware...setApplicationContext()"); this.applicationContext = applicationContext; } }

那Car是如何通过实现ApplicationContextAware接口就能获得ApplicationContext对象呢？答案是通过ApplicationContextAwareProcessor后置处理器来实现，我们来看看ApplicationContextAwareProcessor的源码

class ApplicationContextAwareProcessor implements BeanPostProcessor { private final ConfigurableApplicationContext applicationContext; private final StringValueResolver embeddedValueResolver; /** * Create a new ApplicationContextAwareProcessor for the given context. */ public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) { this.applicationContext = applicationContext; this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory()); } @Override public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException { AccessControlContext acc = null; // 这里bean是Car,它实现了ApplicationContextAware接口 if (System.getSecurityManager() != null && (bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware || bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware || bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) { invokeAwareInterfaces(bean); } return bean; } private void invokeAwareInterfaces(Object bean) { if (bean instanceof Aware) { if (bean instanceof EnvironmentAware) { ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment()); } if (bean instanceof EmbeddedValueResolverAware) { ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver); } if (bean instanceof ResourceLoaderAware) { ((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext); } if (bean instanceof ApplicationEventPublisherAware) { ((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext); } if (bean instanceof MessageSourceAware) { ((MessageSourceAware) bean).setMessageSource(this.applicationContext); } if (bean instanceof ApplicationContextAware) { // 会执行这里回调car重写的setApplicationContext方法，然后将this.applicationContext注入给Car ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext); } } } @Override public Object postProcessAfterInitialization(Object bean, String beanName) { return bean; } }

InitDestroyAnnotationBeanPostProcessor

InitDestroyAnnotationBeanPostProcessor后置处理器是用来处理自定义的初始化方法和销毁方法。Spring中提供了3种自定义初始化和销毁方法：

通过@Bean指定init-method和destroy-method属性
Bean实现InitializingBean（定义初始化逻辑），DisposableBean（定义销毁逻辑）;
@PostConstruct：在bean创建完成并且属性赋值完成；来执行初始化方法@PreDestroy：在容器销毁bean之前通知我们进行清理工作

InitDestroyAnnotationBeanPostProcessor的作用就是让第3种方式生效。先看看如何使用@PostConstruct和@PreDestroy注解。

@Component
public class Car { public Car(){ System.out.println("car instance..."); } /** * 自定义的初始化方法 */ @PostConstruct public void init(){ System.out.println("car ... init..."); } /** * 自定义的销毁方法 */ @PreDestroy public void detory(){ System.out.println("car ... detory..."); } }

InitDestroyAnnotationBeanPostProcessor会在Bean创建的时候通过反射的方式查找包含@PostConstruct和@PreDestroy注解的方法，然后再通过反射执行方法。我们来看看InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization()的源码

public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { // 获取bean的metadata LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass()); try { // 执行@PostConstruct指定的init方法 metadata.invokeInitMethods(bean, beanName); } catch (InvocationTargetException ex) { throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException()); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Failed to invoke init method", ex); } return bean; }

metadata中已经解析出initMethods和destroyMethods，其具体内容如下。

metadata.invokeInitMethods(bean, beanName);就是根据反射执行init方法。

InstantiationAwareBeanPostProcessor

InstantiationAwareBeanPostProcessor是BeanPostProcessor的子接口。

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor { //实例化Bean之前调用 Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException; //实例化Bean之后调用 boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException; //设置Bean对象中的某个属性时调用 PropertyValues postProcessPropertyValues( PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException; }

这两个接口的功能基本类似，不过要注意InstantiationAwareBeanPostProcessor和BeanPostProcessor的方法名区别。InstantiationAwareBeanPostProcessor中是Instantiation，而BeanPostProcessor是Initialization。

BeanPostProcessor详解