前言
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。
参考阅读
https://blog.csdn.net/chjttony/article/details/6301591
Spring开闭原则的表现-BeanPostProcessor扩展点-2
