作用

BeanPostProcessor，顾名思义是bean后置处理器，是 Spring 的扩展点之一。通过自定义 BeanPostProcessor 可以实现对 Bean 的修改，用于在bean实例化后，要初始化的前后对bean进行处理

public interface BeanPostProcessor {/*** 在初始化前调用*/@Nullabledefault Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {return bean;}/*** 在初始化后调用*/@Nullabledefault Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {return bean;}}

执行时机

在bean被实例化出来，并且注入属性后，会调用initializeBean方法去初始化bean，在initializeBean方法中会分别调用BeanPostProcessor接口中的两个方法

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {invokeAwareMethods(beanName, bean);return null;}, getAccessControlContext());}else {// 调用Aware接口方法invokeAwareMethods(beanName, bean);}Object wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) {// 调用bean后置处理器的postProcessBeforeInitialization方法wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}try {// 初始化bean，调用init方法invokeInitMethods(beanName, wrappedBean, mbd);}catch (Throwable ex) {throw new BeanCreationException((mbd != null ? mbd.getResourceDescription() : null),beanName, "Invocation of init method failed", ex);}if (mbd == null || !mbd.isSynthetic()) {// 调用bean后置处理器的postProcessAfterInitialization方法wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;
}

应用

Spring 中有一个类 ApplicationContextAwareProcessor，该类实现了 BeanPostProcessor 接口，是用来判断bean是否实现了各个Aware接口。

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@Nullablepublic Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {AccessControlContext acc = null;if (System.getSecurityManager() != null &&(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {acc = this.applicationContext.getBeanFactory().getAccessControlContext();}if (acc != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {invokeAwareInterfaces(bean);return null;}, acc);}else {invokeAwareInterfaces(bean);}return bean;}private void invokeAwareInterfaces(Object bean) {// 判断是否实现了各个 Aware 接口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) {((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);}}}@Overridepublic Object postProcessAfterInitialization(Object bean, String beanName) {return bean;}}

比如实现EnvironmentAware接口的bean在该Processor内会调用EnvironmentAware接口的setEnvironment方法，并把spring容器内部的ConfigurableEnvironment传递进去。一般最常用的就是bean实现ApplicationContextAware接口，这样ApplicationContextAwareProcessor就会将ApplicationContext设置到该bean里。

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Spring扩展之BeanPostProcessor

作用

执行时机

应用

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