Spring源码组件学习-BeanPostProcessor
定义
BeanPostProcessor称为Bean后置处理器，他是Spring中定义的接口，在Spring容器的创建过程中（具体为Bean初始化前端）会回调BeanPostProcessor中定义的两个方法。
public interface BeanPostProcessor {
Object interface postProcessBeforeInitialization(Object bean,String beanName) throws BeanException;
Object interface postProcessAfterInitialization(Object bean,String beanName) throws BeanException;
}
其中postProcessBeforeInitiallization方法会在每一个bean对象的初始化方法调用之前回调；
其中postProcessAfterInitiallization方法会在每一个bean对象的初始化方法调用之后回调；
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。
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.deCreateBean()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 = applyBeanPostProcessorsAfterInitialzation(wrappedBean, beanName);
}
return wrappedBean;
}
看到在调用初始化方法前后会分别调用applyBeanPostProcessorsBeforeInitializatioan()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 : getBeanPostProcessor()) {
result = beanProcessor.postProcessBeanBeforeInitiallization(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,一旦返回null,就跳出for循环不执行后面的BeanPostProcessor.postProcessBeforeInitialization(),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。applyBeanPostProceorsAfterInitialzation()方法的逻辑与上面一样。
Spring中常见的几个Bean后置处理器
Spring底层的很多功能特性都是借助BeanPostProcessor的子类来实现。
AppliacationContextAwareProcessor
AppliacationContextAwareProcessor作用是当应用程序定义的Bean实现ApplicationContextAware接口时注入ApplicationContext对象。
class ApplicationContextAwareProcessor implements BeanPostProcessor {
private final ConfigurableApplicationCotext applicationContext;
private final StringValueResolver embeddedVakueResolver;
// 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) {
// 会执行这里回调实现类重写的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。
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。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;
}