BeanPostProcessor

February 25, 2020 · View on GitHub

与 BeanFactoryPostProcessor 类似

AutowiredAnnotationBeanPostProcessor 是 InstantiationAwareBeanPostProcessor 的实现类之一，也是 @Autowired 依赖注入的具体实现

InstantiationAwareBeanPostProcessor 实现了 BeanPostProcessor 接口,这里二个类放在一起进行学习。

Where load BeanPostProcessor

// AbstractApplicationContext
// 在执行 refresh 方法的时候注册 BeanPostProcessors
@Override
public void refresh() throws BeansException, IllegalStateException {
// ...
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// ...
}

The hook method in BeanPostProcessor

// BeanPostProcessor 中定义的方法
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;
}
}

// InstantiationAwareBeanPostProcessor
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 postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {

return pvs;
}

}

The load method PostProcessorRegistrationDelegate

初始 BeanPostProcessor 的工具类 (delegate -> 代表)

/**
 * Instantiate and register all BeanPostProcessor beans,
 * respecting explicit order if given.
 * <p>Must be called before any instantiation of application beans.
 */
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}

PostProcessorRegistrationDelegate.registerBeanPostProcessors

public static void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
   //
}

Demo for BeanPostProcessor

InstantiationAwareBeanPostProcessor

在 Bean 被创建的时候，Spring 会扫描所有 InstantiationAwareBeanPostProcessor 的实现类，进行依赖注入

CommonAnnotationBeanPostProcessor 和 AutowiredAnnotationBeanPostProcessor 注入实现类

下面会简单说明

CommonAnnotationBeanPostProcessor

CommonAnnotationBeanPostProcessor 实现了 JAX-WS 规范中的注解的依赖注入，比如javax.annotation.PostConstruct,javax.annotation.PreDestroy,javax.annotation.Resource

AutowiredAnnotationBeanPostProcessor

AutowiredAnnotationBeanPostProcessor 实现了 InstantiationAwareBeanPostProcessorAdapter(本质是 BeanPostProcessor 的加强版本实现) 在 Bean 初始化的时候(getBean) 的时候进行依赖注入，比如我们常用的 Autowired 和 Value 注解

// AutowiredAnnotationBeanPostProcessor
// 主动注入的 注解 Autowired 和 Value
public AutowiredAnnotationBeanPostProcessor() {
  this.autowiredAnnotationTypes.add(Autowired.class);
  this.autowiredAnnotationTypes.add(Value.class);
  try {
    this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
    ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
    logger.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
  }
  catch (ClassNotFoundException ex) {
  // JSR-330 API not available - simply skip.
  }
}

最终会生产 AutowiredMethodElement 和 AutowiredFieldElement 方法注入和字段注入,调用 inject 方法进入依赖的注入

方法调用链:

AbstractAutowireCapableBeanFactory.postProcessPropertyValues
  -> AutowiredAnnotationBeanPostProcessor
   -> postProcessPropertyValues
    -> findAutowiringMetadata
     -> buildAutowiringMetadata
      -> AutowiredMethodElement/AutowiredFieldElement
       -> inject

AbstractAutoProxyCreator

Spring 中的代理也是通过 BeanPostProcessor 的实现类 AbstractAutoProxyCreator 来实现的。这个后续再看