　Spring中的BeanPostProcessor详解

概述

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。

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。

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：1.通过@Bean指定init-method和destroy-method属性;2.Bean实现InitializingBean（定义初始化逻辑），DisposableBean（定义销毁逻辑）; 3.@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

使用

我们定义一个类实现了BeanPostProcessor，默认是会对整个Spring容器中所有的bean进行处理。

public class ConfigValueBeanProcessor implements BeanPostProcessor ,Ordered {
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        ReflectionUtils.doWithLocalFields(bean.getClass(), (field -> {
            System.out.println(field.toString());
            Value valueAnnotation = field.getAnnotation(Value.class);
            if (valueAnnotation != null) {
                if (Modifier.isStatic(field.getModifiers())) {
                    return;
                }

            }
        }));
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

}

//配置类中定义bean
  @Bean
  public ConfigValueBeanProcessor getConfigValueBeanProcessor() {
      return new ConfigValueBeanProcessor();
  }

Spring中的BeanPostProcessor详解

Spring中的BeanPostProcessor详解

概述

执行原理

常见BeanPostProcessor分析

使用

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　Spring中的BeanPostProcessor详解