By krisjin

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       Spring Bean 实例化阶段后处理器主要是BeanPostProcessor接口，它与BeanFactoryPostProcessor容易产生混淆。只要记住BeanPostProcessor是存在于对象实例化阶段，而BeanFactoryPostProcessor则存在于容器启动阶段。这两个概念就比较容易区分了。

       与BeanFacotryPostProcessor通常会处理容器所有符合条件的BeanDefinition类似，BeanPostProcessor会处理容器内所有符合条件的实例化后的对象实例。该接口声明了两个方法，分别在两个不同的时机执行，见下代码定义：

public interface BeanPostProcessor {

/**
 * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
 * or a custom init-method). The bean will already be populated with property values.
 * The returned bean instance may be a wrapper around the original.
 * @param bean the new bean instance
 * @param beanName the name of the bean
 * @return the bean instance to use, either the original or a wrapped one; if
 * {@code null}, no subsequent BeanPostProcessors will be invoked
 * @throws org.springframework.beans.BeansException in case of errors
 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
 */
Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;

/**
 * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
 * or a custom init-method). The bean will already be populated with property values.
 * The returned bean instance may be a wrapper around the original.
 * <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean
 * instance and the objects created by the FactoryBean (as of Spring 2.0). The
 * post-processor can decide whether to apply to either the FactoryBean or created
 * objects or both through corresponding {@code bean instanceof FactoryBean} checks.
 * <p>This callback will also be invoked after a short-circuiting triggered by a
 * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
 * in contrast to all other BeanPostProcessor callbacks.
 * @param bean the new bean instance
 * @param beanName the name of the bean
 * @return the bean instance to use, either the original or a wrapped one; if
 * {@code null}, no subsequent BeanPostProcessors will be invoked
 * @throws org.springframework.beans.BeansException in case of errors
 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
 * @see org.springframework.beans.factory.FactoryBean
 */
Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

postProcessBeforeInitialization方法就如下图中BeanPostProcess前置处理这一步将会执行的方法，postProcessAfterInitialization方法对应下图中BeanPostProcessor后置处理那一步执行的方法。BeanPostProcessor的两个方法中都传入了原理对象实例的引用，这为我们扩展容器的对象实例化过程中的行为提供了极为的便利。我们几乎可以对传入的对象实例执行任何的操作。

       通常比较常见的使用BeanPostProcessor的场景，是处理标记接口实现类，或者为当前对象提供代理实现。ApplicationContext对应的那些Aware接口实际上是通过BeanPostProcessor的方式进行处理的。当ApplicationContext中每个对象实例化过程走到BeanPostProcessor前置处理这一步是，ApplicationContext容器会检测到之前注册到容器的ApplicationContextAwareProcessor这个BeanPostProcessor实现类，然后调用其postProcessBeforeInitialization方法，检查并设置Aware相关依赖。

Bean的实例化过程

BeanPostProcessor类图

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