Spring之BeanPostProcessor接口

BeanPostProcessor接口是Spring IOC容器給我們提供的一個擴展接口，接口定義如下：

/*
  
package org.springframework.beans.factory.config;

import org.springframework.beans.BeansException;
import org.springframework.lang.Nullable;

/**
 * Factory hook that allows for custom modification of new bean instances &mdash;
 * for example, checking for marker interfaces or wrapping beans with proxies.
 *
 * <p>Typically, post-processors that populate beans via marker interfaces
 * or the like will implement {@link #postProcessBeforeInitialization},
 * while post-processors that wrap beans with proxies will normally
 * implement {@link #postProcessAfterInitialization}.
 *
 * <h3>Registration</h3>
 * <p>An {@code ApplicationContext} can autodetect {@code BeanPostProcessor} beans
 * in its bean definitions and apply those post-processors to any beans subsequently
 * created. A plain {@code BeanFactory} allows for programmatic registration of
 * post-processors, applying them to all beans created through the bean factory.
 *
 * <h3>Ordering</h3>
 * <p>{@code BeanPostProcessor} beans that are autodetected in an
 * {@code ApplicationContext} will be ordered according to
 * {@link org.springframework.core.PriorityOrdered} and
 * {@link org.springframework.core.Ordered} semantics. In contrast,
 * {@code BeanPostProcessor} beans that are registered programmatically with a
 * {@code BeanFactory} will be applied in the order of registration; any ordering
 * semantics expressed through implementing the
 * {@code PriorityOrdered} or {@code Ordered} interface will be ignored for
 * programmatically registered post-processors. Furthermore, the
 * {@link org.springframework.core.annotation.Order @Order} annotation is not
 * taken into account for {@code BeanPostProcessor} beans.
 *
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @since 10.10.2003
 * @see InstantiationAwareBeanPostProcessor
 * @see DestructionAwareBeanPostProcessor
 * @see ConfigurableBeanFactory#addBeanPostProcessor
 * @see BeanFactoryPostProcessor
 */
public interface BeanPostProcessor {

    /**
     * Apply this {@code 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.
     * <p>The default implementation returns the given {@code bean} as-is.
     * @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
     */
    @Nullable
    default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

    /**
     * Apply this {@code 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 {@code BeanPostProcessor} callbacks.
     * <p>The default implementation returns the given {@code bean} as-is.
     * @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
     */
    @Nullable
    default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

}

在Spring啟動過程中梭纹，當BeanPostProcessor的接口實現(xiàn)類（如AbstractAutoProxyCreator、DefaultAdvisorAutoProxyCreator等）注冊到Spring IOC容器后進行如下操作：
a.對于IOC容器所創(chuàng)建的每個bean實例在初始化方法（如afterPropertiesSet和init方法）調(diào)用前臭增，將會調(diào)用BeanPostProcessor中的

Object postProcessBeforeInitialization(Object bean, String beanName) ;

方法紧显；
b.在bean實例初始化方法（如afterPropertiesSet和init方法）調(diào)用完成后，則會調(diào)用BeanPostProcessor中的

Object postProcessAfterInitialization(Object bean, String beanName);

方法肥印。

具體過程如下：

1.Spring IOC容器實例化Bean;
2.調(diào)用BeanPostProcessor接口實現(xiàn)類中的postProcessBeforeInitialization方法;
3.調(diào)用bean實例的初始化方法（afterPropertiesSet()和init()方法）;
4.調(diào)用BeanPostProcessor的postProcessAfterInitialization方法。

在實際項目開發(fā)中绝葡，Spring IOC容器中會配置多個BeanPostProcessor接口實現(xiàn)類深碱，在默認情況下Spring IOC容器會根據(jù)BeanPostProcessor接口實現(xiàn)類的定義順序來依次調(diào)用。