這個介面有兩個方法:

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;
	}

}

類實現這個介面，spring每一個bean被建立時都會走這個介面的實現類，從而我們就可以加一下自己的邏輯，對spring中的bean做一些修改。

最好先了解一下spring的Bean的生命週期，我這簡化的說，在spring bean的生命週期中，就和servlet一樣，順序：建構函式例項化->注入屬性->setBeanName->setBeanFactory->postProcessAfterInitialization->afterPropertiesSet->init-method->postProcessBeforeInitialization，大體是這麼個流程。  所以我們可以在生命週期過程中，對bean做修改，完成自己的業務邏輯。

看下一個簡單的例子：

package com.xhx.spring.config;

import com.xhx.spring.domain.User;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.Ordered;

import java.net.UnknownServiceException;

/**
 * xuhaixing
 * 2018/9/28 22:34
 * 後置處理器
 **/
@Configuration
public class MyBeanPostProcessor implements BeanPostProcessor, Ordered {

    @Override
    public int getOrder() {
        return 0;
    }

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

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if(bean instanceof User){
            User user = (User)bean;
            user.setAge(18);
        }
        return bean;
    }
}

在注入User類時，我把年齡給改了。

User bean的初始化：

package com.xhx.spring.config;

import com.xhx.spring.domain.User;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

/**
 * xuhaixing
 * 2018/9/28 22:31
 **/
@Configuration
public class UserConfig {

    @Bean
    public User user(){
        return new User("fdsf-df-32-fa-g","xu",25);
    }
}

我們寫了一個介面，看一下User bean的值：

package com.xhx.spring.controller;

import com.xhx.spring.domain.User;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

/**
 * xuhaixing
 * 2018/9/28 22:32
 **/
@RestController
@RequestMapping(name = "user")
public class UserController {
    @Autowired
    private User user;

    @RequestMapping(name = "getUser")
    public User getUser(){
        return user;
    }
}