（008）spring之BeanPostProcessor的执行时机及作用

　　BeanPostProcessor是一个接口，有两个方法：postProcessBeforeInitialization、postProcessAfterInitialization，这两个方法每个bean创建都会执行一次。

　　postProcessBeforeInitialization在属性设置后，init方法执行前执行，postProcessAfterInitialization在属性设置后，init方法执行后执行。请看示例：

　　pom.xml

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.edu.spring</groupId>
    <artifactId>spring</artifactId>
    <version>1.0.0</version>

    <name>spring</name>
    <!-- FIXME change it to the project's website -->
    <url>http://www.example.com</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>
    
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>4.3.2.RELEASE</version>
        </dependency>
    </dependencies>

</project>

　　User.java

package com.edu.spring;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;

public class User {

private ApplicationContext applicationContext;
    
    public void init(){
        System.out.println("User init()");
    }
    public void show(){
        System.out.println("User show(): "+applicationContext);
    }
    
    @Autowired
    public void setApplicationContext(ApplicationContext applicationContext) {
        System.out.println("------set()-----");
        this.applicationContext = applicationContext;
    }
    
}

　　MyConfig.java

package com.edu.spring;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MyConfig {

    @Bean(initMethod="init")
    public User createUser(){
        return new User();
    }
}

　　EchoBeanPostProcssor.java

package com.edu.spring;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.stereotype.Component;

 
@Component
public class EchoBeanPostProcssor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessBeforeInitialization-------------"+bean.getClass());
        return bean;
    }


    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessAfterInitialization-------------"+bean.getClass());
        return bean;
    }

}

　　App.java

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class App {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext("com.edu.spring");
        context.getBean("createUser",User.class).show();
        context.close();
    }
}

　　运行结果如下：

 　　这两个方法中都不能返回null，否则bean不会创建成功，如下：

　　EchoBeanPostProcssor.java 中postProcessBeforeInitialization方法返回null

package com.edu.spring;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.stereotype.Component;

 
@Component
public class EchoBeanPostProcssor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessBeforeInitialization-------------"+bean.getClass());
        if(bean instanceof User){
            return null;
        }
        return bean;
    }


    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessAfterInitialization-------------"+bean.getClass());
        return bean;
    }

}

　　运行结果如下：

 　　EchoBeanPostProcssor.java 中postProcessAfterInitialization方法返回null

package com.edu.spring;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.stereotype.Component;

 
@Component
public class EchoBeanPostProcssor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessBeforeInitialization-------------"+bean.getClass());
        return bean;
    }


    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessAfterInitialization-------------"+bean.getClass());
        if(bean instanceof User){
            return null;
        }
        return bean;
    }

}

　　运行结果如下：

 　　BeanPostProcessor接口的作用，可以产生代理对象，实现回调，如添加日志等，如下：

　　LogUser.java

package com.edu.spring;

public class LogUser extends User {

    public void show(){
        System.out.println("LogUser start");
        super.show();
        System.out.println("LogUser end");
    }
}

　　EchoBeanPostProcssor.java

package com.edu.spring;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.stereotype.Component;

 
@Component
public class EchoBeanPostProcssor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessBeforeInitialization-------------"+bean.getClass());
        if(bean instanceof User){
            return new LogUser();
        }
        return bean;
    }


    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("-------------postProcessAfterInitialization-------------"+bean.getClass());
        return bean;
    }

}

　　运行结果如下：

 　　利用BeanPostProcessor模拟实现注入ApplicationContext对象

　　SpringContextAware.java

package com.edu.spring;

import org.springframework.context.ApplicationContext;

public interface SpringContextAware {
    public void setApplicationContext(ApplicationContext applicationContext);
}

　　Dog.java

package com.edu.spring;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;

@Component
public class Dog implements SpringContextAware {


    private ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext)
            throws BeansException {
        // TODO Auto-generated method stub
        this.applicationContext=applicationContext;
    }
    
    public void show(){
        System.out.println("Dog :"+applicationContext.getClass());
    }
}

　　ContextBeanPostProcessor.java

package com.edu.spring;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;

@Component
public class ContextBeanPostProcessor implements BeanPostProcessor {

    @Autowired
    private ApplicationContext applicationContext;
    
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        if(bean instanceof SpringContextAware){
            SpringContextAware sa=(SpringContextAware)bean;
            sa.setApplicationContext(applicationContext);
        }
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName)
            throws BeansException {
        // TODO Auto-generated method stub
        return bean;
    }

}

　　App.java

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class App {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext("com.edu.spring");
        context.getBean("dog",Dog.class).show();
        context.close();
    }
}

　　运行结果如下：