代码实现

实现类：

@Component
public class BeanPostProcessorImpl implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        // 返回一个传过来的对象
        // 在初始化方法调用之前进行后置处理工作
        // 什么时候调用呢？在init-method方法之前就调用了
        System.out.println("postProcessBeforeInitialization======" + beanName + "======" + bean);
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("postProcessAfterInitialization======" + beanName + "======" + bean);
        return bean;
    }

}

配置类：

@Configuration
@ComponentScan("com.nmys.story.springCore.springioc.beanpostprocessor_")
public class Config01 {

    @Bean(initMethod = "init",destroyMethod = "destroy")
    public Train train(){
        return new Train();
    }

}

实体类：

/**
 * @author 70KG
 * @Title: Train
 * @Description:
 * @date 2018/7/23下午11:31
 * @From www.nmyswls.com
 */
public class Train {

    private String name = "火车";

    private Integer length = 150;

    public Train() {
        System.out.println("Train构造方法执行。。。。。。");
    }


    public void init() {
        System.out.println("Train的init方法执行。。。。。。");
    }

    public void destroy() {
        System.out.println("Train的destroy方法执行。。。。。。");
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getLength() {
        return length;
    }

    public void setLength(Integer length) {
        this.length = length;
    }
}

测试类：

public class Test01 {
    @Test
    public void test() {
        AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(Config01.class);
        ac.close();
    }
}

打印结果：

Train构造方法执行。。。。。。
postProcessBeforeInitialization======train======com.nmys.story.springCore.springioc.beanpostprocessor_.Train@5aa9e4eb
Train的init方法执行。。。。。。
postProcessAfterInitialization======train======com.nmys.story.springCore.springioc.beanpostprocessor_.Train@5aa9e4eb
Train的destroy方法执行。。。。。。

结果分析：

BeanPostProcessor原理分析

AnnotationConfigApplicationContext构造方法中的refresh();
-> AbstractApplicationContext550行finishBeanFactoryInitialization(beanFactory);完成剩余Bean的初始化工作
-> AbstractApplicationContext869行beanFactory.preInstantiateSingletons();
-> DefaultListableBeanFactory760行getBean(beanName);
-> AbstractBeanFactory317行createBean(beanName, mbd, args);
-> AbstractAutowireCapableBeanFactory503行doCreateBean(beanName, mbdToUse, args);
-> AbstractAutowireCapableBeanFactory580行populateBean(beanName, mbd, instanceWrapper);在执行初始化之前，首先给实例赋值
-> AbstractAutowireCapableBeanFactory581行exposedObject = initializeBean(beanName, exposedObject, mbd);准备执行初始化方法
-> AbstractAutowireCapableBeanFactory1700行wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
-> AbstractAutowireCapableBeanFactory1704行invokeInitMethods(beanName, wrappedBean, mbd);
-> AbstractAutowireCapableBeanFactory1712行wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);

分析AbstractAutowireCapableBeanFactory的applyBeanPostProcessorsBeforeInitialization方法：

@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {
    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        Object current = beanProcessor.postProcessBeforeInitialization(result, beanName);
        if (current == null) {
            return result;
        }
        result = current;
    }
    return result;
}

1. 首先获取到所有的后置处理器getBeanPostProcessors()
2. 在for循环中依次调用后置处理器的方法beanProcessor.postProcessBeforeInitialization(result, beanName);
3. 进入postProcessBeforeInitialization方法(这个方法必须在debug过程中才可以进入)

public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
    AccessControlContext acc = null;
    if (System.getSecurityManager() != null &&
            (bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
                    bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
                    bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
        acc = this.applicationContext.getBeanFactory().getAccessControlContext();
    }
    if (acc != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            invokeAwareInterfaces(bean);
            return null;
        }, acc);
    }
    else {
        invokeAwareInterfaces(bean);
    }
    return bean;
}

<!--more-->

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) {
            ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
        }
    }
}

1. 来到ApplicationContextAwareProcessor类的79行，首先判断此bean是不是各种的Aware，如果是它列举的那几个Aware就获取Bean工厂的权限，可以向容器中导入相关的上下文环境，目的是为了Bean实例能够获取到相关的上下文，如果不是(显然它也不是)它列举的几个Aware，那就调用invokeAwareInterfaces(bean)，向容器中添加相关接口的上下文环境，显然我们自己的实现类也不属于Aware接口，所以这个处理结果直接将Bean返回，不做任何处理。ok，这样初始化之前的方法就分析完了，初始化之后的方法跟前面的流程一模一样。

总结