一 概述

BeanPostProcessor接口是Spring留给用户，用于bean的拓展。在Spring Bean的生命周期中，BeanPostProcessor接口实现类XX的postProcessBeforeInitialization方法执行时间早于Spring Bean实例化，称为前置处理器；XX的postProcessAfterInitialization方法的执行时间晚于Spring Bean实例化，称为后置处理器。

registerBeanPostProcessors方法的主要作用是实例化并注册所有实现BeanPostProcessor接口的bean。

二 应用

2.1 修改Spring的配置文件

添加下面内容
<bean id="myBeanPostProcessor" class="com.service.MyBeanPostProcessor"/>

2.2 添加实现类

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

public class MyBeanPostProcessor implements BeanPostProcessor {

    public MyBeanPostProcessor() {
        System.out.println("这是BeanPostProcessor实现类构造器！！");
    }

    //前置处理器
    @Override
    public Object postProcessAfterInitialization(Object arg0, String arg1)
            throws BeansException {
        System.out
                .println("BeanPostProcessor接口方法postProcessAfterInitialization对属性进行更改！");
        System.out.println("后置处理器： "+ arg0 + ",beanName:" + arg1);

        return arg0;
    }

    //后置处理器
    @Override
    public Object postProcessBeforeInitialization(Object arg0, String arg1)
            throws BeansException {
        System.out
                .println("BeanPostProcessor接口方法postProcessBeforeInitialization对属性进行更改！");
        System.out.println("前置处理器： "+ arg0 + ",beanName:" + arg1);
        return arg0;
    }
}

2.3 日志信息

06:07:13.258 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'myBeanPostProcessor'
这是BeanPostProcessor实现类构造器！！

三 源码

3.1 registerBeanPostProcessors方法

protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {		
      //实例化并注册所有实现BeanPostProcessor接口的 Bean 
            PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
	}   
    
public static void registerBeanPostProcessors(
			ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

        //根据Bean类型查找BeanName
        //此时对应的Bean还没实例化
		String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

		// Register BeanPostProcessorChecker that logs an info message when
		// a bean is created during BeanPostProcessor instantiation, i.e. when
		// a bean is not eligible for getting processed by all BeanPostProcessors.
        
        
        //beanProcessorTargetCount表示所有实现BeanPostProcessor接口的Bean的数量,由三个参数组成。
       // beanFactory.getBeanPostProcessorCount() 表示集合beanPostProcessors的容量，即现在已经初始化的实现BeanPostProcessor接口的Bean的数量  
       //1是后面即将添加的BeanPostProcessorChecker
       //postProcessorNames.length：配置文件中添加的beanPostProcessor接口的实现类，上面配置文件只配置了一个bean，因此数量是1
      
		int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
        
        //添加到beanPostProcessors集合中
        //BeanPostProcessorChecker用于存储信息，如beanPostProcessor接口实现类的数量，beanFactory等
		beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

		// Separate between BeanPostProcessors that implement PriorityOrdered,
		// Ordered, and the rest.
        
        //下面4个容器
        //bean（beanPostProcessor类）实例化的优先级:priorityOrderedPostProcessors  >  orderedPostProcessorNames > nonOrderedPostProcessorNames
		List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
		List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
		List<String> orderedPostProcessorNames = new ArrayList<>();
		List<String> nonOrderedPostProcessorNames = new ArrayList<>();
        
        
		for (String ppName : postProcessorNames) {
          //是否PriorityOrdered类的实例
			if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            //若是，则获取bean，如果bean没用实例化则会触发bean的实例化，即调用实现类的构造函数
				BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
                //添加到priorityOrderedPostProcessors容器中
				priorityOrderedPostProcessors.add(pp);
                //判断是否是MergedBeanDefinitionPostProcessor的实例
				if (pp instanceof MergedBeanDefinitionPostProcessor) {
                //若是添加到internalPostProcessors中
					internalPostProcessors.add(pp);
				}
			}
			else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
				orderedPostProcessorNames.add(ppName);
			}
			else {
				nonOrderedPostProcessorNames.add(ppName);
			}
		}

		// First, register the BeanPostProcessors that implement PriorityOrdered.
        //Bean优先级排序
		sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
		//注册到beanPostProcessors集合中
        registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

		// Next, register the BeanPostProcessors that implement Ordered.
		List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
		for (String ppName : orderedPostProcessorNames) {
			BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
			orderedPostProcessors.add(pp);
			if (pp instanceof MergedBeanDefinitionPostProcessor) {
				internalPostProcessors.add(pp);
			}
		}
		sortPostProcessors(orderedPostProcessors, beanFactory);
		registerBeanPostProcessors(beanFactory, orderedPostProcessors);

		// Now, register all regular BeanPostProcessors.
		List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
		for (String ppName : nonOrderedPostProcessorNames) {
			BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
			nonOrderedPostProcessors.add(pp);
			if (pp instanceof MergedBeanDefinitionPostProcessor) {
				internalPostProcessors.add(pp);
			}
		}
		registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

		// Finally, re-register all internal BeanPostProcessors.
        
        //排序
		sortPostProcessors(internalPostProcessors, beanFactory);
        //将internalPostProcessors集合的中存储的bean重新添加到beanPostProcessors集合中
        //beanPostProcessors集合是List数据结果，重新添加前会判断是否存在，若存在则删除旧的数据；说白了就说将数据插入到list的尾部
        
		registerBeanPostProcessors(beanFactory, internalPostProcessors);

		// Re-register post-processor for detecting inner beans as ApplicationListeners,
		// moving it to the end of the processor chain (for picking up proxies etc).
        //将ApplicationListenerDetector类插入到beanPostProcessors集合尾部
        //这行代码执行完后，beanPostProcessors集合依然只含有4个，因为ApplicationListenerDetector类存在于beanPostProcessors集合中，相当于把它移动到末尾
		beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
	}
         

3.2 addBeanPostProcessor方法

	private final List<BeanPostProcessor> beanPostProcessors = new CopyOnWriteArrayList<>();

@Override
	public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
		Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
		// Remove from old position, if any
        //删除旧的beanPostProcessor
		this.beanPostProcessors.remove(beanPostProcessor);
		// Track whether it is instantiation/destruction aware
        
        //判断是否为InstantiationAwareBeanPostProcessor接口的实现类
		if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {         //若是则将属性修改为true
			this.hasInstantiationAwareBeanPostProcessors = true;
		}
        
           //判断是否为DestructionAwareBeanPostProcessor接口的实现类
		if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
           //若是则将属性修改为true
			this.hasDestructionAwareBeanPostProcessors = true;
		}
		// Add to end of list
        //添加到集合中
		this.beanPostProcessors.add(beanPostProcessor);
	}

四 参考文献

1）JDK7在线文档
https://tool.oschina.net/apidocs/apidoc?api=jdk_7u4
2) JDK8在线文档
https://docs.oracle.com/javase/8/docs/api/
3) Bruce Eckel. Java编程思想，第4版，2007，机械工业出版社
4）方腾飞，魏鹏，程晓明. Java并发编程的艺术，第1版，2015年，机械工业出版社
5）克雷格.沃斯. Spring实战，第5版，2020年，人民邮电出版社