BeanPostProcessor 最佳实践和实现原理。

作用

BeanPostProcessor也称为Bean后置处理器，它是Spring中定义的接口，在Spring容器的创建过程中（具体为Bean初始化前后）会回调BeanPostProcessor中定义的两个方法。

postProcessBeforeInitialization : 初始化bean前调用。
postProcessAfterInitialization : 初始化bean后调用。

源码分析：

1、AbstractBeanFactory

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
    //AbstractBeanFactory 初始化 BeanPostProcessor集合。
    private final List beanPostProcessors = new CopyOnWriteArrayList<>();
}

2、AbstractApplicationContext核心refrsh()方法中调用registerBeanPostProcessors

/**
* 该方法获取所有实现BeanPostProcessor接口的Bean
* 并将对应的Bean分组排序加入到BeanFacotory的BeanPostProcessor数组中。
*/
public static void registerBeanPostProcessors(
            ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
        //通过getBeanNamesForType 方法获取容器中所有bean。
        String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
        int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
        //BeanPostProcessor that logs an info message when a bean is created during
        //系统日志，BeanPostProcessor初始化时打印日志用。
        beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));
     
        /**
        * 对于BeanPostProcessor 分组排序集合申明。
        * priorityOrder：优先级排序接口数组（优先级order排在普通order前）
        * Order： 普通排序接口数组。
        * noOrder：无排序数组。
        * internal: 特殊BeanPostProcessor数组。
        */
        List priorityOrderedPostProcessors = new ArrayList<>();
        List internalPostProcessors = new ArrayList<>();
        List orderedPostProcessorNames = new ArrayList<>();
        List nonOrderedPostProcessorNames = new ArrayList<>();
        //解析分组
        for (String ppName : postProcessorNames) {
            if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
                priorityOrderedPostProcessors.add(pp);
                if (pp instanceof MergedBeanDefinitionPostProcessor) {
                    internalPostProcessors.add(pp);
                }
            }
            else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }
        //优先级数组排序和注入到Beanfactory
        sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
        registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

        List orderedPostProcessors = new ArrayList<>();
        for (String ppName : orderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            orderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        //普通order排序和注入
        sortPostProcessors(orderedPostProcessors, beanFactory);
        registerBeanPostProcessors(beanFactory, orderedPostProcessors);

        List nonOrderedPostProcessors = new ArrayList<>();
        for (String ppName : nonOrderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            nonOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
       //无排序注入
        registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);
       //特殊数组排序和注入
        sortPostProcessors(internalPostProcessors, beanFactory);
        registerBeanPostProcessors(beanFactory, internalPostProcessors);
        //监听器相关处理
        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
    }

3、Bean初始化是调用方法。AbstractAutowireCapableBeanFactory.initializeBean方法

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction