Hook扩展点

• Hook钩子是一种编程思想，也是监听者模式的一种实现。在控制中心的执行过程(可理解也模板方法)中，会调用分阶段或者分层次来调用用户注册进来的Hook方法，用来修改或者监听控制中心的逻辑。
• 如果需要将控制中心的内容暴露出来，可以选择Hook或者Publish Event

Spring框架在org.springframework.context.support.AbstractApplicationContext#refresh的时候，留下了非常多的Hook，用于灵活接口框架内部实现及开发者功能扩展。

Bean生命周期Hook

Bean的实例化会经过如下流程，最终调用AbstractAutowireCapableBeanFactory#createBean(java.lang.Class<T>)实例化Bean

AbstractApplicationContext#refresh -> finishBeanFactoryInitialization为容器填充相关对象,实例化bean（实例化剩余的bean）
实现了后置处理器的bean均不在这里进行注册 -> preInstantiateSingletons生产单例 -> getBean -> doCreateBean。

Hook全部列表

@Component
public class AllBeanPostProcessor implements InstantiationAwareBeanPostProcessor,
        SmartInstantiationAwareBeanPostProcessor,
        MergedBeanDefinitionPostProcessor,
        DestructionAwareBeanPostProcessor {
    // 来源InstantiationAwareBeanPostProcessor  Instantiation-实例化的意思，反射调用构造方法实例化
    @Override
    public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
        return null;
    }

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

    @Override
    public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) throws BeansException {
        return null;
    }

    // 来源BeanPostProcessor  Initialization-初始化，实例化后用户可以手动调用方法初始化
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

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

    // 来源SmartInstantiationAwareBeanPostProcessor
    @Override
    public Class<?> predictBeanType(Class<?> beanClass, String beanName) throws BeansException {
        return SmartInstantiationAwareBeanPostProcessor.super.predictBeanType(beanClass, beanName);
    }

    @Override
    public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, String beanName) throws BeansException {
        return SmartInstantiationAwareBeanPostProcessor.super.determineCandidateConstructors(beanClass, beanName);
    }

    @Override
    public Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
        return SmartInstantiationAwareBeanPostProcessor.super.getEarlyBeanReference(bean, beanName);
    }

    // 来源MergedBeanDefinitionPostProcessor
    @Override
    public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {

    }

    @Override
    public void resetBeanDefinition(String beanName) {
        MergedBeanDefinitionPostProcessor.super.resetBeanDefinition(beanName);
    }

    // 来源DestructionAwareBeanPostProcessor
    @Override
    public void postProcessBeforeDestruction(Object bean, String beanName) throws BeansException {

    }

    @Override
    public boolean requiresDestruction(Object bean) {
        return true;
    }
}

1. 在反射实例化之前，会applyBeanPostProcessorsBeforeInstantiation确定是否在回调中实例化，如果实例化，则applyBeanPostProcessorsAfterInitialization来执行全部BeanPostProcessor#postProcessAfterInitialization来后置处理Bean。

   try {
       // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
       Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
       if (bean != null) {
           return bean;
       }
   }
   if (targetType != null) {
       bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
       if (bean != null) {
           bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
       }
   }
   

2. AbstractAutowireCapableBeanFactory#createBeanInstance会通过determineConstructorsFromBeanPostProcessors，选择合适的Constructor，没有指定则使用autowireConstructor自动推理使用合适的构造方法。

   // Candidate constructors for autowiring?
   Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
   if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
       mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
       return autowireConstructor(beanName, mbd, ctors, args);
   }