淺談Spring中幾個(gè)PostProcessor的區(qū)別與聯(lián)系

更新時(shí)間：2021年08月13日 10:55:09 作者：碩子鴿

這篇文章主要介紹了淺談Spring中幾個(gè)PostProcessor的區(qū)別與聯(lián)系，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教

Spring幾個(gè)PostProcessor的區(qū)別

首先明確 Bean 的生命周期：

首先注冊(cè) Bean 的定義信息；
然后創(chuàng)建 Bean 的實(shí)例；
最后初始化 Bean ，放入容器中。

按照?qǐng)?zhí)行的順序，可以分為以下幾個(gè)步驟：

BeanDefinitionRegistryPostProcessor 是在注冊(cè) Bean 定義信息前后調(diào)用;

BeanFactoryPostProcessor 是在創(chuàng)建 Bean 前后調(diào)用;

BeanPostProcessor 是在初始化 Bean 前后調(diào)用;

其中 BeanDefinitionRegistryPostProcessor 是 BeanFactoryPostProcessor 的子類，所以可以使用前者代替后者實(shí)現(xiàn)功能。

查看 IOC 容器創(chuàng)建時(shí)的調(diào)用流程

refresh 方法的代碼如下：

// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();

其中的 invokeBeanFactoryPostProcessors 就執(zhí)行了注冊(cè)定義信息和創(chuàng)建 Bean 的方法；而 finishBeanFactoryInitialization 執(zhí)行了初始化 Bean 的方法。

具體的執(zhí)行順序大家可以自行打斷點(diǎn)調(diào)試，由于涉及的源碼過多，這里不再展示。

spring-postProcessor的執(zhí)行時(shí)機(jī)

spring bean 的生命周期粗糙的分為以下步驟。

實(shí)例化（創(chuàng)建一個(gè)屬性都為空的對(duì)象）---------》屬性填充（populateBean，下文中這個(gè)步驟我都稱為初始化）-----------》init方法的執(zhí)行（invokerInitMethods，下文稱為init）

postprocessor的方法就是穿插在這三個(gè)大的步驟中。

BeanPostProcessor：

postProcessBeforeInitialization調(diào)用時(shí)機(jī)

向上找調(diào)用者：

繼續(xù)向上：

看以看出populateBean(初始化bean)-------------------》beanpostBeforeInitialization---------------------------------->invokeinitMethods(配置的init-method)

postProcessAfterInitialization調(diào)用時(shí)機(jī)：

向上：

可以看出在init-method方法之后

看以看出populateBean(初始化bean)-------------------》beanpostBeforeInitialization---------------------------------->invokeinitMethods(配置的init-method)------->postProcessAfterInitialization

public class MBeanPostProcessor implements BeanPostProcessor { 
    @Override
    //populateBean之后   invokeinitMethods之前
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("post bean before! ："+beanName);
        return bean;
    }
 
    @Override
    //invokeinitMethods之后
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("post bean after!"+beanName);
        return bean;
    }
}

另一個(gè)重要的是：

InstantiationAwareBeanPostProcessor

postProcessBeforeInstantiation調(diào)用時(shí)機(jī)：

向上找調(diào)用者：

繼續(xù)向上：

可以看出是在實(shí)例化之前：（也就是反射創(chuàng)建對(duì)象之前，如果postProcessBeforeInstantiation創(chuàng)建了一個(gè)非空的對(duì)象，則不會(huì)走實(shí)例化步驟。）

postProcessAfterInstantiation調(diào)用時(shí)機(jī)：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
   PropertyValues pvs = mbd.getPropertyValues();
 
   if (bw == null) {
      if (!pvs.isEmpty()) {
         throw new BeanCreationException(
               mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
      }
      else {
         // Skip property population phase for null instance.
         return;
      }
   }
 
   // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
   // state of the bean before properties are set. This can be used, for example,
   // to support styles of field injection.
   boolean continueWithPropertyPopulation = true;
 
   if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
         if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            //在這里執(zhí)行
            if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
               continueWithPropertyPopulation = false;
               break;
            }
         }
      }
   }
 
   if (!continueWithPropertyPopulation) {
      return;
   }
 省略。。。。。 
   applyPropertyValues(beanName, mbd, bw, pvs);
}

可以看出是在在初始化之前，具體是屬性填充之前。（初始化之前，實(shí)例化之后）如果返回fales，則不會(huì)繼續(xù)初始化，即不會(huì)屬性填充。

postProcessPropertyValues調(diào)用時(shí)機(jī)：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
   PropertyValues pvs = mbd.getPropertyValues(); 
   if (bw == null) {
      if (!pvs.isEmpty()) {
         throw new BeanCreationException(
               mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
      }
      else {
         // Skip property population phase for null instance.
         return;
      }
   }
 
   // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
   // state of the bean before properties are set. This can be used, for example,
   // to support styles of field injection.
   boolean continueWithPropertyPopulation = true;
 
   if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
         if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
               continueWithPropertyPopulation = false;
               break;
            }
         }
      }
   }
 
   if (!continueWithPropertyPopulation) {
      return;
   }
 
   if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
         mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
      MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
 
      // Add property values based on autowire by name if applicable.
      if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
         autowireByName(beanName, mbd, bw, newPvs);
      }
 
      // Add property values based on autowire by type if applicable.
      if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
         autowireByType(beanName, mbd, bw, newPvs);
      } 
      pvs = newPvs;
   }
 
   boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
   boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
 
   if (hasInstAwareBpps || needsDepCheck) {
      PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
      if (hasInstAwareBpps) {
         for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
               InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
               pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
               if (pvs == null) {
                  return;
               }
            }
         }
      }
      if (needsDepCheck) {
         checkDependencies(beanName, mbd, filteredPds, pvs);
      }
   } 
   applyPropertyValues(beanName, mbd, bw, pvs);
}

在postProcessAfterInstantiation之后，applyPropertyValues之前。（屬性填充之前修改屬性值）