浅谈Spring中几个PostProcessor的区别与联系

时间：2021-12-17 10:04:37|栏目：JAVA代码|点击：次

Spring几个PostProcessor的区别

首先明确 Bean 的生命周期：

首先注册 Bean 的定义信息；
然后创建 Bean 的实例；
最后初始化 Bean ，放入容器中。

按照执行的顺序，可以分为以下几个步骤：

BeanDefinitionRegistryPostProcessor 是在注册 Bean 定义信息前后调用;

BeanFactoryPostProcessor 是在创建 Bean 前后调用;

BeanPostProcessor 是在初始化 Bean 前后调用;

其中 BeanDefinitionRegistryPostProcessor 是 BeanFactoryPostProcessor 的子类，所以可以使用前者代替后者实现功能。

查看 IOC 容器创建时的调用流程

refresh 方法的代码如下：

// 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 就执行了注册定义信息和创建 Bean 的方法；而 finishBeanFactoryInitialization 执行了初始化 Bean 的方法。

具体的执行顺序大家可以自行打断点调试，由于涉及的源码过多，这里不再展示。

spring-postProcessor的执行时机

spring bean 的生命周期粗糙的分为以下步骤。

实例化（创建一个属性都为空的对象）---------》属性填充（populateBean，下文中这个步骤我都称为初始化）-----------》init方法的执行（invokerInitMethods，下文称为init）

postprocessor的方法就是穿插在这三个大的步骤中。

BeanPostProcessor：

postProcessBeforeInitialization调用时机

向上找调用者：

继续向上：

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

postProcessAfterInitialization调用时机：

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;
    }
}

另一个重要的是：

InstantiationAwareBeanPostProcessor

postProcessBeforeInstantiation调用时机：

向上找调用者：

继续向上：

可以看出是在实例化之前：（也就是反射创建对象之前，如果postProcessBeforeInstantiation创建了一个非空的对象，则不会走实例化步骤。）

postProcessAfterInstantiation调用时机：

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;
            //在这里执行
            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，具体是属性填充之前。（初始化之前，实例化之后）如果返回fales，则不会继续初始化，即不会属性填充。

postProcessPropertyValues调用时机：

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。（属性填充之前修改属性值）