Spring Bean 的创建过程

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当调用 BeanFactory.getBean 方法获取 Bean 时，如果 Bean 还未创建，则会触发 Bean 的创建，本文主要关注 Bean 的创建过程。

何时创建

在 Spring Bean 的获取过程 有提到 Singleton、Prototype 和 其他 Scope 类型的 Bean 创建，当获取不到所需的 Bean 实例时会触发创建 Bean，而这个创建 Bean 的操作是通过一个 createBean(beanName, mbd, args) 方法实现的。createBean(beanName, mbd, args) 是定义在 AbstractBeanFactory 中的一个方法，并且是在 AbstractAutowireCapableBeanFactory 中实现的，它的方法签名如下：

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Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException

AbstractAutowireCapableBeanFactory 是 AbstractBeanFactory 的子类，具有了 getBean 的能力。另外它还实现了 AutowireCapableBeanFactory 接口，需要提供 Autowire 的能力。

AbstractBeanFactory 定义了 getBean 的流程，而把创建 Bean 的工作交给了子类去实现，AbstractAutowireCapableBeanFactory 是其子类，定义了 createBean 的流程。

当遇到以下几种情况时，都会触发 Bean 的创建

  • 若 Bean 定义为 Singleton，并且从 DefaultSingletonBeanRegistry 的缓存中未获取到 Bean
  • 若 Bean 定义为 Prototype，每一次获取 Bean 时
  • 若 Bean 定义为其他 Scope，从 Scope 上下文中未获取到 Bean

createBean

这是 AbstractAutowireCapableBeanFactory 最核心的一个方法，它的功能包括：创建 bean 实例、填充 bean 的属性、调用 postProcessor 等

解析 Bean Class

这里是为了确保 Bean Class 一定是解析过的

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// 解析 Bean Class
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
   mbdToUse = new RootBeanDefinition(mbd);
   mbdToUse.setBeanClass(resolvedClass);
}

具体的实现如下：

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protected Class<?> resolveBeanClass(final RootBeanDefinition mbd, String beanName, final Class<?>... typesToMatch)
      throws CannotLoadBeanClassException {

   try {
      // beanDefinition中有设置beanClass
      if (mbd.hasBeanClass()) {
         return mbd.getBeanClass();
      }

      // 系统设置了安全策略
      if (System.getSecurityManager() != null) {
         return AccessController.doPrivileged((PrivilegedExceptionAction<Class<?>>) () ->
            doResolveBeanClass(mbd, typesToMatch), getAccessControlContext());
      }
      else {
         // 解析beanClass
         return doResolveBeanClass(mbd, typesToMatch);
      }
   }
   catch (PrivilegedActionException pae) {
      ClassNotFoundException ex = (ClassNotFoundException) pae.getException();
      throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), ex);
   }
   catch (ClassNotFoundException ex) {
      throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), ex);
   }
   catch (LinkageError err) {
      throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), err);
   }
}

通常 BeanDefinition 中有设置 Class，上一步会直接返回。若未设置，则会调用 doResolveBeanClass 方法解析 Bean Class

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private Class<?> doResolveBeanClass(RootBeanDefinition mbd, Class<?>... typesToMatch)
      throws ClassNotFoundException {
   // 获取 beanClassLoader
   ClassLoader beanClassLoader = getBeanClassLoader();
   ClassLoader dynamicLoader = beanClassLoader;
   boolean freshResolve = false;

   // 待匹配类型不为空
   if (!ObjectUtils.isEmpty(typesToMatch)) {
      // 获取 tempClassLoader 并进行处理
      ClassLoader tempClassLoader = getTempClassLoader();
      if (tempClassLoader != null) {
         dynamicLoader = tempClassLoader;
         freshResolve = true;
         if (tempClassLoader instanceof DecoratingClassLoader) {
            DecoratingClassLoader dcl = (DecoratingClassLoader) tempClassLoader;
            for (Class<?> typeToMatch : typesToMatch) {
               dcl.excludeClass(typeToMatch.getName());
            }
         }
      }
   }

   // 获取 className
   String className = mbd.getBeanClassName();
   if (className != null) {
      Object evaluated = evaluateBeanDefinitionString(className, mbd);
      if (!className.equals(evaluated)) {
         // A dynamically resolved expression, supported as of 4.2...
         if (evaluated instanceof Class) {
            return (Class<?>) evaluated;
         }
         else if (evaluated instanceof String) {
            className = (String) evaluated;
            freshResolve = true;
         }
         else {
            throw new IllegalStateException("Invalid class name expression result: " + evaluated);
         }
      }
      if (freshResolve) {
         // When resolving against a temporary class loader, exit early in order
         // to avoid storing the resolved Class in the bean definition.
         if (dynamicLoader != null) {
            try {
               return dynamicLoader.loadClass(className);
            }
            catch (ClassNotFoundException ex) {
               if (logger.isTraceEnabled()) {
                  logger.trace("Could not load class [" + className + "] from " + dynamicLoader + ": " + ex);
               }
            }
         }
         // 动态解析，获取beanClass
         return ClassUtils.forName(className, dynamicLoader);
      }
   }

   // 使用默认的classLoader和beanDefinition中保存的className，获取beanClass
   return mbd.resolveBeanClass(beanClassLoader);
}

准备方法 Overrides

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mbdToUse.prepareMethodOverrides()

具体操作是交给了 AbstractBeanDefinition 的 prepareMethodOverrides 方法

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public void prepareMethodOverrides() throws BeanDefinitionValidationException {
   // Check that lookup methods exists.
   if (hasMethodOverrides()) {
      Set<MethodOverride> overrides = getMethodOverrides().getOverrides();
      synchronized (overrides) {
         for (MethodOverride mo : overrides) {
            prepareMethodOverride(mo);
         }
      }
   }
}

protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
   int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
   if (count == 0) {
      throw new BeanDefinitionValidationException(
            "Invalid method override: no method with name '" + mo.getMethodName() +
            "' on class [" + getBeanClassName() + "]");
   }
   else if (count == 1) {
      // Mark override as not overloaded, to avoid the overhead of arg type checking.
      mo.setOverloaded(false);
   }
}

resolveBeforeInstantiation

给 BeanPostProcessors 一个机会，可以创建 bean 的代理出来，这样就不走后面的创建 bean 逻辑

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Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
   return bean;
}

当符合一定条件时：

  1. 通过 InstantiationAwareBeanPostProcessor 的 postProcessBeforeInstantiation 方法创建 Bean 实例
  2. 若上一步创建的实例不为空，则通过 BeanPostProcessor 的 postProcessAfterInitialization 方法对实例进行处理

整体的流程控制代码如下：

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protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
   Object bean = null;
   if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
      // Make sure bean class is actually resolved at this point.
      if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
         Class<?> targetType = determineTargetType(beanName, mbd);
         if (targetType != null) {
            // 实例化之前的回调，如果返回的不为空，说明已经自定义了实例化及初始化，相当于短路了
            bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
            if (bean != null) {
               // 初始化之后的回调
               bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
            }
         }
      }
      mbd.beforeInstantiationResolved = (bean != null);
   }
   return bean;
}

InstantiationAwareBeanPostProcessor 创建实例的代码：

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protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
   for (BeanPostProcessor bp : getBeanPostProcessors()) {
      if (bp instanceof InstantiationAwareBeanPostProcessor) {
         InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
         Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
         if (result != null) {
            return result;
         }
      }
   }
   return null;
}

创建好实例之后，BeanPostProcessor 的回调

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public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
      throws BeansException {

   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {
      Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {
         return result;
      }
      result = current;
   }
   return result;
}

doCreateBean

若上一步未创建 Bean，则执行下面的 doCreateBean 创建 Bean 流程

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Object beanInstance = doCreateBean(beanName, mbdToUse, args)

createBeanInstance

创建 java bean 并封装到 beanWrapper 中，执行完 beanWrapper 的初始化。

  1. 解析 Bean Class
  2. 判断 Bean Class 是否允许访问（public）
  3. 若设置了 instanceSupplier，则通过它创建 Bean 实例
  4. 若设置了 factoryMethodName，则通过工厂方法创建
  5. 若已经解析过构造函数，再次创建相同的 bean，则进行短路处理
  6. 通过 SmartInstantiationAwareBeanPostProcessor 获取到构造函数，然后创建 Bean（区分有无参数）
  7. 使用默认的无参构造函数创建 Bean
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protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
   // Make sure bean class is actually resolved at this point.
   // 解析beanClass
   Class<?> beanClass = resolveBeanClass(mbd, beanName);

   if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
      throw new BeanCreationException(mbd.getResourceDescription(), beanName,
            "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
   }

   // 通过回调创建
   Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
   if (instanceSupplier != null) {
      return obtainFromSupplier(instanceSupplier, beanName);
   }

   // 通过factoryBean创建
   if (mbd.getFactoryMethodName() != null) {
      return instantiateUsingFactoryMethod(beanName, mbd, args);
   }

   // 重新创建相同的 bean，短路处理
   boolean resolved = false;
   boolean autowireNecessary = false;
   if (args == null) {
      synchronized (mbd.constructorArgumentLock) {
         if (mbd.resolvedConstructorOrFactoryMethod != null) {
            resolved = true;
            autowireNecessary = mbd.constructorArgumentsResolved;
         }
      }
   }
   if (resolved) {
      if (autowireNecessary) {
         // 构造函数autowire
         return autowireConstructor(beanName, mbd, null, null);
      }
      else {
         return instantiateBean(beanName, mbd);
      }
   }

   // 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);
   }

   // Preferred constructors for default construction?
   ctors = mbd.getPreferredConstructors();
   if (ctors != null) {
      return autowireConstructor(beanName, mbd, ctors, null);
   }

   // 使用默认的无参构造函数创建
   return instantiateBean(beanName, mbd);
}

applyMergedBeanDefinitionPostProcessors

调用 MergedBeanDefinitionPostProcessor 处理 beanDefinition。

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protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
   for (BeanPostProcessor bp : getBeanPostProcessors()) {
      // 调用所有的 MergedBeanDefinitionPostProcessor 的后处理方法
      if (bp instanceof MergedBeanDefinitionPostProcessor) {
         MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
         bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
      }
   }
}

这里可以对合并后的 BeanDefinition 进行处理，添加所需的东西，比如解析 Autowire 注解等

addSingletonFactory

如果是 Singleton bean，并且允许循环引用，并且当前 bean 正在创建中，则通过添加 SingletonFactory 的方式提前将 bean 暴露出去

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boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
      isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
   if (logger.isTraceEnabled()) {
      logger.trace("Eagerly caching bean '" + beanName +
            "' to allow for resolving potential circular references");
   }
   // 添加bean对应的ObjectFactory，用于创建singleton bean，以及解决循环依赖
   addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}

暴露出去的 bean 还可以通过 SmartInstantiationAwareBeanPostProcessor 的 getEarlyBeanReference 方法进一步处理

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protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
   Object exposedObject = bean;
   if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
         if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
            SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
            exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
         }
      }
   }
   return exposedObject;
}

populateBean

下面就可以填充 bean 的属性，各种 autowire 都是在这里处理的

postProcessAfterInstantiation

这是在 properties 设置之前的操作，可以对 bean 的状态进行修改。若返回 false 则表示不需要后续的属性填充，直接返回

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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;
         }
      }
   }
}

// 不需要属性填充，直接返回
if (!continueWithPropertyPopulation) {
   return;
}

其中 postProcessAfterInstantiation 的方法签名如下：

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boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException

autowire byName 和 byType

对应 xml 中配置的 autowire="byType" 和 autowire="byName"

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if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
   MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
   // autowire by name
   // Add property values based on autowire by name if applicable.
   if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
      autowireByName(beanName, mbd, bw, newPvs);
   }
   // autowire by type
   // Add property values based on autowire by type if applicable.
   if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
      autowireByType(beanName, mbd, bw, newPvs);
   }
   pvs = newPvs;
}

postProcessProperties

如果存在 InstantiationAwareBeanPostProcessors，则回调 postProcessProperties 方法，Autowire 注解的依赖注入也是在这里处理的

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for (BeanPostProcessor bp : getBeanPostProcessors()) {
   // autowire 注解依赖注入是在这个后处理器里处理的
   if (bp instanceof InstantiationAwareBeanPostProcessor) {
      InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
      // 重点是这里，可以对 bean 进行依赖注入
      PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
      if (pvsToUse == null) {
         if (filteredPds == null) {
            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
         }
         pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
         if (pvsToUse == null) {
            return;
         }
      }
      pvs = pvsToUse;
   }
}

postProcessProperties 方法的签名如下：

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PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) throws BeansException

postProcessPropertyValues 方法的签名如下：

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PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException

applyPropertyValues

应用给定的 property values，解析指向其他 bean 的引用。必须使用深拷贝，这样才不会永久修改引用的属性值。

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if (pvs != null) {
   applyPropertyValues(beanName, mbd, bw, pvs);
}

它的核心代码是：

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protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
  // pvs 为 MutablePropertyValues 实例，并且 mpvs.isConverted() 为 true
  mpvs = (MutablePropertyValues) pvs;
  bw.setPropertyValues(mpvs);
  
  // 否则对 pvs 进行深拷贝，然后封装
  bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}

initializeBean

初始化 bean，包括 aware、postProcessBeforeInitialization、InitializingBean 的 afterPropertiesSet、bean 自定义的 init 方法、postProcessAfterInitialization 等

invokeAwareMethods

调用 aware 方法，包括 BeanNameAware、BeanClassLoaderAware 和 BeanFactoryAware 对应的方法

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private void invokeAwareMethods(final String beanName, final Object bean) {
   if (bean instanceof Aware) {
      // BeanNameAware
      if (bean instanceof BeanNameAware) {
         ((BeanNameAware) bean).setBeanName(beanName);
      }
      // BeanClassLoaderAware
      if (bean instanceof BeanClassLoaderAware) {
         ClassLoader bcl = getBeanClassLoader();
         if (bcl != null) {
            ((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);
         }
      }
      // BeanFactoryAware
      if (bean instanceof BeanFactoryAware) {
         ((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
      }
   }
}

postProcessBeforeInitialization

调用 postProcessBeforeInitialization 方法

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if (mbd == null || !mbd.isSynthetic()) {
   // 调用 postProcessBeforeInitialization 方法
   wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}

它会遍历调用所有的 BeanPostProcessor 的 postProcessBeforeInitialization 方法，如果有一个返回 null，则忽略后续的 BeanPostProcessor。

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public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {
   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {
      Object current = processor.postProcessBeforeInitialization(result, beanName);
      if (current == null) {
         return result;
      }
      result = current;
   }
   return result;
}

这是 bean 初始化之前的扩展点，可以对 bean 的初始化做一些准备工作，比如调用一些其他的 Aware 接口的 set 方法。例如 ApplicationContextAwareProcessor 的 postProcessBeforeInitialization 方法实现如下：

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public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
   // 省略一些权限控制代码
  
   invokeAwareInterfaces(bean);

   return bean;
}

private void invokeAwareInterfaces(Object bean) {
   if (bean instanceof Aware) {
      if (bean instanceof EnvironmentAware) {
         ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
      }
      if (bean instanceof EmbeddedValueResolverAware) {
         ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
      }
      if (bean instanceof ResourceLoaderAware) {
         ((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
      }
      if (bean instanceof ApplicationEventPublisherAware) {
         ((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
      }
      if (bean instanceof MessageSourceAware) {
         ((MessageSourceAware) bean).setMessageSource(this.applicationContext);
      }
      if (bean instanceof ApplicationContextAware) {
         ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
      }
   }
}

invokeInitMethods

调用初始化方法，包括 InitializingBean 的 afterPropertiesSet 方法，以及 bean 自定义的 init 方法

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protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
      throws Throwable {

   boolean isInitializingBean = (bean instanceof InitializingBean);
   if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
      // 省略权限控制代码
     
      // 调用 InitializingBean 的 afterPropertiesSet 方法
      ((InitializingBean) bean).afterPropertiesSet();
   }

   if (mbd != null && bean.getClass() != NullBean.class) {
      String initMethodName = mbd.getInitMethodName();
      if (StringUtils.hasLength(initMethodName) &&
            !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
            !mbd.isExternallyManagedInitMethod(initMethodName)) {
         // 调用自定义的 init 方法
         invokeCustomInitMethod(beanName, bean, mbd);
      }
   }
}

postProcessAfterInitialization

调用 postProcessAfterInitialization 方法

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if (mbd == null || !mbd.isSynthetic()) {
   // 调用 postProcessAfterInitialization 方法
   wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}

这里的控制逻辑与 postProcessBeforeInitialization 类似

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public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException {

   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {
      Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {
         return result;
      }
      result = current;
   }
   return result;
}

这是 bean 初始化之后的扩展点，通常 postProcessAfterInitialization 方法不需要对 bean 做任何操作，直接原样返回

检查是否有冲突

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if (earlySingletonExposure) {
   // 从 singleton 缓存中获取，false 表示只会取缓存
   Object earlySingletonReference = getSingleton(beanName, false);
   // 缓存中没有，表示目前是第一次创建，无需其他检查
   // 否则，需要进行检查
   if (earlySingletonReference != null) {
      // 和当前创建的是同一个 bean，没什么问题
      if (exposedObject == bean) {
         exposedObject = earlySingletonReference;
      }
      // 不是同一个 bean，需要进一步检查
      else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
         String[] dependentBeans = getDependentBeans(beanName);
         Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
         for (String dependentBean : dependentBeans) {
            if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
               actualDependentBeans.add(dependentBean);
            }
         }
         // 当前 beanName 被其他 bean 注入了一个原始的 bean 实例引用，而不是一个最终的版本 bean 实例
         if (!actualDependentBeans.isEmpty()) {
            throw new BeanCurrentlyInCreationException(beanName,
                  "Bean with name '" + beanName + "' has been injected into other beans [" +
                  StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                  "] in its raw version as part of a circular reference, but has eventually been " +
                  "wrapped. This means that said other beans do not use the final version of the " +
                  "bean. This is often the result of over-eager type matching - consider using " +
                  "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
         }
      }
   }
}

registerDisposableBeanIfNecessary

如果当前 bean 需要 destroy，则将其注册到 disposableBeans 中

Singleton 类型的 bean

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if (mbd.isSingleton()) {
   // Register a DisposableBean implementation that performs all destruction
   // work for the given bean: DestructionAwareBeanPostProcessors,
   // DisposableBean interface, custom destroy method.
   registerDisposableBean(beanName,
         new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
}

它实际上是调用了 DefaultSingletonBeanRegistry 的 registerDisposableBean 方法，将自己注册到了 disposableBeans 中

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public void registerDisposableBean(String beanName, DisposableBean bean) {
   synchronized (this.disposableBeans) {
      this.disposableBeans.put(beanName, bean);
   }
}

其他类型的 bean

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Scope scope = this.scopes.get(mbd.getScope());
if (scope == null) {
   throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'");
}
scope.registerDestructionCallback(beanName,
      new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));

它是向 Scope 中注册了一个 DisposableBeanAdapter 类型的回调。DisposableBeanAdapter 封装了要 destroy 的 bean，在其 destroy 方法中进行相关操作

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public void destroy() {
   // destroy 之前的扩展
   if (!CollectionUtils.isEmpty(this.beanPostProcessors)) {
      for (DestructionAwareBeanPostProcessor processor : this.beanPostProcessors) {
         processor.postProcessBeforeDestruction(this.bean, this.beanName);
      }
   }

   if (this.invokeDisposableBean) {
      // 忽略日志
      try {
         // 忽略权限控制代码
         
         // 调用 destroy 方法
         ((DisposableBean) this.bean).destroy();
      }
      catch (Throwable ex) {
         // 忽略日志代码
      }
   }

   // 调用自定义的 destroy 方法
   if (this.destroyMethod != null) {
      invokeCustomDestroyMethod(this.destroyMethod);
   }
   else if (this.destroyMethodName != null) {
      Method methodToCall = determineDestroyMethod(this.destroyMethodName);
      if (methodToCall != null) {
         invokeCustomDestroyMethod(methodToCall);
      }
   }
}

流程图

Spring Bean 创建的主要流程如下：

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