// 我依赖的大哥都好了
// Create bean instance.
if (mbd.isSingleton()) {
   sharedInstance = getSingleton(beanName, () -> {
      try {
         return createBean(beanName, mbd, args);
      } catch (BeansException ex) {
         // 从三级缓存中移除这个 beanName  因为它可能被放进去了 因为放进去三级缓存可以解决 setter 的循环依赖
         destroySingleton(beanName);
         throw ex;
      }
   });
   bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
} 
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public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
   synchronized (this.singletonObjects) {
      // 看看第一级缓存中有没有
      Object singletonObject = this.singletonObjects.get(beanName);
      if (singletonObject == null) {
         // 将 beanName 加入到 singletonsCurrentlyInCreation 中，代表它正在创建中
         beforeSingletonCreation(beanName);
         boolean newSingleton = false;
         try {
            singletonObject = singletonFactory.getObject();
            newSingleton = true;
         } catch (IllegalStateException ex) {
            throw ex;
         } catch (BeanCreationException ex) {
            throw ex;
         } finally {
            // singletonsCurrentlyInCreation 从这里面移除掉
            afterSingletonCreation(beanName);
         }
         if (newSingleton) {
            // 加入缓存中
            addSingleton(beanName, singletonObject);
         }
      }
      return singletonObject;
   }
}
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protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {
   ...........
   ...........
   try {
      // 真正 处理逻辑
      Object beanInstance = doCreateBean(beanName, mbdToUse, args);
      if (logger.isTraceEnabled()) {
         logger.trace("Finished creating instance of bean '" + beanName + "'");
      }
      return beanInstance;
   } catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
      throw ex;
   } catch (Throwable ex) {
      throw new BeanCreationException(xxxx);
   }
}
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protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {
   // Instantiate the bean.
   BeanWrapper instanceWrapper = null;
   if (mbd.isSingleton()) {
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
   }
   if (instanceWrapper == null) {
      // 根据指定 bean 使用对应的策略创建新的实例、如工厂方法、构造函数自动注入、简单初始化
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   final Object bean = instanceWrapper.getWrappedInstance();
   Class<?> beanType = instanceWrapper.getWrappedClass();
  ..........
  .........
   // 是否需要提前曝光、用来解决循环依赖的问题
   // 是单例&允许循环依赖&正在创建中
   boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
         isSingletonCurrentlyInCreation(beanName));
   if (earlySingletonExposure) {
      // 为了避免后期循环依赖、可以在 bean 初始化前将创建实例的ObjectFactory 加入工厂
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
   }
   // Initialize the bean instance.
   Object exposedObject = bean;
   try {
      // 填充属性
      populateBean(beanName, mbd, instanceWrapper);
      // 调用初始方法
      exposedObject = initializeBean(beanName, exposedObject, mbd);
   } catch (Throwable ex) {
      .........
   }
   ........
   .......
   return exposedObject;
}
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protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
    .........
    .........
      if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
         autowireByName(beanName, mbd, bw, newPvs);
      }
      // Add property values based on autowire by type if applicable.
      if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
         autowireByType(beanName, mbd, bw, newPvs);
      }
    .........
    .........
}
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protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
   if (System.getSecurityManager() != null) {
      AccessController.doPrivileged((PrivilegedAction<Object>) () -> {。
          // 调用各种 Aware 接口
         invokeAwareMethods(beanName, bean);
         return null;
      }, getAccessControlContext());
   } else {
       // 调用各种 Aware 接口
      invokeAwareMethods(beanName, bean);
   }
   Object wrappedBean = bean;
   if (mbd == null || !mbd.isSynthetic()) {
       // 调用 BeanPostProcessor postProcessBeforeInitialization
      wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
   }
   try {
       // 调用 InitializingBean 、自定义的初始化方法
      invokeInitMethods(beanName, wrappedBean, mbd);
   } catch (Throwable ex) {
      throw new BeanCreationException(
            (mbd != null ? mbd.getResourceDescription() : null),
            beanName, "Invocation of init method failed", ex);
   }
   if (mbd == null || !mbd.isSynthetic()) {
       //  调用 BeanPostProcessor postProcessAfterInitialization
      wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
   }
   return wrappedBean;
}
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