Spring压轴题：当循环依赖遇上Spring AOP

  protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 从一级缓存获取，key=beanName value=bean
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      synchronized (this.singletonObjects) {
        // 从二级缓存获取，key=beanName value=bean
        singletonObject = this.earlySingletonObjects.get(beanName);
        // 是否允许循环引用
        if (singletonObject == null && allowEarlyReference) {
          /**
           * 三级缓存获取，key=beanName value=objectFactory，objectFactory中存储getObject()方法用于获取提前曝光的实例
           *
           * 而为什么不直接将实例缓存到二级缓存，而要多此一举将实例先封装到objectFactory中？
           * 主要关键点在getObject()方法并非直接返回实例，而是对实例又使用
           * SmartInstantiationAwareBeanPostProcessor的getEarlyBeanReference方法对bean进行处理
           *
           * 也就是说，当spring中存在该后置处理器，所有的单例bean在实例化后都会被进行提前曝光到三级缓存中，
           * 但是并不是所有的bean都存在循环依赖，也就是三级缓存到二级缓存的步骤不一定都会被执行，有可能曝光后直接创建完成，没被提前引用过，
           * 就直接被加入到一级缓存中。因此可以确保只有提前曝光且被引用的bean才会进行该后置处理
           */
          ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
          if (singletonFactory != null) {
            /**
             * 通过getObject()方法获取bean，通过此方法获取到的实例不单单是提前曝光出来的实例，
             * 它还经过了SmartInstantiationAwareBeanPostProcessor的getEarlyBeanReference方法处理过。
             * 这也正是三级缓存存在的意义，可以通过重写该后置处理器对提前曝光的实例，在被提前引用时进行一些操作
             */
            singletonObject = singletonFactory.getObject();
            // 将三级缓存生产的bean放入二级缓存中
            this.earlySingletonObjects.put(beanName, singletonObject);
            // 删除三级缓存
            this.singletonFactories.remove(beanName);
          }
        }
      }
    }
    return singletonObject;
  }

  this.addSingletonFactory(beanName, () -> {
    return this.getEarlyBeanReference(beanName, mbd, bean);
  });

  // AbstractAutoProxyCreator.java
  public Object getEarlyBeanReference(Object bean, String beanName) {
    // 缓存当前bean，表示该bean被提前代理了
    Object cacheKey = getCacheKey(bean.getClass(), beanName);
    this.earlyProxyReferences.put(cacheKey, bean);
    // 对bean进行提前Spring AOP代理
    return wrapIfNecessary(bean, beanName, cacheKey);
  }

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

    Object result = existingBean;
    for (BeanPostProcessor processor : getBeanPostProcessors()) {
      // 调用bean初始化后置处理器处理
      Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {
        return result;
      }
      result = current;
    }
    return result;
  }

  // AbstractAutoProxyCreator.java
  public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
    if (bean != null) {
      // 获取缓存key
      Object cacheKey = getCacheKey(bean.getClass(), beanName);
      // 查看该bean是否被Spring AOP提前代理！而缓存的是原始的bean，因此如果bean被提前代理过，这此处会跳过
      // 如果bean没有被提前代理过，则进入AOP代理
      if (this.earlyProxyReferences.remove(cacheKey) != bean) {
        return wrapIfNecessary(bean, beanName, cacheKey);
      }
    }
    return bean;
  }

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {
    try {
      //
      /**
       * 4. 填充属性
       * 如果@Autowired注解属性，则在上方完成解析后，在这里完成注入
       *
       * @Autowired
       * private Inner inner;
       */
      populateBean(beanName, mbd, instanceWrapper);
      // 5. 初始化
      exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
    catch (Throwable ex) {
      if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
        throw (BeanCreationException) ex;
      }
      else {
        throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
      }
    }

    // 6. 存在提前曝光情况下
    if (earlySingletonExposure) {
      // earlySingletonReference：二级缓存，缓存的是经过提前曝光提前Spring AOP代理的bean
      Object earlySingletonReference = getSingleton(beanName, false);
      if (earlySingletonReference != null) {
        // exposedObject跟bean一样，说明初始化操作没用应用Initialization后置处理器(指AOP操作)改变exposedObject
        // 主要是因为exposedObject如果提前代理过，就会跳过Spring AOP代理，所以exposedObject没被改变，也就等于bean了
        if (exposedObject == bean) {
          // 将二级缓存中的提前AOP代理的bean赋值给exposedObject，并返回
          exposedObject = earlySingletonReference;
        }
        // 引用都不相等了，也就是现在的bean已经不是当时提前曝光的bean了
        else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
          // dependentBeans也就是B, C, D
          String[] dependentBeans = getDependentBeans(beanName);
          Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
          for (String dependentBean : dependentBeans) {
            if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
              actualDependentBeans.add(dependentBean);
            }
          }
          // 被依赖检测异常
          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.");
          }
        }
      }
    }