SpringloC容器的依赖注入源码解析（8）—— 单例循环依赖的解决

这一讨论的前提是要对Spring的doCreateBean方法有所了解，故将其源码放在这里，以供参考：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {

   // Instantiate the bean.
   // bean实例包装类
   BeanWrapper instanceWrapper = null;
   if (mbd.isSingleton()) {
      // 从未完成创建的包装Bean缓存中清理并获取相关中的包装Bean实例，毕竟是单例的，只能存一份
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
   }
   if (instanceWrapper == null) {
      //创建bean的时候，这里创建bean的实例有三种方法
      // 1.工厂方法创建
      // 2.构造方法的方式注入
      // 3.无参构造方法注入
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   final Object bean = instanceWrapper.getWrappedInstance();
   Class<?> beanType = instanceWrapper.getWrappedClass();
   if (beanType != NullBean.class) {
      mbd.resolvedTargetType = beanType;
   }

   // Allow post-processors to modify the merged bean definition.
   // 调用BeanDefinition属性合并完成后的BeanPostProcessor后置处理器
   synchronized (mbd.postProcessingLock) {
      if (!mbd.postProcessed) {
         try {
            // 被@Autowired、@Value标记的属性在这里获取
            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
         }
         catch (Throwable ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                  "Post-processing of merged bean definition failed", ex);
         }
         mbd.postProcessed = true;
      }
   }

   // Eagerly cache singletons to be able to resolve circular references
   // even when triggered by lifecycle interfaces like BeanFactoryAware.
   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");
      }
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
   }

   // Initialize the bean instance.
   Object exposedObject = bean;
   try {
      populateBean(beanName, mbd, instanceWrapper);
      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);
      }
   }

   if (earlySingletonExposure) {
      Object earlySingletonReference = getSingleton(beanName, false);
      if (earlySingletonReference != null) {
         if (exposedObject == bean) {
            exposedObject = earlySingletonReference;
         }
         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);
               }
            }
            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.");
            }
         }
      }
   }

   // Register bean as disposable.
   try {
      registerDisposableBeanIfNecessary(beanName, bean, mbd);
   }
   catch (BeanDefinitionValidationException ex) {
      throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
   }

   return exposedObject;
}

对应的文章《SpringloC容器的依赖注入源码解析（7）—— doCreateBean之剩余逻辑（解决循环依赖的源头）》

现在有A依赖B，B也依赖A，假设A先创建，就会来到doCreateBean方法里，会经过各种包装，在被

instanceWrapper = createBeanInstance(beanName, mbd, args);

处理了之后生成一个没有任何属性的A的实例。

之后在调用

addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));

之前将A实例放入到ObjectFactory里面，然后调用addSingletonFactory将ObjectFactory添加到三级缓存里，此时只有三级缓存保存了A对应的ObjectFactory实例。

随后就会调用

populateBean(beanName, mbd, instanceWrapper);

给属性赋值，由于A依赖于B，在populate里会尝试获取实例B，由于B还没有被创建过，所以又递归的调用doCreateBean方法。

在doCreateBean方法里调用

instanceWrapper = createBeanInstance(beanName, mbd, args);

创建出实例B，将其对应的实例工厂放入到三级缓存中，此时三级缓存中就保存了A和B的实例，随后在B里又会执行

populateBean(beanName, mbd, instanceWrapper);

由于B依赖A，所以在populate里尝试获取A的实例，此时调用的是AbstractBeanFactory的doGetBean方法，在里面调用：

Object sharedInstance = getSingleton(beanName);

从三级缓存里获取A实例对应的ObjectFactory实例 这里getObject()调用的就是getEarlyBeanReference方法 获取到A实例之后就会将A实例放入二级缓存里，同时清空三级缓存里的A

---

此时就回到创建B的doCreateBean的

populateBean(beanName, mbd, instanceWrapper);

A被注入到B里面了，即当B执行完populateBean之后就已经获取到了A，此时B就会执行完剩余逻辑获得到一个完备的B，此时方法返回到doGetBean方法里 在getSingleton方法里面会最终调用

addSingleton(beanName, singletonObject);

在方法里会将实例B添加到一级缓存里，并将B从二三级缓存里移除，表示已经彻底完成了B实例的创建，之后返回B。

B之所以会被创建是因为A调用了

populateBean(beanName, mbd, instanceWrapper);

此时又回到这个地方，A内部已经赋值上了完备的B，之后步骤同上，调用addSingleton将A放入一级缓存里，此时循环依赖的支持完成。

整个的流程图如下图所示，大家可以基于前面的介绍看一下到底有没有掌握 ~：