<bean id="person1" class="com.example.leetcode.spring.bean.Person">
  <property name="parent" ref="person2"></property>
  <property name="name" value="tom"></property>
</bean>

<bean id="person2" class="com.example.leetcode.spring.bean.Person">
  <property name="parent" ref="person1"></property>
  <property name="name" value="jack"></property>
</bean>

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
    @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

  final String beanName = transformedBeanName(name);
  Object bean;

  // 這里會(huì)檢查單例bean是否已經(jīng)在注冊(cè)表，并返回。
  // Eagerly check singleton cache for manually registered singletons.
  Object sharedInstance = getSingleton(beanName);
  if (sharedInstance != null && args == null) {
    if (logger.isTraceEnabled()) {
      if (isSingletonCurrentlyInCreation(beanName)) {
        logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
            "' that is not fully initialized yet - a consequence of a circular reference");
      }
      else {
        logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
      }
    }
    bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
  }
  ...
}

  // 用來(lái)存儲(chǔ)已經(jīng)創(chuàng)建好的單例對(duì)象
  /** Cache of singleton objects: bean name to bean instance. */
  private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

  // 用來(lái)存儲(chǔ)單例beanname到ObjectFactory的映射
  /** Cache of singleton factories: bean name to ObjectFactory. */
  private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

  // 用來(lái)提前存儲(chǔ)還未初始化好的單例對(duì)象
  /** Cache of early singleton objects: bean name to bean instance. */
  private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
  Object singletonObject = this.singletonObjects.get(beanName);
  if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
    synchronized (this.singletonObjects) {
      singletonObject = this.earlySingletonObjects.get(beanName);
      if (singletonObject == null && allowEarlyReference) {
        ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
        if (singletonFactory != null) {
          singletonObject = singletonFactory.getObject();
          this.earlySingletonObjects.put(beanName, singletonObject);
          this.singletonFactories.remove(beanName);
        }
      }
    }
  }
  return singletonObject;
}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {
  ...
  instanceWrapper = createBeanInstance(beanName, mbd, args);
  ...
  // 檢查是否提前將單例bean存入緩存
  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");
    }
    // 這里將beanname與工廠映射放入緩存注冊(cè)表中(也就是上面的singletonFactories)
    addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
  }
  
  ...
  // 注入依賴屬性
  populateBean(beanName, mbd, instanceWrapper);
  ...
        
}