protected Object getSingleton(String beanName, boolean allowEarlyReference) {
 // Quick check for existing instance without full singleton lock
 Object singletonObject = this.singletonObjects.get(beanName);
 if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
  singletonObject = this.earlySingletonObjects.get(beanName);
  if (singletonObject == null && allowEarlyReference) {
   synchronized (this.singletonObjects) {
    // Consistent creation of early reference within full singleton lock
    singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null) {
     singletonObject = this.earlySingletonObjects.get(beanName);
     if (singletonObject == null) {
      ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
      if (singletonFactory != null) {
       singletonObject = singletonFactory.getObject();
       this.earlySingletonObjects.put(beanName, singletonObject);
       this.singletonFactories.remove(beanName);
      }
     }
    }
   }
  }
 }
 return singletonObject;
}

@Component
public class Aoo {
  @Autowired
  private Boo boo;
}
@Component
public class Boo {
  @Autowired
  private Aoo aoo;
}

Object sharedInstance = getSingleton(beanName);

sharedInstance = getSingleton(beanName, () -> {
  return createBean(beanName, mbd, args);
});

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    try {
      singletonObject = singletonFactory.getObject();
      newSingleton = true;
    }
    if (newSingleton) {
      addSingleton(beanName, singletonObject);
    }
    return singletonObject;
}

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args){
   // 省略無關(guān)代碼
   return doCreateBean(beanName, mbdToUse, args);
}
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
 // 創(chuàng)建一個(gè)Aoo的包裝對象。此時(shí)Aoo是一個(gè)空殼對象其中的所有屬性均為空
 BeanWrapper instanceWrapper = null;
 Object bean = instanceWrapper.getWrappedInstance();
 // 判斷是否提前暴露（其實(shí)就是循環(huán)依賴了）
 boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
 if (earlySingletonExposure) {
  // 把Aoo對象加入三級緩存?。?！
  addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
 }
 Object exposedObject = bean;
 try {
  // 給Aoo對象注入屬性
  populateBean(beanName, mbd, instanceWrapper);
  // 初始化Aoo（一些擴(kuò)展點(diǎn)，與循環(huán)依賴關(guān)系不大）
  exposedObject = initializeBean(beanName, exposedObject, mbd);
 }
 if (earlySingletonExposure) {
   // 如果提前暴露對象，則嘗試從緩存中獲取。
  Object earlySingletonReference = getSingleton(beanName, false);
  if (earlySingletonReference != null) {
    exposedObject = earlySingletonReference;
  }
 }
 return exposedObject;
}