Spring为何要用三级缓存来解决循环依赖问题

private final Map<String, Object> singletonObjects = new ConcurrentHashMap(256);

private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

private final Map<String, Object> singletonFactories = new HashMap(16);

public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
  implements AutowireCapableBeanFactory { 
 
  // 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.isDebugEnabled()) {
    logger.debug("Eagerly caching bean '" + beanName +
      "' to allow for resolving potential circular references");
   }
   addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
  }

  /**
  * Add the given singleton factory for building the specified singleton
  * if necessary.
  * <p>To be called for eager registration of singletons, e.g. to be able to
  * resolve circular references.
  * @param beanName the name of the bean
  * @param singletonFactory the factory for the singleton object
  */
 protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
  Assert.notNull(singletonFactory, "Singleton factory must not be null");
  synchronized (this.singletonObjects) {
   if (!this.singletonObjects.containsKey(beanName)) {
    this.singletonFactories.put(beanName, singletonFactory);
    this.earlySingletonObjects.remove(beanName);
    this.registeredSingletons.add(beanName);
   }
  }
 }