spring-ioc-循环依赖

   /** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

   /** Cache of early singleton objects: bean name to bean instance. */
   private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);