Spring_循環依賴

@Component
class A{
	@Resource
	private B b;
}

@Component
class B{
	@Resource
	private A a;
}

Spring在創建單例bean時是如何解決循環依賴？
注：在多例模式下，會拋出BeanCurrentlyInCreationException(beanName);

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {

	/** 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);
	
	/** Set of registered singletons, containing the bean names in registration order. */
	private final Set<String> registeredSingletons = new LinkedHashSet<>(256);

	/** Names of beans that are currently in creation. */
	private final Set<String> singletonsCurrentlyInCreation =
			Collections.newSetFromMap(new ConcurrentHashMap<>(16));

創建過程：

• getSingleton(),A
• doCreateBean(),提前暴露,earlySingletonExplore=true;調用addSingletonFactories()將A工廠添加到singletonFactories 中
• populateBean():給A屬性賦值，發現B
• getSingleton(),B
• doCreateBean(),提前暴露,earlySingletonExplore=true;調用addSingletonFactories()將B工廠添加到singletonFactories 中
• populateBean():給B屬性賦值，發現A
• getSingleton(),A,此時根據集合singletonsCurrentlyInCreation 發現A已經在創建過程中，則從A工廠產出一個 early-A添加到earlySingletonObjects ，並且將singletonFactories中A工廠移除
• 通過addSingleton()將earlySingletonObjects中的 early-A填充到B中
• 將B添加到singletonObjects 中，此時A也完成創建，將A也添加到singletonObjects