Spring_循环依赖

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

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

Spring在创建单例bean时是如何解决循环依赖？
注：在多例模式下，会抛出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));

创建过程：

• getSingleton(),A
• doCreateBean(),提前暴露,earlySingletonExplore=true;调用addSingletonFactories()将A工厂添加到singletonFactories 中
• populateBean():给A属性赋值，发现B
• getSingleton(),B
• doCreateBean(),提前暴露,earlySingletonExplore=true;调用addSingletonFactories()将B工厂添加到singletonFactories 中
• populateBean():给B属性赋值，发现A
• getSingleton(),A,此时根据集合singletonsCurrentlyInCreation 发现A已经在创建过程中，则从A工厂产出一个 early-A添加到earlySingletonObjects ，并且将singletonFactories中A工厂移除
• 通过addSingleton()将earlySingletonObjects中的 early-A填充到B中
• 将B添加到singletonObjects 中，此时A也完成创建，将A也添加到singletonObjects