@Component
public class ServiceA {
    @Autowired
    private ServiceB serviceB;

}
@Component
public class ServiceB {
    @Autowired
    private ServiceA serviceA;

}
@Component
public class ResetServiceABeanPostProcessor implements BeanPostProcessor {
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if(beanName.equals("serviceA")){
            return new ServiceA();
        }
        return bean;
    }
}

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	...
	// 從上至下 分表代表這“三級(jí)緩存”
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); //一級(jí)緩存
	private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); // 二級(jí)緩存
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 三級(jí)緩存
	...
	
	/** Names of beans that are currently in creation. */
	// 這個(gè)緩存也十分重要：它表示bean創(chuàng)建過程中都會(huì)在里面呆著~
	// 它在Bean開始創(chuàng)建時(shí)放值，創(chuàng)建完成時(shí)會(huì)將其移出~
	private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));

	/** Names of beans that have already been created at least once. */
	// 當(dāng)這個(gè)Bean被創(chuàng)建完成后，會(huì)標(biāo)記為這個(gè) 注意：這里是set集合 不會(huì)重復(fù)
	// 至少被創(chuàng)建了一次的  都會(huì)放進(jìn)這里~~~~
	private final Set<String> alreadyCreated = Collections.newSetFromMap(new ConcurrentHashMap<>(256));
}

	/** 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);

	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}