protected Object doCreateBean(final String beanName,final RootBeanDefinition mbd,final Object[] args) throws BeanCreationException {
    ...
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        ...
        addSingletonFactory(beanName,new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
                return getEarlyBeanReference(beanName,mbd,bean);
            }
        });
    }

    Object exposedObject = bean;
    try {
 populateBean(beanName,instanceWrapper); if (exposedObject != null) {
            exposedObject = initializeBean(beanName,exposedObject,mbd);
        }
    }
    ...

    return exposedObject;
}

protected void addSingletonFactory(String beanName,ObjectFactory<?> singletonFactory) {
    ...
    synchronized (this.singletonObjects) {
        if (!this.singletonObjects.containsKey(beanName)) {
            this.singletonFactories.put(beanName,singletonFactory);  this.earlySingletonObjects.remove(beanName); this.registeredSingletons.add(beanName);
        }
    }
}

protected <T> T doGetBean(final String name,final Class<T> requiredType,final Object[] args,boolean typeCheckOnly) throws BeansException {
    ...
 Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) {
        ...
    }

    else {
        ...
    }

    return (T) bean;
}

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 != NULL_OBJECT ? singletonObject : null);
}