Spring Avoid full singleton lock for getSingleton(beanName, singletonFactory)

Quick check for existing instance without full singleton lock.

Comment From: jhoeller

That variant of getSingleton is not exposed in the registry/factory interfaces and is only intended to be used in a complex retrieval algorithm where a quick check should be performed through a plain getSingleton(beanName) call first anyway. Introducing another quick check here would effectively introduce unnecessary overhead in the AbstractBeanFactory.doGetBean algorithm, so I'd rather leave it as-is.