Consider generics when predicting FactoryBean types

[philwebb]

It would be beneficial if we could consider generics when trying to predict the result of FactoryBean.getObjectType(). We currently have difficulty with getBeanNamesForType if the bean being considered is a factory bean and we don't want to initialize it early.

It's possible that we can still deduce the type using generics of the factory method, or the class itself. For example:

@Bean
public FactoryBean<MyBean> myBean() {
    ...
}

public class MyFactoryBean implements FactoryBean<MyBean> {
    ...
}

We have logic to do this already as part of DefaultListableBeanFactory.getTypeForFactoryBean, however, in its current form that method may also cause early initialization of the bean.

[philwebb]

I found an annoying edge case that means any @Configuration class picked up by component scanning won't be detected correctly. This is because the bean definition has a String type not a Class type and hasBeanClass() return false.

I think I have a fix in this branch

[jhoeller]

Resolving a FactoryBean class along the lines of that branch looks ok to me...