how would the container replace the null with serizable handle?
Why would the container need to replace it with anything? When the container activates the bean and realizes it doesn't have the magical serialized state object for one of the fields, it leaves it as null. Simple as that. Another approach would be for the container to replace null's with some kind of NullObject reference. Any of these internals don't affect the bean developer -- null in, null out.
In addition, what is the replacement for beans' home and remote interfaces?
The references to EJBs are already serializable (it's the container generated classes you're using, not the one(s) you typed in). How the container implements this is, again, it's own business and the developer doesn't need to care about it. Probably (if I had to make a guess) the container uses the same
EJB handle mechanism as is available to you as a developer...