Technically, the field is a JGroups Address, which is an interface. I've never seen any implementation other than the JGroups UUID actually be used though. A JGroups UUID is not a subclass of java.lang.UUID.

Why are the bridge implementation classes not part of the public API? I'm assuming that means I shouldn't be using anything in the org.hibernate.search.bridge.builtin.impl package, which seems silly. That means I have to define my own bridge just to use toString(), and it seems like that would be a very common way of mapping a field. I'm also using BuiltinIterableBridge, so I guess I'd need to subclass IterableBridge instead in order to get a bridge that can handle an Iterable and also has a no-arg constructor. But BuiltinIterableBridge already does that, so again, it seems silly to need to define another class to do the exact same thing.