Our company has a large codebase (2500+ classes/interfaces in just the core alone, many more in other projects) for our flagship software product. We've never really hired more than one developer at a time, so we don't have a real training process. We're going to be bringing in 2-5 more developers now, and probably more in the near future (to put things into perspective, we have 7 right now.) Obviously, we would like to get these guys up to speed as soon as possible.

The catch - almost all of our classes (95%+) are completely undocumented. No javadoc, no design docs, basically completely undocumented.

What strategies can we employ to bring the new developers up to speed? I'd like to consider situations that include the existing code getting documented, but it's possible management won't allow for the time to get that done, so I also must consider situations where that won't happen.

Has anyone been there before? What worked well for you?

Thanks!