IIRC the source code for League's engine is originally licensed and derived from a shovelware racing game. Presumably it was rebuilt from scratch, but who knows.
All Wikipedia says is: “A demonstration of League of Legends built in the Warcraft III game engine was completed in four months and then shown at the 2007 Game Developers Conference.”
Maybe Java is used in some backend services here and there, but almost certainly not for the game client, and I'd be very surprised if the game server is written in Java.
No, they made a big deal about rewriting the code a few years back to help make things more stable, easier to maintain, and easier to expand.