alt Hacker News> Unless, of course, it really was a case of "reverse engineer and reimplement Minecraft to make my own server." Not strictly impossible, but a completely unreasonable goal.
Hasn't that already been done several times? There's feather-rs (fairly mature but reduced set of features) and Pumpkin and FerrumC. And, for that matter, Paper - that one is Java and I haven't checked to what degree it's just borrowing from the original implementation, but I do know that Paper is actually used for real servers.
There are quite a few (the protocol is pretty simple and the client is very forgiving of omissions). They are often not as fully featured as the official server, though. Probably the most significant omission on most of them is world generation (I've seen some suggestions that this is kind of an unofficial line in terms of not catching legal threats).