[parenthephobia]

The official server isn't open source, but the Java bytecode can be decompiled into something approximating usable code. The protocol itself is also unencrypted and unobfuscated. All of this together makes it relatively simple to reverse engineer it.

Aside: Documentation obtained this way is available at http://wiki.vg/Protocol.

Some slides about the process: https://www.grahamedgecombe.com/talks/minecraft.pdf