I · Anatomy

Verified against Minecraft 26.2 · Part I · The whole program at once: which threads exist, which loop each runs, and how the two halves talk.

Part I is the program the other twelve parts run inside. Java Minecraft is one codebase running as two programs on four threads worth memorising — a server whose whole life is a tick loop, and a client whose life is a frame loop with ticks inside it — and almost every surprise later in this book is one of those two waiting on the other. A player recognises the split by its symptom: the game that keeps drawing while the world stands still, because the two are not the same loop and never were. Nearly every lane in the sequence diagrams after this part is a class, and the handful that are not stand for a thread — which is the same reason for reading this part first either way.

The shape of the part

Two pages: what the program is, and where it ends. The first is a trace and the second is a boundary, and the boundary is drawn second because a reader decides whether to go on once they can see the whole program.

flowchart LR
    A["Anatomy: four threads, two loops, one wire — from main to a running singleplayer world"]
    B["What this book skips: the fourteen packages the parts do not reach, and why"]
    A -- "now that you can see the whole program, here is the part of it this book does not teach" --> B

Before you start

Nothing. This is the first part, and it assumes only that you have played the game.

Watch in this order

1. Anatomy — from main to a running singleplayer world: the Render thread that is also the game thread, the Server thread that is the world, the Netty threads that run more than bytes, and the one CPU pool that chunk generation, lighting and section meshing all share.
2. What this book skips — the boundary, drawn honestly on the treemap of the jar: save migration, Realms, telemetry, the profiler, the management server, the data generators, and where to start if you need one anyway. It is the second lecture and not the last because a boundary is worth drawing before the investment, not after.

Reference this part uses

Threads — every thread, who makes it and what may run on it; Part I names the four to memorise and this is the rest. Diagram lanes — the abbreviations every sequence diagram uses, needed from the first figure of the first lecture onward. Naming drift — the 1.21-era names that have moved. The atlas — where everything is, and the treemap the second lecture reads.

Where the part stops

Part I says which threads exist and hands each of them on. What the Server thread does with a tick is Part III; what the frame loop does between ticks is Part X; what crosses the wire between them is Part IX. This part owes you only enough of each to read a lane.

Rules: names, never code · how the system works, not how the code reads · newest version only · every backticked name passes tools/verify_names.py.