SHEET 01 — MINECRAFT MOD REV 1.0 · MIT

Learn Java inside Minecraft

A Fabric mod that puts a real code editor in the game window — no alt-tabbing to a browser. It compiles what you type at runtime and runs it against the world you are standing in. Write a loop, press Run, watch a tower go up in front of you.

View source → Run it yourself

JAVA 21 FABRIC 1.21.1 LESSONS 18 TRACKS 3 GRADLE + LOOM

FIG. A — EXECUTION PATH SCALE 1:1

01 Editor In-game screen

source

02 javac In memory

bytes

03 Loader Knot-aware

invoke

04 Playground No MC types

05 Server thread Marshalled

06 The world Blocks move

Compile 5 s timeout Run

02

Why it is built this way

4 NOTES

FIG. 01NO ALT-TAB

The editor is a game screen

Not a localhost page sitting next to the game. You never leave the window, so the gap between writing a line and seeing it happen is one keypress wide.

Open with: G / /codecraft
Panes: List · Code · Console

FIG. 02REAL COMPILER

Real Java, really compiled

Not a block editor and not a scripting DSL. Source goes through javax.tools.JavaCompiler exactly as javac would, so the compiler errors you learn to read are the real ones.

Needs: JDK, not JRE
Timeout: 5 s daemon

FIG. 03CURRICULUM

The whole language

Not limited to the parts that touch Minecraft. Exceptions, generics, lambdas, streams and recursion are all in there — the world is just what makes them visible.

Lessons: 18
Levels: 3

FIG. 04SELF-TEST

Verified by playing itself

A marker file arms an in-game self-test: it opens the screens, exercises the command, scrolls the list, saves screenshots, then compiles and runs every lesson against a live world, timing each.

Coverage: 18 / 18
Slowest: 930 ms

03

The editor

FIG. 05

CODECRAFT — IN-GAME SCREEN TRACK: CODED BEFORE

Core Java

✓Methods

✓Arrays

✓Nested Loops

●Lists

●Classes

●Inheritance

Advanced

●Maps

●Exceptions

●Generics

●Lambdas

public class Lesson { public static void main(String[] args) { int size = 7; int placed = 0; // A loop inside a loop sweeps a 2D area: // every x, and for each x, every z. for (int x = 0; x < size; x++) { for (int z = 0; z < size; z++) { Playground.placeBlock(x, 0, z, "polished_andesite"); placed++; } } System.out.println("placed " + placed + " blocks"); Playground.say("Platform built."); } }

> NESTED LOOPS — CONTROL FLOW Put a loop inside a loop and you sweep a whole 2D area. A 7 by 7 area means the inner body runs 49 times. ▸ compiled in 210 ms placed 49 blocks ✓ finished — lesson complete

Run ▶ Reset 49 blocks placed in front of you

04

What it teaches

18 UNITS

On first open it asks how much Java you already know. Your answer only sets where the lesson list starts — nothing is deleted, and the Track button switches at any time.

**TABLE 01** — EXPERIENCE TRACKS
Track	Starts at	Units
New to code	Fundamentals — what a variable is	18
Coded before	Core Java — methods, arrays, classes	13
Know Java	Advanced — maps, generics, streams	7

01Hello WorldF

02VariablesF

03MathF

04ConditionalsF

05LoopsF

06Nested LoopsC

07MethodsC

08ArraysC

09ListsC

10ClassesC

11InheritanceC

12Maps & CountingA

13ExceptionsA

14GenericsA

15LambdasA

16StreamsA

17RecursionA

18Capstone — a baseA

F — FUNDAMENTALS · C — CORE JAVA · A — ADVANCED

05

Playground API

12 CALLS

Lesson code calls Playground to touch the world. Every position is a relative offset from where you are standing, so (0, 1, 0) is the block above your head.

**TABLE 02** — WORLD CALLS
Call	Effect
say(text)	Chat message to yourself
showTitle(text)	Flash text above the hotbar
placeBlock(dx, dy, dz, id)	Place a block, e.g. `"stone"`
breakBlock(dx, dy, dz)	Replace a block with air
getBlock(dx, dy, dz)	Read a block id back out of the world
spawnEntity(dx, dy, dz, id)	Spawn a mob, e.g. `"cow"`
teleportPlayer(dx, dy, dz)	Move yourself
particles(dx, dy, dz, id, n)	Spray particles
giveItem(id, count)	Put an item in your inventory
playSound(id)	Play a sound at your feet
playerX() / playerY() / playerZ()	Your current world coordinates
isDay()	Whether it is daytime

06

How it works

NOTES

NOTE ACOMPILE

Source is compiled in memory: a ForwardingJavaFileManager captures the .class bytes, a small classloader picks them up, and the program runs on a timeboxed daemon thread.

NOTE BCLASSPATH

A Fabric mod's classes are loaded by Knot, not from java.class.path — so javac cannot find Playground on a normal classpath. The file manager hands those classes to the compiler straight out of the mod's own loader.

NOTE CBOUNDARY

The whole Playground surface is deliberately free of Minecraft types, for the same reason. The Minecraft-aware implementation sits behind an interface, and calls are marshalled onto the server thread before they touch a block.

NOTE DDATA

Lessons are data, not code: a .json of explanation paragraphs beside a real .java starter file. Adding one is two files and an index entry — the level field decides which tracks show it.

07

Run it yourself

SETUP

REQUIREMENTS

Minecraft Java Edition 1.21.1
Fabric Loader 0.19.3+ and Fabric API
A JDK 21 — not just a JRE. CodeCraft compiles at runtime and needs a real compiler on hand.
Join a singleplayer world, then press G or run /codecraft.

FROM SOURCE

$ git clone https://github.com/vanshxnar/codecraft
$ cd codecraft
$ ./gradlew build       # jar in build/libs
$ ./gradlew runClient   # dev client, mod loaded

08

Scope and limits

READ THIS

Singleplayer only. World calls run against the integrated server.
Not sandboxed. Submitted code runs with the game's permissions, guarded only by a timeout. Built for one learner running their own code locally — not a public server accepting code from strangers.
A tight infinite loop cannot be force-killed. It is abandoned as a daemon thread, and you are told so.

LICENCE & TRADEMARKS

MIT. Not affiliated with or endorsed by Mojang or Microsoft. CodeCraft ships no Minecraft assets — it uses only the public Fabric modding API, against a copy of the game you already own.

Source → Back to index