jScratch

jScratch compiles Scratch 3 projects (.sb3) into Java bytecode and executable JARs. Every sprite becomes a JVM class, every script a method, and a small bundled runtime provides the stage, the scheduler, input and rendering.

./gradlew :cli:fatJar
java -jar cli/build/libs/cli-0.1.0-SNAPSHOT-all.jar my-game.sb3
java -jar my-game.jar

# or compile a shared project straight from the Scratch website
java -jar cli/build/libs/cli-0.1.0-SNAPSHOT-all.jar https://scratch.mit.edu/projects/10128407/

Highlights

• Real bytecode generated with ASM: primitive arithmetic, real loops, custom blocks as method calls.
• Self-contained output: the JAR carries the runtime, costumes and sounds, and runs on Java 21 or newer.
• Scratch semantics: value conversion, comparison rules, cooperative script scheduling, broadcasts, clones, "run without screen refresh" and the pen behave like the editor.
• Projects are parsed and validated by scratch-parser, the Scratch Foundation's own JavaScript parser, running in Rhino on the JVM and natively in the browser.
• Scratch 1.4 (.sb) and Scratch 2 (.sb2) projects are converted to Scratch 3 with the Scratch Foundation's own code, scratch-sb1-converter and scratch-vm, then compiled like any other.
• An experimental browser playground, built by translating the compiler to JavaScript with TeaVM.

Status

This is an MVP. Typical projects from the Scratch website compile and run, with gaps listed in docs/blocks.md, such as no extensions besides pen.

Repository

Module	Purpose
runtime/	Scratch runtime bundled into compiled JARs
sb3js/	scratch-parser and the Scratch 1.4 and 2 converter, bundled for Rhino and the browser
compiler/	Platform-neutral core: sb3 model, ASM code generator, JAR writer
compiler-jvm/	Rhino bridge to scratch-parser and the converter, JSON through kotlinx-serialization, end-to-end tests
cli/	jscratch command line tool, including project downloads from scratch.mit.edu
web/	TeaVM playground
samples/	Sample projects, zipped into .sb3 and .sb2 by the build
benchmarks/	CPU-heavy projects timed on jScratch, TurboWarp and scratch-vm
docs/	Documentation site (Zensical)

Useful commands:

./gradlew build               # lint, test and assemble everything
./gradlew :web:assembleSite   # playground in web/build/site
zensical serve                # preview the docs

See the documentation for the CLI options, the supported blocks, the architecture and development notes.

License

jScratch is free software released under the GNU General Public License v3.0. It bundles scratch-parser, scratch-vm, scratch-storage and scratch-sb1-converter, which are licensed under the AGPL-3.0.