A small Kotlin CLI tool that converts a Minecraft world folder (with at least a /region folder inside) into a .polar file.

It’s built with Gradle and uses:

• picocli for command-line parsing
• hollow-cube/polar for the Anvil -> Polar conversion
• Minestom to make Polar conversion work

• JDK 25
• A world folder that contains at least a region/ directory

You can download the latest release from here.

If you want to build the project, use the snippet below. This project produces a single runnable (fat) jar via the Shadow plugin.

./gradlew build

The jar will be in:

build/libs/polar-converter-1.0.0.jar

Run the CLI from the built jar:

java -jar build/libs/polar-converter-1.0.0.jar \
  -i /path/to/world \
  -o /path/to/output \
  -fN myworld

This creates:

/path/to/output/myworld.polar

• -i, --input (required): input world folder (must exist; should contain region/)
• -o, --output (required): output directory (must exist)
• -fN, --fileName (required): name of the output file (without extension)

Example:

java -jar build/libs/polar-converter-1.0.0.jar -i . -o ./out -fN converted

• The output directory must already exist (the tool currently errors if it doesn’t).
• If you get toolchain/JDK errors, ensure JDK 25 is installed and Gradle can find it.