This page explains how to generate a Webstart packaging of muCommander.

The Gradle's shadowJar tasks produces a standalone jar (that includes all the runtime dependencies)

./gradlew shadowJar

Add a new keystore. By default ~/.keystore is used, you can use alternative file with -keystore option

keytool -genkey -alias mucommander

jarsigner -tsa https://timestamp.digicert.com mucommander.jar mucommander

With that, the signed jar is ready for upload.
Some of the information in this page was taken from this tutorial.