Lack of Kotlin support is maybe not a bug as it is most likely that only Java was initially targeted. Kotlin support should be changed to a feature request.

Changes required to the build.gradle file for a Kotlin project:

(1) Replace java plugin with kotlin.jvm plugin

plugins {
        id 'org.jetbrains.kotlin.jvm' version '2.0.21'
        id 'application'
}

(2) Append Kt to mainClass value, i.e. package-name.App becomes package-name.AppKt

(3) Put source files in src/main/kotlin and not src/main/java folder

(4) An empty settings.gradle file is also required to prevent conflict with top-level settings.gradle file

Applied these changes manually, and I could run the Kotlin app with Gradle.

Minimal code changes were needed to add Kotlin support. Testing the functionality and will submit a pull request later.

Tested both a working Java project (to ensure that we do not break Java support) and a working Kotlin project.

This problem is mentioned in #1944, perhaps we should make this issue a sub-issue of it.

Groovy supported as well.

Jbang export gradle support tested with Jbang-enabled Java, Groovy and Kotlin apps generated by the Tulip cli.

$ source test_gradle_export.sh

File - test_gradle_export.sh

# Activate Java 11 JDK
sdk env

# Build and install jbang
unset JBANG_JAVA_OPTIONS
./gradlew installDist

export JBANG="`pwd`/build/install/jbang/bin/jbang"
echo $JANG

mkdir temp
pushd temp

echo "Java"
sleep 5

mkdir Java
pushd Java
jbang run tulip-cli@wfouche init Java
$JBANG export gradle io/tulip/App.java
cp benchmark_config.json App
cd App
gradle run
popd

echo "Groovy"
sleep 5

mkdir Groovy
pushd Groovy
jbang run tulip-cli@wfouche init Groovy
$JBANG export gradle io/tulip/App.groovy
cp benchmark_config.json App
cd App
gradle run
popd

echo "Kotlin"
sleep 5

mkdir Kotlin
pushd Kotlin
jbang run tulip-cli@wfouche init Kotlin
$JBANG export gradle io/tulip/App.kt
cp benchmark_config.json App
cd App
gradle run
popd

popd