Kotlin

Detection

migetpacks detects a Kotlin application when any of the following conditions are met:

settings.gradle.kts exists
.kt files exist in src/ directory
build.gradle.kts contains the kotlin plugin
build.gradle (Groovy DSL) contains a Kotlin plugin reference

Kotlin detection takes priority over Java/Gradle detection. If your project has build.gradle.kts with Kotlin plugins, it will be identified as Kotlin rather than Java.

Version Detection

Kotlin Version

Kotlin version is resolved in the following priority order:

Source	Example
`.kotlin-version`	`2.1.0`
`gradle.properties` (`kotlin.version`)	`kotlin.version=2.1.0`
`build.gradle.kts` (plugin version)	`kotlin("jvm") version "2.1.0"`
Default	`2.1.0`

Java Version

The JDK version used for compilation and runtime is read from:

Source	Example
`system.properties`	`java.runtime.version=21`
Default	`21`

Build Process

migetpacks generates a multi-stage Dockerfile using Eclipse Temurin JDK with Gradle:

# Build stage
FROM gradle:jdk21 AS builder
WORKDIR /build

# Kotlin build environment
ENV JAVA_TOOL_OPTIONS="-Dfile.encoding=UTF-8"

# Layer caching: copy Gradle build files first
COPY gradlew ./
COPY gradle/ gradle/
COPY build.gradle* settings.gradle* ./

# Download dependencies (cached if build files unchanged)
RUN ./gradlew dependencies --no-daemon || true

# Copy source code
COPY . .

# Build and cleanup
RUN ./gradlew build -x test --no-daemon \
    && rm -rf src/ gradle/ gradlew* build.gradle* settings.gradle*

# Runtime stage
FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=builder --chown=1000:1000 /build /app

Gradle Wrapper

If gradlew is present in the project root, migetpacks uses the Gradle Wrapper for correct version compatibility. Otherwise, it falls back to the system Gradle.

Run Command

The default run command for Kotlin applications:

java -jar build/libs/*.jar

Override with RUN_COMMAND or a Procfile:

web: java -jar build/libs/myapp-1.0-all.jar

JVM Configuration

The following environment variables are set for production:

Variable	Value
`JAVA_OPTS`	`-Dfile.encoding=UTF-8 -XX:MaxRAMPercentage=80.0`
`JAVA_TOOL_OPTIONS`	`-Dfile.encoding=UTF-8`

Caching

Layer Caching

Dependencies are cached by copying Gradle build files before the full source:

Cached Files	Purpose
`build.gradle.kts` / `build.gradle`	Build configuration
`settings.gradle.kts` / `settings.gradle`	Project settings
`gradlew` + `gradle/`	Gradle Wrapper

Scenario	Dependency Install
First build	Runs
Source code change only	Cached
Build file change	Runs

BUILD_CACHE_DIR

When BUILD_CACHE_DIR is set, Gradle caches persist at:

Cache Path	Contents
`/cache/gradle`	Gradle caches and wrapper downloads

DHI Support

Docker Hardened Images are supported for Kotlin builds.

Stage	Image
Build	`dhi.io/eclipse-temurin:21-jdk-dev`
Runtime	`dhi.io/eclipse-temurin:21`

DHI Notes

Java version is read from system.properties (java.runtime.version), defaulting to 21.
Eclipse Temurin DHI JDK images include the JDK but Gradle must be available (via wrapper or system install).
Runtime images are distroless (no shell).
The nonroot user is used instead of the miget user.

Example with DHI

docker run --rm \
  -v /path/to/kotlin-app:/workspace/source \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e OUTPUT_IMAGE=my-kotlin-app:latest \
  -e USE_DHI=true \
  miget/migetpacks:latest

Example

docker run --rm \
  -v /path/to/kotlin-app:/workspace/source \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e OUTPUT_IMAGE=my-kotlin-app:latest \
  miget/migetpacks:latest

With build cache

docker run --rm \
  -v /path/to/kotlin-app:/workspace/source \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v /tmp/build-cache:/cache \
  -e OUTPUT_IMAGE=my-kotlin-app:latest \
  -e BUILD_CACHE_DIR=/cache \
  miget/migetpacks:latest

Getting Started

Guides

Configuration

Languages

Deploying

Reference

Detection

Version Detection

Kotlin Version

Java Version

Build Process

Gradle Wrapper

Run Command

JVM Configuration

Caching

Layer Caching

BUILD_CACHE_DIR

DHI Support

DHI Notes

Example with DHI

Example

With build cache

​Detection

​Version Detection

​Kotlin Version

​Java Version

​Build Process

​Gradle Wrapper

​Run Command

​JVM Configuration

​Caching

​Layer Caching

​BUILD_CACHE_DIR

​DHI Support

​DHI Notes

​Example with DHI

​Example

​With build cache

Detection

Version Detection

Kotlin Version

Java Version

Build Process

Gradle Wrapper

Run Command

JVM Configuration

Caching

Layer Caching

BUILD_CACHE_DIR

DHI Support

DHI Notes

Example with DHI

Example

With build cache