Create a Docker Image for a Kotlin http4k backend with gradle as a build tool

Question

Iam trying to build a docker image for a Kotlin http4k backend but i cant get it quite working. I can't create a fat jar so my dependencies are missing when i try to run the image.

So i get a ClassNotFound exception.

Here is my build.gradle file:

buildscript {
    ext.kotlinVersion = "1.4.31"
    ext.http4kVersion = "4.5.0.1"

    repositories {
        mavenCentral()
        maven {
            url "https://plugins.gradle.org/m2/"
        }
    }

    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlinVersion}"
    }
}

repositories {
    mavenCentral()
}

apply plugin: 'kotlin'
apply plugin: 'application'

compileKotlin.kotlinOptions.jvmTarget = "11"
compileTestKotlin.kotlinOptions.jvmTarget = "11"

mainClassName = 'com.scalangular.LauncherKt'
sourceCompatibility = JavaVersion.VERSION_11
targetCompatibility = JavaVersion.VERSION_11

test {
    useJUnitPlatform()
}

compileTestKotlin.kotlinOptions {
    jvmTarget = "11"
}

dependencies {
    implementation "org.http4k:http4k-contract:${http4kVersion}"
    implementation "org.http4k:http4k-core:${http4kVersion}"
    implementation "org.http4k:http4k-format-jackson:${http4kVersion}"
    implementation "org.http4k:http4k-server-jetty:${http4kVersion}"
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.4.30"
    // testImplementation ...
}

jar {
    manifest {
        attributes 'Main-Class': mainClassName
    }
}

And here is my Dockerfile:

FROM gradle:latest as builder

COPY --chown=gradle:gradle . /home/gradle/src
WORKDIR /home/gradle/src
RUN gradle build --no-daemon 

FROM openjdk:latest
EXPOSE 9000
RUN mkdir /app
COPY --from=builder /home/gradle/src/build/libs/*.jar /app/ShoppingListApi.jar

CMD [ "java", "-jar", "-Djava.security.egd=file:/dev/./urandom", "/app/ShoppingListApi.jar" ]

I also tryed some gradle plugins which should enable me to build a fatjar but i didnt get it working.

Answer 1

You need to use the shadowjar plugin to create a FatJar. The easiest thing here is to use the http4k toolbox to generate a template project using Shadow and then just copy in the gradle magic from there: https://toolbox.http4k.org/