Kotlin-multiplatform: ShadowJar gradle plugin creates empty jar - kotlin

Question

I tried to use ShadowJar gradle pluging to pack my ktor app into fat jar. But as result of shadowJar task i get every time almost empty jar. It contains only manifest (main class is properly set).
Gradle configuration (groovy):
import org.gradle.jvm.tasks.Jar
buildscript {
ext.kotlin_version = '1.3.72'
ext.ktor_version = '1.3.2'
ext.serialization_version = '0.20.0'
ext.sl4j_version = '1.6.1'
repositories { jcenter() }
dependencies {
classpath 'com.github.jengelman.gradle.plugins:shadow:5.2.0'
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
classpath "org.jetbrains.kotlin:kotlin-serialization:$kotlin_version"
}
}
plugins {
id 'org.jetbrains.kotlin.multiplatform' version '1.3.61'
}
apply plugin: 'kotlinx-serialization'
apply plugin: 'application'
apply plugin: 'java'
mainClassName = 'com.example.JvmMainKt'
apply plugin: 'com.github.johnrengelman.shadow'
repositories {
mavenCentral()
jcenter()
}
group 'com.example'
version '0.0.1'
apply plugin: 'maven-publish'
kotlin {
jvm {
}
js {
browser {
}
nodejs {
}
}
// For ARM, should be changed to iosArm32 or iosArm64
// For Linux, should be changed to e.g. linuxX64
// For MacOS, should be changed to e.g. macosX64
// For Windows, should be changed to e.g. mingwX64
mingwX64("mingw") {
binaries {
executable {
// Change to specify fully qualified name of your application's entry point:
entryPoint = 'main'
// Specify command-line arguments, if necessary:
runTask?.args('')
}
}
}
sourceSets {
commonMain {
dependencies {
implementation kotlin('stdlib-common')
implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime-common:$serialization_version"
implementation "io.ktor:ktor-client-core:$ktor_version"
}
}
commonTest {
dependencies {
implementation kotlin('test-common')
implementation kotlin('test-annotations-common')
}
}
jvmMain {
dependencies {
implementation kotlin('stdlib-jdk8')
implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime:$serialization_version"
implementation "io.ktor:ktor-serialization:$ktor_version"
implementation "io.ktor:ktor-server-netty:$ktor_version"
implementation "org.slf4j:slf4j-simple:$sl4j_version"
}
}
jvmTest {
dependencies {
implementation kotlin('test')
implementation kotlin('test-junit')
}
}
jsMain {
dependencies {
implementation kotlin('stdlib-js')
implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime-js:$serialization_version"
}
}
jsTest {
dependencies {
implementation kotlin('test-js')
}
}
mingwMain {
dependencies {
implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime-native:$serialization_version"
}
}
mingwTest {
}
}
}
shadowJar {
mainClassName = 'com.example.JvmMainKt'
mergeServiceFiles()
}

Answer 1

marcu's answer is correct but he doesn't explain why that snipped solved his issue, and he's code snippet cannot be directly converted to build.gradle.kts because a cast is needed to see runtimeDependencyFiles from Kotlin. In groovy, his snippet works because groovy supports duck typing while Kotlin doesnt.
I needed this solution in Gradle Kotlin, so I'm sharing it this way.
The com.github.johnrengelman.shadow gradle plugin is designed to work with regular the java gradle plugin, which builds a single jar by default, so it automatically generates a single fat-jar based on that jar classpath.
The Kotlin-Multiplatform gradle plugin works differently, it creates jar files for each jvm target based on a lot of custom settings that is set up using a methodology that is unique to Kotlin-Multiplatform, that's why the default shadowJar task does not work out of the box on Kotlin-Multiplatform.
To solve this problem, we have to create a new ShadowJar task for each jvm target that we need a fat-jar manually, we can do it after we declare the targets (as marcu's example is showing), or during the creation of them. I'm going to show both ways, but I recommend doing during the creation to avoid having to cast objects.
Another thing is that I created function to apply the needed configuration because I have a 8 JVM targets, so this way I don't need to copy-paste the snipped 8 times, I only call the function instead.
During the creation of the target
The explanations are commented on the code:
// Add this imports on top of your build.gradle.kts file
import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
import org.jetbrains.kotlin.gradle.targets.jvm.KotlinJvmTarget
// Since you need a main class, I added this default constant
// which you can change as you need:
val defaultMainClassName: String? = null
// Here I declare the function that will register the new ShadowJar task for the target
// If you need another mainClassName, you can pass it as parameter
fun KotlinJvmTarget.registerShadowJar(mainClassName: String? = defaultMainClassName) {
// We get the name from the target here to avoid conflicting
// with the name of the compilation unit
val targetName = name
// Access the main compilation
// We only want to create ShadowJar
// for the main compilation of the target, not the test
compilations.named("main") {
// Access the tasks
tasks {
// Here we register our custom ShadowJar task,
// it's being prefixed by the target name
val shadowJar = register<ShadowJar>("${targetName}ShadowJar") {
// Allows our task to be grouped alongside with other build task
// this is only for organization
group = "build"
// This is important, it adds all output of the build to the fat-jar
from(output)
// This tells ShadowJar to merge all jars in runtime environment
// into the fat-jar for this target
configurations = listOf(runtimeDependencyFiles)
// Here we configure the name of the resulting fat-jar file
// appendix makes sure we append the target name before the version
archiveAppendix.set(targetName)
// classifier is appended after the version,
// it's a common practice to set this classifier to fat-jars
archiveClassifier.set("all")
// Apply the main class name attribute
if (mainClassName != null) {
manifest {
attributes("Main-Class" to mainClassName)
}
}
// This instruction tells the ShadowJar plugin to combine
// ServiceLoader files together, this is needed because
// a lot of Kotlin libs uses service files and
// they would break without this instruction
mergeServiceFiles()
}
// Finally, we get the normal jar task for this target
// and tells kotlin to execute our recently created ShadowJar task
// after the normal jar task completes
getByName("${targetName}Jar") {
finalizedBy(shadowJar)
}
}
}
}
kotlin {
// Here we create a JVM target
jvm("jvm8") {
// We can configure anything we want
compilations.all {
kotlinOptions.jvmTarget = "1.8"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
// If we need a ShadowJar for it,
// all we have to do now is call
// our custom function
// Here's an example of what I'm saying:
// https://stackoverflow.com/a/57951962/804976
registerShadowJar()
}
// Another one just to illustrate the example
jvm("jvm16") {
compilations.all {
kotlinOptions.jvmTarget = "16"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
registerShadowJar()
}
}
With comments removed
import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
import org.jetbrains.kotlin.gradle.targets.jvm.KotlinJvmTarget
val defaultMainClassName: String? = null
fun KotlinJvmTarget.registerShadowJar(mainClassName: String? = defaultMainClassName) {
val targetName = name
compilations.named("main") {
tasks {
val shadowJar = register<ShadowJar>("${targetName}ShadowJar") {
group = "build"
from(output)
configurations = listOf(runtimeDependencyFiles)
archiveAppendix.set(targetName)
archiveClassifier.set("all")
if (mainClassName != null) {
manifest {
attributes("Main-Class" to mainClassName)
}
}
mergeServiceFiles()
}
getByName("${targetName}Jar") {
finalizedBy(shadowJar)
}
}
}
}
kotlin {
jvm("jvm8") {
compilations.all {
kotlinOptions.jvmTarget = "1.8"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
registerShadowJar()
}
jvm("jvm16") {
compilations.all {
kotlinOptions.jvmTarget = "16"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
registerShadowJar()
}
}
After the creation of the target
Now, I'm going to comment only the changes:
import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
import org.jetbrains.kotlin.gradle.targets.jvm.KotlinJvmTarget
kotlin {
jvm("jvm8") {
compilations.all {
kotlinOptions.jvmTarget = "1.8"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
// Not registering on creation this time,
// we are going to register the task later
}
jvm("jvm16") {
compilations.all {
kotlinOptions.jvmTarget = "16"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
}
}
val defaultMainClassName: String? = null
// Instead of having KotlinJvmTarget as receiver,
// we are going to cast the target to it.
// We are also getting the target name from
// the function parameter
fun registerShadowJar(targetName: String, mainClassName: String? = defaultMainClassName) {
// Get the target casting to KotlinJvmTarget in the process
kotlin.targets.named<KotlinJvmTarget>(targetName) {
// Access the main compilation
compilations.named("main") {
tasks {
val shadowJar = register<ShadowJar>("${targetName}ShadowJar") {
group = "build"
from(output)
configurations = listOf(runtimeDependencyFiles)
archiveAppendix.set(targetName)
archiveClassifier.set("all")
if (mainClassName != null) {
manifest {
attributes("Main-Class" to mainClassName)
}
}
mergeServiceFiles()
}
getByName("${targetName}Jar") {
finalizedBy(shadowJar)
}
}
}
}
}
// Now we call the method for each JVM target that we need to create a ShadowJar
registerShadowJar("jvm8")
registerShadowJar("jvm16")
With comments removed
import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
import org.jetbrains.kotlin.gradle.targets.jvm.KotlinJvmTarget
kotlin {
jvm("jvm8") {
compilations.all {
kotlinOptions.jvmTarget = "1.8"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
}
jvm("jvm16") {
compilations.all {
kotlinOptions.jvmTarget = "16"
}
testRuns["test"].executionTask.configure {
useJUnitPlatform()
}
}
}
val defaultMainClassName: String? = null
fun registerShadowJar(targetName: String, mainClassName: String? = defaultMainClassName) {
kotlin.targets.named<KotlinJvmTarget>(targetName) {
compilations.named("main") {
tasks {
val shadowJar = register<ShadowJar>("${targetName}ShadowJar") {
group = "build"
from(output)
configurations = listOf(runtimeDependencyFiles)
archiveAppendix.set(targetName)
archiveClassifier.set("all")
if (mainClassName != null) {
manifest {
attributes("Main-Class" to mainClassName)
}
}
mergeServiceFiles()
}
getByName("${targetName}Jar") {
finalizedBy(shadowJar)
}
}
}
}
}
registerShadowJar("jvm8")
registerShadowJar("jvm16")

Answer 2

I've faced the same issue in the past, the syntax that was working (for groovy gradle) for me was:
shadowJar {
mergeServiceFiles()
manifest {
attributes 'Main-Class': 'com.example.JvmMainKt'
}
}

Answer 3

I finally found solution to my problem.
import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
//...
task shadowJar2(type: ShadowJar) {
def target = kotlin.targets.jvm
from target.compilations.main.output
def runtimeClasspath = target.compilations.main.runtimeDependencyFiles
manifest {
attributes 'Main-Class': mainClassName
}
configurations = [runtimeClasspath]
}