'Building a self-executable JAR with Gradle and Kotlin

I've written a simple Kotlin source file in order to get started, and a Gradle script file. But I can't figure out how to add the main function to the manifest, so that the JAR could be self-executable.

Here is my build.gradle script :

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:0.9.66'
    }
}
apply plugin: "kotlin"
repositories {
    mavenCentral()
}
dependencies {
    compile 'org.jetbrains.kotlin:kotlin-stdlib:0.9.66'
}

jar {
    manifest {
        attributes 'Main-Class': 'com.loloof64.kotlin.exps.ExpsPackage'
    }
}

Here is my com.loloof64.kotlin.exps.Multideclarations.kt:

package com.loloof64.kotlin.exps

class Person(val firstName: String, val lastName: String) {
    fun component1(): String {
        return firstName
    }
    fun component2(): String {
        return lastName
    }
}

fun main(args: Array < String > ) {
    val(first, last) = Person("Laurent", "Bernabé")
    println("First name : $first - Last name : $last")
}

When I launch the JAR from terminal (java -jar MYJar.jar) I get the following stack trace, saying that the Kotlin reflection library classes are missing, and indeed they have not been added to the JAR. It seems that I am missing the kotlin-compiler artifact classes from the final JAR, and also the kotlin-stdlib sources, but I don't know how to adapt the Gradle build.

$> java -jar build/libs/kotlin_exps.jar 

Exception in thread "main" java.lang.NoClassDefFoundError: kotlin/reflect/jvm/internal/InternalPackage
    at com.loloof64.kotlin.exps.ExpsPackage.<clinit>(MultiDeclarations.kt)
Caused by: java.lang.ClassNotFoundException: kotlin.reflect.jvm.internal.InternalPackage
    at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 1 more

I am using Kotlin 0.9.66 and Gradle 2.1.

Solution 1:^[1]

Add the plugin application, then set the mainClassName as

mainClassName = '[your_namespace].[your_arctifact]Kt'

For instance, suppose you have placed the following code in a file named main.kt:

package net.mydomain.kotlinlearn

import kotlin
import java.util.ArrayList

fun main(args: Array<String>) {

    println("Hello!")

}

your build.gradle should be:

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

mainClassName = "net.mydomain.kotlinlearn.MainKt"

In fact Kotlin is building a class to encapsulate your main function named with the same name of your file - with Title Case.

Solution 2:^[2]

None of the above solution worked for me while building Artifact.

IDE version IntelliJ IDEA 2019.1.1.

To fix the issue, do the following

Steps

Step 1 - Create Artifact

Go to File -> Project Structure -> Artifacts
Click the + -> JAR -> From modules with dependencies

Select your program's Main Class

Step 2 - Change MANIFEST Path

Change value of Directory for META-INF/MANIFEST.MF to your project root.

For example , from /your/project/directory/src/main/kotlin to /your/project/directory

Press OK,then Press Apply and OK.

Step 3 - Build Artifact

Finally, Go to Build -> Build Artifacts -> [your-artifact-name] -> Build.

The generated JAR file can be found in the out/artifact/[your-artifact-name] directory. (y)

Solution 3:^[3]

In case you happen to be dumb, like me:

Don't create your IntelliJ run configuration as an Application. IntelliJ will assume it's Java & it will never work. Instead, use the "Kotlin" entry.

Solution 4:^[4]

Thanks, for me it worked adding the jar section

jar {
    manifest {
        attributes 'Main-Class': 'com.photofiles.Application'
    }

    from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
}

No need for the application plugin.

Solution 5:^[5]

If using Gradle with the Kotlin DSL, then my duplicate question has an answer of:

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar

plugins {
    kotlin("jvm") version "1.2.51"
    id("com.github.johnrengelman.shadow") version "2.0.4"
}

group = "xxx.yyy"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    implementation(kotlin("stdlib-jdk8"))
}

tasks.withType<KotlinCompile> {
    kotlinOptions.jvmTarget = "1.8"
}

tasks.withType<ShadowJar> {

    manifest.attributes.apply {
        put("Implementation-Title", "Gradle Jar File Example")
        //put("Implementation-Version" version)
        put("Main-Class", "HelloKotlinWorld.App")
    }

Which is, I think, the simplest solution. Oh, perhaps you're using just Kotlin itself and not the DSL.

Solution 6:^[6]

For anyone using the Kotlin MP plugin here is your code

jvm{
    jvmJar {
        manifest{
            attributes 'Main-Class':'Class path here'
        }
    }

}

Solution 7:^[7]

This is for Kotlin DSL (build.gradle.kts).
Note that the first 3 methods here modify the existing Jar task of Gradle.

Method 1: Placing library files beside the result JAR

This method does not need application or any other plugins.

tasks.jar {
    manifest.attributes["Main-Class"] = "com.example.MyMainClass"
    manifest.attributes["Class-Path"] = configurations
        .runtimeClasspath
        .get()
        .joinToString(separator = " ") { file ->
            "libs/${file.name}"
        }
}

Note that Java requires us to use relative URLs for the Class-Path attribute. So, we cannot use the absolute path of Gradle dependencies (which is also prone to being changed and not available on other systems). If you want to use absolute paths, maybe this workaround will work.

Create the JAR with the following command:

./gradlew jar

The result JAR will be created in build/libs/ directory by default.

After creating your JAR, copy your library JARs in libs/ sub-directory of where you put your result JAR. Make sure your library JAR files do not contain space in their file name (their file name should match the one specified by ${file.name} variable above in the task).

Method 2: Embedding the libraries in the result JAR file (fat or uber JAR)

This method too does not need any Gradle plugin.

tasks.jar {
    manifest.attributes["Main-Class"] = "com.example.MyMainClass"
    val dependencies = configurations
        .runtimeClasspath
        .get()
        .map(::zipTree) // OR .map { zipTree(it) }
    from(dependencies)
    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
}

Creating the JAR is exactly the same as the previous method.

Method 3: Using the Shadow plugin (to create a fat or uber JAR)

plugins {
    id("com.github.johnrengelman.shadow") version "6.0.0"
}
// Shadow task depends on Jar task, so these configs are reflected for Shadow as well
tasks.jar {
    manifest.attributes["Main-Class"] = "org.example.MainKt"
}

Create the JAR with this command:

./gradlew shadowJar

See Shadow documentations for more information about configuring the plugin.

Method 4: Creating a new task (instead of modifying the `Jar` task)

tasks.create("MyFatJar", Jar::class) {
    group = "my tasks" // OR, for example, "build"
    description = "Creates a self-contained fat JAR of the application that can be run."
    manifest.attributes["Main-Class"] = "com.example.MyMainClass"
    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
    val dependencies = configurations
        .runtimeClasspath
        .get()
        .map(::zipTree)
    from(dependencies)
    with(tasks.jar.get())
}

Running the created JAR

java -jar my-artifact.jar

The above solutions were tested with:

Java 17
Gradle 7.1 (which uses Kotlin 1.4.31 for .kts build scripts)

See the official Gradle documentation for creating uber (fat) JARs.
For more information about manifests, see Oracle Java Documentation: Working with Manifest files.
For difference between tasks.create() and tasks.register() see this post.

Note that your resource files will be included in the JAR file automatically (assuming they were placed in /src/main/resources/ directory or any custom directory set as resources root in the build file). To access a resource file in your application, use this code (note the / at the start of names):

Kotlin

val vegetables = MyClass::class.java.getResource("/vegetables.txt").readText()
// Alternative ways:
// val vegetables = object{}.javaClass.getResource("/vegetables.txt").readText()
// val vegetables = MyClass::class.java.getResourceAsStream("/vegetables.txt").reader().readText()
// val vegetables = object{}.javaClass.getResourceAsStream("/vegetables.txt").reader().readText()

Java

var stream = MyClass.class.getResource("/vegetables.txt").openStream();
// OR var stream = MyClass.class.getResourceAsStream("/vegetables.txt");

var reader = new BufferedReader(new InputStreamReader(stream));
var vegetables = reader.lines().collect(Collectors.joining("\n"));

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution	Source
Solution 1	StefMa
Solution 2	Community
Solution 3	Tim Keating
Solution 4	paucls
Solution 5	Eric Aya
Solution 6	Alaa Sbateen
Solution 7	Mahozad

'Building a self-executable JAR with Gradle and Kotlin

Solution 1:[1]

Solution 2:[2]

Solution 3:[3]

Solution 4:[4]

Solution 5:[5]

Solution 6:[6]

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