"No main manifest attribute" when creating Kotlin jar using IntelliJ IDEA

Question

When creating a jar from my Kotlin code and running it, it says "No main manifest attribute". When looking at the manifest.mf, it has this content:

Manifest-Version: 1.0

When looking at the file in the source, it has this content:

Manifest-Version: 1.0
Main-Class: MyMainClass

When manually copying the source manifest to the jar, it runs perfectly.

Screenshot of my artifact settings

Ambulator

Answer 1

If any of the dependent jars has a MANIFEST.MF file, it will override your custom one which defines the Main-Class.

In order to address this problem you should do the following:

• Disable the alphabetical ordering
• Change items ordering so that item which has META-INF/MANIFEST.MF file is the first in the list
• Your custom MANIFEST.MF will be picked up by IntelliJ IDEA and displayed for the jar artifact.

See the related issue for more details.

You can also use Gradle or Maven to generate the fat jar instead.

Continental

Thanks, it worked! I marked your answer as accepted answer I took the gradle route using this page: mkyong.com/gradle/gradle-create-a-jar-file-with-dependencies – Ambulator

Answer 2

I got this error with Gradle and Kotlin. I had to add in my build.gradle.kts an explicit manifest attribute:

tasks.withType<Jar> {
    manifest {
        attributes["Main-Class"] = "com.example.MainKt"
    }
}

From the gradle documentation, it's better to create a fatJar task to englobe all of the runtime dependencies in case you encounter java.lang.NoClassDefFoundError errors

Lozengy

Answer 3

So far the simplest and best solution I've found is using Ktor Gradle Plugin

plugins {
    id("io.ktor.plugin") version "2.2.3" // Check if it's latest
}

application {
    mainClass.set("com.example.ApplicationKt")
}

ktor {
    fatJar {
        archiveFileName.set("fat.jar")
    }
}

Run it with ./gradlew buildFatJar

Lindsey

Answer 4

1.Add the following task definition in the build script

tasks.jar {
manifest {
    attributes["Main-Class"] = "MainKt"
}
configurations["compileClasspath"].forEach { file: File ->
    from(zipTree(file.absoluteFile))
}
}

2. Then the jar tasks (Tasks | build | jar) again from the right hand sidebar.

Always

Answer 5

For Spring boot apps:

What worked for me (gradle kotlin) in build.gradle.kts

1. add spring boots plugin &. apply dependency management

plugins {
    id("org.springframework.boot") version "2.6.7"
}
apply(plugin = "io.spring.dependency-management")

2. set your main class

springBoot {
    mainClass.set("com.example.Application")
}

Found this all by reading up on spring-boot docs found here

Shoreward