How to execute Kotlin Multiplatform Jar

Question

I have a full stack Kotlin Multiplatform web app with Kotlin/JVM backend and Kotlin/JS frontend.

The problem I'm having is that, when I go to execute the JAR, there is no Main-Class manifest entry and I get this error:

$ java -jar shoppinglist-jvm-1.0-SNAPSHOT.jar
no main manifest attribute, in shoppinglist-jvm-1.0-SNAPSHOT.jar

I assume that this is, for some reason, by design and that I'm missing something essential.

Further Details:

• I am using the Ktor example project provided by Jetbrains, but this is also occurring my own project in the same way.
• I am executing the jvm JAR in the libs folder, but I also attempted it on both the js and metadata JARs with the same result.
• I am using gradle build within IntelliJ Run Configurations to compile.

Extracting the JARs with 7-Zip reveals that js and metadata don't contain Java bytecode; so, I'm, mostly, ignoring those for now.

However, jvm has interesting contents:

• The manifest file contains the version without the Main-Class.
• There is a .kotlin_module file that does contain some kind of class list, but I'm not sure how that can be used.
• The bytecode .class and .js files are all in the expected locations.
• There is a .tar and .zip file that contains all the lib JARs along with a script that seems as though it is supposed to start the application, but it's all in an archive.
  • In my project, these are included in the JAR itself.
  • In other projects, these are included in the distributions directory.

I'm thinking this is something simple that I'm missing, but I can't seem to find documentation or questions about this specifically anywhere.

Answer 1

You have 3 options to make it work.

1. Out of box option

# Build from the root dir
./gradlew clean build

cd build/distributions/

# unzip
tar -xvf shoppinglist-1.0-SNAPSHOT.tar

cd shoppinglist-1.0-SNAPSHOT/bin

# Execute launch script (generated by application plugin)
./shoppinglist

# Result
Hello, JVM!

2. Distribution option (My Fork with changes)

We need to add our custom distribution and edit manifest if you really want to execute it like java -jar shoppinglist-jvm-1.0-SNAPSHOT.jar

1. Replace application plugin with distribution plugin
2. Remove application configuration block
3. Remove this

tasks.getByName<JavaExec>("run") {
    classpath(tasks.getByName<Jar>("jvmJar")) // so that the JS artifacts generated by `jvmJar` can be found and served
}

4. Replace distribution configuration block with

distributions {
    main {
        distributionBaseName.set("shoppinglist")
        contents {
            into("") {
                val jvmJar by tasks.getting
                from(jvmJar)
            }
            into("lib/") {
                val main by kotlin.jvm().compilations.getting
                from(main.runtimeDependencyFiles)
            }
        }
    }
}

tasks.withType<Jar> {
    doFirst {
        manifest {
            val main by kotlin.jvm().compilations.getting
            attributes(
                "Main-Class" to "ServerKt",
                "Class-Path" to main.runtimeDependencyFiles.files.joinToString(" ") { "lib/" + it.name }
            )
        }
    }
}

Now you can launch it like so

# Build from the root dir
./gradlew clean build

cd build/distributions/

# unzip
tar -xvf shoppinglist-1.0-SNAPSHOT.tar

# Run
java -jar shoppinglist-jvm-1.0-SNAPSHOT.jar

# Result
Hello, JVM!

3. uberJar (fatJar) option (My Fork with changes)

1. Remove `applicaion` plugin
2. Remove `distributions` and `application` configuration blocks
3. Remove `stage` and `run` tasts
4. Add uberJar task

tasks.withType<Jar> {
    doFirst {
        duplicatesStrategy = DuplicatesStrategy.EXCLUDE
        val main by kotlin.jvm().compilations.getting
        manifest {
            attributes(
                "Main-Class" to "ServerKt",
            )
        }
        from({
            main.runtimeDependencyFiles.files.filter { it.name.endsWith("jar") }.map { zipTree(it) }
        })
    }
}