Error: Main method is not static in class. Kotlin Project

Question

I need to create a runnable Jar file which is a console app from the Kotlin project. I set up this project using gradle init. I tried all solutions I could find online by moving the main method out of the class, wrapping it in a companion object, and using JvmStatic annotation but I still keep getting an error message "Error: Main method is not static in class com.yk.ConsoleGradle.App, please define the main method as: public static void main(String[] args)"

I am using ./gradlew jar to build the project

class App {
    val greeting: String
        get() {
            return "Hello World!"
        }
   
    fun main(args: Array<String>){
        println("Hello kotlin")
    }
}

application {
    // Define the main class for the application.
    mainClass.set("com.yk.ConsoleGradle.AppKt")
}
tasks.jar{
    manifest {
        attributes(mapOf("Implementation-Title" to project.name,
                "Implementation-Version" to project.version, "Main-Class" to "com.yk.ConsoleGradle.App" ))

    }
}

Here is the output of jar tf

jar tf /ConsoleGradle/app/build/libs/app-1.0.1.jar 
META-INF/
META-INF/MANIFEST.MF
META-INF/app.kotlin_module
com/
com/yk/
com/yk/ConsoleGradle/
com/yk/ConsoleGradle/AppKt.class
com/yk/ConsoleGradle/App.class

Any help appreciated.

Answer 1

If you don't want to declare the main function inside the App class you could just create an App.kt file with the following code:

class App {
    val greeting: String
        get() {
            return "Hello World!"
        }
}

fun main(args: Array<String>){
    println("Hello kotlin")
}

also you need to update your gradle jar task as:

jar {
    manifest {
        attributes "Implementation-Title": project.name
        attributes "Implementation-Version": project.version
        attributes "Main-Class": "com.yk.ConsoleGradle.AppKt"
    }
}

notice the Main-Class is specified as AppKt instead of App.
This is because the main function is a top-level function and for top-level functions inside App.kt the Kotlin compiler creates a class called AppKt.class.

Answer 2

I don't know if this is the right approach for what you're doing, but how about

class App {
    val greeting: String
        get() {
            return "Hello World!"
        }
    companion object {
        @JvmStatic
        fun main(args: Array<String>){
            println("Hello kotlin")
        }
    }
}

The companion object akes the function accessible through the class instead of through an instance, and the JvmStatic annotation generates a static method for Java interop