Why is @JVMStatic annotation necessary for Kotlin main function?

Recently started learning Kotlin. I noticed an interesting behaviour when declaring the main function in Kotlin. It uses a @JvmStatic annotation even when in an object class. I looked around and did not find a relevant post.

object Main {
    @JvmStatic
    fun main(args: Array<String>) {
        println("Hello World")
    }
}

Why does the main function in Kotlin require a @JvmStatic annotation?

Coming from a Java and Scala background, it seems redundant and unnecessary, so I am curious what the background context and decision making was that made Kotlin adopt this convention.

Solution

@JvmStatic is not required if you declare main as a top-level function. That is, if the entirety of your Kotlin source file is:

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

You do not need @JvmStatic in this case - all top-level functions are translated to a static method in Java, in a class called XXXKt by default, where XXX is your Kotlin file name.

Note that for parameterless top-level mains in Kotlin, the compiler actually generates a proper Java main that takes String[] args, and delegate to your Kotlin function.

// the Kotlin main gets converted to something like this...
public static void main$1() {
    // your kotlin code here...
}

// A proper Java main generated by Kotlin
public static void main(String[] args) {
    main$1();
}

You do need @JvmStatic when you put the main method inside an object, because objects are translated to a singleton class in Java. All the members in the object are non-static by default.

object MyObject {
    fun main(args: Array<String>) {

    }
}

is translated to something like this:

public final class MyObject {
    private MyObject() {}

    public static final MyObject INSTANCE = new MyObject();

    public void main(String[] args) { }
}

When you do MyObject.main(...) in Kotlin, it is actually MyObject.INSTANCE.main(...) in Java. Obviously, such a main method cannot be used as the entry point of a Java program, because it is not static.