How to write a package-level static initializer in Kotlin?

Question

A previous question shows how to put a static initializer inside a class using its companion object. I'm trying to find a way to add a static initializer at the package level, but it seems packages have no companion object.

// compiler error: Modifier 'companion' is not applicable inside 'file'
companion object { init { println("Loaded!") } }
fun main(args: Array<String>) { println("run!") }

I've tried other variations that might've made sense (init on its own, static), and I know as a workaround I can use a throwaway val as in

val static_init = {
    println("ugly workaround")
}()

but is there a clean, official way to achieve the same result?

Edit: As @mfulton26's answer mentions, there is no such thing as a package-level function really in the JVM. Behind the scenes, the kotlin compiler is wrapping any free functions, including main in a class. I'm trying to add a static initializer to that class -- the class being generated by kotlin for the free functions declared in the file.

Because I picked a bad toy example. Imagine a file full of related free functions that all depend on some shared initialization. Arguably I could just wrap those in a named `object`, but a lot of the beauty of kotlin is generally not being forced to do stuff like that. — Dan, Aug 16 '16 at 18:30
Got it. Unfortunately I don't know of a way to do that other than your workaround or a variant. e.g. `val static_init = run { println("workaround") }` or `val static_init = object { init { println("workaround") } }`. — mfulton26, Aug 16 '16 at 19:35

score 8 · Accepted Answer · answered Aug 17 '16 at 08:53

Currently there is no way to add code to the static constructor generated for Kotlin file classes, only top-level property initializers are getting there. This sounds like a feature request, so now there is an issue to track this: KT-13486 Package-level 'init' blocks

Another workaround is to place initialization in top-level private/internal object and reference that object in those functions that depend on the effect of that initialization. Objects are initialized lazily, when they are referenced first time.

fun dependsOnState(arg: Int) = State.run {
    arg + value
}

private object State {
    val value: Int
    init {
        value = 42
        println("State was initialized")
    }
}

In Kotlin 1.1, you can use Kotlin Script (.kts) which will run code without needing a declaration. This is kind of similar to what the asker wants. — Jire, Aug 18 '16 at 07:11

score 3 · Answer 2 · answered Aug 17 '16 at 08:07

3

As you mentioned, you need a property with something that would run on initialisation:

val x = run {
    println("The package class has loaded")
}

answered Aug 17 '16 at 08:07

voddan

31,956
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Eric Liu · Answer 3 · 2017-06-09T06:33:53.900

0

I got around it by using a Backing Property on the top-level, under the Kotlin file. Kotlin Docs: Backing Properties

private var _table: Map<String, Int>? = null
public val table: Map<String, Int>
    get() {
        if (_table == null) {
            _table = HashMap() // Type parameters are inferred
           // .... some other initialising code here
        }
        return _table ?: throw AssertionError("Set to null by another thread")
    }

edited Jun 09 '17 at 06:33

answered Jun 09 '17 at 06:11

Eric Liu

1,516
13
19

If all you need is a single, lazily-initialized value like this, Kotlin has a builtin for you: `val table: MutableMap by lazy(::HashMap)` – Dan Jun 21 '17 at 15:46

How to write a package-level static initializer in Kotlin?

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