Kotlin thread safe native lazy singleton with parameter

Question

In java we can write thead-safe singletons using double Checked Locking & volatile:

    public class Singleton {         


        

Answer 1

I recently wrote an article on that topic. TL;DR Here's the solution I came up to:

1) Create a SingletonHolder class. You only have to write it once:

open class SingletonHolder(creator: (A) -> T) {
    private var creator: ((A) -> T)? = creator
    @Volatile private var instance: T? = null

    fun getInstance(arg: A): T {
        val i = instance
        if (i != null) {
            return i
        }

        return synchronized(this) {
            val i2 = instance
            if (i2 != null) {
                i2
            } else {
                val created = creator!!(arg)
                instance = created
                creator = null
                created
            }
        }
    }
}

2) Use it like this in your singletons:

class MySingleton private constructor(arg: ArgumentType) {
    init {
        // Init using argument
    }

    companion object : SingletonHolder(::MySingleton)
}

The singleton initialization will be lazy and thread-safe.