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

Object declaration is exactly for this purpose:

object Singleton {
    //singleton members
}

It is lazy and thread-safe, it initializes upon first call, much as Java's static initializers.

You can declare an object at top level or inside a class or another object.

For more info about working with objects from Java, please refer to this answer.

---

As to the parameter, if you want to achieve exactly the same semantics (first call to getInstance takes its argument to initialize the singleton, following calls just return the instance, dropping the arguments), I would suggest this construct:

private object SingletonInit { //invisible outside the file
    lateinit var arg0: String
}

object Singleton {
    val arg0: String = SingletonInit.arg0
}

fun Singleton(arg0: String): Singleton { //mimic a constructor, if you want
    synchronized(SingletonInit) {
        SingletonInit.arg0 = arg0
        return Singleton
    }
}

The main flaw of this solution is that it requires the singleton to be defined in a separate file to hide the object SingletonInit, and you cannot reference Singleton directly until it's initialized.

Also, see a similar question about providing arguments to a singleton.