How Singleton pattern is implemented in Kotlin.

KotlinSingleton.md

Singleton pattern in Kotlin

Regular object class in Kotlin provides the Singleton behavior (not allowed to have any constructor inside the object class!). It's thread-safe and instantiated lazily, when we access it for the first time.

Thread-safe and lazy:

object Singleton {
    // code to be executed
}

How to make a class Singleton in Kotlin?

Double-Check Locking. Thread safe and lazy:

class Singleton private constructor() {

    companion object {

        @Volatile private var instance: Singleton? = null //Volatile modifier is necessary

        fun getInstance() =
            instance ?: synchronized(this) {
                instance ?: Singleton().also { instance = it }
            }
    }
}

Simple, thread safe and lazy:

class Singleton private constructor() {

    companion object {
    
        val INSTANCE: Singleton by lazy(LazyThreadSafetyMode.SYNCHRONIZED) { Singleton() }
    }

}