Creating singletons without locking

[dvas0004]

Typically Java singletons are created using double-checked locking (i.e. check if the singleton instance is null, only if it is, go into synchronized code, check if null again and create instance). However, a more elegant way of doing this is to use static code blocks, which are also thread safe. For example:

public class ourSingleton{

    public static ourSingleton INSTANCE;
    private static String demo;

    private ourSingleton(){

        demo = "ourSingleton"; // an example of initializing a variable
        this.INSTANCE = this;
        System.out.println("init done");

    }

    static { // static code block
        new ourSingleton();
    }

    public void printDemo(){
        System.out.println(this); // show that same instance of class is used
        System.out.println(demo); // get access to the initialized variable
    }

}

// [...]

// note that now ourSingleton.INSTANCE can't be null due to the static code block, eliminating
// the need for the typical double-check locking
ourSingleton demo1 = ourSingleton.INSTANCE; 
ourSingleton demo2 = ourSingleton.INSTANCE;

demo1.printDemo();
demo2.printDemo();

/*
outputs:
"
init done               <---- init only called once
ourSingleton@5594a1b5   <---- same output for both instances of singleton
ourSingleton
ourSingleton@5594a1b5
ourSingleton
 "
 */

In kotlin this is even easier by using an object:

object ourSingleton{
    init{
        println("init done");
   }
}

[dvas0004]

https://medium.com/@BladeCoder/kotlin-singletons-with-argument-194ef06edd9e

[dvas0004]

http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html