Java: Lazy Initializing Singleton

Question

The pattern to create singletons seems to be something like:

public class Singleton {
    private static final Singleton instance = new Singleton();
    priv         


        

Answer 1

You can use an enum as a Singleton

enum Singleton {
    INSTANCE;
}

Say your singleton does something undesirable in unit tests, you can;

// in the unit test before using the Singleton, or any other global flag.
System.setProperty("unit.testing", "true");

Singleton.INSTANCE.doSomething();

enum Singleton {
    INSTANCE;
    {
        if(Boolean.getBoolean("unit.testing")) {
           // is unit testing.
        } else {
           // normal operation.
        }
    }
}

Note: there is no synchronised blocks or explicit lock needed. The INSTANCE will not be loaded until the .class is accessed and not initialised until a member is used. provided you only use Singleton.INSTANCE and not Singleton.class there won't be a problem with the value used to initialise changing later.

---

Edit: if you use just the Singleton.class this might not initialise the class. It doesn't in this example on Java 8 update 112.

public class ClassInitMain {
    public static void main(String[] args) {
        System.out.println("Printing a class reference");
        Class clazz = Singleton.class;
        System.out.println("clazz = " + clazz);
        System.out.println("\nUsing an enum value");
        Singleton instance = Singleton.INSTANCE;
    }

    static enum Singleton {
        INSTANCE;

        Singleton() {
            System.out.println(getClass() + " initialised");
        }
    }
}

prints

Printing a class reference
clazz = class ClassInitMain$Singleton

Using an enum value
class ClassInitMain$Singleton initialised