Implementing Singleton with an Enum (in Java)

I have read that it is possible to implement Singleton in Java using an Enum such as:

public enum MySingleton {
     INSTANCE;   
}

But, how does the above work? Specifically, an Object has to be instantiated. Here, how is MySingleton being instantiated? Who is doing new MySingleton()?

Solution 1:

This,

public enum MySingleton {
  INSTANCE;   
}

has an implicit empty constructor. Make it explicit instead,

public enum MySingleton {
    INSTANCE;
    private MySingleton() {
        System.out.println("Here");
    }
}

If you then added another class with a main() method like

public static void main(String[] args) {
    System.out.println(MySingleton.INSTANCE);
}

You would see

Here
INSTANCE

enum fields are compile time constants, but they are instances of their enum type. And, they're constructed when the enum type is referenced for the first time.

Solution 2:

An enum type is a special type of class.

Your enum will actually be compiled to something like

public final class MySingleton {
    public final static MySingleton INSTANCE = new MySingleton();
    private MySingleton(){} 
}

When your code first accesses INSTANCE, the class MySingleton will be loaded and initialized by the JVM. This process initializes the static field above once (lazily).