Implementing Singleton with an Enum (in Java)

Question

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

public enum MySingleton {
     INSTANCE;   
}         


        

Answer 1

Like all enum instances, Java instantiates each object when the class is loaded, with some guarantee that it's instantiated exactly once per JVM. Think of the INSTANCE declaration as a public static final field: Java will instantiate the object the first time the class is referred to.

The instances are created during static initialization, which is defined in the Java Language Specification, section 12.4.

For what it's worth, Joshua Bloch describes this pattern in detail as item 3 of Effective Java Second Edition.