Java singleton pattern

1.1 A class with a private constructor

The first singleton implementation I discuss relies on a private constructor, a static instance and a static method for retrieving this instance.

The file Singleton.java contains an abstract version of such a singleton implementation:

package nl.mauritssilvis.patterns.java.singleton.hide.general;

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

    private Singleton() {
    }

    public static Singleton getInstance() {
        return INSTANCE;
    }
}

Unit tests verify the singleton nature of the above class.

I used eager initialization of the singleton instance to guarantee thread safety without additional checks. Additionally, the keyword final is only applied to the class to remind us that extending a class with a private constructor is impossible. This keyword is not essential for the singleton pattern.

A specific singleton implementation that makes use of a private constructor and that represents a global configuration class is provided by the file Configuration.java.

1.2 An enum

Another Java implementation of the singleton design pattern benefits from the property that enum values are unique.

An abstract version of such an implementation is given by another file called Singleton.java:

package nl.mauritssilvis.patterns.java.singleton.enumerate.general;

public enum Singleton {
    INSTANCE
}

Again, unit tests verify that an enum forms a valid implementation of the singleton pattern.

A specific singleton implementation of the enum type is given by Configuration.java.

2.1 A class with a protected constructor

A particular example of a faulty singleton implementation is given by classes using a protected constructor. Although protected constructors serve the purpose of making class extension possible, they also allow child classes and classes that are part of the same package to instantiate additional objects.

An abstract version of a faulty singleton implementation is given by the file NoSingleton.java:

package nl.mauritssilvis.patterns.java.singleton.protect.general;

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

    protected NoSingleton() {
    }

    public static NoSingleton getInstance() {
        return INSTANCE;
    }
}

Unit tests verify that child classes and classes within the same package can create additional class instances.