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Singletons. This article describes the Design Pattern "Singleton" and its usage in the programming language Java.

1. The singletons pattern in Java

1.1. Overview

A singleton in Java is a class for which only one instance can be created. It provides a global point of access this instance. The singleton pattern describe how this can be archived.

Singletons are useful to provide a unique source of data or functionality to other Java Objects. For example you may use a singleton to access your data model from within your application or to define logger which the rest of the application can use.

1.2. Code Example

The possible implementation of Java depends on the version of Java you are using. As of Java 6 you can singletons with a single-element enum type. This way is currently the best way to implement a singleton in Java 1.6 or later according to the book Effective Java from Joshua Bloch.

package mypackage;

public enum MyEnumSingleton {
        INSTANCE;

        // other useful methods here
}

Before Java 1.6 a class which should be a singleton can be defined like the following.

public class Singleton {
        private static Singleton uniqInstance;

        private Singleton() {
        }

        public static Singleton getInstance() {
                if (uniqInstance == null) {
                synchronized(this) {
                        if (uniqInstance == null)
                                uniqInstance = new Singleton();
                        }
                }
                return uniqInstance;
        }
        // other useful methods here
}

1.3. Evaluation

A static class with static method would result in the same functionality as a singleton. As singletons are define using an object orientated approach it is in general advised to work with singletons. Singleton violate the "One Class, one responsibility" principle as they are used to manage its one instance and the functionality of the class. A singleton cannot be subclassed as the constructor is declared private. If you are using multiple classloaders then several instances of the singleton can get created.

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3. Links and Literature

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