Getting a Grip on the Singleton Pattern in Software Design: Ensuring a Single Instance

Around the globe design patterns are being widely accepted by software developers which in turn makes it a fundamental part of the software development process.

It cannot be ignored that design patterns are the must-have tool for solving software programming problems and moreover, best practices for the writing of a maintainable and robust code have been developed.

What makes the Singleton Pattern smart and convenient is that it solves the issue with global state access and the lack of a clear identification of the class instance.

A concrete example of the Singleton Pattern as an object-oriented programming (OOP) concept is that it is a simplifying method. In this article, we shall go through the basics of the Singleton Pattern, some of its main features, and finally, we will show you the various programming languages that you can implement the Singleton Pattern using.

What is the Singleton Pattern?
The Singleton pattern is a creational design pattern, which effectively ensures that a class has only one instance and serves as a global point of access to that instance. It’s a simple yet powerful concept in object-oriented programming, widely used for managing shared resources and coordinating actions across a system.

Key Characteristics:

• Single instance: The class allows only one instance to be created.
• Global access: Provides a method to access the instance from anywhere in the application.
• Lazy initialization: The instance is typically created only when it’s first requested.
• Reduced Namespace Pollution: By using a singleton, you avoid creating a global variable.
• Resource Management: Useful for managing shared resources like database connections or configuration settings.
• Cross-System Coordination: It is useful for coordinating actions across a system, such as managing a configuration object, logging, or managing a connection pool.

Implementation:

Here’s a basic implementation in Dart:

class Singleton{
  // private static variable
  static Singleton _instance = Singleton._internal();

  // private constructor
  Singleton._internal();

  // static method to get the instance
  static Singleton get instance => _instance;

  // public variable
  String name = 'Singleton';
}

Here’s a basic implementation in Python:

class Singleton:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

    def some_business_logic(self):
        # ...

Here’s a basic implementation in Java:

public class Singleton {
    private static Singleton instance;

    private Singleton() {
        // private constructor to prevent instantiation
    }

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

Here’s a basic implementation in JavaScript:

class Singleton {
    constructor() {
        if (!Singleton.instance) {
            Singleton.instance = this;
        }
        return Singleton.instance;
    }
}

// Usage
const instance1 = new Singleton();
const instance2 = new Singleton();

console.log(instance1 === instance2);  // true

Use Cases

• Database connections
• Configuration managers
• Logging services
• Thread pools

Advantages

• Ensures a single instance, saving memory
• Provides a global access point
• Allows lazy initialization

Disadvantages

• Can make unit testing more difficult
• Violates the Single Responsibility Principle
• Can be overused, leading to tight coupling

Best Practices

• Use Singletons sparingly and only when truly necessary
• Consider alternatives like dependency injection for better testability
• Ensure thread safety in multi-threaded environments

The Singleton pattern, which can be helpful in some cases, should be applied with caution. Before you start implementing this pattern, you should always analyze the unique requirements of your application and possible future requirements.