Singleton Design Pattern

Real-World Analogy
Sun of our solar system is an excellent example of the singleton pattern. All the planets in our solar system have one sun and it has a global point of acess.

What is it?
Singleton is a creational design pattern that lets you ensure that a class has
only one instance and the instance has a global access point

The singleton design pattern solves problems like:

• How can it be ensured that a class has only one instance?
• How can the sole instance of a class be accessed easily?
• How can a class control its instantiation?
• How can the number of instances of a class be restricted?

Solution:
All implementations of the Singleton have these steps in common:

• Make default constructor private to control class instantiation
• Create a static creation method that underhood the hood uses the private constructor to create an instance then stores it in a static field and all subsequent calls to this method return cached object

Class Diagram:

Example:

The above code will work for single threaded applications . But there's a potential that multiple objects get created in a multithreading environment. Here's one example scenario:

• Thread A calls the method getInstance and finds the onlyInstance to be null but before it can actually new-up the instance it gets context switched out.
• Now thread B comes along and calls the getInstance method and goes on to new-up the instance and returns the Sun object.
• When thread A is scheduled again, is when the mischief begins. The thread was already past the if null condition check and will proceed to new-up another object of Sun and assign it to onlyInstance. Now there are two different Sun objects out in the wild, one with thread A and one with thread B

There are two trivial ways to fix this race condition:

• One is to add synchronized to the getInstance() method.
  • synchronized public static AirforceOne getInstance()
• The other is to undertake static initialization of the instance, which is guaranteed to be thread-safe.
  • private static AirforceOne onlyInstance = new AirforceOne();

Pusedo Code:

Advatntages:

• A class has only a single instance.
• You gain a global access point to that instance.
• The singleton object is initialized only first time.

DisAdvantages

• Violates the Single Responsibility Principle.
• The pattern requires special treatment in a multithreaded environment so that multiple threads won’t create a singleton object several times.
• Difficult to unit test

Github Code Repository:
The code for the Singleton pattern can be found at following github repository:
Github repository link: Singleton pattern