What is Singleton Design Pattern and How many ways to implement? | #SingletonDesignPattern

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#DesignPattern #SingletonDesignPattern #EagerSingleton #StaticblockSingleton #BillPughSingleton #ThreadSafeSingleton #EnumSingleton

#LazySingleton
A singleton implementation may use lazy initialization, where the instance is created when the static method is first invoked. If the static method might be called from multiple threads simultaneously, measures may need to be taken to prevent race conditions that could result in the creation of multiple instances of the class. The following is a thread-safe sample implementation, using lazy initialization with double-checked locking, written in Java.

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one "single" instance. This is useful when exactly one object is needed to coordinate actions across the system. The term comes from the mathematical concept of a singleton.

Critics consider the singleton to be an anti-pattern in that it is frequently used in scenarios where it is not beneficial, introduces unnecessary restrictions in situations where a sole instance of a class is not actually required, and introduces global state into an application.

The singleton design pattern is one of the twenty-three well-known "Gang of Four" design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

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?
How can a global variable be accessed?
The singleton design pattern describes how to solve such problems:

Hide the constructor of the class.
Define a public static operation (getInstance()) that returns the sole instance of the class.
The key idea in this pattern is to make the class itself responsible for controlling its instantiation (that it is instantiated only once).
The hidden constructor (declared private) ensures that the class can never be instantiated from outside the class.

The abstract factory, factory method, builder, and prototype patterns can use singletons in their implementation.
Facade objects are often singletons because only one facade object is required.
State objects are often singletons.
Singletons are often preferred to global variables because:
They do not pollute the global namespace (or, in languages with nested namespaces, their containing namespace) with unnecessary variables.
They permit lazy allocation and initialization, whereas global variables in many languages will always consume resources.

Singleton classes break object-orientated design principles
It cannot be inherited from. To add new functionality, a new class cannot be descended to contain that functionality, breaking Separation of Concern.
No control over creation. It is impossible to tell if a reference is of an existing instance or a new instance.
Prevents dependency injection. As there is only a single instance of the class, a dependency cannot be injected into it. If done via a property, the dependency is changed for all references to that instance.

Singleton classes do not allow for test-driven development (TDD).
As there is no control over creation, a "clean" instance of the object cannot be used for each test.
Without dependency injection mock objects cannot be used in tests.

An implementation of the singleton pattern must:

ensure that only one instance of the singleton class ever exists; and
provide global access to that instance.
Typically, this is done by:

declaring all constructors of the class to be private; and
providing a static method that returns a reference to the instance.
The instance is usually stored as a private static variable; the instance is created when the variable is initialized, at some point before the static method is first called. The following is a sample implementation written in Java.