Singleton pattern

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to a singular instance. One of the well-known "Gang of Four" design patterns, which describe how to solve recurring problems in object-oriented software, the pattern is useful when exactly one object is needed to coordinate actions across a system.

More specifically, the singleton pattern allows objects to:
* Ensure they only have one instance
* Provide easy access to that instance
* Control their instantiation (for example, hiding the constructors of a class)

The term comes from the mathematical concept of a singleton.

Common uses

Singletons are often preferred to global variables because they do not pollute the global namespace (or their containing namespace). Additionally, they permit lazy allocation and initialization, whereas global variables in many languages will always consume resources.

The singleton pattern can also be used as a basis for other design patterns, such as the abstract factory, factory method, builder and prototype patterns. Facade objects are also often singletons because only one facade object is required.

Logging is a common real-world use case for singletons, because all objects that wish to log messages require a uniform point of access and conceptually write to a single source.

Implementations

Implementations of the singleton pattern ensure that only one instance of the singleton class ever exists and typically provide global access to that instance.

Typically, this is accomplished by:
* Declaring all constructors of the class to be private, which prevents it from being instantiated by other objects
* 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 when the static method is first called.

The following demonstrates an example implementation in Java:

public class Coin

Lazy initialization

A singleton implementation may use lazy initialization in which the instance is created when the static method is first invoked. In multithreaded programs, this can cause race conditions that result in the creation of multiple instances. The following Java example is a thread-safe implementation, using lazy initialization with double-checked locking.

public class Singleton

Criticism

Some consider the singleton to be an anti-pattern that introduces global state into an application, often unnecessarily. This introduces a potential dependency on the singleton by other objects, requiring analysis of implementation details to determine whether a dependency actually exists. This increased coupling can introduce difficulties with unit testing. In turn, this places restrictions on any abstraction that uses the singleton, such as preventing concurrent use of multiple instances.Steve Yegge
Singletons considered stupid
September 2004Hevery, Miško,

, ''Clean Code Talks'', 21 November 2008.

Singletons also violate the single-responsibility principle because they are responsible for enforcing their own uniqueness along with performing their normal functions.

See also

* Initialization-on-demand holder idiom
* Multiton pattern
* Software design pattern

References

External links

* Complete article
Java Singleton Pattern Explained

* Four different ways to implement singleton in Java
Ways to implement singleton in Java

* Book extract
Implementing the Singleton Pattern in C#
by Jon Skeet

Singleton at Microsoft patterns & practices Developer Center
* IBM article
Double-checked locking and the Singleton pattern
by Peter Haggar
*

Google Singleton Detector
(analyzes Java bytecode to detect singletons)

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