Multiton pattern: Difference between revisions

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{{Short description|Software engineering design pattern}}

{{Plain image with caption|Image:Multiton.svg|UML diagram of the multiton}}

In [[software engineering]], the '''multiton pattern''' is a [[design pattern (computer science)|design pattern]] which generalizes the [[singleton pattern]]. Whereas the [[singleton pattern | singleton]] allows only one instance of a class to be created, the multiton pattern allows for the controlled creation of multiple instances, which it manages through the use of a [[associative array|map]].

Rather than having a single instance ''per application'' (e.g. the {{Javadoc:SE|package=java.lang|java/lang|Runtime}} object in the [[Java (programming language)|Java programming language]]) the multiton pattern instead ensures a single instance ''per key''.

The multiton pattern does not explicitly appear as a pattern in the highly regarded [[object-oriented programming]] textbook ''[[Design Patterns]]''.<ref>{{cite book |last1=O'Docherty |first1=Mike |title=Object-oriented analysis and design: understanding system development with UML 2.0 |date=2005 |publisher=Wiley |___location=Chichester |isbn=0470092408 |page=341}}</ref> However, the book describes using a '''registry of singletons''' to allow subclassing of singletons,<ref>{{cite book |title=Design patterns: elements of reusable object-oriented software |date=2011 |publisher=Addison-Wesley |___location=Boston, Mass. Munich |isbn=0-201-63361-2 |page=130}}</ref> which is essentially the multiton pattern.{{Citation needed|date=April 2012}}

Most people and textbooks consider this a singleton pattern{{Citation needed|date=April 2012}}. For example, multiton does not explicitly appear in the highly regarded [[object-oriented programming]] textbook ''[[Design Patterns (book)|Design Patterns]]'' (it appears as a more flexible approach named '''registry of singletons''').

==Description==

While it may appear that the multiton is a [[hash table]] with synchronized access there are two important distinctions. First, the multiton does not allow clients to add mappings. Secondly, the multiton never returns a [[Null (computer programming)wikt:null|null]] or empty reference; instead, it creates and stores a multiton instance on the first request with the associated key. Subsequent requests with the same key return the original instance. A hash table is merely an implementation detail and not the only possible approach. The pattern simplifies retrieval of shared objects in an application.

Since the object pool is created only once, being a member associated with the class (instead of the instance), the multiton retains its flat behavior rather than evolving into a [[Tree (data structure)|tree structure]].

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==Drawbacks==

This pattern, like the [[Singleton pattern]], makes [[unit testing]] far more difficult,<ref>{{Cite web | url=httphttps://googletesting.blogspot.com/2008/11/clean-code-talks-global-state-and.html |title = Clean Code Talks - Global State and Singletons}}</ref> as it introduces [[global variables|global state]] into an application.

With garbage collected languages it may become a source of memory leaks as it introduces global strong references to the objects.

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public override string ToString()

{

return $"My type is " + {this.type}";

}

// Sample usage

public static void Main(string[] args)

{

varMultiton m0 = Multiton.GetInstance(MultitonType.Zero);

varMultiton m1 = Multiton.GetInstance(MultitonType.One);

varMultiton m2 = Multiton.GetInstance(MultitonType.Two);

Console.WriteLine(m0);

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* [https://github.com/PureMVC/puremvc-as3-multicore-framework/blob/master/src/org/puremvc/as3/multicore/patterns/facade/Facade.as Multiton usage in PureMVC Framework for ActionScript 3]

* [http://gen5.info/q/2008/07/25/the-multiton-design-pattern/ Article with a C# Multiton implementation, example of use, and discussion of memory issues]

{{Design Patterns patterns}}

[[Category:Software design patterns]]