Dependency injection: Difference between revisions

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{{Short description|Software programming technique}}

[[File:DependencyInjectionServiceProvider.png|alt=A diagram of an archetypical dependency injection container for the .NET platform.|thumb|Dependency injection is often used alongside specialized frameworks, known as 'containers', to facilitate program composition.]]

In [[software engineering]], '''dependency injection''' is a programming technique in which an [[Object (computer science)|object]] or [[Subroutine|function]] receives other objects or functions that it requires, as opposed to creating them internally. Dependency injection aims to [[separation of concerns|separate the concerns]] of constructing objects and using them, leading to [[Loose coupling|loosely coupled]] programs.<ref>{{Cite web |last=Seemann |first=Mark |title=Dependency Injection is Loose Coupling |url=http://blog.ploeh.dk/2010/04/07/DependencyInjectionisLooseCoupling/ |access-date=2015-07-28 |website=blog.ploeh.dk}}</ref><ref name="MarkSeeman2011P4" /><ref>Niko Schwarz, Mircea Lungu, Oscar Nierstrasz, “Seuss: Decoupling responsibilities from static methods for fine-grained configurability”, Journal of Object Technology, Volume volume&nbsp;11, no. &nbsp;1 (April 2012), pp. &nbsp;3:1-231–23.</ref> The pattern ensures that an object or function that wants to use a given [[Service (systems architecture)|service]] should not have to know how to construct those services. Instead, the receiving '"[[Client (computing)|client]]'" (object or function) is provided with its dependencies by external code (an '"injector'"), which it is not aware of.<ref name="HollywoodPrinciple.c2">{{Cite web |title=HollywoodPrinciple |url=http://c2.com/cgi/wiki?HollywoodPrinciple |access-date=2015-07-19 |website=c2.com}}</ref> Dependency injection makes implicit dependencies explicit and helps solve the following problems:<ref>{{cite web |title=The Dependency Injection design pattern -&nbsp;– Problem, Solution, and Applicability |url=http://w3sdesign.com/?gr=u01&ugr=proble |access-date=2017-08-12 |website=w3sDesign.com}}</ref>

* How can a [[Class (computer programming)|class]] be independent from the creation of the objects it depends on?

* How can an application, and the objects it uses support different configurations?

Dependency injection is often used to keep code in-line with the [[dependency inversion principle]].<ref>{{Cite web |last=Erez |first=Guy |date=2022-03-09 |title=Dependency Inversion vs. Dependency Injection |url=https://betterprogramming.pub/straightforward-simple-dependency-inversion-vs-dependency-injection-7d8c0d0ed28e |access-date=2022-12-06 |website=Medium |language=en}}</ref><ref>{{Cite web |last=Mathews |first=Sasha |date=2021-03-25 |title=You are Simply Injecting a Dependency, Thinking that You are Following the Dependency Inversion… |url=https://levelup.gitconnected.com/you-are-simply-injecting-a-dependency-thinking-that-you-are-following-the-dependency-inversion-32632954c208 |access-date=2022-12-06 |website=Medium |language=en }}</ref>

In [[statically typed language]]s using dependency injection means that a client only needs to declare the [[Interface (computing)|interfaces]] of the services it uses, rather than their concrete implementations, making it easier to change which services are used at runtime without recompiling.

Application frameworks often combine dependency injection with [[inversion of control]]. Under inversion of control, the framework first constructs an object (such as a controller), and then passes [[control flow]] to it. With dependency injection, the framework also instantiates the dependencies declared by the application object (often in the constructor method's parameters), and passes the dependencies into the object.<ref>{{cite web |title=Spring IoC Container |url=https://docs.spring.io/spring-framework/docs/3.2.x/spring-framework-reference/html/beans.html |access-date=2023-05-23 |language=en }}</ref>