Dependency inversion principle
Encyclopedia
In object-oriented programming
, the dependency inversion principle refers to a specific form of decoupling where conventional dependency relationships established from high-level, policy-setting modules to low-level, dependency modules are inverted (i.e. reversed) for the purpose of rendering high-level modules independent of the low-level module implementation details. The principle states:
The goal of the dependency inversion principle is to decouple high-level components from low-level components such that reuse with different low-level component implementations becomes possible. This is facilitated by the separation of high-level components and low-level components into separate packages/libraries, where interfaces
defining the behavior/services required by the high-level component are owned by, and exist within the high-level component's package. The implementation of the high-level component's interface by the low level component requires that the low-level component package depend upon the high-level component for compilation, thus inverting the conventional dependency relationship. Various patterns such as Plugin, Service Locator
, or Dependency Injection
are then employed to facilitate the run-time provisioning of the chosen low-level component implementation to the high-level component.
Applying the dependency inversion principle can also be seen as applying the Adapter pattern
, i.e. the high-level class defines its own adapter interface which is the abstraction that the high-level class depends on. The adaptee implementation also depends on the adapter interface abstraction (of course, since it implements its interface) while it can be implemented by using code from within its own low-level module. The high-level has no dependency to the low-level module since it only uses the low-level indirectly through the adapter interface by invoking polymorphic methods to the interface which are implemented by the adaptee and its low-level module.
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
, the dependency inversion principle refers to a specific form of decoupling where conventional dependency relationships established from high-level, policy-setting modules to low-level, dependency modules are inverted (i.e. reversed) for the purpose of rendering high-level modules independent of the low-level module implementation details. The principle states:
-
- A. High-level modules should not depend on low-level modules. Both should depend on abstractions.
- B. Abstractions should not depend upon details. Details should depend upon abstractions.
Description
In conventional application architecture, lower-level components are designed to be consumed by higher-level components which enable increasingly complex systems to be built. In this composition, higher-level components depend directly upon lower-level components to achieve some task. This dependency upon lower-level components limits the reuse opportunities of the higher-level components.The goal of the dependency inversion principle is to decouple high-level components from low-level components such that reuse with different low-level component implementations becomes possible. This is facilitated by the separation of high-level components and low-level components into separate packages/libraries, where interfaces
Interface (computer science)
In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
defining the behavior/services required by the high-level component are owned by, and exist within the high-level component's package. The implementation of the high-level component's interface by the low level component requires that the low-level component package depend upon the high-level component for compilation, thus inverting the conventional dependency relationship. Various patterns such as Plugin, Service Locator
Service locator pattern
The service locator pattern is a design pattern used in software development to encapsulate the processes involved in obtaining a service with a strong abstraction layer...
, or Dependency Injection
Dependency injection
Dependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
are then employed to facilitate the run-time provisioning of the chosen low-level component implementation to the high-level component.
Applying the dependency inversion principle can also be seen as applying the Adapter pattern
Adapter pattern
In computer programming, the adapter pattern is a design pattern that translates one interface for a class into a compatible interface...
, i.e. the high-level class defines its own adapter interface which is the abstraction that the high-level class depends on. The adaptee implementation also depends on the adapter interface abstraction (of course, since it implements its interface) while it can be implemented by using code from within its own low-level module. The high-level has no dependency to the low-level module since it only uses the low-level indirectly through the adapter interface by invoking polymorphic methods to the interface which are implemented by the adaptee and its low-level module.
History
The dependency inversion principle was postulated by Robert C. Martin and described in several publications including the paper Object Oriented Design Quality Metrics: an analysis of dependencies, an article appearing in the C++ Report in May 1996 entitled The Dependency Inversion Principle, and the books Agile Software Development, Principles, Patterns, and Practices, and Agile Principles, Patterns, and Practices in C#.See also
- SOLID
- Inversion of ControlInversion of ControlIn software engineering, Inversion of Control is an abstract principle describing an aspect of some software architecture designs in which the flow of control of a system is inverted in comparison to procedural programming....
- InterfaceInterface (computer science)In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
- Dependency InjectionDependency injectionDependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
- Service locator patternService locator patternThe service locator pattern is a design pattern used in software development to encapsulate the processes involved in obtaining a service with a strong abstraction layer...
- Plug-in (computing)
- Adapter patternAdapter patternIn computer programming, the adapter pattern is a design pattern that translates one interface for a class into a compatible interface...