Dependency injection (DI) is a programming architectural model, sometimes also referred to as inversion of control or IOC although, technically speaking, dependency injection specifically refers to an implementation of a particular form of IOC. This article or section is in need of attention from an expert on the subject. ...

Description

Dependency Injection describes the situation where one object uses a second object to provide a particular capacity. For example, being passed a database connection as an argument to the constructor instead of creating one internally.

The pattern seeks to establish a level of abstraction via a public interface, and to remove dependency on components by (for example) supplying a plug-in architecture. The architecture links the components rather than the components linking themselves or being linked together. Dependency injection is a pattern in which responsibility for object creation and object linking is removed from the objects themselves and transferred to a factory. Dependency injection therefore is inverting the control for object creation and linking, and can be seen to be a form of IoC. In computer science, abstraction is a mechanism and practice to reduce and factor out details so that one can focus on a few concepts at a time. ... The factory method pattern is an object-oriented design pattern. ...

There are three common forms of dependency injection: setter-, constructor- and interface-based injection. In computer science, a mutator method is a method used to control changes to a variable. ... In object-oriented programming, a constructor (sometimes shortened to ctor) in a class is a special block of statements called when an object is created, either when it is declared (statically constructed on the stack, possible in C++ but not in Java and other object-oriented languages) or dynamically constructed... An interface defines the communication boundary between two entities, such as a piece of software, a hardware device, or a user. ...

Dependency injection is a way to achieve loose coupling. The technique results in highly testable objects, particularly when applying test-driven development using mock objects: Avoiding dependencies on the implementations of collaborating classes (by depending only on interfaces that those classes adhere to) makes it possible to produce controlled unit tests that focus on exercising the behavior of, and only of, the class under test. To achieve this, dependency injection is used to cause instances of the class under test to interact with mock collaborating objects, whereas, in production, dependency injection is used to set up associations with bona fide collaborating objects. Loosely coupled describes a resilient relationship between two or more computer systems that are exchanging data. ... Test-Driven Development (TDD) is a software development technique that involves repeatedly first writing a test case and then implementing only the code necessary to pass the test. ... Mock objects are simulated objects that mimic the behavior of real objects in controlled ways. ... In computer programming, a unit test is a procedure used to validate that a particular module of source code is working properly. ...

Existing frameworks

Dependency Injection frameworks exist for a number of platforms and languages including:

ActionScript

• Syringe

C++

• Autumn Framework
• QtIocContainer

ColdFusion

• ColdSpring Framework

Java

• Azuki
• iPOJO
• Google Guice
• HiveMind
• JBoss Microcontainer
• PicoContainer
• Seasar
• Spring Framework
• J2EE 5 / EJB 3

.NET Seasar2 is an open source application framework similar to the Spring Framework (Java). ... The Spring Framework (or Spring for short) is an open source application framework for the Java platform. ...

• Castle MicroKernel/Windsor
• ObjectBuilder
• PicoContainer.NET
• Puzzle.NFactory
• Spring.NET
• StructureMap
• Ninject

Perl

• The IOC Module

Python

• PyContainer

Ruby

• Copland
• Needle

External links

• Inversion of Control Containers and the Dependency Injection pattern - Martin Fowler.
• Dependency Injection & Testable Objects: Designing loosely coupled and testable objects - Jeremy Weiskotten; Dr. Dobb's Journal, May 2006.
• Design Patterns: Dependency Injection -- MSDN Magazine, September 2005
• Writing More Testable Code with Dependency Injection -- Developer.com, October 2006

Martin Fowler is a famous author and international speaker on software architecture, specializing in object-oriented analysis and design, UML, Patterns, and agile software development methodologies, including Extreme Programming. ... Dr. Dobbs Journal of Computer Calisthenics & Orthodontia with the subtitle Running Light without Overbyte was the full title of the pioneer microcomputer hobbyist newsletter published from early 1976 by Bob Albrecht and Dennis Allisons Peoples Computer Company. ...

References