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In software engineering, dependency injection is a design pattern in which an
object or function receives other objects or functions that it depends on. A
form of inversion of control, dependency injection aims to separate the concerns
of constructing objects and using them, leading to loosely coupled programs.
The pattern ensures that an object or function which wants to use a given service
should not have to know how to construct those services. Instead, the receiving
'client' (object or function) is provided with its dependencies by external code
(an 'injector'), which it is not aware of. Dependency injection helps by
making implicit dependencies explicit and helps solve the following problems:
# How can a class be independent from the creation of the objects it depends on?
# How can an application, and the objects it uses support different configurations?
# How can the behavior of a piece of code be changed without editing it directly
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Dependency injection is a programming technique that makes a class independent
of its dependencies. That enables you to replace dependencies without changing
the class that uses them.
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Dependency injection is basically providing the objects that an object needs
(its dependencies) instead of having it construct them itself.
It's a very useful technique for testing, since it allows dependencies
to be mocked or stubbed out.
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- Dependency Injection
- Classes should not instantiate their dependency or create new object
- Classes should only use the object that is given to them (Creating and using are two different things)
- There should be a separate injector class who will create object and then give the created object to the class who needs that object.
- So the injector class will be responsible for passing or injecting a dependency.
- This is called dependency injection.
- three types:
1. constructor injection : Dependency is passed via constructor
2. setter injection : Dependency is passed via setter
3. method injection : Dependency is passed via method
- Inversion of Control
- the container maintains all the classes including the dependency classes
- objects are inserted during runtime or startup by Appconfig.java or appContext.xml
- Instead of pushing the dependency, the object accepts the dependency during it's creation through constructor or setter
- Constructor injection : If the class cannot operate without dependency, it should be through constructor injection
- Setter injection : If the class can treat the dependency as optional or can accept multiple variable types of dependency, it should be through setter injection
- Without dependency injection
- You want contstruct main
- Main needs dependency of Class A
- So you need to create object of class A and provide that to main
- Main needs dependency of Class B
- So you need to create object of class B and provide that to main
- Class B needs dependency of class X and class Y
- You need to create onject of class X and object of class Y and provide them inside the object of class B
- You then need to give the constructed object of class B to class Main
- With dependency injection
- You want to construct main
- IoC container manages all classes
- Main has dependencies
- Instead of constructing the objects, main only takes constructor arguments
- IoC container handles the construction of the objects which main needs by looking at the constructor arguments
- Framework is in control of the injection of these dependencies according to needs
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In object-oriented programming (OOP) software design, dependency injection (DI) is the process of supplying a resource that a given piece of code requires.
The required resource, which is often a component of the application itself, is called a dependency.
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In object-oriented programming (OOP) software design, dependency injection (DI) is the process of supplying a resource that a given piece of code requires.
The required resource, which is often a component of the application itself, is called a dependency.
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Dependency injection is a programming technique that makes a class independent
of its dependencies. That enables you to replace dependencies without changing
the class that uses them.
Dependency injection is basically providing the objects that an object needs
(its dependencies) instead of having it construct them itself.
It's a very useful technique for testing, since it allows dependencies
to be mocked or stubbed out.
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In general, dependency injection can be done in three ways, namely :
Constructor Injection
Setter Injection
Interface Injection
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public interface IHelloWorldService
{
string SaysHello();
}
public class HelloWorldService: IHelloWorldService
{
public string SaysHello()
{
return "Hello ";
}
}
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public void ConfigureServices(IServiceCollection services)
{
….
…
services.AddTransient<IHelloWorldService, HelloWorldService>();
…
…
}