he dependency injection pattern, also knows as Inversion of Control, is one of the most popular design paradigms today. It facilitates the design and implementation of loosely coupled, reusable, and testable objects in your software designs by removing dependencies that often inhibit reuse. Dependency injection can help you design your applications so that the architecture links the components rather than the components linking themselves.
This article presents an overview of the dependency injection pattern, the advantages of using dependency injection in your designs, the different types of dependency injection, and the pros and cons of each of these types, with code examples where appropriate.
Object Dependency Explained
When an object needs another object to operate properly, we say that the former is dependent on the latter. This behavior is transitive in nature. Consider three objects, namely, A, B, and C. If object A is coupled to object B, and B is in turn coupled to C, then object A is effectively coupled to object Cit is dependent
on C. I've used the terms coupling and dependency interchangeably in this article.
Objects can be coupled in two ways: tight coupling and loose coupling. When an object is loosely coupled with another object, you can change the coupling with ease; when the coupling is tight, the objects are not independently reusable and hence are difficult to use effectively in unit test scenarios.
Here's an example of tight coupling. Consider two classes, C1 and C2, where C1 is tightly coupled with C2 and requires it to operate. In this case C1 is dependent on C2, as shown below:
public class C2
public class C1
C2 bObject = new C2();
The tight coupling between the two classes shown above occurs because C1 (which is dependent on C2) creates and contains an instance of the class C2. It's "tight" because you can eliminate or change the dependency only by modifying the container class (C1). This is where dependency injection fits in.
What is Dependency Injection?
Dependency injection eliminates tight coupling between objects to make both the objects and applications that use them more flexible, reusable, and easier to test. It facilitates the creation of loosely coupled objects and their dependencies. The basic idea behind Dependency Injection is that you should isolate the implementation of an object from the construction of objects on which it depends. Dependency Injection is a form of the Inversion of Control Pattern where a factory object carries the responsibility for object creation and linking. The factory object ensures loose coupling between the objects and promotes seamless testability.
Advantages and Disadvantages of Dependency Injection
The primary advantages of dependency injection are:
- Loose coupling
- Centralized configuration
- Easily testable
Code becomes more testable because it abstracts and isolates class dependencies.
However, the primary drawback of dependency injection is that wiring instances together can become a nightmare if there are too many instances and many dependencies that need to be addressed.