Dependency Injection
Intent
Dependency Injection is a software design pattern in which one or more dependencies (or services) are injected, or passed by reference, into a dependent object (or client) and are made part of the client’s state. The pattern separates the creation of a client’s dependencies from its own behavior, which allows program designs to be loosely coupled and to follow the inversion of control and single responsibility principles.
Explanation
Real world example
The old wizard likes to fill his pipe and smoke tobacco once in a while. However, he doesn’t want to depend on a single tobacco brand only but likes to be able to enjoy them all interchangeably.
In plain words
Dependency Injection separates creation of client’s dependencies from its own behavior.
Wikipedia says
In software engineering, dependency injection is a technique in which an object receives other objects that it depends on. These other objects are called dependencies.
Programmatic Example
Let’s first introduce the Tobacco interface and the concrete brands.
1@Slf4j
2public abstract class Tobacco {
3
4 public void smoke(Wizard wizard) {
5 LOGGER.info("{} smoking {}", wizard.getClass().getSimpleName(),
6 this.getClass().getSimpleName());
7 }
8}
9
10public class SecondBreakfastTobacco extends Tobacco {
11}
12
13public class RivendellTobacco extends Tobacco {
14}
15
16public class OldTobyTobacco extends Tobacco {
17}
Next here’s the Wizard class hierarchy.
1public interface Wizard {
2
3 void smoke();
4}
5
6public class AdvancedWizard implements Wizard {
7
8 private final Tobacco tobacco;
9
10 public AdvancedWizard(Tobacco tobacco) {
11 this.tobacco = tobacco;
12 }
13
14 @Override
15 public void smoke() {
16 tobacco.smoke(this);
17 }
18}
And lastly we can show how easy it is to give the old wizard any brand of tobacco.
1 var advancedWizard = new AdvancedWizard(new SecondBreakfastTobacco());
2 advancedWizard.smoke();
Class diagram
Applicability
Use the Dependency Injection pattern when:
- When you need to remove knowledge of concrete implementation from object.
- To enable unit testing of classes in isolation using mock objects or stubs.