Dependency Injection in Java: Your ultimate guide

In this blog post, we’ll explain Dependency Injection in simple terms and go over the different types of dependency injection, their benefits and how to implement them. Imagine you’re developing a Java application and carefully designing your classes and methods. Suddenly you hit a wall: managing dependencies becomes tedious, your code feels tightly coupled and making changes seems like a nightmare. If this sounds familiar, don’t worry — you’re not alone. Dependency injection (DI) is here to save the day, and by the end of this guide, you’ll be a pro at understanding its types and using it effectively in Java.
Are you ready to improve your Java skills? Then let’s dive in!

What is Dependency Injection?

Before we get into the different types of DI, let’s first understand what it actually is. Dependency injection is a Java EE design pattern that helps you achieve Inversion of Control (IoC). This means that you can hand over responsibility for managing your dependencies (e.g. objects that your classes rely on) to an external entity — usually a framework or container.
Instead of a class creating its own dependencies, you inject them from the outside. Think of it like ordering a pizza: Instead of baking it yourself, you have a delivery person bring it to your door. This approach makes your code more modular, easier to test and easier to maintain.

Why should you care about dependency injection?

Here you can find out why DI changes the game:

1. Decoupling code: Your classes no longer depend directly on each other, but on abstractions. This makes it easier to swap implementations.
2. Improves testability: Mocking dependencies become child’s play during unit testing.
3. Increases maintainability: You can update part of the code without breaking everything else.
4. Simplifies object management: Frameworks like Spring take care of object creation and lifecycle management for you.
   Got the gist of it? Great! Then let’s explore the different types of dependency injection you’ll encounter.

Types of Dependency Injection in Java

DI comes in three main variants: Constructor Injection, Setter Injection, and Interface Injection. Each has its own use cases and benefits. We’ll unpack each type and learn how you can use them effectively.

1. Constructor based dependency injection

Imagine you are building a machine and you get all the components handed out in advance. That’s what constructor injection essentially does. It adds dependencies to a class via its constructor.
And this is how it works:

• When you create an instance of a class, you pass its dependencies via the constructor.
• This ensures that the class is always initialized with everything it needs.

Code example:

class Engine {
public void start() {
System.out.println("Engine started.");
}
}
class Car {
private Engine engine;
// The dependency is injected by the constructor
public Car(Engine engine) {
this.engine = engine;
}
public void drive() {
engine.start();
System.out.println("The car is driving.");
}
}
public class Main {
public static void main(String[] args) {
Engine engine = new Engine(); // Create dependency
Car = new car(engine); // Insert dependency via constructor
car.drive();
}
}

Why use Constructor Injection?

• Inalterability: Dependencies are provided at object creation time, making the class immutable (if you want).
• Mandatory Dependencies: Ensures that the required dependencies are always available.

When to use:

Use constructor injection when a dependency is essential for the proper functioning of the class.

2. Setter injection

Imagine the following: You buy a smartphone and later decide to add accessories like a case or screen protector. This is how setter injection works: you can “set” dependencies after the object has been created.
And this is how it works:

• Dependencies are injected via public setter methods.
• This ensures flexibility, as you can change or replace the dependencies later.

Code example:

class Engine {
public void start() {
System.out.println("Engine started.");
}
}
class Car {
private Engine Motor;
// The dependency is injected via a setter method
public void setEngine(Engine engine) {
this.engine = engine;
}
public void drive() {
if (motor != null) {
motor.start();
System.out.println("The car is driving.");
} else {
System.out.println("No motor found.");
}
}
}
public class Main {
public static void main(String[] args) {
Engine engine = new Engine(); // create dependency
Car = new Car();
car.setEngine(engine); // Insert dependency via setter
car.drive();
}
}

Why use setter injection?

• Flexibility: Dependencies can be set or changed after object creation.
• Optional dependencies: Allows you to initialize optional dependencies only when needed.

When to use:

Use setter injection for optional dependencies or when you need to change dependencies dynamically.

3. Interface injection

Interface injection takes DI to a new level by using an interface to enforce dependency injection. Think of it as a kind of ground rule: any class that implements the interface must adhere to the injection mechanism defined by the interface.
And this is how it works:

• You create an interface with a method for injecting the dependency.
• Classes that implement the interface must provide their own implementation of this method.

Code example:

interface EngineInjector {
void injectEngine(Engine engine);
}
class Engine {
public void start() {
System.out.println("Engine started.");
}
}
class Car implements EngineInjector {
private Engine Engine;
// Interface method for dependency injection
@Override
public void injectEngine(Engine engine) {
this.engine = engine;
}
public void drive() {
if (engine != null) {
engine.start();
System.out.println("The car is driving.");
} else {
System.out.println("No engine found.");
}
}
}
public class Main {
public static void main(String[] args) {
Engine engine = new Engine(); // create dependency
Car auto = new Auto();
car.injectEngine(engine); // Inject dependency via interface
car.drive();
}
}

Why use interface injection?

• Contract enforcement: Ensures that dependency injection is part of the design contract.
• Extensibility: Facilitates the extension of functions in large systems.

When to use it:

Interface injection is less common, but it is useful in systems where standardization and extensibility are important.

Comparison of the types of dependency injection

Now that you’ve learned about the different types, let’s compare them to understand when each of them has its strengths:

Aspect	Constructor Injection	Setter Injection	Interface Injection
Initialization	On Object Creation	After Object Creation	After Object Creation
Flexibility	Low	High	High
Enforce dependencies	Strong	Weak	Moderate
Complexity	Low	Moderate	High
Common Use Case	Mandatory Dependencies	Optional Dependencies	Standardized Contracts

Dependency Injection in Action: The Use of Dependency Injection Framework

You might be asking yourself: “Do I have to write all this standard code myself?” Not at all! Frameworks like Spring and Guice do the DI for you. Let’s take a look at how Spring simplifies DI with annotations:

Spring Dependency Injection Example:

@Component class Engine {
public void start() {
System.out.println("Engine started.");
}
}
@Component class Auto {
private final Engine Motor;
// constructor injection with @Autowired
@Autowired
public Car(Engine engine) {
this.engine = engine;
}
public void drive() {
engine.start();
System.out.println("The car is driving.");
}
}
@SpringBootApplication public class Main {
public static void main(String[] args) {
ApplicationContext context = SpringApplication.run(Main.class, args);
Car Auto = context.getBean(Car.class);
car.drive();
}
}

With Spring, you focus on your business logic while the framework manages the dependencies in the background. Cool, right?

Tips for mastering dependency injection

Here are some practical tips to make DI your best friend:

1. Prefer Constructor Injection: It is the most robust and ensures that dependencies are always available.
2. Keep dependencies simple: Avoid injecting too many dependencies into a single class.
3. Use frameworks wisely: Use frameworks like Spring to avoid unnecessary code.
4. Design for interfaces: Program on abstractions, not implementations, to keep your code flexible.

Wrapping Up

Dependency injection may sound fancy, but at its core it’s about writing clean, modular and maintainable code. Whether you use constructor injection for immutability, setter injection for flexibility or interface injection for standardization depends on your use case.
If you master DI, you will not only improve your Java skills, but you will also develop better software. So go ahead, refactor your tightly coupled code and harness the power of dependency injection!