What is Dependency Injection in Java?

Dependency Injection (DI) is a design pattern used to achieve loose coupling between classes by providing required objects (dependencies) from outside instead of creating them inside the class.

In simple terms, a class does not create its dependencies — they are injected into it by a framework like Spring.

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🔹 Why Dependency Injection?

Without Dependency Injection, classes become tightly coupled and difficult to maintain or test.

Without DI (Tight Coupling):

class Engine {
}

class Car {
    Engine engine = new Engine(); // object created internally
}

Here, Car is tightly dependent on Engine.

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🔹 With Dependency Injection (Loose Coupling)

class Engine {
}

class Car {
    Engine engine;

    Car(Engine engine) {
        this.engine = engine;
    }
}

Now the dependency is provided from outside, making the code flexible and testable.

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🔹 Types of Dependency Injection

1️⃣ Constructor Injection – Dependency provided through constructor
2️⃣ Setter Injection – Dependency provided using setter method
3️⃣ Field Injection – Dependency injected directly into fields (commonly used in Spring)

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🔹 Example in Spring Boot

@Service
class Engine {
}

@Service
class Car {

    private final Engine engine;

    @Autowired
    public Car(Engine engine) {
        this.engine = engine;
    }
}

Spring automatically injects the required object.

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🔹 Advantages

✅ Loose coupling between classes
✅ Easier testing and maintenance
✅ Better code reusability
✅ Improved scalability

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✅ Conclusion

Dependency Injection is a core concept in Spring and Spring Boot that helps build flexible, maintainable, and scalable applications. It is widely used in modern enterprise Java development.

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