What is Dependency Injection in Java
💡 Concept: Dependency Injection
Dependency Injection (DI) is a design pattern used to implement IoC (Inversion of Control), where objects receive their dependencies from an external source rather than creating them internally.
📘 Quick Intro
DI helps reduce coupling between components and promotes greater flexibility in testing and maintenance. It allows components to remain unaware of how their dependencies are created.
🧠 Analogy
Think of a car. A driver does not need to know how the engine is built; they just need to know how to start the car. Similarly, with DI, the driver (client) does not need to know how the dependencies (engine) are instantiated.
🔧 Technical Explanation
Constructor Injection: Dependencies are passed through the constructor.Setter Injection: Dependencies are set through setter methods after the object is created.Interface Injection: The dependency provides an injector method that will inject the dependency into the client class.- DI frameworks like Spring automate the process of injecting dependencies, making it easier to manage complex systems.
🎯 Use Cases
- ✅ Decoupling of application components.
- ✅ Making components more testable by injecting mock dependencies.
- ✅ Managing complex systems and frameworks like Spring and Java EE.
💻 Example: Constructor Injection
class Car {
private Engine engine;
// Constructor Injection
public Car(Engine engine) {
this.engine = engine;
}
public void start() {
engine.run();
}
}
class Engine {
public void run() {
System.out.println("Engine running...");
}
}
public class Main {
public static void main(String[] args) {
Engine engine = new Engine();
Car car = new Car(engine); // DI is done here
car.start();
}
}
❓ Interview Q&A
Q1: What is Dependency Injection?
A: A design pattern where an object receives its dependencies from an external source.
Q2: What are the types of Dependency Injection?
A: Constructor Injection, Setter Injection, Interface Injection.
Q3: Why is Dependency Injection important?
A: It reduces coupling between components and promotes flexibility and testability.
Q4: Can you explain the advantages of Dependency Injection?
A: It makes code more modular, easier to test, and easier to maintain.
Q5: What is the difference between Constructor Injection and Setter Injection?
A: Constructor Injection passes dependencies during object creation, while Setter Injection uses setter methods after object creation.
Q6: How does Spring implement Dependency Injection?
A: Spring uses annotations or XML configuration to inject dependencies automatically.
Q7: What are some popular frameworks that use Dependency Injection?
A: Spring, Java EE, Google Guice.
Q8: How do you test classes that use Dependency Injection?
A: By injecting mock or fake dependencies to isolate functionality.
Q9: What is IoC (Inversion of Control)?
A: IoC is the principle where the control of object creation and management is transferred to a framework or container.
Q10: Is Dependency Injection used only in large frameworks like Spring?
A: No, it can be used in any application to manage dependencies and improve code maintainability.
📝 MCQs
Q1. What is Dependency Injection?
- A way to call methods
- A design pattern that provides dependencies to objects externally
- A way to initialize classes
- A design pattern for threading
Q2. What are the types of Dependency Injection?
- Constructor Injection, Setter Injection, Interface Injection
- Constructor Injection, Static Injection
- Constructor Injection, Singleton
- None of the above
Q3. What is the benefit of Dependency Injection?
- Improves performance
- Improves testability and decouples components
- Requires less memory
- Makes the code complex
Q4. Which Dependency Injection type uses setters?
- Constructor Injection
- Setter Injection
- Interface Injection
- Static Injection
Q5. How does Dependency Injection help in testing?
- By running tests in parallel
- By injecting mock dependencies
- By improving performance
- By using multiple threads
Q6. Which framework uses Dependency Injection extensively?
- Hibernate
- Spring
- Struts
- JSF
Q7. What does IoC stand for?
- Inversion of Communication
- Inversion of Classes
- Inversion of Control
- Intelligence of Control
Q8. Can Dependency Injection be used without a framework?
- No
- Yes
- Only in large applications
- Only in Java EE
Q9. What is Constructor Injection?
- Setting dependencies after object creation
- Passing dependencies during object creation
- Using static methods
- Injecting dependencies via interfaces
Q10. How is Dependency Injection configured in Spring?
- Using Java interfaces
- Using annotations or XML configuration
- Using reflection
- By default
💡 Bonus Insight
While DI frameworks like Spring make it easier to manage dependencies, it's important to balance the complexity introduced by DI with its benefits in modularity and testability.
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