Module 1: Design Pattern: Dependency Injection

Learning Objectives

• Understand loose vs tight-coupling and identify how to remedy tight-coupling
• Understand the concept of dependency injection and its benefits
  • What problems does it solve for us
  • What are the advantages
• Learn how to implement dependency injection manually in Java applications
  • Constructor injection: passing dependencies through constructors
  • Field injection: setting dependencies directly into fields
  • Method injection: passing dependencies through setter methods
• Understand what a dependency graph (or chain) is and how it relates to DI
• Understand what a DI framework is and how it helps simplify dependency management
• Explore how the Spring framework helps with implementing dependency injection
  • What is @Autowire annotation
  • What are @Components and their specializations
  • What kinds of components we use in Spring applications
• Practice using Spring DI annotations effectively
• Understand ApplicationContext and its role in Spring's DI container

Code Example: Dependency Injection

// Without Dependency Injection (tight coupling)
public class OrderService {
    private final DatabaseRepository repository = new MySQLRepository(); // Tightly coupled
    
    public void createOrder(Order order) {
        repository.save(order);
    }
}

// With Dependency Injection (loose coupling)
public class OrderService {
    private final Repository repository; // Interface reference
    
    // Constructor injection
    public OrderService(Repository repository) {
        this.repository = repository;
    }
    
    public void createOrder(Order order) {
        repository.save(order);
    }
}

// With Spring Framework
@Service
public class OrderService {
    private final Repository repository;
    
    @Autowired // Spring handles the injection
    public OrderService(Repository repository) {
        this.repository = repository;
    }
    
    public void createOrder(Order order) {
        repository.save(order);
    }
}