What are the advantages of using constructor-based dependency injection in Spring?

Dependency injection through constructor in Spring has several advantages:

• Ensuring immutability: dependencies are passed during object creation and cannot be changed later, which increases code safety and predictability.
• Mandatory dependencies: if a dependency is not provided, the object will not be created, preventing errors related to missing necessary components.
• Simplified testing: mock objects or stubs can be easily substituted through the constructor when writing unit tests.
• Explicit declaration of dependencies: it is clear from the constructor what the class depends on, improving code readability and maintainability.

Example:

@Component
public class UserService {
    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    // class methods
}