Why is it preferable to implement dependency injection through the constructor in Spring?

Dependency injection through constructor in Spring is preferred for several reasons:

• Immutability of the object: All dependencies are set during object creation, making its state more predictable and safe.
• Mandatory dependencies: The constructor guarantees that all necessary dependencies are provided; otherwise, the object will not be created.
• Simplified testing: It is easy to manually create objects with the required dependencies in tests.
• No hidden dependencies: Injection through fields or setters can lead to dependencies being not obvious, whereas through the constructor they are explicitly visible.

Example of constructor injection:

@Component
public class UserService {
    private final UserRepository userRepository;

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

    // methods
}