Constructor Injection

This is a very common and recommended way to handle dependencies. Spring will automatically resolve and inject the required dependencies through the constructor of your class.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
 
@Service
public class UserService {
 
    private final UserRepository userRepository;
 
    @Autowired
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
 
    public User getUserById(Long id) {
        return userRepository.findById(id).orElse(null);
    }
}
 
@Service
public class AnotherService {
 
    private final UserService userService;
 
    @Autowired
    public AnotherService(UserService userService) {
        this.userService = userService;
    }
 
    public void processUser(Long id) {
        User user = userService.getUserById(id);
        // ... do something with the user
    }
}
 
@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    // ... your repository methods
}
 
@Entity
public class User {
    @Id
    private Long id;
    private String name;
    // ... other properties and getters/setters
}

In this example:

• UserService depends on UserRepository. Instead of UserService creating an instance of UserRepository, Spring creates an instance of UserRepository (because of the @Repository annotation) and passes it into the constructor of UserService (due to the @Autowired annotation on the constructor).
• Similarly, AnotherService depends on UserService, and Spring manages this dependency as well.