Constructor Injection is considered the recommended and most reliable way of injecting beans in Spring. It ensures that all required dependencies are provided when an object is created, making the class immutable, easier to test, and less prone to runtime errors.
Key Points: • Constructor Injection makes dependencies mandatory and ensures the object is created in a valid state. • It promotes immutability by allowing dependencies to be declared as final. • Dependencies are clearly visible through the constructor, improving code readability. • It simplifies unit testing because dependencies can be easily provided using mocks or stubs. • Spring automatically performs constructor injection when a class has a single constructor.
Example: A UserService requires a UserRepository to perform database operations. With Constructor Injection, the UserRepository is provided during object creation, ensuring that UserService can never exist without its required dependency.
Code Example:
@Service
public class UserService {
private final UserRepository userRepository;
public UserService(UserRepository userRepository) {
this.userRepository = userRepository;
}
}Interview Tip: A concise interview answer is:
"Constructor Injection is the preferred way to inject beans in Spring because it makes dependencies mandatory, supports immutability, improves testability, and ensures objects are created with all required dependencies."