Why Constructor Injection preferred Over Field and Setter Injection in Dependency Injection

Understanding Dependency Injection

Before we delve into the specifics of constructor injection, let’s briefly revisit the concept of dependency injection. Dependency injection is a design pattern used to achieve loose coupling between components in a software system. Instead of creating dependencies within a class, dependencies are injected from external sources, typically through constructor parameters, setter methods, or directly into public fields.

The Case for Constructor Injection

Constructor Injection 
public class UserService {
    private final UserRepository userRepository;

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

    public void addUser(User user) {
        userRepository.save(user);
    }
}


Field Injection
@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    public void addUser(User user) {
        userRepository.save(user);
    }
}


Setter Injection

@Service
public class UserService {
    private UserRepository userRepository;

    @Autowired
    public void setUserRepository(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public void addUser(User user) {
        userRepository.save(user);
    }
}

• Explicit Dependencies: Constructor injection explicitly declares a class’s dependencies as parameters in its constructor. This makes it immediately clear what dependencies a class requires, enhancing readability and maintainability.
• Immutable Dependencies: When dependencies are injected via constructor parameters, they are typically assigned to final fields, making them immutable once initialized. Immutable dependencies help enforce the principle of immutability, reducing the risk of unintended changes and enhancing thread safety.
• Compile-Time Safety: Constructor injection provides compile-time safety by ensuring that all required dependencies are provided at the time of object creation. This helps catch missing dependencies or misconfigurations early in the development process, reducing the likelihood of runtime errors.
• Testability: Constructor injection facilitates easier unit testing by allowing dependencies to be mocked or stubbed during testing. Since dependencies are passed in as parameters, test cases can easily provide mock implementations or test doubles without the need for complex frameworks or reflection.
• Constructor Chaining: Constructor injection supports constructor chaining, allowing dependencies to be passed down through multiple layers of abstraction. This promotes a hierarchical structure and facilitates better separation of concerns within the codebase.

Drawbacks of Field and Setter Injection

While field and setter injection are viable alternatives to constructor injection, they come with their own set of drawbacks:

• Hidden Dependencies: Field and setter injection can lead to hidden dependencies, as dependencies are not explicitly declared in the class’s signature. This can make code harder to understand and maintain, especially for developers who are not familiar with the implementation details.
• Mutable Dependencies: With field and setter injection, dependencies are often mutable, leading to potential issues with state management and thread safety. Mutable dependencies can be modified externally, making it harder to reason about the behavior of the class.
• Runtime Errors: Field and setter injection rely on reflection or framework magic to inject dependencies at runtime. This can lead to runtime errors if dependencies are not properly configured or if their types are incompatible with the class’s expectations.

Happy Coding !