[Clean Code] 19장. SOLID 원칙 리팩토링 실습

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// 실습 대상: 다섯 가지 책임이 뒤섞인 God Class
public class UserManager {
    private Connection dbConnection;
    private EmailService emailService;
    private Map<String, User> userCache = new HashMap<>();

    public UserManager() {
        this.dbConnection = DriverManager.getConnection("jdbc:mysql://localhost/userdb");
        this.emailService = new EmailService();
    }

    public boolean isValidUser(User user) {
        return user.getEmail() != null && user.getEmail().matches("^[A-Za-z0-9+_.-]+@(.+)$");
    }

    public void saveUser(User user) throws SQLException {
        String sql = "INSERT INTO users (name, email) VALUES (?, ?)";
        PreparedStatement stmt = dbConnection.prepareStatement(sql);
        stmt.setString(1, user.getName());
        stmt.setString(2, user.getEmail());
        stmt.executeUpdate();
    }

    public void sendWelcomeEmail(User user) {
        emailService.send(user.getEmail(), "Welcome!", "Thanks for joining.");
    }

    public User getCachedUser(String email) {
        return userCache.get(email);
    }
}

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// 리팩토링 결과: 책임별로 분리된 클래스들
public class UserValidator {
    public boolean isValid(User user) {
        return user.getEmail() != null && user.getEmail().matches("^[A-Za-z0-9+_.-]+@(.+)$");
    }
}

public class UserRepository {
    private final Connection dbConnection;
    public UserRepository(Connection dbConnection) { this.dbConnection = dbConnection; }

    public void save(User user) throws SQLException {
        String sql = "INSERT INTO users (name, email) VALUES (?, ?)";
        PreparedStatement stmt = dbConnection.prepareStatement(sql);
        stmt.setString(1, user.getName());
        stmt.setString(2, user.getEmail());
        stmt.executeUpdate();
    }
}

public class UserNotifier {
    private final EmailService emailService;
    public UserNotifier(EmailService emailService) { this.emailService = emailService; }

    public void sendWelcomeEmail(User user) {
        emailService.send(user.getEmail(), "Welcome!", "Thanks for joining.");
    }
}

public class UserCache {
    private final Map<String, User> cache = new HashMap<>();
    public void put(User user) { cache.put(user.getEmail(), user); }
    public User get(String email) { return cache.get(email); }
}

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// UserService는 구체적인 구현이 아니라 필요한 협력자를 주입받아 조립한다
public class UserService {
    private final UserValidator validator;
    private final UserRepository repository;
    private final UserNotifier notifier;
    private final UserCache cache;

    public UserService(UserValidator validator, UserRepository repository,
                        UserNotifier notifier, UserCache cache) {
        this.validator = validator;
        this.repository = repository;
        this.notifier = notifier;
        this.cache = cache;
    }

    public void registerUser(User user) throws SQLException {
        if (!validator.isValid(user)) {
            throw new IllegalArgumentException("Invalid user: " + user.getEmail());
        }
        repository.save(user);
        cache.put(user);
        notifier.sendWelcomeEmail(user);
    }
}