Beanの配線：@Component、@Service、@Repository

ステレオタイプアノテーション

@Component: 汎用Bean

@Component
public class EmailFormatter {
    public String format(String to, String subject) {
        return "To: " + to + "\nSubject: " + subject;
    }
}

@Service: ビジネスロジック層

@Service
public class OrderService {
    private final OrderRepository repo;
    public OrderService(OrderRepository repo) { this.repo = repo; }
    public Order placeOrder(Cart cart) { /* business logic */ return repo.save(new Order(cart)); }
}

@Repository: データアクセス層

@Repository
public class JdbcUserRepository {
    private final JdbcTemplate jdbc;
    public JdbcUserRepository(JdbcTemplate jdbc) { this.jdbc = jdbc; }
    public User findById(long id) {
        return jdbc.queryForObject("SELECT * FROM users WHERE id=?",
            new BeanPropertyRowMapper<>(User.class), id);
    }
}

@Configurationクラスの@Bean

@Configuration
public class AppConfig {
    @Bean
    public ObjectMapper objectMapper() {
        return new ObjectMapper().registerModule(new JavaTimeModule());
    }
    @Bean
    public RestTemplate restTemplate() { return new RestTemplate(); }
}

コンポーネントスキャン

// Explicit scan (rarely needed in Spring Boot):
@SpringBootApplication
@ComponentScan(basePackages = {"com.example.app", "com.example.shared"})
public class MyApp { ... }

Beanの名前付け

@Service("orderSvc")
public class OrderService { ... }
// Inject by name:
@Autowired @Qualifier("orderSvc")
private OrderService svc;

Beanスコープ

@Service
@Scope("prototype")
public class ReportBuilder {
    private List<String> lines = new ArrayList<>();
    public void addLine(String line) { lines.add(line); }
}

@Primaryと@Qualifierによる曖昧性の解消

@Service @Primary
public class CacheUserService implements UserService { ... }
@Service
public class DbUserService implements UserService { ... }
// Inject specific one:
@Autowired @Qualifier("dbUserService")
private UserService userService;

遅延初期化

@Service @Lazy
public class HeavyReportService {
    public HeavyReportService() { System.out.println("Created on first use"); }
}

@PostConstructと@PreDestroy

@Service
public class CacheService {
    @PostConstruct
    public void init() { System.out.println("Cache warming up..."); }
    @PreDestroy
    public void cleanup() { System.out.println("Cache cleared."); }
}

クイックチェック

まとめ