Spring Beans

// Data Access Layer
@Repository
public class UserRepository {
    // Spring adds exception translation: SQL exceptions -> Spring DataAccessException
    public User findById(Long id) { /* JDBC/JPA code */ return null; }
    public void save(User user) { /* JDBC/JPA code */ }
}

// Service Layer
@Service
public class UserService {
    private final UserRepository userRepository;

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

    public User getUser(Long id) {
        return userRepository.findById(id);
    }
}

// Web Layer
@RestController
@RequestMapping("/api/users")
public class UserController {
    private final UserService userService;

    @Autowired
    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return userService.getUser(id);
    }
}

// Generic component
@Component
public class DataInitializer implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) {
        System.out.println("Application started, initializing data...");
    }
}

import org.springframework.context.annotation.*;
import org.springframework.web.context.annotation.*;
import javax.annotation.*;

// Singleton (default) - one instance
@Service
// @Scope("singleton") // This is the default, no need to specify
public class SingletonService {
    private int counter = 0;
    public int increment() { return ++counter; }
}

// Prototype - new instance each time
@Component
@Scope("prototype")
public class PrototypeBean {
    private final String id = java.util.UUID.randomUUID().toString();
    public String getId() { return id; }
}

// Request scope (web only)
@Component
@RequestScope
public class RequestContext {
    private String requestId = java.util.UUID.randomUUID().toString();
    public String getRequestId() { return requestId; }
}

// Session scope (web only)
@Component
@SessionScope
public class UserSession {
    private String username;
    private List<String> cart = new ArrayList<>();
    // getters/setters
}

// Bean lifecycle callbacks
@Service
public class DatabaseService {
    @PostConstruct // Called after bean is created and dependencies injected
    public void init() {
        System.out.println("DatabaseService initialized");
    }

    @PreDestroy // Called before bean is destroyed
    public void cleanup() {
        System.out.println("DatabaseService cleanup");
    }
}

// Lazy initialization - bean created only when first requested
@Service
@Lazy
public class HeavyService {
    public HeavyService() {
        System.out.println("HeavyService created (lazy)");
    }
}

@Configuration
public class AppConfig {

    // Singleton bean (default)
    @Bean
    public ModelMapper modelMapper() {
        return new ModelMapper();
    }

    // Prototype bean via @Bean
    @Bean
    @Scope("prototype")
    public HttpClient httpClient() {
        return HttpClient.newBuilder()
                .connectTimeout(Duration.ofSeconds(10))
                .build();
    }

    // Conditional bean - only created if property is set
    @Bean
    @ConditionalOnProperty(name = "feature.cache.enabled", havingValue = "true")
    public CacheManager cacheManager() {
        return new ConcurrentMapCacheManager("users", "products");
    }

    // Bean with init and destroy methods
    @Bean(initMethod = "start", destroyMethod = "stop")
    public SchedulerService schedulerService() {
        return new SchedulerService();
    }
}