package com.example.{module}.{layer};

import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;
import org.springframework.stereotype.{Annotation};
import org.springframework.transaction.annotation.Transactional;

/**
 * {Class purpose and responsibility}
 *
 * <p>Key features:
 * <ul>
 *   <li>Feature 1</li>
 *   <li>Feature 2</li>
 * </ul>
 *
 * @author Enterprise Java Developer
 * @since {Version}
 */
@Slf4j
@RequiredArgsConstructor
@{Annotation}
public class {ClassName} {
    
    private final Dependency dependency;
    
    /**
     * {Method purpose}
     *
     * @param param parameter description
     * @return return value description
     * @throws BusinessException when business rules violated
     */
    @Transactional(rollbackFor = Exception.class)
    public Result methodName(Param param) {
        log.info("Method started, param: {}", param);
        
        try {
            // 1. Validate input
            validateParam(param);
            
            // 2. Execute business logic
            Result result = executeBusinessLogic(param);
            
            // 3. Return result
            log.info("Method completed successfully");
            return result;
            
        } catch (BusinessException e) {
            log.error("Business error: {}", e.getMessage(), e);
            throw e;
        } catch (Exception e) {
            log.error("System error occurred", e);
            throw new SystemException("Unexpected error", e);
        }
    }
    
    private void validateParam(Param param) {
        if (param == null) {
            throw new IllegalArgumentException("Param cannot be null");
        }
        // Additional validations...
    }
    
    private Result executeBusinessLogic(Param param) {
        // Implementation...
        return new Result();
    }
}

controller/    - HTTP endpoints, request/response handling
├── dto/       - Data Transfer Objects
└── vo/        - View Objects

service/       - Business logic, orchestration
├── impl/      - Service implementations

repository/    - Data access layer
├── entity/    - JPA entities
└── mapper/    - MyBatis mappers

domain/        - Domain models (DDD)
├── model/     - Domain objects
├── service/   - Domain services
└── event/     - Domain events

config/        - Configuration classes
exception/     - Custom exceptions
util/          - Utility classes
constant/      - Constants and enums

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// Improved version

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[Component A] --> [Component B]
[Component B] --> [Component C]

// Key entities

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// Before
public List<User> getUsersWithOrders() {
    List<User> users = userRepository.findAll();
    users.forEach(user -> user.getOrders().size()); // N queries
    return users;
}

// After
public List<User> getUsersWithOrders() {
    return userRepository.findAllWithOrders(); // 1 query
}

// Repository
@Query("SELECT u FROM User u LEFT JOIN FETCH u.orders")
List<User> findAllWithOrders();

@Cacheable(value = "users", key = "#userId")
public User getUser(Long userId) {
    return userRepository.findById(userId)
        .orElseThrow(() -> new UserNotFoundException(userId));
}

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java.lang.OutOfMemoryError: Java heap space
  at ArrayList.grow()
  at OrderService.exportAllOrders()

// Problematic code
public List<Order> exportAllOrders() {
    return orderRepository.findAll(); // Loads 1M+ records
}

-Xmx4g -Xms4g

public void exportAllOrders(OutputStream output) {
    int pageSize = 1000;
    int page = 0;
    
    Page<Order> orderPage;
    do {
        orderPage = orderRepository.findAll(
            PageRequest.of(page++, pageSize)
        );
        
        writeToStream(orderPage.getContent(), output);
        
    } while (orderPage.hasNext());
}

@Scheduled(fixedRate = 60000)
public void checkMemoryUsage() {
    MemoryMXBean memoryBean = ManagementFactory.getMemoryMXBean();
    long used = memoryBean.getHeapMemoryUsage().getUsed();
    long max = memoryBean.getHeapMemoryUsage().getMax();
    
    if (used > max * 0.8) {
        log.warn("High memory usage: {}%", (used * 100 / max));
    }
}

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// ❌ Bad
try {
    service.process();
} catch (Exception e) {
    e.printStackTrace();
}

// ✅ Good
try {
    service.process();
} catch (BusinessException e) {
    log.warn("Business validation failed: {}", e.getMessage());
    throw e;
} catch (Exception e) {
    log.error("Unexpected error in process", e);
    throw new SystemException("Processing failed", e);
}

// ❌ Bad
public String getUserName(User user) {
    return user.getName();
}

// ✅ Good
public String getUserName(User user) {
    return Optional.ofNullable(user)
        .map(User::getName)
        .orElse("Unknown");
}

// ❌ Bad
InputStream is = new FileInputStream(file);
// forgot to close

// ✅ Good
try (InputStream is = new FileInputStream(file)) {
    // use stream
} // automatically closed

// ❌ Bad
private static final String API_URL = "http://api.example.com";

// ✅ Good
@Value("${api.url}")
private String apiUrl;

// ❌ Bad
System.out.println("User: " + user);
log.debug("Processing order: " + order.getId());

// ✅ Good
log.info("User operation started, userId: {}", user.getId());
log.debug("Processing order, orderId: {}", order.getId());

// ❌ Wrong: Transaction in loop
public void updateUsers(List<User> users) {
    for (User user : users) {
        updateUser(user); // Each call opens/closes transaction
    }
}

// ✅ Correct: Single transaction
@Transactional
public void updateUsers(List<User> users) {
    for (User user : users) {
        userRepository.save(user);
    }
}

// ❌ LazyInitializationException
@Transactional
public User getUser(Long id) {
    return userRepository.findById(id).orElse(null);
}
// Later: user.getOrders() fails - no session

// ✅ Fetch needed data
@Transactional
public User getUserWithOrders(Long id) {
    return userRepository.findByIdWithOrders(id).orElse(null);
}

// ❌ Stale cache after update
@Cacheable("users")
public User getUser(Long id) { ... }

public void updateUser(User user) {
    userRepository.save(user);
    // Cache still has old data!
}

// ✅ Invalidate cache
@CacheEvict(value = "users", key = "#user.id")
public void updateUser(User user) {
    userRepository.save(user);
}