理解REST API的无状态性：避免跨请求内存状态管理的陷阱

// 1. 用户实体类
public class User {
    private String id;
    private String name;
    private String email;

    // 构造函数, getters and setters
    public User() {}

    public User(String id, String name, String email) {
        this.id = id;
        this.name = name;
        this.email = email;
    }

    public String getId() { return id; }
    public void setId(String id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public String getEmail() { return email; }
    public void setEmail(String email) { this.email = email; }

    @Override
    public String toString() {
        return "User{id='" + id + "', name='" + name + "', email='" + email + "'}";
    }
}

// 2. 用户数据访问接口 (Repository 模拟数据库操作)
interface UserRepository {
    void save(User user);
    User findById(String id);
    // ... 其他数据库操作，如 findAll, delete
}

// 3. 用户数据访问实现 (模拟数据库操作，实际项目中会使用JPA/MyBatis等)
class InMemoryUserRepository implements UserRepository {
    // 实际项目中这里会连接数据库，此处仅为演示，不应在生产环境使用内存Map
    private final java.util.Map<String, User> users = new java.util.concurrent.ConcurrentHashMap<>();

    @Override
    public void save(User user) {
        if (user.getId() == null || user.getId().isEmpty()) {
            user.setId(java.util.UUID.randomUUID().toString()); // 为新用户生成ID
        }
        users.put(user.getId(), user);
        System.out.println("User saved to 'database': " + user);
    }

    @Override
    public User findById(String id) {
        User user = users.get(id);
        System.out.println("User retrieved from 'database': " + (user != null ? user : "null"));
        return user;
    }
}

// 4. 用户服务层 (业务逻辑处理)
class UserService {
    private final UserRepository userRepository;

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

    public User createUser(User user) {
        // 可以在这里添加业务校验逻辑
        userRepository.save(user);
        return user;
    }

    public User getUserById(String id) {
        return userRepository.findById(id);
    }
}

// 5. REST API 控制器 (模拟Spring Boot或其他框架的API层)
class UserApiController {
    private final UserService userService;

    public UserApiController(UserService userService) {
        this.userService = userService;
    }

    // 模拟 POST /users API
    public User saveUser(User newUserRequest) {
        System.out.println("\nAPI Call: POST /users - Request to save user: " + newUserRequest);
        // 在实际API中，newUserRequest可能不包含ID，由Service层或Repository生成
        return userService.createUser(newUserRequest);
    }

    // 模拟 GET /users/{id} API
    public User getUser(String userId) {
        System.out.println("\nAPI Call: GET /users/" + userId + " - Request to get user.");
        return userService.getUserById(userId);
    }
}

// 6. 应用程序入口 (模拟Spring Boot启动和请求处理)
public class Application {
    public static void main(String[] args) {
        // 依赖注入 (通常由框架完成，这里手动创建)
        UserRepository userRepository = new InMemoryUserRepository(); // 实际会是数据库实现
        UserService userService = new UserService(userRepository);
        UserApiController userApiController = new UserApiController(userService);

        // 模拟第一次API调用：保存用户
        User user1 = new User(null, "Alice", "alice@example.com");
        User savedUser1 = userApiController.saveUser(user1);
        String aliceId = savedUser1.getId();

        // 模拟第二次API调用：获取用户
        User retrievedUser1 = userApiController.getUser(aliceId);
        if (retrievedUser1 != null) {
            System.out.println("Retrieved user: " + retrievedUser1);
        } else {
            System.out.println("User with ID " + aliceId + " not found.");
        }

        // 模拟保存另一个用户
        User user2 = new User(null, "Bob", "bob@example.com");
        User savedUser2 = userApiController.saveUser(user2);
        String bobId = savedUser2.getId();

        // 模拟获取另一个用户
        User retrievedUser2 = userApiController.getUser(bobId);
        if (retrievedUser2 != null) {
            System.out.println("Retrieved user: " + retrievedUser2);
        } else {
            System.out.println("User with ID " + bobId + " not found.");
        }

        // 模拟获取一个不存在的用户
        userApiController.getUser("nonExistentId");
    }
}