REST API设计原则：理解无状态性与持久化数据管理

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

    // 构造函数、Getter和Setter
    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 + "'}";
    }
}

// UserRepository.java - 数据访问接口
import java.util.List;
import java.util.Optional;

public interface UserRepository {
    User save(User user);
    Optional<User> findById(String id);
    List<User> findAll();
    void deleteById(String id);
}

// InMemoryUserRepository.java - 仅为演示，实际应使用JDBC/JPA/ORM框架实现数据库操作
// 注意：此实现仅用于说明接口，在生产环境中不应作为REST API的持久化层
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.concurrent.ConcurrentHashMap;

public class InMemoryUserRepository implements UserRepository {
    private final Map<String, User> users = new ConcurrentHashMap<>();

    @Override
    public User save(User user) {
        // 实际应用中，id可能由数据库生成
        if (user.getId() == null || user.getId().isEmpty()) {
            user.setId("user-" + (users.size() + 1)); // 简单生成ID
        }
        users.put(user.getId(), user);
        System.out.println("Saved user to in-memory: " + user);
        return user;
    }

    @Override
    public Optional<User> findById(String id) {
        System.out.println("Finding user by id from in-memory: " + id);
        return Optional.ofNullable(users.get(id));
    }

    @Override
    public List<User> findAll() {
        System.out.println("Getting all users from in-memory.");
        return new ArrayList<>(users.values());
    }

    @Override
    public void deleteById(String id) {
        System.out.println("Deleting user by id from in-memory: " + id);
        users.remove(id);
    }
}

// UserService.java - 业务逻辑层，通过UserRepository操作数据
public 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逻辑，如验证
        return userRepository.save(user);
    }

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

    public List<User> getAllUsers() {
        return userRepository.findAll();
    }

    public void deleteUser(String id) {
        userRepository.deleteById(id);
    }
}

// 假设这是一个简单的REST控制器（伪代码，Spring Boot等框架会简化很多）
public class UserController {
    private final UserService userService;

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

    // POST /users
    public User createUser(User newUser) {
        // 实际中newUser的ID可能为空，由数据库生成
        return userService.createUser(newUser);
    }

    // GET /users/{id}
    public User getUser(String id) {
        return userService.getUserById(id)
                          .orElseThrow(() -> new RuntimeException("User not found"));
    }

    // GET /users
    public List<User> getAllUsers() {
        return userService.getAllUsers();
    }
}

// 模拟应用启动和使用
public class Application {
    public static void main(String[] args) {
        // 在实际应用中，这里会注入一个真正的数据库UserRepository实现
        // 例如：JdbcUserRepository 或 JpaUserRepository
        UserRepository userRepository = new InMemoryUserRepository(); // 仅用于演示
        UserService userService = new UserService(userRepository);
        UserController userController = new UserController(userService);

        // 模拟API调用
        System.out.println("\n--- 模拟 POST /users ---");
        User user1 = userController.createUser(new User(null, "Alice", "alice@example.com"));
        User user2 = userController.createUser(new User(null, "Bob", "bob@example.com"));

        System.out.println("\n--- 模拟 GET /users ---");
        List<User> allUsers = userController.getAllUsers();
        allUsers.forEach(System.out::println);

        System.out.println("\n--- 模拟 GET /users/{id} ---");
        User foundUser = userController.getUser(user1.getId());
        System.out.println("Found user: " + foundUser);

        // 模拟另一个API调用，获取之前保存的用户
        System.out.println("\n--- 模拟 GET /users/{id} (第二次调用) ---");
        User anotherFoundUser = userController.getUser(user2.getId());
        System.out.println("Found user: " + anotherFoundUser);

        // 如果是真正的数据库，即使应用重启，数据也依然存在。
        // 而In-Memory版本，重启后数据会丢失。
    }
}