请写出带注释的Java的SpringCloud微信登录的插入和查询后端代码分为controller层，service接口，service实现类，mapper接口，mapper配置文件,调用微信API类

Controller层代码：

@RestController
@RequestMapping("/api")
public class UserController {

    @Autowired
    private UserService userService;

    @PostMapping("/login")
    public ResponseEntity<?> login(@RequestBody User user) {
        try {
            // 调用微信API获取openid和session_key
            WeChatUtil weChatUtil = new WeChatUtil();
            JSONObject json = weChatUtil.getSessionKeyOrOpenId(user.getCode());
            String openid = json.getString("openid");
            String sessionKey = json.getString("session_key");

            // 判断用户是否已存在
            User userFromDB = userService.getUserByOpenId(openid);
            if (userFromDB != null) {
                // 更新session_key
                userFromDB.setSessionKey(sessionKey);
                userService.updateUser(userFromDB);
                return new ResponseEntity<>(userFromDB, HttpStatus.OK);
            } else {
                // 新建用户
                user.setOpenId(openid);
                user.setSessionKey(sessionKey);
                return new ResponseEntity<>(userService.insertUser(user), HttpStatus.OK);
            }
        } catch (Exception e) {
            e.printStackTrace();
            return new ResponseEntity<>("登录失败", HttpStatus.BAD_REQUEST);
        }
    }
}

Service接口代码：

public interface UserService {

    User insertUser(User user);

    void updateUser(User user);

    User getUserByOpenId(String openId);
}

Service实现类代码：

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    private UserMapper userMapper;

    @Override
    public User insertUser(User user) {
        userMapper.insertUser(user);
        return user;
    }

    @Override
    public void updateUser(User user) {
        userMapper.updateUser(user);
    }

    @Override
    public User getUserByOpenId(String openId) {
        return userMapper.getUserByOpenId(openId);
    }
}

Mapper接口代码：

@Mapper
public interface UserMapper {

    @Insert("insert into user(open_id,session_key,nick_name,gender,city,province,country,avatar_url,create_time) " +
            "values(#{openId},#{sessionKey},#{nickName},#{gender},#{city},#{province},#{country},#{avatarUrl},#{createTime})")
    void insertUser(User user);

    @Update("update user set session_key=#{sessionKey} where open_id=#{openId}")
    void updateUser(User user);

    @Select("select * from user where open_id=#{openId}")
    User getUserByOpenId(String openId);
}

Mapper配置文件：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.demo.mapper.UserMapper">
</mapper>

调用微信API类代码：

public class WeChatUtil {

    public static final String APP_ID = "your_app_id";
    public static final String APP_SECRET = "your_app_secret";

    public JSONObject getSessionKeyOrOpenId(String code) {
        String requestUrl = "https://api.weixin.qq.com/sns/jscode2session?appid=" + APP_ID + "&secret=" + APP_SECRET
                + "&js_code=" + code + "&grant_type=authorization_code";
        JSONObject jsonObject = null;
        try {
            CloseableHttpClient httpClient = HttpClients.createDefault();
            HttpGet httpGet = new HttpGet(requestUrl);
            CloseableHttpResponse response = httpClient.execute(httpGet);
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                String result = EntityUtils.toString(entity, "UTF-8");
                jsonObject = JSON.parseObject(result);
            }
            httpGet.releaseConnection();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return jsonObject;
    }
}

注：需要替换APP_ID和APP_SECRET为自己的微信小程序的信息。