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

1. controller层代码：

@RestController
@RequestMapping("/api/wechat")
public class WechatController {

    @Autowired
    private WechatService wechatService;

    @PostMapping("/login")
    public String login(@RequestParam String code) {
        return wechatService.login(code);
    }

    @GetMapping("/user")
    public WechatUser getUser(@RequestParam String openid) {
        return wechatService.getUser(openid);
    }
}

2. service接口代码：

public interface WechatService {

    String login(String code);

    WechatUser getUser(String openid);
}

3. service实现类代码：

@Service
public class WechatServiceImpl implements WechatService {

    @Autowired
    private WechatMapper wechatMapper;

    @Autowired
    private WechatApi wechatApi;

    @Override
    public String login(String code) {
        String openid = wechatApi.getOpenid(code);
        WechatUser user = wechatMapper.selectUserByOpenid(openid);
        if (user == null) {
            user = wechatApi.getUserInfo(openid);
            wechatMapper.insertUser(user);
        }
        return openid;
    }

    @Override
    public WechatUser getUser(String openid) {
        return wechatMapper.selectUserByOpenid(openid);
    }
}

4. mapper接口代码：

@Mapper
public interface WechatMapper {

    void insertUser(WechatUser user);

    WechatUser selectUserByOpenid(String openid);
}

5. 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.WechatMapper">

    <insert id="insertUser" parameterType="com.example.demo.entity.WechatUser">
        INSERT INTO wechat_user(openid, nickname, avatar_url)
        VALUES(#{openid}, #{nickname}, #{avatarUrl})
    </insert>

    <select id="selectUserByOpenid" parameterType="String" resultType="com.example.demo.entity.WechatUser">
        SELECT openid, nickname, avatar_url
        FROM wechat_user
        WHERE openid = #{openid}
    </select>

</mapper>

6. 调用微信API类代码：

@Component
public class WechatApi {

    @Value("${wechat.appid}")
    private String appid;

    @Value("${wechat.secret}")
    private String secret;

    public String getOpenid(String code) {
        String url = "https://api.weixin.qq.com/sns/jscode2session?appid=" + appid
                + "&secret=" + secret
                + "&js_code=" + code
                + "&grant_type=authorization_code";
        String json = restTemplate.getForObject(url, String.class);
        return JsonPath.read(json, "$.openid");
    }

    public WechatUser getUserInfo(String openid) {
        String url = "https://api.weixin.qq.com/cgi-bin/user/info?access_token=" + getAccessToken() + "&openid=" + openid;
        String json = restTemplate.getForObject(url, String.class);
        String nickname = JsonPath.read(json, "$.nickname");
        String avatarUrl = JsonPath.read(json, "$.headimgurl");
        return new WechatUser(openid, nickname, avatarUrl);
    }

    private String getAccessToken() {
        String url = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=" + appid + "&secret=" + secret;
        String json = restTemplate.getForObject(url, String.class);
        return JsonPath.read(json, "$.access_token");
    }
}

注：在以上代码中，使用了RestTemplate和JsonPath，需要在pom.xml中添加相应的依赖。