1.全局配置文件没配好？

检查全局配置文件application.properties或application.yml是否配置扫描mapper包的文件路径

#mybatis配置mapper文件路径
#mybatis.mapper-locations=classpath:/mapper/*.xml
#mybatis-plus配置mapper文件路径
mybatis-plus.mapper-locations=classpath:/mapper/*.xml

2.启动类没加包扫描注解 ？

检查启动类上是否添加包扫描注解

@SpringBootApplication
@MapperScan("项目路径.mapper")
public class DormitoryBackendApplication {public static void main(String[] args) {SpringApplication.run(DormitoryBackendApplication.class, args);
}

3.没使用@Mapper注解？

XXXmapper.java类是否使用@Mapper注解

@Mapper
public interface AttendanceMapper extends BaseMapper<Attendance> {}

4.命名空间不正确？

检查XXXmapper.xml的命名空间是否正确，是否能跳转到对应的XXXmapper.java

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dormitory.dormitory_backend.mapper.AttendanceMapper"></mapper>

5.检查mybatis-plus和spirngboot版本是否对应