//import org.mybatis.spring.annotation.MapperScan;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication

@MapperScan("com.example.mybatisplus.mapper")  //把这个加上，别写上正确的路径，如果是模块化一般路径为@MapperScan("com.kfit.*.mapper")  

public class Application {

    public static void main(String[] args){

        System.out.println("hahahaha");

         SpringApplication.run(Application.class, args);

    }

}

package com.example.mybatisplus.mapper;

//import org.apache.ibatis.annotations.Mapper;

import com.baomidou.mybatisplus.mapper.BaseMapper;

import com.example.mybatisplus.entity.Person;

@Mapper     //每个文件配置这个
public interface PersonMapper extends BaseMapper<Person> { 
  Integer listCount(); 
  Person findPersonById(Integer id); 
}