我遇到这个错的原因是 测试类没有加上主程序启动类和没有进行mapperscan扫描

package com.example.test;

import com.itheima.dao.UserDao;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
@SpringBootTest(classes = {com.example.demo.DemoApplication.class}, properties = {"application.yaml"})
public class DemoApplication {
    @Autowired
    private UserDao userDao;


    @Test
    public  void test() {
        userDao.selectList(null);
    }

}

com.example.demo.DemoApplication.class 为主程序启动类