springboot mybatis搭建

@RestController
public class UserController {
    @RequestMapping("/hello")
    public String index() {
        return "Hello World";
    }

    @Autowired
    UserService userService;

    @RequestMapping(value = "/list", method = RequestMethod.GET)
    public List<User> getAccounts() {
        return userService.getUserList();
    }
}

@Mapper
public interface UserMapper {

    @Select("SELECT * FROM tb_user WHERE id = #{id}")
    User getUserById(Integer id);

    @Select("SELECT * FROM tb_user")
    public List<User> getUserList();

}

@Service
public class UserService {
    @Autowired
    private UserMapper userMapper;

    public List<User> getUserList() {
        return userMapper.getUserList();
    }
}

public class User {
    private int id;
    private String username;
    private int age;

    public User(int id, String username, int age) {
        this.id = id;
        this.username = username;
        this.age = age;
    }
    public User(Long id, String username, Long age) {
        this.id = Math.toIntExact(id);
        this.username = username;
        this.age = Math.toIntExact(age);
    }
  //不知道为啥报错，就按照提示来呗
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

spring.datasource.url=jdbc:mysql://localhost:3306/test
spring.datasource.username=root
spring.datasource.password=123456
spring.datasource.driver-class-name=com.mysql.jdbc.Driver