1.修改UserService

@Service
public class UserServiceImpl extends ServiceImpl<UserMapper, UserDO> implements UserService {
    public UserRespDTO getUserByUsername(String username) {

    LambdaQueryWrapper<UserDO> queryWrapper = Wrappers.lambdaQuery(UserDO.class)
        .eq(UserDO::getUsername,username);
    UserDO userDO = baseMapper.selectOne(queryWrapper);

    if (userDO != null) {
        UserRespDTO result = new UserRespDTO();
        BeanUtils.copyProperties(userDO, result);
        return result;
    } else {
        // UserDO是null，无法找到用户
        return null;
        }
    }
}

2.完善UserContorller的为空报错

@RestController
@RequiredArgsConstructor
public class UserController {

private final UserService userService;
/**
*根据用户名查询用户信息
*/
    @GetMapping("/api/shortlink/v1/user/{username}")
    public Result<UserRespDTO> getUserByUsername(@PathVariable("username") String username){
        UserRespDTO result = userService.getUserByUsername(username);

        if(result==null){
            return new Result<UserRespDTO>().setCode("-1").setMessage("用户查询为空");
        }else {
            return new Result<UserRespDTO>().setCode("0").setData(result);
        }
    }
}

3.注意UserController中返回值的变化

String->UserRespDTO->Result<UserRespDTO>