我想通过mybatis先测试一下能不能从数据库访问ProductCategory表，参数都固定好了，结果一直抛出空指针，mybatis用的不熟，最后的测试类一直抛出空指针

public interface ProductCategoryDao {
    //List<ProductCategory> queryProductCategory(@Param("productId") long ProductId);
    List<ProductCategory> queryProductCategory();
}

public interface ProductCategoryService {
    List<ProductCategory> getProductCategoryList();
}

@Service
public class ProductCategoryServiceImpl implements ProductCategoryService {
    @Autowired
    private ProductCategoryDao productCategoryDao;

    @Override
    public List<ProductCategory> getProductCategoryList() {
        List<ProductCategory> productCategories = productCategoryDao.queryProductCategory();
        return productCategories;
    }
}

<mapper namespace="com.imooc.o2o.dao.ShopCategoryDao">
    <select id="queryProductCategory" resultType="com.imooc.o2o.entity.ProductCategory" >
        select
        pc.priority,
        pc.product_category_name,
        pc.shop_id
        from
        tb_product_category as pc,
        where pc.shop_id=1
    </select>
</mapper>

public class ProductCategoryDaoTest {
    @Autowired
    ProductCategoryService productCategoryService;
    @Test
    public void testQueryProductCategory() {
        List<ProductCategory> productCategoryList = productCategoryService.getProductCategoryList();
        System.out.println(productCategoryList);
    }
}