查询总数

  @Test
    public void testCount(){
        //1.获取entityManager对象
        EntityManager entityManager = JpaUtils.getEntityManager();
        //2.开启事务
        EntityTransaction transaction = entityManager.getTransaction();
        transaction.begin();
        //3.查询全部
        //i.根据jpql语句创建查询对象
        String jpql = "select count(custId) from Customer";
        Query query = entityManager.createQuery(jpql);
        //ii.对参数赋值
        //iii.发送查询，封装结果集
        /**
         * getResultList:直接将查询结果封装为list集合
         * getSingleResult：得到唯一的结果集
         */
        Object singleResult = query.getSingleResult();
        System.out.println(singleResult);
        //4.提交事务
        transaction.commit();
        //5.释放资源
        entityManager.close();
    }