jpa入门案例----使用jpa对数据库进行查询、删除、修改操作

配置文件、依赖、实体类的创建方法在上一篇博客《使用jpa对数据库进行添加数据》的博客中写的有

一、查询操作

 @Test
    //根据id查找
    public void find(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("myjpa");
        EntityManager entityManager = factory.createEntityManager();
        Customer customer = entityManager.getReference(Customer.class, 6L);//第一个参数是实体类名，第二个参数是id
//也可以使用下面这个方法查询
        Customer customer1 = entityManager.find(Customer.class, 8L);
    }


二、删除操作
要先查询在删除

 @Test
    public void testDelete(){
        //创建工厂
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("myjpa");
        //创建一个EntityManager对象
        EntityManager entityManager = factory.createEntityManager();
        //使用entityManager对象开启事物
        EntityTransaction transaction = entityManager.getTransaction();
        transaction.begin();
        Customer customer = entityManager.find(Customer.class, 1L);
//        System.out.println(customer);
        //执行删除操作
        entityManager.remove(customer);
        transaction.commit();
    }

三、修改
同样要先进查询操作，然后再进行修改操作

//修改
@Test
public void update(){
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("myjpa");
    EntityManager entityManager = factory.createEntityManager();
    EntityTransaction transaction = entityManager.getTransaction();
    //开启事物
    transaction.begin();
    //先查询
    Customer customer = entityManager.find(Customer.class, 8L);
    //修改
    customer.setCustName("aaa");
    //执行修改命令
    entityManager.persist(customer);
    //提交事物
    transaction.commit();
    //释放资源
    entityManager.close();
    factory.close();
    
}