解密DDD：高内聚对象组的维护之道( 六 ) _DDD

3.3.2. 延迟加载延迟加载是指在进行数据库查询时，并不会立即查询关联表数据，而是要等到使用时才会去查，这样可以避免不必要的数据库查询，提高查询效率。
延迟加载又分为两种情况：

表间的延迟加载：在表关联情况下，进行数据库查询时，并不会立即查询关联表，而是要等到使用时才会去查数据库；
表中属性的延迟加载：比如大型字段blob，需要等到使用时才加载，这样可以避免不必要的数据库查询，提高查询效率；

在此，重点介绍表间关联的延迟加载：
实体代码如下所示：

@Entity@Table(name = "order_info")public class Order{@Id@GeneratedValue(strategy = GenerationType.IDENTITY)private Long id;@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)@JoinColumn(name = "order_id")private List<OrderItem> orderItems = new ArrayList<>();@OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)private Pay pay;@OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)private Address address;// 忽略其他字段和方法}

查询代码如下：

Order order = this.orderRepository.findById(this.order.getId()).get();Assertions.assertNotNull(order);System.out.println("访问 item");Assertions.assertEquals(3, order.getOrderItems().size());System.out.println("访问 address");Assertions.assertNotNull(order.getAddress().getDetail());System.out.println("访问 pay");Assertions.assertNotNull(order.getPay().getPrice());

控制台输出如下：

Hibernate: select order0_.id as id1_3_0_, order0_.address_id as address_6_3_0_, order0_.pay_id as pay_id7_3_0_, order0_.status as status2_3_0_, order0_.total_price as total_pr3_3_0_, order0_.total_selling_price as total_se4_3_0_, order0_.user_id as user_id5_3_0_ from order_info order0_ where order0_.id=?访问 itemHibernate: select orderitems0_.order_id as order_id6_4_0_, orderitems0_.id as id1_4_0_, orderitems0_.id as id1_4_1_, orderitems0_.price as price2_4_1_, orderitems0_.product_id as product_3_4_1_, orderitems0_.quantity as quantity4_4_1_, orderitems0_.selling_price as selling_5_4_1_ from order_item orderitems0_ where orderitems0_.order_id=?访问 addressHibernate: select address0_.id as id1_2_0_, address0_.detail as detail2_2_0_ from address address0_ where address0_.id=?访问 payHibernate: select pay0_.id as id1_5_0_, pay0_.price as price2_5_0_ from pay_info pay0_ where pay0_.id=?

从日志输出可知，关联实体只有在属性被访问时才会触发自动加载。
延迟加载在聚合更新时极为重要，面对一个大聚合，每次修改只会涉及少量相关联的实体，由于延迟加载机制的保障，对于那些没有必要访问的实体并不会执行实际的加载操作，从而大幅提升性能。
3.4. 按需更新简单理解按需更新，就是只有在有必要时才会对数据进行更新。
按需更新可以分为两个场景：

只更新变更实体：在保存一组对象时，只对状态发生变化的实体进行更新；
只更新变更字段：保存一个实体时，只对状态发生变化的字段进行更新；

3.4.1. 只更新变更实体在数据保存时，JPA 会自动识别发生变更的实体，仅对变更实体执行 update 语句。
测试代码如下：

Order order = this.orderRepository.findById(this.order.getId()).get();order.getOrderItems().size(); // 获取未更新order.getPay().getPrice(); // 获取未更新order.getAddress().setDetail("新地址"); // 获取并更新System.out.println("更新数据");this.orderRepository.save(order);

控制台输出如下：

Hibernate: select order0_.id as id1_3_0_, order0_.address_id as address_6_3_0_, order0_.pay_id as pay_id7_3_0_, order0_.status as status2_3_0_, order0_.total_price as total_pr3_3_0_, order0_.total_selling_price as total_se4_3_0_, order0_.user_id as user_id5_3_0_ from order_info order0_ where order0_.id=?Hibernate: select orderitems0_.order_id as order_id6_4_0_, orderitems0_.id as id1_4_0_, orderitems0_.id as id1_4_1_, orderitems0_.price as price2_4_1_, orderitems0_.product_id as product_3_4_1_, orderitems0_.quantity as quantity4_4_1_, orderitems0_.selling_price as selling_5_4_1_ from order_item orderitems0_ where orderitems0_.order_id=?Hibernate: select pay0_.id as id1_5_0_, pay0_.price as price2_5_0_ from pay_info pay0_ where pay0_.id=?Hibernate: select address0_.id as id1_2_0_, address0_.detail as detail2_2_0_ from address address0_ where address0_.id=?更新数据Hibernate: update address set detail=? where id=?

从日志输出可见：