Hibernate中的Fetch(数据抓取)

Hibernate抓取策略(fetch)

一、单端关联上的fetch[取值：select(默认)/join]

测试用例：

TBook b = (TBook)session.get(TBook.class,1);System.out.println("图书名称："+b.getBName());System.out.println("图书类别："+b.getCategory().getCategory());

1）保持默认，相当于fetch="select"

如：<many-to-one name="category" class="cn.bdqn.pojo.Category" cascade="all" fetch="select">

数据抓取过程中，select抓取会另外发送一条select语句来抓取与当前对象相关联的实体或集合。

执行结果：2条语句

Hibernate: select tbook0_.id as id1_1_0_, tbook0_.category_Id as category2_1_0_, tbook0_.author as author3_1_0_, tbook0_.b_name as b_name4_1_0_, tbook0_.b_price as b_price5_1_0_, tbook0_.pubDate as pubDate6_1_0_ from admindb.t_book tbook0_ where tbook0_.id=?图书名称：红楼梦Hibernate: select category0_.id as id1_0_0_, category0_.category as category2_0_0_ from admindb.category category0_ where category0_.id=?图书类别：文学

2）设置fetch="join"

如：<many-to-one name="category" class="cn.bdqn.pojo.Category" cascade="all"fetch="join">

数据抓取过程中，join抓取会通过select语句使用外连接来加载其关联实体或集合。

执行结果：1条语句

Hibernate:     select        tbook0_.id as id1_1_0_,        tbook0_.category_Id as category2_1_0_,        tbook0_.author as author3_1_0_,        tbook0_.b_name as b_name4_1_0_,        tbook0_.b_price as b_price5_1_0_,        tbook0_.pubDate as pubDate6_1_0_,        category1_.id as id1_0_1_,        category1_.category as category2_0_1_     from        admindb.t_book tbook0_     left outer join        admindb.category category1_             on tbook0_.category_Id=category1_.id     where        tbook0_.id=?图书名称：红楼梦图书类别：文学

我们发现此时懒加载已经失效。

二、集合端的fetch[取值：select(默认)/join/subselect]

测试用例：

@Testpublic void test() {Session session = null;try {session = SessionUtil.getSession();Category c = (Category)session.load(Category.class,1);System.out.println("图书类别："+c.getCategory());Set<TBook> tBooks = c.getTBooks();System.out.println(tBooks.size());Iterator<TBook> it = tBooks.iterator();while(it.hasNext()){System.out.println(it.next().getBName());}} catch (Exception e) {e.printStackTrace();} finally {SessionUtil.close(session);}}

1）保持默认，相当于fetch="select"

如：<set name="TBooks" inverse="true" fetch="select">

数据抓取过程中，select抓取会另外发送一条select语句来抓取与当前对象相关联的实体或集合。

执行结果：2条语句

Hibernate:     select        category0_.id as id1_0_0_,        category0_.category as category2_0_0_     from        admindb.category category0_     where        category0_.id=?图书类别：文学Hibernate:     select        tbooks0_.category_Id as category2_0_0_,        tbooks0_.id as id1_1_0_,        tbooks0_.id as id1_1_1_,        tbooks0_.category_Id as category2_1_1_,        tbooks0_.author as author3_1_1_,        tbooks0_.b_name as b_name4_1_1_,        tbooks0_.b_price as b_price5_1_1_,        tbooks0_.pubDate as pubDate6_1_1_     from        admindb.t_book tbooks0_     where        tbooks0_.category_Id=?2西游记红楼梦

2）设置fetch="join"
如：<set name="TBooks" inverse="true" fetch="join">

数据抓取过程中，join抓取会通过select语句使用外连接来加载其关联实体或集合。

执行结果：1条语句

Hibernate:     select        category0_.id as id1_0_0_,        category0_.category as category2_0_0_,        tbooks1_.category_Id as category2_0_1_,        tbooks1_.id as id1_1_1_,        tbooks1_.id as id1_1_2_,        tbooks1_.category_Id as category2_1_2_,        tbooks1_.author as author3_1_2_,        tbooks1_.b_name as b_name4_1_2_,        tbooks1_.b_price as b_price5_1_2_,        tbooks1_.pubDate as pubDate6_1_2_     from        admindb.category category0_     left outer join        admindb.t_book tbooks1_             on category0_.id=tbooks1_.category_Id     where        category0_.id=?图书类别：文学2西游记红楼梦

3）设置fetch="subselect"
如：<set name="TBooks" inverse="true" fetch="subselect">

使用subselect进行数据抓取过程中， 抓取数据相关联的集合数据时会一次性完成数据的抓取,也就是说会另外发送一条select语句抓取在前面查询到的所有实体对象的关联集。

测试用例：

@Testpublic void testSubSelect() {Session session = null;Transaction tran = null;try {session = SessionUtil.getSession();tran = session.beginTransaction();List<Category> list = session.createQuery("from Category where id in (1,2,3)").list();for (Category c : list) {System.out.println("图书类别：" + c.getCategory());System.out.println("图书数量：" + c.getTBooks().size());}tran.commit();} catch (Exception e) {e.printStackTrace();} finally {SessionUtil.close(session);}}

当不设置fetch="subselect"时，也就是默认，或者设置成select。

结果如下：会产生额外的三条sql

Hibernate:     select        category0_.id as id1_0_,        category0_.category as category2_0_     from        admindb.category category0_     where        category0_.id in (            1 , 2 , 3        )图书类别：文学Hibernate:     select        tbooks0_.category_Id as category2_0_0_,        tbooks0_.id as id1_1_0_,        tbooks0_.id as id1_1_1_,        tbooks0_.category_Id as category2_1_1_,        tbooks0_.author as author3_1_1_,        tbooks0_.b_name as b_name4_1_1_,        tbooks0_.b_price as b_price5_1_1_,        tbooks0_.pubDate as pubDate6_1_1_     from        admindb.t_book tbooks0_     where        tbooks0_.category_Id=?图书数量：2图书类别：历史Hibernate:     select        tbooks0_.category_Id as category2_0_0_,        tbooks0_.id as id1_1_0_,        tbooks0_.id as id1_1_1_,        tbooks0_.category_Id as category2_1_1_,        tbooks0_.author as author3_1_1_,        tbooks0_.b_name as b_name4_1_1_,        tbooks0_.b_price as b_price5_1_1_,        tbooks0_.pubDate as pubDate6_1_1_     from        admindb.t_book tbooks0_     where        tbooks0_.category_Id=?图书数量：1图书类别：武侠Hibernate:     select        tbooks0_.category_Id as category2_0_0_,        tbooks0_.id as id1_1_0_,        tbooks0_.id as id1_1_1_,        tbooks0_.category_Id as category2_1_1_,        tbooks0_.author as author3_1_1_,        tbooks0_.b_name as b_name4_1_1_,        tbooks0_.b_price as b_price5_1_1_,        tbooks0_.pubDate as pubDate6_1_1_     from        admindb.t_book tbooks0_     where        tbooks0_.category_Id=?图书数量：1

当设置成fetch="subselect"，即：<set name="TBooks" inverse="true" fetch="subselect">时:

结果如下：只会额外产生一条sql(一条包含子查询的sql)

Hibernate:     select        category0_.id as id1_0_,        category0_.category as category2_0_     from        admindb.category category0_     where        category0_.id in (            1 , 2 , 3        )图书类别：文学Hibernate:     select        tbooks0_.category_Id as category2_0_1_,        tbooks0_.id as id1_1_1_,        tbooks0_.id as id1_1_0_,        tbooks0_.category_Id as category2_1_0_,        tbooks0_.author as author3_1_0_,        tbooks0_.b_name as b_name4_1_0_,        tbooks0_.b_price as b_price5_1_0_,        tbooks0_.pubDate as pubDate6_1_0_     from        admindb.t_book tbooks0_     where        tbooks0_.category_Id in (            select                category0_.id             from                admindb.category category0_             where                category0_.id in (                    1 , 2 , 3                )        )图书数量：2图书类别：历史图书数量：1图书类别：武侠图书数量：1