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：

@Test
public 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：

@Test
public 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