检索策略
类级别检索
默认检索策略:默认延迟加载, 可以使用lazy属性来进行改变.
session.get(clazz,object)默认立即加载
@Test//测试左外连接查询public void test13(){Session session = HibernateUtils.getSession();Transaction bt = session.beginTransaction();Customer customer = session.get(Customer.class,1);bt.commit();session.close();}
session.load(clazz,object)默认延迟加载 可以使用Hibernate.initialize(customer)初始化数据;
@Test//load延迟加载public void test14(){Session session = HibernateUtils.getSession();Transaction bt = session.beginTransaction();Customer customer = session.load(Customer.class,1);bt.commit();session.close();}关联级别检索
一对一<one-to-one>
一对多/多对一 <set>下有<one-to-many> <many-to-one>
多对多<many-to-many>
我们主要是在set下one-to-many或many-to-one设置lazy和fetch
查询一个客户下的订单
set上
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未进行设置则默认为懒加载
@Testpublic void test15(){Session session = HibernateUtils.getSession();Transaction bt = session.beginTransaction();String hql="from Customer c right outer join c.orders";Query query = session.createQuery(hql);List<Object[]> list = query.list();bt.commit();session.close();}sql打印:
Hibernate: ????select ???????customer0_.c_id as c_id1_0_, ???????customer0_.name as name2_0_ ????from ???????test.c_customer customer0_
打印结果证明只进行查询了customer对象
其中Order对象未进行加载 这样就是在调用时会发送sql语句进行查询 为了解决这一事件 我们让Customer立即加载
修改其配置文件:
?<set lazy="false" > //设置立即加载
再次执行会sql打印
Hibernate: ????select ???????customer0_.c_id as c_id1_0_, ???????customer0_.name as name2_0_ ????from ???????test.c_customer customer0_Hibernate: ????select ???????orders0_.o_customer_id as o_custom4_1_0_, ???????orders0_.o_id as o_id1_1_0_, ???????orders0_.o_id as o_id1_1_1_, ???????orders0_.o_money as o_money2_1_1_, ???????orders0_.o_receiverInfo as o_receiv3_1_1_, ???????orders0_.o_customer_id as o_custom4_1_1_ ????from ???????test.o_order orders0_ ????where ???????orders0_.o_customer_id=?Hibernate: ????select ???????orders0_.o_customer_id as o_custom4_1_0_, ???????orders0_.o_id as o_id1_1_0_, ???????orders0_.o_id as o_id1_1_1_, ???????orders0_.o_money as o_money2_1_1_, ???????orders0_.o_receiverInfo as o_receiv3_1_1_, ???????orders0_.o_customer_id as o_custom4_1_1_ ????from ???????test.o_order orders0_ ????where ???????orders0_.o_customer_id=?
这样在查询customer时会进行查询order
测试lazy=extra属性
<set lazy="true" > //设置延迟加载
执行以下方法
@Testpublic void test15(){Session session = HibernateUtils.getSession();Transaction bt = session.beginTransaction();String hql="from Customer";Query query = session.createQuery(hql);List<Customer> list = query.list();/*int size = list.size();System.out.println(size);*/for (Customer customer : list) {System.out.println(customer.getOrders().size());}//操作bt.commit();session.close();}sql打印:
Hibernate: ????select ???????customer0_.c_id as c_id1_0_, ???????customer0_.name as name2_0_ ????from ???????test.c_customer customer0_Hibernate: ????select ???????orders0_.o_customer_id as o_custom4_1_0_, ???????orders0_.o_id as o_id1_1_0_, ???????orders0_.o_id as o_id1_1_1_, ???????orders0_.o_money as o_money2_1_1_, ???????orders0_.o_receiverInfo as o_receiv3_1_1_, ???????orders0_.o_customer_id as o_custom4_1_1_ ????from ???????test.o_order orders0_ ????where ???????orders0_.o_customer_id=?1Hibernate: ????select ???????orders0_.o_customer_id as o_custom4_1_0_, ???????orders0_.o_id as o_id1_1_0_, ???????orders0_.o_id as o_id1_1_1_, ???????orders0_.o_money as o_money2_1_1_, ???????orders0_.o_receiverInfo as o_receiv3_1_1_, ???????orders0_.o_customer_id as o_custom4_1_1_ ????from ???????test.o_order orders0_ ????where ???????orders0_.o_customer_id=?100
<set lazy="extra" > //设置加强延迟加载
sql打印为:
Hibernate: ????select ???????customer0_.c_id as c_id1_0_, ???????customer0_.name as name2_0_ ????from ???????test.c_customer customer0_Hibernate: ????select ???????count(o_id) ????from ???????test.o_order ????where ???????o_customer_id =?1Hibernate: ????select ???????count(o_id) ????from ???????test.o_order ????where ???????o_customer_id =?100
总结: ???对于懒加载和加强懒加载区别:都是是在调用时才会产生,但是区别在于发送sql语句的意义;懒加载在发送sql语句时会发送查询全部的语句,返回为每一列,而加强懒加载发送的是count(*),我需要什么加强懒加载会给我什么直接查询,而懒加载不会.
测试fetch 只是sql的生成方式不同而已
<set name="orders" ?lazy="false" fetch="subselect" > ?//subselect生成子查询
@Testpublic void test15(){Session session = HibernateUtils.getSession();Transaction bt = session.beginTransaction();String hql="from Customer";Query query = session.createQuery(hql);List<Customer> list = query.list();bt.commit();session.close();}sql打印:
Hibernate: ????select ???????customer0_.c_id as c_id1_0_, ???????customer0_.name as name2_0_ ????from ???????test.c_customer customer0_Hibernate: ????select ???????orders0_.o_customer_id as o_custom4_1_1_, ???????orders0_.o_id as o_id1_1_1_, ???????orders0_.o_id as o_id1_1_0_, ???????orders0_.o_money as o_money2_1_0_, ???????orders0_.o_receiverInfo as o_receiv3_1_0_, ???????orders0_.o_customer_id as o_custom4_1_0_ ????from ???????test.o_order orders0_ ????where ???????orders0_.o_customer_id in ( //子查询 ???????????select ???????????????customer0_.c_id ????????????from ???????????????test.c_customer customer0_ ???????)
在<many-to-one>或<one-to-one>如果去查询对方
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hibernate检索策略(抓取策略)
原文地址：http://www.cnblogs.com/fjkgrbk/p/hibernate_lazy.html