所谓懒加载(lazy)就是延时加载，就是当在真正需要数据的时候，才真正执行数据加载操作
至于为什么要用懒加载呢，就是当我们要访问的数据量过大时，明显用缓存不太合适，因为内存容量有限 ，为了减少并发量，减少系统资源的消耗，我们让数据在需要的时候才进行加载，这时我们就用到了懒加载。
1.类级别的懒加载(session.load()方法)
get方法:没有任何策略.调用即立即查询数据库加载数据.
load方法: 应用类级别的加载策略
1.get方法测试，没有任何延迟加载策略
@Test
// get方法 : 立即加载.执行方法时立即发送sql语句查询结果
public void fun1(){
Session session = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer customer = session.get(Customer.class, 1l);//执行完此句即查询发出SQL
System.out.println(customer);
//----------------------------------------------------
tx.commit();
session.close();
}
没有延迟加载的get方法执行时候就发出SQL请求，且生成的对象是原生的JavaBean对象。
2.load方法测试，测试懒加载策略
修改配置文件懒加载设为true，默认就是开启懒加载的.比如我们查询customer的时候不会查询linkman，而我们调用get方法获取linkman的时候才去获取linkman。
lazy(默认值):true, 查询类时,会返回代理对象.会在使用属性时,根据关联的session查询数据库.加载数据.
lazy:false. load方法会与get方法没有任何区别.调用时即加载数据.
结论:为了提高效率.建议使用延迟加载(懒加载)
原理:
延迟加载是生成javaassist代理对象，生成代理对象的作用是依赖于session在使用对象的时候利用session去查询数据库，所以在使用懒加载时要确保,调用属性加载数据时,session还是打开的.不然会抛出异常
测试代码:
@Test
// load方法(默认):是在执行时,不发送任何sql语句.返回一个对象.使用该对象时,才执行查询.
// 延迟加载: 仅仅获得没有使用.不会查询.在使用时才进行查询.
// 是否对类进行延迟加载: 可以通过在class元素上配置lazy属性来控制.
//lazy:true 加载时,不查询.使用时才查询
//lazy:false 加载时立即查询.
public void fun2(){
Session session = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer customer = session.load(Customer.class, 1l);
System.out.println(customer);
//----------------------------------------------------
tx.commit();
session.close();
}
查看生成的代理对象:(代理对象的名称后面带有$符号)
,如下面的代码是错误的:
public void fun2(){
Session session = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer customer = session.load(Customer.class, 1l);
//----------------------------------------------------
tx.commit();
session.close();
System.out.println(customer);
}
结果会报错:
2.关联级别的查询(fetch、lazy、batch-size属性)
1.集合策略(根据客户关联查询联系人集合)
配置简介:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.qlq.domain" >
<class name="Customer" table="cst_customer" >
<id name="cust_id" >
<generator class="native"></generator>
</id>
<property name="cust_name" column="cust_name" >
</property>
<property name="cust_source" column="cust_source" ></property>
<property name="cust_industry" column="cust_industry" ></property>
<property name="cust_level" column="cust_level" ></property>
<property name="cust_linkman" column="cust_linkman" ></property>
<property name="cust_phone" column="cust_phone" ></property>
<property name="cust_mobile" column="cust_mobile" ></property>
<!--
lazy属性: 决定是否延迟加载
true(默认值): 延迟加载,懒加载
false: 立即加载
extra: 极其懒惰
fetch属性: 决定加载策略.使用什么类型的sql语句加载集合数据
select(默认值): 单表查询加载
join: 使用多表查询加载集合
subselect:使用子查询加载集合
-->
<!-- batch-size: 抓取集合的数量为3.
抓取客户的集合时,一次抓取几个客户的联系人集合.
-->
<set name="linkMens">
<key column="lkm_cust_id" ></key>
<one-to-many class="LinkMan" />
</set>
</class>
</hibernate-mapping>
(1)fetch为select的测试:
- lazy=true的测试
// 集合级别的关联
// fetch:select 单表查询
// lazy:true 使用时才加载集合数据.
@Test
public void fun1() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
Customer c = session.get(Customer.class, 7l);
Set<LinkMan> linkMens = c.getLinkMens();// 关联级别
System.out.println(linkMens);//打印SQL
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL:
Hibernate:
select
customer0_.cust_id as cust_id1_0_0_,
customer0_.cust_name as cust_nam2_0_0_,
customer0_.cust_source as cust_sou3_0_0_,
customer0_.cust_industry as cust_ind4_0_0_,
customer0_.cust_level as cust_lev5_0_0_,
customer0_.cust_linkman as cust_lin6_0_0_,
customer0_.cust_phone as cust_pho7_0_0_,
customer0_.cust_mobile as cust_mob8_0_0_
from
cst_customer customer0_
where
customer0_.cust_id=?
Hibernate:
select
linkmens0_.lkm_cust_id as lkm_cus10_1_0_,
linkmens0_.lkm_id as lkm_id1_1_0_,
linkmens0_.lkm_id as lkm_id1_1_1_,
linkmens0_.lkm_gender as lkm_gend2_1_1_,
linkmens0_.lkm_name as lkm_name3_1_1_,
linkmens0_.lkm_phone as lkm_phon4_1_1_,
linkmens0_.lkm_email as lkm_emai5_1_1_,
linkmens0_.lkm_qq as lkm_qq6_1_1_,
linkmens0_.lkm_mobile as lkm_mobi7_1_1_,
linkmens0_.lkm_memo as lkm_memo8_1_1_,
linkmens0_.lkm_position as lkm_posi9_1_1_,
linkmens0_.lkm_cust_id as lkm_cus10_1_1_
from
cst_linkman linkmens0_
where
linkmens0_.lkm_cust_id=?
[LinkMan [lkm_id=3, lkm_gender=null, lkm_name=王八, lkm_phone=null, lkm_email=null, lkm_qq=null, lkm_mobile=null, lkm_memo=null, lkm_position=null, customer=Customer [cust_id=7, cust_name=测试名称1]], LinkMan [lkm_id=4, lkm_gender=null, lkm_name=田七, lkm_phone=null, lkm_email=null, lkm_qq=null, lkm_mobile=null, lkm_memo=null, lkm_position=null, customer=Customer [cust_id=7, cust_name=测试名称1]]]
- lazy=false的测试(会立即加载)
// 集合级别的关联
// fetch:select 单表查询
// lazy:false 立即记载集合数据
@Test
public void fun2() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
Customer c = session.get(Customer.class, 7l);//加载所有SQL，包括查询linkman
Set<LinkMan> linkMens = c.getLinkMens();// 关联级别
System.out.println(linkMens);
// ----------------------------------------------------
tx.commit();
session.close();
}
- lazy=xetra极其懒惰，与懒加载效果基本一致. 如果只获得集合的size.只查询集合的size(count语句)
// 集合级别的关联
// fetch:select 单表查询
// lazy:extra 极其懒惰.与懒加载效果基本一致. 如果只获得集合的size.只查询集合的size(count语句)
@Test
public void fun3() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
Customer c = session.get(Customer.class, 7l);
Set<LinkMan> linkMens = c.getLinkMens();// 关联级别
System.out.println(linkMens.size());//count SQL
System.out.println(linkMens);//查询SQL
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL：
Hibernate:
select
customer0_.cust_id as cust_id1_0_0_,
customer0_.cust_name as cust_nam2_0_0_,
customer0_.cust_source as cust_sou3_0_0_,
customer0_.cust_industry as cust_ind4_0_0_,
customer0_.cust_level as cust_lev5_0_0_,
customer0_.cust_linkman as cust_lin6_0_0_,
customer0_.cust_phone as cust_pho7_0_0_,
customer0_.cust_mobile as cust_mob8_0_0_
from
cst_customer customer0_
where
customer0_.cust_id=?
Hibernate:
select
count(lkm_id)
from
cst_linkman
where
lkm_cust_id =?
2
Hibernate:
select
linkmens0_.lkm_cust_id as lkm_cus10_1_0_,
linkmens0_.lkm_id as lkm_id1_1_0_,
linkmens0_.lkm_id as lkm_id1_1_1_,
linkmens0_.lkm_gender as lkm_gend2_1_1_,
linkmens0_.lkm_name as lkm_name3_1_1_,
linkmens0_.lkm_phone as lkm_phon4_1_1_,
linkmens0_.lkm_email as lkm_emai5_1_1_,
linkmens0_.lkm_qq as lkm_qq6_1_1_,
linkmens0_.lkm_mobile as lkm_mobi7_1_1_,
linkmens0_.lkm_memo as lkm_memo8_1_1_,
linkmens0_.lkm_position as lkm_posi9_1_1_,
linkmens0_.lkm_cust_id as lkm_cus10_1_1_
from
cst_linkman linkmens0_
where
linkmens0_.lkm_cust_id=?
(2)fetch为join的多表查询: 延迟策略失效
// 集合级别的关联
// fetch:join 多表查询
// lazy:true|false|extra 失效.立即加载.
@Test
public void fun4() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
Customer c = session.get(Customer.class, 7l);
Set<LinkMan> linkMens = c.getLinkMens();// 关联级别
System.out.println(linkMens.size());
System.out.println(linkMens);
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL：
select
customer0_.cust_id as cust_id1_0_0_,
customer0_.cust_name as cust_nam2_0_0_,
customer0_.cust_source as cust_sou3_0_0_,
customer0_.cust_industry as cust_ind4_0_0_,
customer0_.cust_level as cust_lev5_0_0_,
customer0_.cust_linkman as cust_lin6_0_0_,
customer0_.cust_phone as cust_pho7_0_0_,
customer0_.cust_mobile as cust_mob8_0_0_,
linkmens1_.lkm_cust_id as lkm_cus10_1_1_,
linkmens1_.lkm_id as lkm_id1_1_1_,
linkmens1_.lkm_id as lkm_id1_1_2_,
linkmens1_.lkm_gender as lkm_gend2_1_2_,
linkmens1_.lkm_name as lkm_name3_1_2_,
linkmens1_.lkm_phone as lkm_phon4_1_2_,
linkmens1_.lkm_email as lkm_emai5_1_2_,
linkmens1_.lkm_qq as lkm_qq6_1_2_,
linkmens1_.lkm_mobile as lkm_mobi7_1_2_,
linkmens1_.lkm_memo as lkm_memo8_1_2_,
linkmens1_.lkm_position as lkm_posi9_1_2_,
linkmens1_.lkm_cust_id as lkm_cus10_1_2_
from
cst_customer customer0_
left outer join
cst_linkman linkmens1_
on customer0_.cust_id=linkmens1_.lkm_cust_id
where
customer0_.cust_id=?
(3)fetch为subselect (只有在批量查询的时候才有用，如果是查询单个的话与select的作用一样)
- lazy为true
@Test
// fetch: subselect 子查询
// lazy: true 懒加载
public void fun5() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
String hql = "from Customer";
Query query = session.createQuery(hql);
List<Customer> list = query.list();
for (Customer c : list) {
System.out.println(c);
System.out.println(c.getLinkMens().size());
System.out.println(c.getLinkMens());
}
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL：
Hibernate:
select
customer0_.cust_id as cust_id1_0_,
customer0_.cust_name as cust_nam2_0_,
customer0_.cust_source as cust_sou3_0_,
customer0_.cust_industry as cust_ind4_0_,
customer0_.cust_level as cust_lev5_0_,
customer0_.cust_linkman as cust_lin6_0_,
customer0_.cust_phone as cust_pho7_0_,
customer0_.cust_mobile as cust_mob8_0_
from
cst_customer customer0_
Customer [cust_id=7, cust_name=测试名称1]
Hibernate:
select
linkmens0_.lkm_cust_id as lkm_cus10_1_1_,
linkmens0_.lkm_id as lkm_id1_1_1_,
linkmens0_.lkm_id as lkm_id1_1_0_,
linkmens0_.lkm_gender as lkm_gend2_1_0_,
linkmens0_.lkm_name as lkm_name3_1_0_,
linkmens0_.lkm_phone as lkm_phon4_1_0_,
linkmens0_.lkm_email as lkm_emai5_1_0_,
linkmens0_.lkm_qq as lkm_qq6_1_0_,
linkmens0_.lkm_mobile as lkm_mobi7_1_0_,
linkmens0_.lkm_memo as lkm_memo8_1_0_,
linkmens0_.lkm_position as lkm_posi9_1_0_,
linkmens0_.lkm_cust_id as lkm_cus10_1_0_
from
cst_linkman linkmens0_
where
linkmens0_.lkm_cust_id in (
select
customer0_.cust_id
from
cst_customer customer0_
)
- lazy为false，立即加载
@Test
// fetch: subselect 子查询
// lazy: false 立即加载
public void fun6() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
String hql = "from Customer";
Query query = session.createQuery(hql);
List<Customer> list = query.list();
for (Customer c : list) {
System.out.println(c);
System.out.println(c.getLinkMens().size());
System.out.println(c.getLinkMens());
}
// ----------------------------------------------------
tx.commit();
session.close();
}
- lazy为extra
@Test
// fetch: subselect 子查询
// lazy: extra 极其懒惰
public void fun7() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
String hql = "from Customer";
Query query = session.createQuery(hql);
List<Customer> list = query.list();
for (Customer c : list) {
System.out.println(c);
System.out.println(c.getLinkMens().size());
System.out.println(c.getLinkMens());
}
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL：
Hibernate:
select
customer0_.cust_id as cust_id1_0_,
customer0_.cust_name as cust_nam2_0_,
customer0_.cust_source as cust_sou3_0_,
customer0_.cust_industry as cust_ind4_0_,
customer0_.cust_level as cust_lev5_0_,
customer0_.cust_linkman as cust_lin6_0_,
customer0_.cust_phone as cust_pho7_0_,
customer0_.cust_mobile as cust_mob8_0_
from
cst_customer customer0_
Customer [cust_id=7, cust_name=测试名称1]
Hibernate:
select
linkmens0_.lkm_cust_id as lkm_cus10_1_1_,
linkmens0_.lkm_id as lkm_id1_1_1_,
linkmens0_.lkm_id as lkm_id1_1_0_,
linkmens0_.lkm_gender as lkm_gend2_1_0_,
linkmens0_.lkm_name as lkm_name3_1_0_,
linkmens0_.lkm_phone as lkm_phon4_1_0_,
linkmens0_.lkm_email as lkm_emai5_1_0_,
linkmens0_.lkm_qq as lkm_qq6_1_0_,
linkmens0_.lkm_mobile as lkm_mobi7_1_0_,
linkmens0_.lkm_memo as lkm_memo8_1_0_,
linkmens0_.lkm_position as lkm_posi9_1_0_,
linkmens0_.lkm_cust_id as lkm_cus10_1_0_
from
cst_linkman linkmens0_
where
linkmens0_.lkm_cust_id in (
select
customer0_.cust_id
from
cst_customer customer0_
)
2.关联属性懒加载(先查询联系人，跟据联系人反查顾客)
XML配置:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.qlq.domain" >
<class name="LinkMan" table="cst_linkman" >
<id name="lkm_id" >
<generator class="native"></generator>
</id>
<property name="lkm_gender" ></property>
<property name="lkm_name" ></property>
<property name="lkm_phone" ></property>
<property name="lkm_email" ></property>
<property name="lkm_qq" ></property>
<property name="lkm_mobile" ></property>
<property name="lkm_memo" ></property>
<property name="lkm_position" ></property>
<!--
fetch 决定加载的sql语句
select: 使用单表查询
join : 多表查询 (会导致lazy失效)
lazy 决定加载时机
false: 立即加载
proxy: 由customer的类级别加载策略决定.
-->
<many-to-one name="customer" column="lkm_cust_id" class="Customer" fetch="select" lazy="proxy"></many-to-one>
</class>
</hibernate-mapping>
1.fetch为select,lazy为proxy,Customer类的lazy为true
测试代码:
// 集合级别的关联
// fetch:select 单表查询
// lazy:proxy 代理，也就是由Customer的lazy属性决定是否懒加载
//Customer的lazy为true
@Test
public void fun1() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
LinkMan linkMan = session.get(LinkMan.class, 5l);
System.out.println("-------1------------");
Customer customer = linkMan.getCustomer();
System.out.println("-------2------------");
System.out.println(customer);// 打印SQL
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL：
Hibernate:
select
linkman0_.lkm_id as lkm_id1_1_0_,
linkman0_.lkm_gender as lkm_gend2_1_0_,
linkman0_.lkm_name as lkm_name3_1_0_,
linkman0_.lkm_phone as lkm_phon4_1_0_,
linkman0_.lkm_email as lkm_emai5_1_0_,
linkman0_.lkm_qq as lkm_qq6_1_0_,
linkman0_.lkm_mobile as lkm_mobi7_1_0_,
linkman0_.lkm_memo as lkm_memo8_1_0_,
linkman0_.lkm_position as lkm_posi9_1_0_,
linkman0_.lkm_cust_id as lkm_cus10_1_0_
from
cst_linkman linkman0_
where
linkman0_.lkm_id=?
-------1------------
-------2------------
Hibernate:
select
customer0_.cust_id as cust_id1_0_0_,
customer0_.cust_name as cust_nam2_0_0_,
customer0_.cust_source as cust_sou3_0_0_,
customer0_.cust_industry as cust_ind4_0_0_,
customer0_.cust_level as cust_lev5_0_0_,
customer0_.cust_linkman as cust_lin6_0_0_,
customer0_.cust_phone as cust_pho7_0_0_,
customer0_.cust_mobile as cust_mob8_0_0_
from
cst_customer customer0_
where
customer0_.cust_id=?
Customer [cust_id=9, cust_name=测试名称3]
2.fetch为select,lazy为proxy,Customer类的lazy为false
测试代码:
// 集合级别的关联
// fetch:select 单表查询
// lazy:proxy 代理，也就是由Customer的lazy属性决定是否懒加载
//Customer的lazy为false
@Test
public void fun2() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
LinkMan linkMan = session.get(LinkMan.class, 5l);
System.out.println("-------1------------");
Customer customer = linkMan.getCustomer();
System.out.println("-------2------------");
System.out.println(customer);// 打印SQL
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL：
Hibernate:
select
linkman0_.lkm_id as lkm_id1_1_0_,
linkman0_.lkm_gender as lkm_gend2_1_0_,
linkman0_.lkm_name as lkm_name3_1_0_,
linkman0_.lkm_phone as lkm_phon4_1_0_,
linkman0_.lkm_email as lkm_emai5_1_0_,
linkman0_.lkm_qq as lkm_qq6_1_0_,
linkman0_.lkm_mobile as lkm_mobi7_1_0_,
linkman0_.lkm_memo as lkm_memo8_1_0_,
linkman0_.lkm_position as lkm_posi9_1_0_,
linkman0_.lkm_cust_id as lkm_cus10_1_0_
from
cst_linkman linkman0_
where
linkman0_.lkm_id=?
Hibernate:
select
customer0_.cust_id as cust_id1_0_0_,
customer0_.cust_name as cust_nam2_0_0_,
customer0_.cust_source as cust_sou3_0_0_,
customer0_.cust_industry as cust_ind4_0_0_,
customer0_.cust_level as cust_lev5_0_0_,
customer0_.cust_linkman as cust_lin6_0_0_,
customer0_.cust_phone as cust_pho7_0_0_,
customer0_.cust_mobile as cust_mob8_0_0_
from
cst_customer customer0_
where
customer0_.cust_id=?
-------1------------
-------2------------
Customer [cust_id=9, cust_name=测试名称3]
3.fetch为join,lazy属性失效
代码:
// 集合级别的关联
// join 单表查询
// lazy: 属性失效
@Test
public void fun3() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
LinkMan linkMan = session.get(LinkMan.class, 5l);
System.out.println("-------1------------");
Customer customer = linkMan.getCustomer();
System.out.println("-------2------------");
System.out.println(customer);// 打印SQL
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL:
Hibernate:
select
linkman0_.lkm_id as lkm_id1_1_0_,
linkman0_.lkm_gender as lkm_gend2_1_0_,
linkman0_.lkm_name as lkm_name3_1_0_,
linkman0_.lkm_phone as lkm_phon4_1_0_,
linkman0_.lkm_email as lkm_emai5_1_0_,
linkman0_.lkm_qq as lkm_qq6_1_0_,
linkman0_.lkm_mobile as lkm_mobi7_1_0_,
linkman0_.lkm_memo as lkm_memo8_1_0_,
linkman0_.lkm_position as lkm_posi9_1_0_,
linkman0_.lkm_cust_id as lkm_cus10_1_0_,
customer1_.cust_id as cust_id1_0_1_,
customer1_.cust_name as cust_nam2_0_1_,
customer1_.cust_source as cust_sou3_0_1_,
customer1_.cust_industry as cust_ind4_0_1_,
customer1_.cust_level as cust_lev5_0_1_,
customer1_.cust_linkman as cust_lin6_0_1_,
customer1_.cust_phone as cust_pho7_0_1_,
customer1_.cust_mobile as cust_mob8_0_1_
from
cst_linkman linkman0_
left outer join
cst_customer customer1_
on linkman0_.lkm_cust_id=customer1_.cust_id
where
linkman0_.lkm_id=?
-------1------------
-------2------------
Customer [cust_id=9, cust_name=测试名称3]
总结:
为了提高效率.fetch的选择上应选择select. lazy的取值应选择 true. 全部使用默认值.
3.批量抓取策略(batch-size属性)
此次略用于获取客户联系人的时候，一次抓取几个客户的联系人。此策略用于避免多次访问数据库，没有此策略是一次获取一个客户联系人，有了此策略可以一次获取多个客户的，减少访问数据库次数。
例如
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.qlq.domain" >
<class name="Customer" table="cst_customer">
<id name="cust_id" >
<generator class="native"></generator>
</id>
<property name="cust_name" column="cust_name" >
</property>
<property name="cust_source" column="cust_source" ></property>
<property name="cust_industry" column="cust_industry" ></property>
<property name="cust_level" column="cust_level" ></property>
<property name="cust_linkman" column="cust_linkman" ></property>
<property name="cust_phone" column="cust_phone" ></property>
<property name="cust_mobile" column="cust_mobile" ></property>
<!--
lazy属性: 决定是否延迟加载
true(默认值): 延迟加载,懒加载
false: 立即加载
extra: 极其懒惰
fetch属性: 决定加载策略.使用什么类型的sql语句加载集合数据
select(默认值): 单表查询加载
join: 使用多表查询加载集合
subselect:使用子查询加载集合
-->
<!-- batch-size: 5 抓取集合的数量为5.
抓取客户的集合时,一次抓取几个客户的联系人集合.
-->
<set name="linkMens" batch-size="5">
<key column="lkm_cust_id" ></key>
<one-to-many class="LinkMan" />
</set>
</class>
</hibernate-mapping>
测试代码:
public void fun5() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
// ----------------------------------------------------
String hql = "from Customer";
Query query = session.createQuery(hql);
List<Customer> list = query.list();
for (Customer c : list) {
System.out.println(c.getLinkMens());
}
// ----------------------------------------------------
tx.commit();
session.close();
}
SQL:
Hibernate:
select
customer0_.cust_id as cust_id1_0_,
customer0_.cust_name as cust_nam2_0_,
customer0_.cust_source as cust_sou3_0_,
customer0_.cust_industry as cust_ind4_0_,
customer0_.cust_level as cust_lev5_0_,
customer0_.cust_linkman as cust_lin6_0_,
customer0_.cust_phone as cust_pho7_0_,
customer0_.cust_mobile as cust_mob8_0_
from
cst_customer customer0_
Hibernate:
select
linkmens0_.lkm_cust_id as lkm_cus10_1_1_,
linkmens0_.lkm_id as lkm_id1_1_1_,
linkmens0_.lkm_id as lkm_id1_1_0_,
linkmens0_.lkm_gender as lkm_gend2_1_0_,
linkmens0_.lkm_name as lkm_name3_1_0_,
linkmens0_.lkm_phone as lkm_phon4_1_0_,
linkmens0_.lkm_email as lkm_emai5_1_0_,
linkmens0_.lkm_qq as lkm_qq6_1_0_,
linkmens0_.lkm_mobile as lkm_mobi7_1_0_,
linkmens0_.lkm_memo as lkm_memo8_1_0_,
linkmens0_.lkm_position as lkm_posi9_1_0_,
linkmens0_.lkm_cust_id as lkm_cus10_1_0_
from
cst_linkman linkmens0_
where
linkmens0_.lkm_cust_id in (
?, ?, ?, ?, ?
)
[LinkMan [lkm_id=4, lkm_gender=null, lkm_name=田七, lkm_phone=null, lkm_email=null, lkm_qq=null, lkm_mobile=null, lkm_memo=null, lkm_position=null, customer=Customer [cust_id=7, cust_name=测试名称1]], LinkMan [lkm_id=3, lkm_gender=null, lkm_name=王八, lkm_phone=null, lkm_email=null, lkm_qq=null, lkm_mobile=null, lkm_memo=null, lkm_position=null, customer=Customer [cust_id=7, cust_name=测试名称1]]]
[]
[LinkMan [lkm_id=5, lkm_gender=null, lkm_name=99, lkm_phone=null, lkm_email=null, lkm_qq=null, lkm_mobile=null, lkm_memo=null, lkm_position=null, customer=Customer [cust_id=9, cust_name=测试名称3]]]
[]
[]
Hibernate:
select
linkmens0_.lkm_cust_id as lkm_cus10_1_1_,
linkmens0_.lkm_id as lkm_id1_1_1_,
linkmens0_.lkm_id as lkm_id1_1_0_,
linkmens0_.lkm_gender as lkm_gend2_1_0_,
linkmens0_.lkm_name as lkm_name3_1_0_,
linkmens0_.lkm_phone as lkm_phon4_1_0_,
linkmens0_.lkm_email as lkm_emai5_1_0_,
linkmens0_.lkm_qq as lkm_qq6_1_0_,
linkmens0_.lkm_mobile as lkm_mobi7_1_0_,
linkmens0_.lkm_memo as lkm_memo8_1_0_,
linkmens0_.lkm_position as lkm_posi9_1_0_,
linkmens0_.lkm_cust_id as lkm_cus10_1_0_
from
cst_linkman linkmens0_
where
linkmens0_.lkm_cust_id in (
?, ?, ?
)
通过SQL看出获取客户联系人的时候一次获取5个客户的联系人，最后一次由于没有五个客户所以会查询剩余3个客户的联系人。
问题: no-session问题解决: 扩大session的作用范围.
代理对象)经常放在request域中带到页面进行显示，也就是到了页面才使用对象(查询数据库)，此时session已经关闭，如果使用了懒加载策略会报no-session异常。解决办法:过滤器扩大session作用范围(filter可以在请求执行前后都执行一些代码)，如下图逻辑:
其实SSH整合的时候spring已经做了过滤器来实现此功能，会在搭建SSH环境的时候介绍。
【当你用心写完每一篇博客之后,你会发现它比你用代码实现功能更有成就感!】
