Hibernate延迟加载策略
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)
注意:我们在使用懒加载的时候不能再关闭session之后再使用对象，因为懒加载的代理对象依赖session再使用对象的时候查询数据库,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.
在懒加载的策略中，我们通常是在Service层打开事务、session，service代码执行完毕只会进行提交或者回滚。但是我们的对象(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!】
