1、延迟加载
//懒加载|延迟加载
public class Demo {
@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 c = session.get(Customer.class, 2l);
System.out.println(c);
//----------------------------------------------------
tx.commit();
session.close();
}
@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b
//lazy:false 加载时立即查询.
public void fun2(){
Session session = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer c = session.load(Customer.class, 2l);
//----------------------------------------------------
tx.commit();
session.close();
System.out.println(c);
}
}结论:为了提高效率.建议使用延迟加载(懒加载)。
注意: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。
2、抓取策略
//关联级别 延迟加载 & 抓取策略
public class Demo {
//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 = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer c = session.get(Customer.class, 2l);
Set<LinkMan> linkMens = c.getLinkMens();//关联级别
System.out.println(linkMens);
//----------------------------------------------------
tx.commit();
session.close();
}
//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 = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer c = session.get(Customer.class, 2l);
Set<LinkMan> linkMens = c.getLinkMens();//关联级别
System.out.println(linkMens);
//----------------------------------------------------
tx.commit();
session.close();
}
//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 = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer c = session.get(Customer.class, 2l);
Set<LinkMan> linkMens = c.getLinkMens();//关联级别
System.out.println(linkMens.size());
System.out.println(linkMens);
//----------------------------------------------------
tx.commit();
session.close();
}
//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 = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
Customer c = session.get(Customer.class, 2l);
Set<LinkMan> linkMens = c.getLinkMens();//关联级别
System.out.println(linkMens.size());
System.out.println(linkMens);
//----------------------------------------------------
tx.commit();
session.close();
}
@Test
//fetch: subselect 子查询
//lazy: true 懒加载
public void fun5(){
Session session = HibernateUtils.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();
}
@Test
//fetch: subselect 子查询
//lazy: false 立即加载
public void fun6(){
Session session = HibernateUtils.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();
}
@Test
//fetch: subselect 子查询
//lazy: extra 极其懒惰
public void fun7(){
Session session = HibernateUtils.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();
}
}
//关联级别 延迟加载 & 抓取策略
public class Demo2 {
@Test
//fetch:select 单表查询
//lazy:proxy
//customer-true 懒加载
public void fun1(){
Session session = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
LinkMan lm = session.get(LinkMan.class, 3l);
Customer customer = lm.getCustomer();
System.out.println(customer);
//----------------------------------------------------
tx.commit();
session.close();
}
@Test
//fetch:join 多表
//lazy: 失效
public void fun3(){
Session session = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
LinkMan lm = session.get(LinkMan.class, 3l);
Customer customer = lm.getCustomer();
System.out.println(customer);
//----------------------------------------------------
tx.commit();
session.close();
}
@Test
//fetch:select 单表查询
//lazy:proxy
//customer-false 立即加载
public void fun2(){
Session session = HibernateUtils.openSession();
Transaction tx = session.beginTransaction();
//----------------------------------------------------
LinkMan lm = session.get(LinkMan.class, 3l);
Customer customer = lm.getCustomer();
System.out.println(customer);
//----------------------------------------------------
tx.commit();
session.close();
}
}结论: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。
no-session问题解决: 扩大session的作用范围（使用Filter）。
3、批量抓取
public class Demo {
@Test
public void fun1(){
Session session = HibernateUtils.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();
}
}