Hibernate -- lazy加载

Hibernate -- lazy加载

hibernate类级别懒加载:

Hibernate -- lazy加载

lazy:true(默认)

//类级别懒加载
//load方法
//class lazy属性
//默认值:true load获得时,会返回代理对象,不查询数据库,使用时才查询
public void fun1() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Customer load = (Customer) session.load(Customer.class, 17);
System.out.println("----------------sql语句未发送");
System.out.println(load.getName());
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

lazy :false

//类级别懒加载
//load方法
//class lazy属性
//默认值:false load方法执行就会发送sql语句,和get方法一致
public void fun2() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Customer load = (Customer) session.load(Customer.class, 19);
System.out.println("----------------sql语句发送");
System.out.println(load.getName());
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

hibernate (一对多)加载策略

关联级别懒加载的配置以及查询方式:

Hibernate -- lazy加载

lazy:false

  //关联级别懒加载
//lazy:false
public void fun2() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Customer load = (Customer) session.get(Customer.class, 19); System.out.println("所有的--------sql语句发送");
for (Order o : load.getOrders()) {
System.out.println(o.getName());
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

fetch:

fetch:join  / lazy:false  立刻使用select语句加载集合数据
fetch:join  / lazy:true  立刻使用select语句加载集合数据
fetch:join  / lazy:extra  立刻使用select语句加载集合数据

两种相同:会立刻加载出集合数据

fetch:join  / lazy:ture  查询集合时使用表连接查询,会立刻加载出集合数据 //关联级别懒加载
//lazy:false
//fetch :join
public void fun3() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Customer load = (Customer) session.get(Customer.class, 19); System.out.println("左外连接查询--------sql语句发送");
for (Order o : load.getOrders()) {
System.out.println(o.getName());
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

select:(默认)表连接语句查询集合数据  lazy:false

fetch:subselect  / lazy:false  立刻使用select语句加载集合数据

一次加载多个customer的订单数据

    //关联级别懒加载
//lazy:false
//fetch :subselect
public void fun4() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
List<Customer> list = session.createQuery("from com.huhu.domain.Customer").list();
for (Customer c : list) {
System.out.println(c.getName() + c.getOrders().size());
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

fetch:subselect  / lazy:true  立刻使用select语句加载集合数据
    //关联级别懒加载
//lazy:true
//fetch :subselect
public void fun5() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
List<Customer> list = session.createQuery("from com.huhu.domain.Customer").list();
System.out.println("使用order时才去数据库查询");
for (Customer c : list) {
System.out.println(c.getName() + c.getOrders().size());
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

fetch:select  / lazy:true (默认)普通的查询语句
//关联级别懒加载
//lazy:true
//fetch :select
public void fun6() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
System.out.println("customer--------sql语句发送");
Customer load = (Customer) session.get(Customer.class, 17); System.out.println("order--------sql语句发送");
for (Order o : load.getOrders()) {
System.out.println(o.getName());
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}
Hibernate -- lazy加载
fetch:select  / lazy:false  立刻使用select语句加载集合数据
 //关联级别懒加载
//lazy:false
//fetch :select
public void fun7() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Customer load = (Customer) session.get(Customer.class, 17); System.out.println("所有的--------sql语句发送");
for (Order o : load.getOrders()) {
System.out.println(o.getName());
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

关联级别懒加载:

由两个属性控制,在关联属性<set>中的两个属性:lazy和fecth,他们的对应值分别为 true,false,extra/select,subselect,join,这样一共有9中方法。仔细看看关于extra。

true/select:(默认值)会在使用集合是加载,普通的select语句

true/subselect:会在使用集合时加载,子查询语句,是延迟加载

true/join和false/join和extra/join:查询时使用表链接查询,会立刻加载集合数据,都没有延时加载

false/select:立刻使用select语句加载集合数据

false/subselect:会在查询customer时,  立即使用子查询加载客户的订单数据

extra:特别懒惰,如果时用集合时,之调用size()方法查询数量时,Hibernate发送count语句,只查询数量,不查询集合内数据
        extra/select
 //关联级别懒加载
//lazy:extra
//fetch :select
public void fun8() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Customer load = (Customer) session.get(Customer.class, 17); System.out.println("-------查询数量count()------");
System.out.println(load.getOrders().size());
System.out.println("----------查询customer--------");
for (Order o : load.getOrders()) {
System.out.println(o.getName());
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

extra/subselect

//关联级别懒加载
//lazy:extra
//fetch :subselect
public void fun9() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
List<Customer> list = session.createQuery("from com.huhu.domain.Customer").list();
for (Customer c : list) {
System.out.println(c.getName() + "------------" + c.getOrders().size());
}
for (Customer cs : list) {
for (Order o : cs.getOrders()) {
System.out.println(cs.getName() + "------------" + o.getName());
}
}
//--------------------------------------
session.getTransaction().commit();
session.close();
}

Hibernate -- lazy加载

hibernate:加载策略多对一

多对一:
lazy
  false 加载订单时,会立即加载客户
  proxy 看客户对象的类加载策略来决定
  no-proxy : 不做研究. 
  fetch=
select  : (默认值)使用普通select加载

join : 使用表链接加载数据

Hibernate -- lazy加载

测试:

package com.huhu.b_lazy;

import com.huhu.domain.Customer;
import com.huhu.domain.Order;
import com.huhu.utils.HibernateUtils;
import org.hibernate.Session; import java.util.List; /**
* 加载策略:多对一
*/
public class HibernateDemo3 { //fetch:select
//lazy:false
//默认:与我关联的数据时,在使用时才会加载。
public void fun1() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Order order = (Order) session.get(Order.class, 17);
System.out.println(order.getCustomer().getName());
//--------------------------------------
session.getTransaction().commit();
session.close();
} //fetch:select
//lazy:proxy
//Customer类加载策略:lazy :false
//默认:与我关联的数据时,在使用时才会加载。
public void fun2() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Order order = (Order) session.get(Order.class, 17); //--------------------------------------
session.getTransaction().commit();
session.close();
} //fetch:select
//lazy:proxy
//Customer类加载策略:lazy :true
//默认:与我关联的数据时,在使用时才会加载。
public void fun3() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Order order = (Order) session.get(Order.class, 19);
System.out.println(order.getCustomer().getName());
//--------------------------------------
session.getTransaction().commit();
session.close();
} //fetch:join
//lazy:proxy|false
//Customer类加载策略:lazy :true
//默认:与我关联的数据时,在使用时才会加载。
public void fun4() {
Session session = HibernateUtils.openSession();
session.beginTransaction();
//--------------------------------------
Order order = (Order) session.get(Order.class, 19);
//--------------------------------------
session.getTransaction().commit();
session.close();
} public static void main(String[] args) {
HibernateDemo3 d = new HibernateDemo3();
d.fun4();
}
}

如果想看代码得话可以复制以上代码测试。

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