懒加载和抓取策略

懒加载用于何时发出sql语句

抓取策略用于如何发出sql语句，

join连接形式的 当查出的一条的记录情况时候有效，子查询不起作用，select  * from  table1这是隐士子查询

 还是 select  n+1语句 还是 subselect子查询2条语句  select * from table1 where (select * in (table2))

在 <set>那里添加这两个属性。

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="cn.itcast.hiberate.sh.domain.Classes">
        <id name="cid" length="5" type="java.lang.Long">
            <generator class="increment"></generator>
        </id>
        <property name="cname" length="20" type="java.lang.String"></property>
        
        <property name="description" length="100" type="java.lang.String"></property>
        <!-- 
            set元素对应类中的set集合
            通过set元素使classes表与student表建立关联
               key是通过外键的形式让两张表建立关联
               one-to-many是通过类的形式让两个类建立关联
            
            cascade 级联
               save-update
                   1、当 保存班级的时候，对学生进行怎么样的操作
                        如果学生对象在数据库中没有对应的值，这个时候会执行save操作
                        如果学生对象在数据库中有对应的值，这个时候会执行update操作
               delete
               all
            inverse  维护关系
               true      不维护关系     
               false     维护关系
               default   false
         -->
        <set name="students" cascade="save-update" inverse="true" lazy="false" fetch="join">
            <!-- 
                key是用来描述外键
             -->
            <key>
                <column name="cid"></column>
            </key>
            <one-to-many class="cn.itcast.hiberate.sh.domain.Student"/>
        </set>
    </class>
</hibernate-mapping>

 懒加载

package cn.itcast.hibernate.sh.test;

import java.util.Set;

import org.hibernate.Session;
import org.junit.Test;

import cn.itcast.hiberate.sh.domain.Classes;
import cn.itcast.hiberate.sh.domain.Student;
import cn.itcast.hibernate.sh.utils.HiberanteUtils;

public class LazyTest extends HiberanteUtils{
    static{
        url = "hibernate.cfg.xml";
    }
    /**
     * 类的懒加载
     */
    @Test
    public void testLoad(){
        Session session = sessionFactory.openSession();
        Classes classes = (Classes)session.load(Classes.class, 1L);
        session.close();
        classes.getCname();
    }
    
    /**
     * 集合的延迟加载
     */
    @Test
    public void testSet(){
        Session session = sessionFactory.openSession();
        Classes classes = (Classes)session.get(Classes.class, 1L);
        Set<Student> students = classes.getStudents();
        for(Student student:students){//这个时候才要发出sql语句
            System.out.println(student.getSname());
        }
        session.close();
    }
    
    /**
     * 集合的延迟加载
     */
    @Test
    public void testSet_EXTRA(){
        Session session = sessionFactory.openSession();
        Classes classes = (Classes)session.get(Classes.class, 1L);
        Set<Student> students = classes.getStudents();
        System.out.println(students.size());
        session.close();
    }
}

抓取策略

package cn.itcast.hibernate.sh.test;

import java.util.List;
import java.util.Set;

import org.hibernate.Session;
import org.junit.Test;

import cn.itcast.hiberate.sh.domain.Classes;
import cn.itcast.hiberate.sh.domain.Student;
import cn.itcast.hibernate.sh.utils.HiberanteUtils;

public class FetchTest extends HiberanteUtils{
    static{
        url = "hibernate.cfg.xml";
    }
    
    /**
     * n+1的问题
     *    解决问题的方案：子查询  fetch="subselect"
     */
    @Test
    public void testAll_Classes(){
        Session session = sessionFactory.openSession();
        List<Classes> cList = session.createQuery("from Classes").list();
        for(Classes classes:cList){
            Set<Student> students = classes.getStudents();
            for(Student student:students){
                System.out.println(student.getSname());
            }
        }
        session.close();
    }
    
    /**
     * n+1的问题
     *    解决问题的方案：子查询  fetch="subselect"
     */
    @Test
    public void testClasses_Some(){
        Session session = sessionFactory.openSession();
        List<Classes> cList = session.createQuery("from Classes where cid in(1,2,3)").list();
        for(Classes classes:cList){
            Set<Student> students = classes.getStudents();
            for(Student student:students){
                System.out.println(student.getSname());
            }
        }
        session.close();
    }
    
    @Test
    public void testQueryClasses_Id(){
        Session session = sessionFactory.openSession();
        Classes classes = (Classes)session.get(Classes.class, 1L);
        Set<Student> students = classes.getStudents();
        for(Student student:students){
            System.out.println(student.getSname());
        }
        session.close();
    }
}