Hibernate抓取策略二
在Classes与Student一对多映射中，我们将Set里面的fetch属性设置为subselect来实现子查询。
如下示例
首先看一下set中fetch="select"即默认情况下：
生成数据类：
package com.bjsxt.hibernate;
import org.hibernate.Session;
public class InitData {
public static void main(String[] args) {
Session session = HibernateUtils.getSession();
try {
session.beginTransaction();
for(int i=0; i<10; i++){
Classes classes = new Classes();
classes.setName("班级"+i);
session.save(classes);
for(int j=0; j<10; j++){
Student student = new Student();
student.setName("班级"+i+"的学生"+j);
//在内存中建立由student指向classes的引用
student.setClasses(classes);
session.save(student);
}
}
session.getTransaction().commit();
} catch (Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
} finally{
HibernateUtils.closeSession(session);
}
}
}
测试类：
package com.bjsxt.hibernate;
import java.io.Serializable;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import org.hibernate.Session;
import junit.framework.TestCase;
public class FetchTest extends TestCase {
public void testFetch1() {
Session session = null;
try {
session = HibernateUtils.getSession();
Classes classes = (Classes)session.load(Classes.class, 1);
System.out.println("班级:" + classes.getName());
Set students = classes.getStudents();
for (Iterator iter = students.iterator(); iter.hasNext();) {
Student student = (Student)iter.next();
System.out.println(student.getName());
}
}catch(Exception e) {
e.printStackTrace();
}finally {
HibernateUtils.closeSession(session);
}
}
public void testFetch2() {
Session session = null;
try {
session = HibernateUtils.getSession();
List Classes = session.createQuery("select c from Classes c where c.id in(1, 2, 3)").list();
for (Iterator iter = Classes.iterator(); iter.hasNext();) {
Classes cls = (Classes)iter.next();
System.out.println("班级：" + cls.getName());
for (Iterator iter1 = cls.getStudents().iterator(); iter1.hasNext();) {
Student student = (Student)iter1.next();
System.out.println(student.getName());
}
}
}catch(Exception e) {
e.printStackTrace();
}finally {
HibernateUtils.closeSession(session);
}
}
}
分别列出方法一与方法二的测试结果：
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from t_classes classes0_ where classes0_.id=? 班级:班级0 Hibernate: select students0_.classid as classid0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.classid as classid1_0_ from t_student students0_ where students0_.classid=? 班级0的学生8 班级0的学生1 班级0的学生7 班级0的学生5 班级0的学生3 班级0的学生4 班级0的学生0 班级0的学生6 班级0的学生9 班级0的学生2
Hibernate: select classes0_.id as id0_, classes0_.name as name0_ from t_classes classes0_ where classes0_.id in (1 , 2 , 3) 班级：班级0 Hibernate: select students0_.classid as classid0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.classid as classid1_0_ from t_student students0_ where students0_.classid=? 班级0的学生7 班级0的学生2 班级0的学生3 班级0的学生1 班级0的学生0 班级0的学生9 班级0的学生4 班级0的学生8 班级0的学生6 班级0的学生5 班级：班级1 Hibernate: select students0_.classid as classid0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.classid as classid1_0_ from t_student students0_ where students0_.classid=? 班级1的学生9 班级1的学生0 班级1的学生6 班级1的学生8 班级1的学生7 班级1的学生2 班级1的学生5 班级1的学生3 班级1的学生1 班级1的学生4 班级：班级2 Hibernate: select students0_.classid as classid0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.classid as classid1_0_ from t_student students0_ where students0_.classid=? 班级2的学生6 班级2的学生9 班级2的学生3 班级2的学生1 班级2的学生2 班级2的学生7 班级2的学生8 班级2的学生4 班级2的学生0 班级2的学生5
下面更改fetch="subselect"
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="com.bjsxt.hibernate.Classes" table="t_classes"> <id name="id"> <generator class="native"/> </id> <property name="name"/> <!-- <set name="students" cascade="all" order-by="id"> --> <set name="students" inverse="true" fetch="subselect"> <key column="classid"/> <one-to-many class="com.bjsxt.hibernate.Student"/> </set> </class> </hibernate-mapping>
同样，将两个方法的测试结果列出如下：
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from t_classes classes0_ where classes0_.id=? 班级:班级0 Hibernate: select students0_.classid as classid0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.classid as classid1_0_ from t_student students0_ where students0_.classid=? 班级0的学生3 班级0的学生1 班级0的学生9 班级0的学生7 班级0的学生5 班级0的学生0 班级0的学生6 班级0的学生2 班级0的学生8 班级0的学生4
Hibernate: select classes0_.id as id0_, classes0_.name as name0_ from t_classes classes0_ where classes0_.id in (1 , 2 , 3) 班级：班级0 Hibernate: select students0_.classid as classid0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.classid as classid1_0_ from t_student students0_ where students0_.classid in (select classes0_.id from t_classes classes0_ where classes0_.id in (1 , 2 , 3)) 班级0的学生5 班级0的学生1 班级0的学生0 班级0的学生8 班级0的学生7 班级0的学生2 班级0的学生9 班级0的学生6 班级0的学生4 班级0的学生3 班级：班级1 班级1的学生5 班级1的学生1 班级1的学生0 班级1的学生7 班级1的学生4 班级1的学生9 班级1的学生8 班级1的学生3 班级1的学生6 班级1的学生2 班级：班级2 班级2的学生0 班级2的学生5 班级2的学生1 班级2的学生8 班级2的学生6 班级2的学生9 班级2的学生4 班级2的学生7 班级2的学生3 班级2的学生2
比较两次测试的结果我们发现，方法二的输出结果不同，当fetch="subselect"的时候，方法二进行了子查询
如：
Hibernate: select students0_.classid as classid0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.classid as classid1_0_ from t_student students0_ where students0_.classid in (select classes0_.id from t_classes classes0_ where classes0_.id in (1 , 2 , 3))