hibernate抓取策略fetch详解
一、hibernate抓取策略（单端代理的批量抓取fetch=select(默认）/join）
测试用例：
Student student = (Student)session.get(Student.class, 1); System.out.println(student.getName()); System.out.println(student.getClasses().getName());
1）保持默认，同fetch="select",如：
<many-to-one name="classes" column="classesid" fetch="select"/>
fetch="select",另外发送一条select语句抓取当前对象关联实体或集合
执行结果：2条语句
Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_, student0_.class_id as class3_1_0_ from student_join student0_ where student0_.id=?
学生1
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes_join classes0_ where classes0_.id=?
高一(1)班
2）设置fetch="join",如：
<many-to-one name="classes" column="classesid" fetch="join"/>
fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合
此时lazy会失效
执行结果：一条join语句
Hibernate: select student0_.id as id1_1_, student0_.name as name1_1_, student0_.class_id as class3_1_1_, classes1_.id as id0_0_, classes1_.name as name0_0_ from student_join student0_ left
outer join classes_join classes1_ on student0_.class_id=classes1_.id where student0_.id=?
学生1
高一(1)班
二、hibernate抓取策略（集合代理的批量抓取，fetch=select（默认）/join/subselect）
测试用例：
Classes c = (Classes) session.load(Classes.class, new Integer(1));
System.out.println("Class.name=" + c.getName());
Set stuSet = c.getStudents();
System.out.println(stuSet.size());
if(stuSet != null && !stuSet.isEmpty()){
for(Iterator it = stuSet.iterator(); it.hasNext();){
Student s = (Student) it.next();
System.out.println("student.name=" + s.getName());
}
}
保持默认，同fetch="select",如：
<set name="students" inverse="true" fetch="select">
fetch="select",另外发送一条select语句抓取当前对象关联实体或集合
测试结果：2条独立的查询语句
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes_join classes0_ where classes0_.id=?
Class.name=高一(1)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
student.name=学生7
student.name=学生3
student.name=学生1
student.name=学生8
(2)设置fetch="join",如：
<set name="students" inverse="true" fetch="join">
fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合
此时lazy会失效
测试结果：1条独立的join查询语句
Hibernate: select classes0_.id as id0_1_, classes0_.name as name0_1_, students1_.class_id as class3_3_, students1_.id as id3_, students1_.id as id1_0_, students1_.name as name1_0_, students1_.class_id
as class3_1_0_ from classes_join classes0_ left outer join student_join students1_ on classes0_.id=students1_.class_id where classes0_.id=?
Class.name=高一(1)班
student.name=学生3
student.name=学生8
student.name=学生1
student.name=学生5
(3)设置fetch="subselect",如：用在查询语句中
<set name="students" inverse="true" fetch="subselect">
fetch="subselect",另外发送一条select语句抓取在前面查询到的所有实体对象的关联集合
测试用例：
List classList = session.createQuery("from Classes where id in (1,2,3)").list();
for(Iterator iter = classList.iterator(); iter.hasNext();){
Classes c = (Classes)iter.next();
System.out.println("Class.name=" + c.getName());
Set stuSet = c.getStudents();
System.out.println(stuSet.size());
if(stuSet != null && !stuSet.isEmpty()){
for(Iterator it = stuSet.iterator(); it.hasNext();){
Student s = (Student) it.next();
System.out.println("student.name=" + s.getName());
}
}
}
当不设fetch="subselect" ,即：<set name="students" inverse="true">,结果如下：
执行了3条查询语句
Hibernate: select classes0_.id as id0_, classes0_.name as name0_ from classes_join classes0_ where classes0_.id in (1 , 2 , 3)
Class.name=高一(1)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
3
student.name=学生4
student.name=学生6
student.name=学生2
Class.name=高一(2)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
4
student.name=学生3
student.name=学生4
student.name=学生1
student.name=学生2
Class.name=高一(3)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
0
当设fetch="subselect" ,即：<set name="students" inverse="true" fetch="subselect">,结果如下：
执行了1条查询语句（嵌套子查询）
Hibernate: select classes0_.id as id0_, classes0_.name as name0_ from classes_join classes0_ where classes0_.id in (1 , 2 , 3)
Class.name=高一(1)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id in (select classes0_.id from classes_join
classes0_ where classes0_.id in (1 , 2 , 3))
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总结：
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1.hibernate抓取策略（单端代理的批量抓取） 保持默认，同fetch="select",如： fetch="select",另外发送一条select语句抓取当前对象关联实体或集合
2.hibernate抓取策略（单端代理的批量抓取） 设置fetch="join",如： fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合 此时lazy会失效
3.hibernate抓取策略（集合代理的批量抓取） 保持默认，同fetch="select",如： fetch="select",另外发送一条select语句抓取当前对象关联实体或集合
4.hibernate抓取策略（集合代理的批量抓取） 设置fetch="join",如： fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合 此时lazy会失效
5.hibernate抓取策略（集合代理的批量抓取） 设置fetch="subselect",如： fetch="subselect",另外发送一条select语句抓取在前面查询到的所有实体对象的关联集合
6.hibernate抓取策略，,batch-szie在<class>上的应用 batch-size属性，可以批量加载实体类，参见：Classes.hbm.xml 当查classes对象时发出9条hql语句配置过后batch-size=3后会之发9/3=3条hql语句，提高性能
7.hibernate抓取策略,batch-szie在集合上的应用
当查students对象时发出10条hql语句配置过后batch-size=5后会之发10/5=2条hql语句，提高性能
总体上分析：默认是fetch="select" 当配置fetch="join"时直接查询包含的对象或者集合lazy失效。 |
