Hibernate的fetch

作者: phlsheji
发布时间:2015-08-07 10:42:09

 

hibernate抓取策略fetch详细解释
一、hibernate抓取策略(单机端代理批量抓取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

hibernate抓取策略fetch具体解释
运行结果: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语句使用外连接来载入其关联实体或集合

hibernate抓取策略fetch具体解释
此时lazy会失效

运行结果: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)

測试用例:

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());
}
}

1)hibernate抓取策略fetch具体解释
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

測试结果: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=?


9
student.name=学生7
student.name=学生3
student.name=学生1
student.name=学生8
student.name=学生2
student.name=学生4
student.name=学生5
student.name=学生9
student.name=学生6

(2)设置fetch="join",如:
<set name="students" inverse="true" fetch="join">

fetch="join",hibernate会通过select语句使用外连接来载入其关联实体或集合

此时lazy会失效

測试结果: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
9
student.name=学生6
student.name=学生4
student.name=学生9
student.name=学生7
student.name=学生2
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全部实体对象的关联集合

測试用例:

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">,结果例如以下:

运行了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=?


9
student.name=学生8
student.name=学生5
student.name=学生3
student.name=学生9
student.name=学生7
student.name=学生1
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


当不设fetch="subselect" ,即:<set name="students" inverse="true" fetch="subselect">,结果例如以下:

运行了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))

 

 

------------------------------------------------------

总结:

hibernate抓取策略(单端代理的批量抓取)

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

 

2.hibernate抓取策略(单端代理的批量抓取)

设置fetch="join",如:
<many-to-one name="classes" column="classesid" fetch="join"/>

fetch="join",hibernate会通过select语句使用外连接来载入其关联实体或集合

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:
<set name="students" inverse="true" cascade="all" 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

 

4.hibernate抓取策略(集合代理的批量抓取)

设置fetch="join",如:
<set name="students" inverse="true" cascade="all" fetch="join">

fetch="join",hibernate会通过select语句使用外连接来载入其关联实体或集合

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:
<set name="students" inverse="true" cascade="all" fetch="subselect">

fetch="subselect",另外发送一条select语句抓取在前面查询到的全部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,能够批量载入实体类。參见:Classes.hbm.xml
<class name="Classes" table="t_classes" batch-size="3">

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


batch-size属性。能够批量载入实体类,參见:Classes.hbm.xml
<set name="students" inverse="true" cascade="all" batch-size="5">

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

 

 

总体分析:缺省值是fetch="select"       当配置fetch="join"直接查询包括对象或集合lazy失败。

 

标签: Hibernate
来源:http://www.cnblogs.com/bhlsheji/p/4584423.html

推荐: