Hibernate的fetch

hibernate抓取策略fetch具体解释
一、hibernate抓取策略（单端代理的批量抓取fetch=select(默认）/join）
測试用例：
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

整体上分析：默认是fetch="select"      
当配置fetch="join"时直接查询包括的对象或者集合lazy失效。