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）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))

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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。