Hibernate的fetch

 

hibernate抓取策略fetch具體解釋
1、hibernate抓取策略（單端代理的批量抓取fetch=select(默認）/join）
測試用例：
Student student = (Student)session.get(Student.class, 1);
System.out.println(student.getName());
System.out.println(student.getClasses().getName());

1）保持默認，同fetch="select",如：
<many-to-one name="classes" column="classesid" fetch="select"/>

fetch="select",另外發送一條select語句抓取當前對象關聯實體或集合

hibernate抓取策略fetch具體解釋
運行結果：2條語句

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=?
學生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）設置fetch="join",如：
<many-to-one name="classes" column="classesid" fetch="join"/>

fetch="join",hibernate會經過select語句使用外鏈接來載入其關聯實體或集合

hibernate抓取策略fetch具體解釋
此時lazy會失效

運行結果：一條join語句

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=?
學生1
高一(1)班
2、hibernate抓取策略（集合代理的批量抓取，fetch=select（默認）/join/subselect）

測試用例：

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具體解釋
保持默認，同fetch="select",如：
<set name="students" inverse="true" fetch="select">

fetch="select",另外發送一條select語句抓取當前對象關聯實體或集合

測試結果：2條獨立的查詢語句

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

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

fetch="join",hibernate會經過select語句使用外鏈接來載入其關聯實體或集合

此時lazy會失效

測試結果：1條獨立的join查詢語句

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=學生6
student.name=學生4
student.name=學生9
student.name=學生7
student.name=學生2
student.name=學生3
student.name=學生8
student.name=學生1
student.name=學生5
(3)設置fetch="subselect",如：用在查詢語句中
<set name="students" inverse="true" fetch="subselect">

fetch="subselect",另外發送一條select語句抓取在前面查詢到的所有實體對象的關聯集合

測試用例：

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());
}
}
}

當不設fetch="subselect" ,即：<set name="students" inverse="true">,結果例如如下：

運行了3條查詢語句

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

當不設fetch="subselect" ,即：<set name="students" inverse="true" fetch="subselect">,結果例如如下：

運行了1條查詢語句（嵌套子查詢）

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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總結：

hibernate抓取策略（單端代理的批量抓取） 保持默認，同fetch="select",如： <many-to-one name="classes" column="classesid" fetch="select"/> fetch="select",另外發送一條select語句抓取當前對象關聯實體或集合   2.hibernate抓取策略（單端代理的批量抓取） 設置fetch="join",如： <many-to-one name="classes" column="classesid" fetch="join"/> fetch="join",hibernate會經過select語句使用外鏈接來載入其關聯實體或集合 此時lazy會失效   3.hibernate抓取策略（集合代理的批量抓取） 保持默認，同fetch="select",如： <set name="students" inverse="true" cascade="all" fetch="select"> fetch="select",另外發送一條select語句抓取當前對象關聯實體或集合   4.hibernate抓取策略（集合代理的批量抓取） 設置fetch="join",如： <set name="students" inverse="true" cascade="all" fetch="join"> fetch="join",hibernate會經過select語句使用外鏈接來載入其關聯實體或集合 此時lazy會失效 5.hibernate抓取策略（集合代理的批量抓取） 設置fetch="subselect",如： <set name="students" inverse="true" cascade="all" fetch="subselect"> fetch="subselect",另外發送一條select語句抓取在前面查詢到的所有實體對象的關聯集合   6.hibernate抓取策略，,batch-szie在<class>上的應用 batch-size屬性，可以批量載入實體類，參見：Classes.hbm.xml <class name="Classes" table="t_classes" batch-size="3"> 當查classes對象時發出9條hql語句配置事後batch-size=3後會之發9/3=3條hql語句，提升性能   7.hibernate抓取策略,batch-szie在集合上的應用 batch-size屬性，可以批量載入實體類，參見：Classes.hbm.xml <set name="students" inverse="true" cascade="all" batch-size="5"> 當查students對象時發出10條hql語句配置事後batch-size=5後會之發10/5=2條hql語句，提升性能     總體上分析：默認是fetch="select"       當配置fetch="join"時直接查詢包括的對象或者集合lazy失效。