一、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
執行結果：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會通過select語句使用外連接來加載其關聯實體或集合
此時lazy會失效
執行結果：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
======================================================
二、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）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會通過select語句使用外連接來加載其關聯實體或集合
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">,結果如下：
執行了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">,結果如下：
執行了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))
9
student.name=學生8
student.name=學生4
student.name=學生5
student.name=學生9
student.name=學生6
student.name=學生2
student.name=學生3
student.name=學生1
student.name=學生7
Class.name=高一(2)班
4
student.name=學生3
student.name=學生4
student.name=學生2
student.name=學生1
Class.name=高一(3)班
