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(fā)送一條select語句抓取當(dāng)前對象關(guān)聯(lián)實體或集合

執(zhí)行結(jié)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(xué)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(shè)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(guān)聯(lián)實體或集合

此時lazy會失效

執(zhí)行結(jié)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(xué)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(fā)送一條select語句抓取當(dāng)前對象關(guān)聯(lián)實體或集合

測試結(jié)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(xué)生7
student.name=學(xué)生3
student.name=學(xué)生1
student.name=學(xué)生8
student.name=學(xué)生2
student.name=學(xué)生4
student.name=學(xué)生5
student.name=學(xué)生9
student.name=學(xué)生6

(2)設(shè)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(guān)聯(lián)實體或集合

此時lazy會失效

測試結(jié)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(xué)生6
student.name=學(xué)生4
student.name=學(xué)生9
student.name=學(xué)生7
student.name=學(xué)生2
student.name=學(xué)生3
student.name=學(xué)生8
student.name=學(xué)生1
student.name=學(xué)生5

(3)設(shè)置fetch="subselect",如：用在查詢語句中
<set name="students" inverse="true" fetch="subselect">

fetch="subselect",另外發(fā)送一條select語句抓取在前面查詢到的所有實體對象的關(guān)聯(lián)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(dāng)不設(shè)fetch="subselect" ,即：<set name="students" inverse="true">,結(jié)果如下：

執(zhí)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(xué)生8
student.name=學(xué)生5
student.name=學(xué)生3
student.name=學(xué)生9
student.name=學(xué)生7
student.name=學(xué)生1
student.name=學(xué)生4
student.name=學(xué)生6
student.name=學(xué)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(xué)生3
student.name=學(xué)生4
student.name=學(xué)生1
student.name=學(xué)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