抓取策略（fetching strategy） 是指：當應用程序需要在（Hibernate實體對象圖的）關聯關
系間進行導航的時候， Hibernate如何獲取關聯對象的策略。抓取策略可以在O/R映射的元數據中聲明，
也可以在特定的HQL 或條件查詢（Criteria Query）中重載聲明。隻影響get load 方法，對hql是不影響的，除了有一個之外。
例如，經典的班級對學生是一對多關聯，我配置的時候採取的是一對多雙向關聯。主要是它的關聯對象的加載。
（1）：單端代理的批量抓取
如果沒有配置抓取策略，默認的fetch="select"，即：另外發送一條 SELECT 語句抓取當前對象的關聯實體或集合
Student stu = (Student) session.load(Student.class, 1);
System.out.println(stu.getName());
System.out.println(stu.getClasses().getName());
看執行後發的SQL語句：
Hibernate: select student0_.id as id0_0_, student0_.name as name0_0_, student0_.classesid as classesid0_0_ from t_student student0_ where student0_.id=?
班級0學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
班級0
從發的SQL語句可以看出，第一條語句是通過students表來查詢的，
當我們想去加載它的關聯對象classes時，發出第二條語句，是從classes表裏查尋得，
如果我設置抓取策略爲join，即：<many-to-one name="classes" column="classesid" fetch="join"></many-to-one>
執行之後，你會發現發出的SQL語句：
Hibernate: select student0_.id as id0_1_, student0_.name as name0_1_, student0_.classesid as classesid0_1_, classes1_.id as id1_0_, classes1_.name as name1_0_ from t_student student0_ left outer join t_classes classes1_ on
student0_.classesid=classes1_.id where student0_.id=?
班級0學生0
班級0
fetch="join"的意思是 ：Hibernate通過 在SELECT語句使用OUTER JOIN（外連接）來 獲得對象的關聯實例或者關聯集合
你看我們上面發出的SQL語句，就是個左外連接查詢，把學生對應得班級也加載了，所以，它的lazy就失效了。
（2）集合上的抓取策略：（有三種策略，select，join,subselect）
Classes classes = (Classes) session.load(Classes.class, 1);
System.out.println(classes.getName());
for(Iterator iter = classes.getStudents().iterator();iter.hasNext();){
Student stu = (Student) iter.next();
System.out.println(stu.getName());
}
如果沒有配置抓取策略，默認的fetch="select"，
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
班級0
Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, students0_.id as id0_0_, students0_.name as name0_0_, students0_.classesid as classesid0_0_ from t_student students0_ where students0_.classesid=?
班級0學生5
班級0學生1
默認發出兩條select語句。
當設置抓取策略fetch="join"：<set name="students" inverse="true" fetch="join">
Hibernate: select classes0_.id as id1_1_, classes0_.name as name1_1_, students1_.classesid as classesid3_, students1_.id as id3_, students1_.id as id0_0_, students1_.name as name0_0_, students1_.classesid as classesid0_0_ from t_classes classes0_ left outer
join t_student students1_ on classes0_.id=students1_.classesid where classes0_.id=?
班級0
班級0學生7
班級0學生3
會發現只發出一條外連接語句，加載它的關聯實體，此時，lazy也失效了。
如果設置抓取策略爲：<set name="students" inverse="true" fetch="subselect">
另外發送一條SELECT 語句抓取在前面查詢到（或者抓取到）的所有實體對象的關聯集合。（注意，它是從前面查詢到的）
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
班級0
Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, students0_.id as id0_0_, students0_.name as name0_0_, students0_.classesid as classesid0_0_ from t_student students0_ where students0_.classesid=?
班級0學生5
班級0學生1
仔細看它發出的SQL語句：跟我們默認的select策略一模一樣，但是，它會影響我們的HQL查詢，如：
List classesList = session.createQuery("select c from Classes c where c.id in(1, 2, 3)").list();
for (Iterator iter=classesList.iterator(); iter.hasNext();) {
Classes classes = (Classes)iter.next();
System.out.println("classes.name=" + classes.getName());
for (Iterator iter1=classes.getStudents().iterator(); iter1.hasNext();) {
Student student = (Student)iter1.next();
System.out.println("student.name=" + student.getName());
}
}
看它發的SQL語句，另外分別發了三條select語句，
Hibernate: select classes0_.id as id1_, classes0_.name as name1_ from t_classes classes0_ where classes0_.id in (1 , 2 , 3)
classes.name=班級0
Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, students0_.id as id0_0_, students0_.name as name0_0_, students0_.classesid as classesid0_0_ from t_student students0_ where students0_.classesid=?
student.name=班級0學生6
student.name=班級0學生0
classes.name=班級1
Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, students0_.id as id0_0_, students0_.name as name0_0_, students0_.classesid as classesid0_0_ from t_student students0_ where students0_.classesid=?
student.name=班級1學生5
student.name=班級1學生9
st
classes.name=班級2
Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, students0_.id as id0_0_, students0_.name as name0_0_, students0_.classesid as classesid0_0_ from t_student students0_ where students0_.classesid=?
student.name=班級2學生0
student.name=班級2學生7
如果設置<set name="students" inverse="true" fetch="subselect">
Hibernate: select classes0_.id as id1_, classes0_.name as name1_ from t_classes classes0_ where classes0_.id in (1 , 2 , 3)
classes.name=班級0
Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, students0_.id as id0_0_, students0_.name as name0_0_, students0_.classesid as classesid0_0_ from t_student students0_ where students0_.classesid in (select
classes0_.id from t_classes classes0_ where classes0_.id in (1 , 2 , 3))
student.name=班級0學生4
student.name=班級0學生3
另外只需發一條SQL語句，仔細看看他的SQL語句，從from之後看：用了一個子查詢，from t_student students0_ where students0_.classesid in (select classes0_.id from t_classes classes0_ where classes0_.id in (1 , 2 , 3))。
下面講解下批量抓取策略：
List students = session.createQuery("select s from Student s where s.id in(:ids)")
.setParameterList("ids", new Object[]{1,11,21,31,41,51,61,71,81,91})
.list();
for (Iterator iter=students.iterator(); iter.hasNext();) {
Student stu =(Student)iter.next();
System.out.println("classes.name=" + stu.getName());
System.out.println("student.name=" + stu.getClasses().getName());
}
我發出的SQL語句有10條
Hibernate: select student0_.id as id0_, student0_.name as name0_, student0_.classesid as classesid0_ from t_student student0_ where student0_.id in (? , ? , ? , ? , ? , ? , ? , ? , ? , ?)
classes.name=班級0學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級0
classes.name=班級1學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級1
classes.name=班級2學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級2
classes.name=班級3學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級3
classes.name=班級4學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級4
classes.name=班級5學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級5
classes.name=班級6學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級6
classes.name=班級7學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級7
classes.name=班級8學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級8
classes.name=班級9學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id=?
student.name=班級9
如果我在班級那裏設置一下：<class name="Classes" table="t_classes" batch-size="5">
結果，看看效果就知道了，它會5條一次抓取，而不會一次一條的從數據庫裏取。
Hibernate: select student0_.id as id0_, student0_.name as name0_, student0_.classesid as classesid0_ from t_student student0_ where student0_.id in (? , ? , ? , ? , ? , ? , ? , ? , ? , ?)
classes.name=班級0學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id in (?, ?, ?, ?, ?)
student.name=班級0
classes.name=班級1學生0
student.name=班級1
classes.name=班級2學生0
student.name=班級2
classes.name=班級3學生0
student.name=班級3
classes.name=班級4學生0
student.name=班級4
classes.name=班級5學生0
Hibernate: select classes0_.id as id1_0_, classes0_.name as name1_0_ from t_classes classes0_ where classes0_.id in (?, ?, ?, ?, ?)
student.name=班級5
classes.name=班級6學生0
student.name=班級6
classes.name=班級7學生0
student.name=班級7
classes.name=班級8學生0
student.name=班級8
classes.name=班級9學生0
student.name=班級9