Fetch抓取策略

1 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

设置fetch="join",如：
<many-to-one name="classes" column="classesid" fetch="join"/>

fetch="join",hibernate会通过select语句使用左外连接来加载其关联实体或集合

此时lazy会失效


2 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


设置fetch="join",如：
<set name="students" inverse="true" cascade="all" fetch="join">

fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合

此时lazy会失效


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所有实体对象的关联集合

/**
	 * Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, 
	 * students0_.id as id1_0_, students0_.name as name1_0_, 
	 * students0_.classesid as classesid1_0_ 
	 * from t_student students0_ 
	 * where students0_.classesid in (select classes0_.id from t_classes classes0_ where classes0_.id in (1 , 2 , 3))
	 */

try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			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());
				}
			}

HQL 支持LAZY,但是不支持映射文件中的fetch，即使设置fetch="join",也不会采用join。只有在hql中显示利用fetch才能利用，如：
select s from Student s join fetch s.classes c