Hibernate试验03_06.批量延迟加载.doc

实践1 批量延迟加载实验目的帮助学生理解掌握批量延迟加载的使用方式及其含义。技术要点l SQL基础知识；l Hibernate基础知识。实验步骤1. 打开HibernateProject03，下面我们演示批量加载。2. 假设班级和学生是一对多关联，现在采用批量延迟加载得到班级，班级对象会通过类关联取得所有的学生数据，学生和身份证采用延迟加载策略。此时，我们Student.hbm.xml、Team.hbm.xml、Certificate.hbm.xml，如下：Student.hbm.xml：Team.hbm.xml：Certificate.hbm.xml：3. 修改TeamDAO的getAllTeam()方法，修改如下：TeamDAO.java： /*-取得所有Team-*/ public static List getAllTeam() List list = null; try session = HibernateUtil.currentSession(); / 开启连接 tx = session.beginTransaction(); / 开启事务 list = (List) session.createQuery(from Team t).list(); mit(); HibernateUtil.closeSession(session); Team team = (Team)list.get(0); System.out.println(team.getStudents().size(); catch (HibernateException e) / 捕捉例外 e.printStackTrace(); tx.rollback(); return list; 4. 运行TeamManager，控制台执行结果如下：Eclipse Console：Hibernate: select team0_.id as id2_, team0_.teamName as teamName2_ from team team0_ERROR main (LazyInitializationException.java:19) - failed to lazily initialize a collection of role: com.qrsx.hibernate.model.Team.students, no session or session was closed5. 上述程序在Session关闭后试图打印延迟加载的学生集合，于是弹出了例外，这说明批量延迟加载是在“延迟”的基础上运行批量加载的。6. 重新修改TeamDAO的getAllTeam()方法，修改如下：TeamDAO.java： /*-取得所有Team-*/ public static List getAllTeam() List list = null; try session = HibernateUtil.currentSession(); / 开启连接 tx = session.beginTransaction(); / 开启事务 list = (List) session.createQuery(from Team t).list(); Team team1 = (Team)list.get(0); System.out.println(team1.getTeamName()+的学生数量:+team1.getStudents().size(); Team team2 = (Team)list.get(1); System.out.println(team2.getTeamName()+的学生数量:+team2.getStudents().size(); Team team3 = (Team)list.get(2); System.out.println(team3.getTeamName()+的学生数量:+team3.getStudents().size(); Team team4 = (Team)list.get(3); System.out.println(team4.getTeamName()+的学生数量:+team4.getStudents().size(); Team team5 = (Team)list.get(4); System.out.println(team5.getTeamName()+的学生数量:+team5.getStudents().size(); mit(); HibernateUtil.closeSession(session); catch (HibernateException e) / 捕捉例外 e.printStackTrace(); tx.rollback(); return list; 7. 运行TeamManager，控制台执行结果如下：Eclipse Console：Hibernate: * from team team0_Hibernate: select * from student s where s.team_id in (?, ?) Hibernate: select * from student s where s.team_id in (?, ?)计