Hibernate 延迟加载原理

    如何简单的理解延迟加载？开发中常见的org.hibernate.LazyInitializationException no session错误又是怎么产生的？下面通过一个简单的例子来解析一下。

load VS. get

    load是Hibernate利用延迟加载获取对象的方法，当调用load时返回的是一个代理对象，并且该对象只包含主键ID。只有当你想主动获取该对象的其他非主键属性时，才会去数据库中获取对应的属性返回。如果这时该对象对应的数据库记录被删除了或不存在则会报抛出ObjectNotFoundException异常。

    get是Hibernate获取对象的一个普通方法，没有使用延迟加载，调用get时hibernate直接返回数据库中对应的记录信息。如果数据库中不存在该对象的记录，会抛出空指针异常。

程序所示

load:

   1:  Session session = null;

   2:          try {

   3:              session = HibernateUtil.openSession();

   4:              //使用load，利用延迟加载获取对象其他属性

   5:              Object object = session.load(Member.class, 121L);

   6:              String mobile = ((Member) object).getMobile();//<---这里当调用getMobile()时，才会去数据库查询对应的记录

   7:              System.out.println(mobile);

   8:          } catch (HibernateException e) {

   9:              e.printStackTrace();

  10:          } finally {

  11:              if (session != null)

  12:                  session.close();

  13:          }

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get:

   1:  Session session = null;

   2:          try {

   3:              session = HibernateUtil.openSession();

   4:              //使用get，直接从数据库中查询对应的记录

   5:              Object object = session.get(Member.class, 121L);

   6:              String mobile = ((Member) object).getMobile();//<---这里没有使用延迟加载直接获取mobile

   7:              System.out.println(mobile);

   8:          } catch (HibernateException e) {

   9:              e.printStackTrace();

  10:          } finally {

  11:              if (session != null)

  12:                  session.close();

  13:          }

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使用延迟加载时，常见的org.hibernate.LazyInitializationException no session异常，又是怎么产生的呢？

这是因为当使用延迟加载获取对象属性时如果这时session已经关闭，则无法进行数据库操作，就会报出上述异常，我们用程序模拟一下：

get:

   1:  Session session = null;

   2:          try {

   3:              session = HibernateUtil.openSession();

   4:              //使用get，直接从数据库中查询对应的记录

   5:              Object object = session.get(Member.class, 121L);

   6:              session.close();//这时关闭session

   7:              String mobile = ((Member) object).getMobile();//可以正常获取，因为mobile是从对象中直接获取的

   8:              System.out.println(mobile);

   9:          } catch (HibernateException e) {

  10:              e.printStackTrace();

  11:          } 

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load:

   1:  Session session = null;

   2:          try {

   3:              session = HibernateUtil.openSession();

   4:              //使用get，直接从数据库中查询对应的记录

   5:              Object object = session.load(Member.class, 121L);

   6:              session.close();//这时关闭session

   7:              String mobile = ((Member) object).getMobile();//使用load的情况，获取mobile是从数据库中查询的，但此时session已经关闭，故调用getMobile时会抛出org.hibernate.LazyInitializationException no session异常

   8:              System.out.println(mobile);

   9:          } catch (HibernateException e) {

  10:              e.printStackTrace();

  11:          } 

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