Java Hibernate中会话关闭时的延迟加载

输出：仔细查看输出有两个select查询，一个用于用户，另一个用于调用

getListOfAddresses（）

时的地址表

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现在，如果您更改为该类文件的一些代码行，以验证代理对象。 在调用getListOfAddresses（）方法关闭会话之前，会发生这种情况

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).
log4j:WARN Please initialize the log4j system properly.
Hibernate: insert into USER_DETAIL (USER_NAME) values (?)
Hibernate: insert into USER_ADDRESS (USER_ID, CITY_NAME, PIN_CODE, STATE_NAME, STREET_NAME) values (?, ?, ?, ?, ?)
Hibernate: insert into USER_ADDRESS (USER_ID, CITY_NAME, PIN_CODE, STATE_NAME, STREET_NAME) values (?, ?, ?, ?, ?)
Hibernate: select userdetail0_.USER_ID as USER1_0_0_, userdetail0_.USER_NAME as USER2_0_0_ from USER_DETAIL userdetail0_ where userdetail0_.USER_ID=?
Hibernate: select lisofaddre0_.USER_ID as USER1_0_, lisofaddre0_.CITY_NAME as CITY2_0_, lisofaddre0_.PIN_CODE as PIN3_0_, lisofaddre0_.STATE_NAME as STATE4_0_, lisofaddre0_.STREET_NAME as STREET5_0_ from USER_ADDRESS lisofaddre0_ where lisofaddre0_.USER_ID=?
2

现在输出：

         session = sessionFactory.openSession(); // again create another session object  
         user = null;
         user = (UserDetails) session.get(UserDetails.class, 1); 
          session.close(); // close the session before calling collection getter 
         System.out.println(user.getLisOfAddresses().size());

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如上所述，代码运行失败，因为当我们使用获取对象的惰性类型策略时，hibernate会话返回代理对象，如果我们关闭会话，它存在于会话中，那么惰性加载不会发生

对于即时抓取：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).
log4j:WARN Please initialize the log4j system properly.
Hibernate: insert into USER_DETAIL (USER_NAME) values (?)
Hibernate: insert into USER_ADDRESS (USER_ID, CITY_NAME, PIN_CODE, STATE_NAME, STREET_NAME) values (?, ?, ?, ?, ?)
Hibernate: insert into USER_ADDRESS (USER_ID, CITY_NAME, PIN_CODE, STATE_NAME, STREET_NAME) values (?, ?, ?, ?, ?)
Hibernate: select userdetail0_.USER_ID as USER1_0_0_, userdetail0_.USER_NAME as USER2_0_0_ from USER_DETAIL userdetail0_ where userdetail0_.USER_ID=?
Exception in thread "main" org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.sdnext.hibernate.tutorial.dto.UserDetails.lisOfAddresses, no session or session was closed
    at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException
(AbstractPersistentCollection.java:380)
    at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected
(AbstractPersistentCollection.java:372)
    at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:119)
    at org.hibernate.collection.PersistentBag.size(PersistentBag.java:248)
    at com.sdnext.hibernate.tutorial.HibernateTestDemo.main(HibernateTestDemo.java:48)

输出：

 SessionFactory sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
        Session session = sessionFactory.openSession();
        session.beginTransaction();
        session.save(user);
        session.getTransaction().commit();
        session.close();

         session = sessionFactory.openSession();
         user = null;
         user = (UserDetails) session.get(UserDetails.class, 1);
         session.close(); //closing the session before calling collection getter
         System.out.println(user.getLisOfAddresses().size());
      }
}

二,****************************************************************************** 请看，这段代码正在成功运行，因为我们使用的是急切抓取策略，所以在这个策略会话中，返回原始对象，所有字段都在加载到内存时初始化。 在上面的输出字符串中，请仔细查看，只有一个带有join子句的select语句

我只想知道，即使在明确结束会议之后

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).
log4j:WARN Please initialize the log4j system properly.
Hibernate: insert into USER_DETAIL (USER_NAME) values (?)
Hibernate: insert into USER_ADDRESS (USER_ID, CITY_NAME, PIN_CODE, STATE_NAME, STREET_NAME) values (?, ?, ?, ?, ?)
Hibernate: insert into USER_ADDRESS (USER_ID, CITY_NAME, PIN_CODE, STATE_NAME, STREET_NAME) values (?, ?, ?, ?, ?)
Hibernate: select userdetail0_.USER_ID as USER1_0_0_, userdetail0_.USER_NAME as USER2_0_0_, lisofaddre1_.USER_ID as USER1_2_, lisofaddre1_.CITY_NAME as CITY2_2_, lisofaddre1_.PIN_CODE as PIN3_2_, lisofaddre1_.STATE_NAME as STATE4_2_, lisofaddre1_.STREET_NAME as STREET5_2_ from USER_DETAIL userdetail0_ left outer join USER_ADDRESS lisofaddre1_ on userdetail0_.USER_ID=lisofaddre1_.USER_ID where userdetail0_.USER_ID=?

执行良好。 因此，HIBERNATE从何处加载上述集合。它来自一级缓存。 即使在即时抓取中关闭会话后，对象是否仍保留在那里（我知道依赖对象也会加载）？延迟加载是什么情况？由于集合未加载，这就是

 System.out.println(user.getLisOfAddresses().size());

在会话关闭后显示延迟加载错误，并且在会话关闭后调用GETTER？ 有人能反思一下吗？

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我知道什么是懒惰和急切的获取？

一个问题上的问题太多了。 嗯，Hibernate正在做你要求它做的事情。 您需要知道

get

和

load

之间的区别。当您确实获取时，Hibernate从DB获取对象。当您执行加载操作时，Hibernate生成并代理对象，并且在您真正需要它之前不会加载该对象（直到您调用getter或类似的东西）

是的，关闭会话不会使您的初始化（从db对象读取）变为null，而是使这些对象变为null。请参阅hibernate文档中分离的含义。但对象本身将一直保留，直到GC将其销毁

I JUST WISH TO KNOW THAT EVEN AFTER CLOSING THE SESSION EXPLICITLY,

 System.out.println(user.getLisOfAddresses().size());

EXECUTES FINE.

这正是分离对象+延迟加载的原因。您的

lisofaddress

未加载，并且

user

对象已分离（这意味着未附加任何hibernate会话）。因此，它有一个代理，它会在假定

user

对象连接到hibernate会话的情况下尝试命中db，这在您的情况下是错误的，但在正确的情况下失败了

System.out.println(user.getLisOfAddresses().size()); 

I JUST WISH TO KNOW THAT EVEN AFTER CLOSING THE SESSION EXPLICITLY,

 System.out.println(user.getLisOfAddresses().size());

EXECUTES FINE.

 `SINCE THE COLLECTION IS NOT LOADED, IS IT THIS VERY REASON THAT

System.out.println(user.getLisOfAddresses().size()); 

SHOWS ERROR IN LAZY LOADING AFTER SESSION IS CLOSED AND GETTER IS CALLED AFTER CLOSING THE SESSION