延遲加載

-1

SessionFactory sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory(); 
     Session session = sessionFactory.openSession(); // create session object 
     session.beginTransaction(); // start transaction object 
     session.save(user); // save the user to database 
     session.getTransaction().commit(); // commit the transaction 
     session.close(); // closing session 

     session = sessionFactory.openSession(); 
     user = null; 
     user = (UserDetails) session.get(UserDetails.class, 1); 
     System.out.println(user.getLisOfAddresses().size()); 
    } 
}

OUTPUT：看保健充分的輸出有兩個選擇查詢一個是用戶，另一個是地址表時，我們稱之爲getListOfAddresses();延遲加載

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

現在，如果你改變一些代碼行該類文件以驗證代理對象。在調用getListOfAddresses（）方法關閉會話之前，會發生這種情況。

  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());

現在輸出：

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)

正如上面的代碼失敗的運行，因爲當我們使用抓取物體的延遲型戰略Hibernate的Session返回代理對象，它在會話中如果我們正在關閉會話，延遲加載不會發生。

對於預先抓取：

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()); 
     } 
}

OUTPUT：

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=?

2 ************************* ************************************************** *** 查看此代碼是否成功運行，因爲我們使用EAGER獲取策略，因此在此策略會話中返回原始對象，並且所有字段在加載到內存時都被初始化。在上面的輸出字符串中，完全看起來只有一個帶連接子句的select語句。

我只是想知道，即使在顯式關閉SESSION，

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

執行罰款。因此，從上述收藏處休息。來自第一級CACHE。即使在EAGER放棄會議後（即我知道依賴的對象也加載），對象是否仍然存在？什麼是懶惰負荷的情況？因爲收藏品都是沒有加載，難道是這個原因，

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

顯示錯誤在遲緩裝載後會話關閉和getter被調用後關閉會話？ COULS有人反映了這個？我知道什麼是懶惰和EAGER抓取？

來源

2014-05-11 user3625546

對一個問題的問題太多。那麼，Hibernate正在做你剛纔要求它做的事情。您需要知道的get和load之間的區別。當你得到的時候，Hibernate從DB獲取並且反對。當你做load Hibernate製作和代理對象，並不會加載對象，直到你真的需要它（直到你打電話給它或類似的東西）。

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

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

EXECUTES FINE.

是它，閉幕會議不會使你的初始化（從數據庫對象讀取）爲null，而是讓這些對象detached。查看從hibernate文檔中分離出來的意思。但是物體本身會一直存在，直到GC破壞它爲止。

`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

這正是分離對象+延遲加載的原因。您的LisOfAddresses未加載，並且user對象已分離（表示未附加任何休眠會話）。所以它有代理，試圖打數據庫假設user對象連接到休眠會話這是錯誤的在你的情況下，失敗，這是正確的。

來源

2014-05-11 16:20:03 Elbek

回答

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