Hibernate學習筆記 merge()方法–add對象

merge

add操作

背景：
    Account 和 Group 兩個對象，設定了雙向的many-to-many關係，lazy=true
    不使用open session in view 模式
    不使用hibernate二級緩衝

    考慮web應用情境，設定account和group關聯時，只需要group和account的id就夠了。
    資料庫中存在兩個group: 1.administrators, 2.engineers
    而po對象中，group資訊為：1.invalid, 2.any one

代碼A：
Account account = (Account) getHibernateTemplate().merge(po);<br />Long id = account.getId();<br />System.out.println("/tGet obj after added in dao start ...");<br />Account readAccount = (Account) getHibernateTemplate().get(<br />Account.class, id);<br />System.out.println("/tGet obj after added in dao end ...");<br />System.out.println("/tIs po==readAccount ? " + (po == readAccount));<br />System.out.println("/tShow detai of po: " + po.toDetailString());<br />System.out.println("/tShow detai of readAccount: " + readAccount.toDetailString());<br />

   其中，為po設定了兩個group

輸出結果：
Hibernate: select ... from SYS_GROUPS where ID=?<br />Hibernate: select ... from SYS_GROUPS where ID=?<br />Get obj after added in dao start ...<br />Get obj after added in dao end ...<br />Is po==readAccount ? false<br />Show detai of po: Account[0.account_22, groups[2.any one 1.invalid ]]<br />Show detai of readAccount: Account[22.account_22, groups[2.engineers 1.administrators ]]<br />Hibernate: insert into SYS_ACCOUNTS (...) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)<br />Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)<br />Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)<br />

 

代碼B：

Long id = (Long) getHibernateTemplate().save(po);<br />System.out.println("/tGet obj after added in dao start ...");<br />Group group = (Group)getHibernateTemplate().get(Group.class,new Long(1));<br />System.out.println("/tGroup detai:" + group.toString());<br />Account readAccount = (Account) getHibernateTemplate().get(<br />Account.class, id);<br />System.out.println("/tGet obj after added in dao end ...");<br />System.out.println("/tIs po==readAccount ? " + (po == readAccount));<br />System.out.println("/tShow detai of po: " + po.toDetailString());<br />System.out.println("/tShow detai of readAccount: "<br />+ readAccount.toDetailString());</p><p>getHibernateTemplate().merge(readAccount);<br />Account readAgain = (Account) getHibernateTemplate().get(Account.class,<br />id);<br />System.out.println("/tIs po==readAgain ? " + (readAgain == po));<br />System.out.println("/tIs readAgain== readAccount? "<br />+ (readAgain == readAccount));<br />System.out.println("/tShow detai again: " + readAgain.toDetailString());<br />

輸出結果:
Get obj after added in dao start ...<br />Hibernate: select ... from SYS_GROUPS where ID=?<br />Group detai:Group 1. administrators<br />Get obj after added in dao end ...<br />Is po==readAccount ? true<br />Show detai of po: Account[27.account_27, groups[1.invalid 2.any one ]]<br />Show detai of readAccount: Account[27.account_27, groups[1.invalid 2.any one ]]<br />Hibernate: select ... from SYS_GROUPS where ID=?<br />Is po==readAgain ? true<br />Is readAgain== readAccount? true<br />Show detai again: Account[27.account_27, groups[1.administrators 2.engineers ]]<br />Hibernate: insert into SYS_ACCOUNTS (...) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)<br />Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)<br />Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)<br />  

結論：

 1. merge()方法會導致執行查詢group對象的select語句，在調用merge()命令時立即執行（條件：目標group對象沒有被緩衝）

 2. 無論merger()或save()方法，insert語句都在最後執行，並非在調用相應命令時立即執行

 3. 直接調用merge()方法時，會返回一個新的instance，原po保持不變

 4. save()之後，po中的group對象並沒有被關聯到session，因此查詢group(id=1)會觸發select語句

 5. save()之後，po對象被關聯到session，重新查詢，不會觸發select語句，並且不會檢查group對象是否被關聯到session

 6. save()之後再調用merge，返回的是同一個instance，但其關聯group對象會被更新

    如果在add一個對象之後，如果存在關聯對象，並且需要再同一個hibernate session中進行回顯，則建議使用merge()方法。

參考：
    Hibernate Session.merge() javadoc
    open session in view 模式