Hibernate學習筆記 merge()方法--update對象

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

測試邏輯：
    從數據庫獲取一個account對象（存在兩個group: 1.administrators, 2.engineers）
    修改這個對象的group信息爲：1.invalid, 2.any one
    然後調用DAO的update方法（這部分代碼如下所示）
   

代碼 v1 只使用merge()
getHibernateTemplate().merge(po); System.out.println("/tGet obj after update in dao start ..."); Account readAccount = (Account) getHibernateTemplate().get( Account.class, po.getId()); System.out.println("/tGet obj after update in dao end ..."); System.out.println("/tIs po==readAccount ? " + (po == readAccount)); System.out.println("/tShow detai of po: " + po.toDetailString()); System.out.println("/tShow detai of readAccount: " + readAccount.toDetailString());

 

輸出結果：

Hibernate: select ... from SYS_ACCOUNTS where ID=? Hibernate: select T1.ACCOUNT_ID, T1.GROUP_ID, T2.ID, T2.NAME from TB_GROUP_MEMBER T1 left outer join TB_GROUPS T2 on T1.GROUP_ID=T2.ID where T1.ACCOUNT_ID=? Get obj after update in dao start ... Get obj after update in dao end ... Is po==readAccount ? false Show detai of po: Account[27.account_27, groups[1.invalid 2.any one ]] Show detai of readAccount: Account[27.account_27, groups[2.engineers 1.administrators ]]

注意：沒有update/insert/delete語句出現

代碼 v2 只使用update()

getHibernateTemplate().update(po); System.out.println("/tGet obj after added in dao start ..."); Group group = (Group) getHibernateTemplate().get(Group.class, new Long(1)); System.out.println("/tGroup detai:" + group.toString()); Account readAccount = (Account) getHibernateTemplate().get( Account.class, po.getId()); System.out.println("/tGet obj after added in dao end ..."); System.out.println("/tIs po==readAccount ? " + (po == readAccount)); System.out.println("/tShow detai of po: " + po.toDetailString()); System.out.println("/tShow detai of readAccount: " + readAccount.toDetailString());

輸出結果：

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

注意：出現了一句update, 一句delete，兩句insert

代碼 v3 先執行update()，再執行merge()
getHibernateTemplate().update(po); System.out.println("/tGet obj after added in dao start ..."); Group group = (Group) getHibernateTemplate().get(Group.class, new Long(1)); System.out.println("/tGroup detai:" + group.toString()); Account readAccount = (Account) getHibernateTemplate().get( Account.class, po.getId()); System.out.println("/tGet obj after added in dao end ..."); System.out.println("/tIs po==readAccount ? " + (po == readAccount)); System.out.println("/tShow detai of po: " + po.toDetailString()); System.out.println("/tShow detai of readAccount: " + readAccount.toDetailString()); getHibernateTemplate().merge(po); Account readAgain = (Account) getHibernateTemplate().get(Account.class, po.getId()); System.out.println("/tIs readAgain == po ? " + (readAgain == po)); System.out.println("/tIs readAgain == readAccount? " + (readAgain == readAccount)); System.out.println("/tShow detai again: " + readAgain.toDetailString());

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

注意：
    1. update/delete/insert語句依然被執行
    2. 由於代碼中添加了對group(id=1)的查找，出現了兩句select來查找group表，而不是一句sql通過left outer join來完成
       如果把修改前後的accout都調整爲有3個group，結果不確定：
           出現過兩種情況：
               一條對group表的select，另外一句select還是會使用left outer join
               三條對group表的select語句
       在代碼v1中也有同樣的效果，同樣的調整，測試結果都是一句select group，一句left outer join，但還不能認爲是100%如此，猜測跟account是否已經被關聯到session有關。

結論：
    1. merge()方法，會根據根據對象是否進行了實質性修改，來決定是否執行相應的update/delete/update語句，而upate()則不會進行比較，只用給定的對象信息覆蓋原有信息

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