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本文教你簡單學會Hibernate merge() add操作，具體的實現(xiàn)過程如下：
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背景：
◆Account 和 Group 兩個對象，設置了雙向的many-to-many關(guān)系，lazy=true
◆不使用open session in view 模式
◆不使用hibernate二級緩
◆考慮web應用場景，設置account和group關(guān)聯(lián)時，只需要group和account的id就夠了。
◆數(shù)據(jù)庫中存在兩個group: 1.administrators, 2.engineers而po對象中，group信息為：1.invalid, 2.any one
代碼A：
- Account account = (Account) getHibernateTemplate().merge(po);
- Long id = account.getId();
- System.out.println("\tGet obj after added in dao start ...");
- Account readAccount = (Account) getHibernateTemplate().get(
- Account.class, id);
- 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());
其中，為po設置了兩個group 輸出結(jié)果：
- Hibernate: select ... from SYS_GROUPS where ID=?
- Hibernate: select ... from SYS_GROUPS where ID=?
- Get obj after added in dao start ...
- Get obj after added in dao end ...
- Is po==readAccount ? false
- Show detai of po: Account[0.account_22, groups[2.any one 1.invalid ]]
- Show detai of readAccount: Account[22.account_22, groups[2.engineers 1.administrators ]]
- Hibernate: insert into SYS_ACCOUNTS (...) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
- Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)
- Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)
代碼B：
- Long id = (Long) getHibernateTemplate().save(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, id);
- 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(readAccount);
- Account readAgain = (Account) getHibernateTemplate().get(Account.class,
- id);
- System.out.println("\tIs po==readAgain ? " + (readAgain == po));
- System.out.println("\tIs readAgain== readAccount? "
- + (readAgain == readAccount));
- System.out.println("\tShow detai again: " + readAgain.toDetailString());
輸出結(jié)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: select ... from SYS_GROUPS where ID=?
- Is po==readAgain ? true
- Is readAgain== readAccount? true
- Show detai again: Account[27.account_27, groups[1.administrators 2.engineers ]]
- Hibernate: insert into SYS_ACCOUNTS (...) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
- Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)
- Hibernate: insert into SYS_GROUP_MEMBER (ACCOUNT_ID, GROUP_ID) values (?, ?)
結(jié)論：
1. Hibernate merge()方法會導致執(zhí)行查詢group對象的select語句，在調(diào)用merge()命令時立即執(zhí)行（條件：目標group對象沒有被緩存）
2. 無論Hibernate merge()或save()方法，insert語句都在***執(zhí)行，并非在調(diào)用相應命令時立即執(zhí)行
3. 直接調(diào)用merge()方法時，會返回一個新的instance，原po保持不變
4. save()之后，po中的group對象并沒有被關(guān)聯(lián)到session，因此查詢group(id=1)會觸發(fā)select語句
5. save()之后，po對象被關(guān)聯(lián)到session，再次查詢，不會觸發(fā)select語句，并且不會檢查group對象是否被關(guān)聯(lián)到session
6. save()之后再調(diào)用merge，返回的是同一個instance，但其關(guān)聯(lián)group對象會被更新
如果在add一個對象之后，如果存在關(guān)聯(lián)對象，并且需要再同一個hibernate session中進行回顯，則建議使用Hibernate merge()方法。
參考：
Hibernate Session.merge() javadoc
open session in view 模式
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名稱欄目：Hibernate中merge()方法解析
文章來源：http://www.dlmjj.cn/article/cogjcjj.html
