Hibernate学习笔记 merge()方法--update对象

 背景：
    Account 和 Group 两个对象，设置了双向的many-to-many关系，lazy=true
    不使用open session in view 模式
    不使用hibernate二级缓存

测试逻辑：
    从数据库获取一个account对象（存在两个group: 1.administrators, 2.engineers）
    修改这个对象的group信息为：1.invalid, 2.any one
    然后调用DAO的update方法（这部分代码如下所示）
   

代码 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());

 

输出结果：

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 ? falseShow 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：没有update/insert/delete语句出现

代码 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());

输出结果：

Get obj after added in dao start ...Hibernate: select ... from SYS_GROUPS where ID=?Group detai:Group 1. administratorsGet obj after added in dao end ...Is po==readAccount ? trueShow 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：出现了一句update, 一句delete，两句insert

代码 v3 先执行update()，再执行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());

输出结果：
Get obj after added in dao start ...Hibernate: select ... from SYS_GROUPS where ID=?Group detai:Group 1. administratorsGet obj after added in dao end ...Is po==readAccount ? trueShow 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 ? trueIs readAgain == readAccount? trueShow 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语句依然被执行
    2. 由于代码中添加了对group(id=1)的查找，出现了两句select来查找group表，而不是一句sql通过left outer join来完成
       如果把修改前后的accout都调整为有3个group，结果不确定：
           出现过两种情况：
               一条对group表的select，另外一句select还是会使用left outer join
               三条对group表的select语句
       在代码v1中也有同样的效果，同样的调整，测试结果都是一句select group，一句left outer join，但还不能认为是100%如此，猜测跟account是否已经被关联到session有关。

结论：
    1. merge()方法，会根据根据对象是否进行了实质性修改，来决定是否执行相应的update/delete/update语句，而upate()则不会进行比较，只用给定的对象信息覆盖原有信息

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