转载地址：http://littie1987.iteye.com/blog/1039082
下面来讲讲Hibernate的merge方法。我打算按照hibernate对象生命周期的三个状态来讲。
1：如果POJO对象处于游离态，我所说的游离态是指该对象的id值为空。hibernate判断一个对象在数据库中是否存在不是看对象的其他信息，而是判断该id在数据库中是不是存在。如果id为空，那自然是不存在，所以当我们调用merge方法的时候，就会直接执行插入操作。这一点有点像saveorupdate()方法。看一段代码：
- User user = new User();
- //user.setId(4);
- user.setUsername("heyuanling2");
- user.setAge(23);
- user.setSex("w");
- user.setPassword("heyuanling");
- Session session = this.getSession();
- Transaction tr = session.beginTransaction();
- //User exituser = (User)session.get(User.class, new Integer(1));
- session.merge(user);
- tr.commit();
User user = new User();
//user.setId(4);
user.setUsername("heyuanling2");
user.setAge(23);
user.setSex("w");
user.setPassword("heyuanling");
Session session = this.getSession();
Transaction tr = session.beginTransaction();
//User exituser = (User)session.get(User.class, new Integer(1));
session.merge(user);
tr.commit();
再看hibernate的sql语句：
- Hibernate:
- select
- max(id)
- from
- user_
- Hibernate:
- insert
- into
- user_
- (username, password, sex, age, birthday, other, id)
- values
- (?, ?, ?, ?, ?, ?, ?)
Hibernate:
select
max(id)
from
user_
Hibernate:
insert
into
user_
(username, password, sex, age, birthday, other, id)
values
(?, ?, ?, ?, ?, ?, ?)
二：脱管态：如果我们把上面代码里//user.setId(4);的注释去掉，那么它就变成了脱管的对象了(其实从游离到脱管就这么简单，没有官方说的那么邪乎...)。这是我们再来看控制台的sql打印：
- Hibernate:
- select
- user0_.id as id4_0_,
- user0_.username as username4_0_,
- user0_.password as password4_0_,
- user0_.sex as sex4_0_,
- user0_.age as age4_0_,
- user0_.birthday as birthday4_0_,
- user0_.other as other4_0_
- from
- user_ user0_
- where
- user0_.id=?
Hibernate:
select
user0_.id as id4_0_,
user0_.username as username4_0_,
user0_.password as password4_0_,
user0_.sex as sex4_0_,
user0_.age as age4_0_,
user0_.birthday as birthday4_0_,
user0_.other as other4_0_
from
user_ user0_
where
user0_.id=?
看到没有，因为id不为空了，所以hibernate就不会再insert了。由于该对象的信息和数据库里的一模一样，所以hibernate只执行了一个select语句，并没有update，如果我们把字段的值做稍微的变动，那么控制台打印的sql语句还应该有一条update语句。就这一点来说，merge还有和saveorupdate()方法一样。
三：持久态：持久态更好理解。如果我们从数据库里get一条记录，那么这条记录就处于持久态，如果再调用merge，那么hibernate就会先判断该记录是否被修改，没有则什么也不干，修改了就update。这一点还是和saveorupdate()有点像。
- Session session = this.getSession();
- Transaction tr = session.beginTransaction();
- User exituser = (User)session.get(User.class,
new Integer(1));
- exituser.setAge(11);
- session.merge(exituser);
- tr.commit();
- session.close();
Session session = this.getSession(); Transaction tr = session.beginTransaction(); User exituser = (User)session.get(User.class, new Integer(1)); exituser.setAge(11); session.merge(exituser); tr.commit(); session.close();
再看控制台打印结果：
- Hibernate:
- select
- user0_.id as id4_0_,
- user0_.username as username4_0_,
- user0_.password as password4_0_,
- user0_.sex as sex4_0_,
- user0_.age as age4_0_,
- user0_.birthday as birthday4_0_,
- user0_.other as other4_0_
- from
- user_ user0_
- where
- user0_.id=?
- Hibernate:
- update
- user_
- set
- username=?,
- password=?,
- sex=?,
- age=?,
- birthday=?,
- other=?
- where
- id=?
Hibernate:
select
user0_.id as id4_0_,
user0_.username as username4_0_,
user0_.password as password4_0_,
user0_.sex as sex4_0_,
user0_.age as age4_0_,
user0_.birthday as birthday4_0_,
user0_.other as other4_0_
from
user_ user0_
where
user0_.id=?
Hibernate:
update
user_
set
username=?,
password=?,
sex=?,
age=?,
birthday=?,
other=?
where
id=?
如果没有对记录进行修改则不会有后面的那条update语句。
那么merge和saveorupdate()到底有什么区别呢？看一段代码：
- Session session = this.getSession();
- Transaction tr = session.beginTransaction();
- User exituser = (User)session.get(User.class,
new Integer(1));
- tr.commit();
- session.close();
- session = getSession();
- tr = session.beginTransaction();
- User exituser2 = (User)session.get(User.class,
new Integer(1));
- session.update(exituser);
- tr.commit();
- session.close();
Session session = this.getSession(); Transaction tr = session.beginTransaction(); User exituser = (User)session.get(User.class, new Integer(1)); tr.commit(); session.close(); session = getSession(); tr = session.beginTransaction(); User exituser2 = (User)session.get(User.class, new Integer(1)); session.update(exituser); tr.commit(); session.close();
运行上面的代码，hibernate给我们报了一个错误：a different object with the same identifier value was already associated with the session。意思是，在session缓存中以两个标识相同的对象，这是不可以的。那么，吧update改成merge会怎么样呢？改为merge后，一切OK，运行正常。其实merge在执行更新之前会将两个标识符相同的对象进行合并，具体合并的方向是向exituser2合并。
注意：merge方法在执行之前都回去缓存中找是不是有相应的记录，也就是会有一条select语句，执行改语句的目的是为了判断该对象是否被修改了。而update就不管这些，直接就一条update语句。