乐观锁与悲观锁
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version乐观锁
1 操作员 A 此时将其读出( version=1 ),并从其帐户余额中扣除 $50( $100-$50 )。
2 在操作员 A 操作的过程中,操作员B 也读入此用户信息( version=1 ),并从其帐户余额中扣除 $20 ( $100-$20 )。
3 操作员 A 完成了修改工作,将数据版本号加一( version=2 ),连同帐户扣除后余额( balance=$50 ),提交至数据库更新,此时由于提交数据版本大于数据库记录当前版本,数据被更新,数据库记录 version 更新为 2 。
4 操作员 B 完成了操作,也将版本号加一( version=2 )试图向数据库提交数据( balance=$80 ),但此时比对数据库记录版本时发现,操作员 B 提交的数据版本号为 2 ,数据库记录当前版本也为 2 ,不满足 “ 提交版本必须大于记录当前版本才能执行更新 “ 的乐观锁策略,因此,操作员 B 的提交被驳回。
悲观锁
一个典型的倚赖数据库的悲观锁调用:
select * from account where name="Erica" for update
这条 sql 语句锁定了 account 表中所有符合检索条件( name="Erica" )的记录。 本次事务提交之前(事务提交时会释放事务过程中的锁),外界无法修改这些记录。
ibatis 乐观锁使用示例
Java代码 sqlmap
- <insert id="updateUser" parameterClass="user">
- UPDATE
- T_USER
- SET
- NAME = #name#,
- VERSION = #version# + 1
- WHERE
- ID = #id#
- AND VERSION = (#version#)
- </insert>
Java代码
- public Object update(final String namespace, Object object) {
- Assert.notNull(object, "The argument [object] can't be null!");
- if(1>this.getSqlMapClientTemplate().update(fullname(namespace, SUFFIX_UPDATE), object)) {
- throw new OptimisticLockException(String.valueOf(object));
- }
- return object;
- }
hibernate 悲观锁使用示例
1 String hqlStr = " from TUser as user where user.name=’Erica’ " ;
2 Query query = session.createQuery(hqlStr);
3 query.setLockMode( " user " ,LockMode.UPGRADE); // 加锁
4 List userList = query.list(); // 执行查询,
2 Query query = session.createQuery(hqlStr);
3 query.setLockMode( " user " ,LockMode.UPGRADE); // 加锁
4 List userList = query.list(); // 执行查询,
Sql:
1 select tuser0_.id as id, tuser0_.name as name, tuser0_.group_id as group_id, tuser0_.user_type as user_type, tuser0_.sex as sex from t_user tuser0_ where (tuser0_.name = ’Erica’ ) for update
hibernate 乐观锁使用示例
1. 首先为TUser的class描述符添加optimistic-lock属性:
代码内容
< hibernate - mapping >
< class
name = " org.hibernate.sample.TUser "
table = " t_user "
dynamic - update = " true "
dynamic - insert = " true "
optimistic - lock = " version "
>
……
</ class >
</ hibernate - mapping >
< class
name = " org.hibernate.sample.TUser "
table = " t_user "
dynamic - update = " true "
dynamic - insert = " true "
optimistic - lock = " version "
>
……
</ class >
</ hibernate - mapping >
添加一个Version属性描述符
代码内容
1 < hibernate - mapping >
2 < class
3 name = " org.hibernate.sample.TUser "
4 table = " t_user "
5 dynamic - update = " true "
6 dynamic - insert = " true "
7 optimistic - lock = " version "
8 >
9 < id
10 name = " id "
11 column = " id "
12 type = " java.lang.Integer "
13 >
14 < generator class = " native " >
15 </ generator >
16 </ id >
17 < version
18 column = " version "
19 name = " version "
20 type = " java.lang.Integer "
21 />
22 ……
23 </ class >
24 </ hibernate - mapping >
2 < class
3 name = " org.hibernate.sample.TUser "
4 table = " t_user "
5 dynamic - update = " true "
6 dynamic - insert = " true "
7 optimistic - lock = " version "
8 >
9 < id
10 name = " id "
11 column = " id "
12 type = " java.lang.Integer "
13 >
14 < generator class = " native " >
15 </ generator >
16 </ id >
17 < version
18 column = " version "
19 name = " version "
20 type = " java.lang.Integer "
21 />
22 ……
23 </ class >
24 </ hibernate - mapping >
1 Criteria criteria = session.createCriteria(TUser. class );
2 criteria.add(Expression.eq( " name " , " Erica " ));
3 List userList = criteria.list();
4 TUser user = (TUser)userList.get( 0 );
5 Transaction tx = session.beginTransaction();
6 user.setUserType( 1 ); // 更新UserType字段
7 tx.commit();
2 criteria.add(Expression.eq( " name " , " Erica " ));
3 List userList = criteria.list();
4 TUser user = (TUser)userList.get( 0 );
5 Transaction tx = session.beginTransaction();
6 user.setUserType( 1 ); // 更新UserType字段
7 tx.commit();
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