乐观并发控制,可以有三种方式。
1,Version版本号
2,时间戳
3,自动版本控制。
这里不建议在新的应用程序中定义没有版本或者时间戳列的版本控制:它更慢,更复杂,如果你正在使用脱管对象,它则不会生效。
以下信息来自 : http://esffor.javaeye.com/blog/168243
通过在表中及POJO中增加一个version字段来表示记录的版本,来达到多用户同时更改一条数据的冲突
数据库脚本:
create
table
studentVersion (id
varchar
(
32
),name
varchar
(
32
),ver
int
);
POJO
package
Version;
public
class
Student
{
private String id;
private String name;
private int version;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getVersion() {
return version;
}
public void setVersion(int version) {
this.version = version;
}
}
Student.hbm.xml
xml version="1.0" encoding="utf-8"
?>
DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"
>
<
hibernate-mapping
>
<
class
name
="Version.Student"
table
="studentVersion"
>
<
id
name
="id"
unsaved-value
="null"
>
<
generator
class
="uuid.hex"
>
generator
>
id
>
<
version
name
="version"
column
="ver"
type
="int"
>
version
>
<
property
name
="name"
type
="string"
column
="name"
>
property
>
class
>
hibernate-mapping
>
hibernate.cfg.xml
xml version='1.0' encoding='UTF-8'
?>
DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"
>
<
hibernate-configuration
>
<
session-factory
>
<
property
name
="connection.username"
>
root
property
>
<
property
name
="connection.url"
>
jdbc:mysql://localhost:3306/schoolproject?characterEncoding=gb2312
&
useUnicode=true
property
>
<
property
name
="dialect"
>
org.hibernate.dialect.MySQLDialect
property
>
<
property
name
="myeclipse.connection.profile"
>
mysql
property
>
<
property
name
="connection.password"
>
1234
property
>
<
property
name
="connection.driver_class"
>
com.mysql.jdbc.Driver
property
>
<
property
name
="hibernate.dialect"
>
org.hibernate.dialect.MySQLDialect
property
>
<
property
name
="hibernate.show_sql"
>
true
property
>
<
property
name
="current_session_context_class"
>
thread
property
>
<
property
name
="jdbc.batch_size"
>
15
property
>
<
mapping
resource
="Version/Student.hbm.xml"
/>
session-factory
>
hibernate-configuration
>
测试代码:
package
Version;
import
java.io.File;
import
java.util.Iterator;
import
java.util.Set;
import
org.hibernate.Session;
import
org.hibernate.SessionFactory;
import
org.hibernate.Transaction;
import
org.hibernate.cfg.Configuration;
public
class
Test
{
public static void main(String[] args) {
String filePath=System.getProperty("user.dir")+File.separator+"src/Version"+File.separator+"hibernate.cfg.xml";
File file=new File(filePath);
System.out.println(filePath);
SessionFactory sessionFactory=new Configuration().configure(file).buildSessionFactory();
Session session=sessionFactory.openSession();
Transaction t=session.beginTransaction();
Student stu=new Student();
stu.setName("tom11");
session.save(stu);
t.commit();
/*
* 模拟多个session操作student数据表
*/
Session session1=sessionFactory.openSession();
Session session2=sessionFactory.openSession();
Student stu1=(Student)session1.createQuery("from Student s where s.name='tom11'").uniqueResult();
Student stu2=(Student)session2.createQuery("from Student s where s.name='tom11'").uniqueResult();
//这时候,两个版本号是相同的
System.out.println("v1="+stu1.getVersion()+"--v2="+stu2.getVersion());
Transaction tx1=session1.beginTransaction();
stu1.setName("session1");
tx1.commit();
//这时候,两个版本号是不同的,其中一个的版本号递增了
System.out.println("v1="+stu1.getVersion()+"--v2="+stu2.getVersion());
Transaction tx2=session2.beginTransaction();
stu2.setName("session2");
tx2.commit();
}
}
运行结果:
Hibernate: insert into studentVersion (ver, name, id) values (?, ?, ?)
Hibernate: select student0_.id as id0_, student0_.ver as ver0_, student0_.name as name0_ from studentVersion student0_ where student0_.name='tom11'
Hibernate: select student0_.id as id0_, student0_.ver as ver0_, student0_.name as name0_ from studentVersion student0_ where student0_.name='tom11'
v1=0--v2=0
Hibernate: update studentVersion set ver=?, name=? where id=? and ver=?
v1=1--v2=0
Hibernate: update studentVersion set ver=?, name=? where id=? and ver=?
Exception in thread "main" org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [Version.Student#4028818316cd6b460116cd6b50830001]
可以看到,第二个“用户”session2修改数据时候,记录的版本号已经被session1更新过了,所以抛出了红色的异常,我们可以在实际应用 中处理这个异常,例如在处理中重新读取数据库中的数据,同时将目前的数据与数据库中的数据展示出来,让使用者有机会比较一下,或者设计程序自动读取新的数 据
注意:
要注意的是,由于乐观锁定是使用系统中的程式来控制,而不是使用资料库中的锁定机制,因而如果有人特意自行更新版本讯息来越过检查,则锁定机制就会无效,例如在上例中自行更改stu的version属性,使之与资料库中的版本号相同的话就不会有错误,像这样版本号被更改,或是由于资料是由外部系统而来,因而版本资讯不受控制时,锁定机制将会有问题,设计时必须注意。
如果手工设置stu.setVersion()自行更新版本以跳过检查,则这种乐观锁就会失效,应对方法可以将Student.Java的setVersion设置成private