多个用户同时访问一个数据库，则当他们的事务同时使用相
同的数据时可能会发生问题。由于并发操作带来的数据不一致性包括：丢失数据
修改、读
”
脏
”
数据（脏读）、不可重复读、产生幽灵数据
多个用户同时访问一个数据库，当他们的事务同时使用相同的数据时可能会发生问题。由于并发操作带来的数据不一致性包括丢失数据、修改、脏读、不可重复读、产生幽灵数据。为了解决并发问题hibernate提出了悲观锁和乐观锁来解决问题。
悲观锁
新建一个java项目，项目结构如下
User类
public class User {
private int id;
private String name;
private String pwd;
//省略get/set
}User.hbm.xml配置
<hibernate-mapping package="com.test.pojo">
<class name="User" table="user">
<id name="id">
<generator class="identity" />
</id>
<property name="name" />
<property name="pwd" />
</class>
</hibernate-mapping> public class HibernateTest {
@Test
public void testCreateDB(){
Configuration cfg=new Configuration().configure();
SchemaExport se=new SchemaExport(cfg);
se.create(true, true);
}
@Test
public void testSave(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
User user=new User();
user.setName("张三");
user.setPwd("1111");
session.save(user);
tx.commit();
HibernateUtil.closeSession();
}
@Test
public void testGet(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
User user=(User) session.get(User.class, 1, LockOptions.UPGRADE);
System.out.println(user.getName());
user.setName("李四");
session.update(user);
tx.commit();
HibernateUtil.closeSession();
}
@Test
public void testGet1(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
User user=(User) session.get(User.class, 1,LockOptions.UPGRADE);
System.out.println(user.getName());
user.setName("李四");
session.update(user);
tx.commit();
HibernateUtil.closeSession();
}
}User user=(User) session.get(User.class, 1, LockOptions.UPGRADE);乐观锁
乐观锁和悲观锁不同的是，乐观锁不是排他性，而是使用了一个版本version来记录，当数据被更改后，version会更改。
新建java项目，项目结构如下：
User代码
public class User {
private int id;
private String name;
private String pwd;
private int version;
//省略get/set
}<hibernate-mapping package="com.test.pojo">
<class name="User" table="user">
<id name="id">
<generator class="identity" />
</id>
<version name="version"/>
<property name="name" />
<property name="pwd" />
</class>
</hibernate-mapping> HibernateTest测试代码
public class HibernateTest {
@Test
public void testCreateDB(){
Configuration cfg=new Configuration().configure();
SchemaExport se=new SchemaExport(cfg);
se.create(true, true);
}
@Test
public void testSave(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
User user=new User();
user.setName("张三");
user.setPwd("1111");
session.save(user);
tx.commit();
HibernateUtil.closeSession();
}
@Test
public void testGet(){
Session session = null;
Transaction tx = null;
User user = null;
try {
session = HibernateUtil.getSession();
tx = session.beginTransaction();
user = (User) session.get(User.class, 1) ;
user.setName("李四") ;
Session session1 = HibernateUtil.getSession() ;
Transaction tx1 = session1.beginTransaction() ;
User user1 = (User) session.get(User.class, 1) ;
user1.setName("李四2") ;
session1.update(user1) ;
tx1.commit() ;
session.update(user) ;
tx.commit();
} catch (Exception e) {
e.printStackTrace();
} finally {
HibernateUtil.closeSession();
}
}
@Test
public void testGet1(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
User user=(User) session.get(User.class, 1);
System.out.println(user.getName());
user.setName("李四");
session.update(user);
tx.commit();
HibernateUtil.closeSession();
}
}
执行testSave方法，控制台打印信息
Hibernate: insert into user (version, name, pwd) values (?, ?, ?)
执行testGet1，控制台打印信息
Hibernate: select user0_.id as id1_0_0_, user0_.version as version2_0_0_, user0_.name as name3_0_0_,
user0_.pwd as pwd4_0_0_ from user user0_ where user0_.id=?
张三
Hibernate: update user set version=?, name=?, pwd=? where id=? and version=?数据库表信息
由图可以看出id和version都发生了改变。
再来测试testGet，控制台打印信息
Hibernate: select user0_.id as id1_0_0_, user0_.version as version2_0_0_, user0_.name as name3_0_0_, user0_.pwd as pwd4_0_0_ from user user0_ where user0_.id=?
Hibernate: update user set version=?, name=?, pwd=? where id=? and version=?
Hibernate: update user set version=?, name=?, pwd=? where id=? and version=?
org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) : [com.test.pojo.User#1]
at org.hibernate.persister.entity.AbstractEntityPersister.check(AbstractEntityPersister.java:2541)
at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3285)
at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:3183)
at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3525)
at org.hibernate.action.internal.EntityUpdateAction.execute(EntityUpdateAction.java:159)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:465)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:351)
at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:350)
at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:56)
at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1258)
at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:425)
at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:177)
at com.test.test.HibernateTest.testGet(HibernateTest.java:52)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
数据库表信息如下
由数据库中数据得知是user1数据更新到了数据库中，user的数据没有更新到数据库，报错了，这是因为user和user1都从数据库中查询出了version是1的数据，但是user1先更新了数据，然后提交到数据库中，此时该数据库中该数据的version变为了2，等到user也更新数据时，where条件中的version没有找到version是1的对应的数据，所以就报错了。
总结：
1、悲观锁：安全，并发效率低
2、乐观锁：安全性比悲观锁低，并发效率高
如果数据需要大量修改，适用悲观锁。
如果数据是用来读取的，适用乐观锁。
