http://blog.csdn.net/a19881029/article/details/20665663
如果需要保证数据访问的排它性，则需对目标数据加“锁”，使其无法被其它程序修改
一，悲观锁
对数据被外界（包括本系统当前的其它事务和来自外部系统的事务处理）修改持保守态度，通过数据库提供的锁机制实现
最常用的，是对查询进行加锁（LockMode.UPGRADE和LockMode.UPGRADE_NOWAIT）：
public class Test {public static void main(String[] args) {Configuration conf = new Configuration(); SessionFactory sessionFactory = conf.configure().buildSessionFactory(); Session sess = sessionFactory.openSession();Transaction tran = sess.beginTransaction();String hql = "from User where id = 1";Query query = sess.createQuery(hql);query.setLockOptions(LockOptions.UPGRADE);List<User> list = query.list();for(User user : list){System.out.print(user.getName()+" ");}System.out.println();tran.commit();sess.close(); }
}
Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.age as age0_ from TEST_USER user0_ where user0_.id=1 for update
longlong
通过for update子句，这条SQL锁定了TEST_USER表中符合检索条件的记录，本次事务提交前，外界无法修改这些记录，事务提交时会释放事务过程中的锁
Hibernate提供了2个锁对象，LockMode和LockOptions：
通过LockOptions的源代码，可以发现LockOptions只是LockMode的简单封装（在LockMode的基础上提供了timeout和scope）：
......
/*** NONE represents LockMode.NONE (timeout + scope do not apply)*/
public static final LockOptions NONE = new LockOptions(LockMode.NONE);/*** READ represents LockMode.READ (timeout + scope do not apply)*/
public static final LockOptions READ = new LockOptions(LockMode.READ);/*** UPGRADE represents LockMode.UPGRADE (will wait forever for lock and* scope of false meaning only entity is locked)*/
public static final LockOptions UPGRADE = new LockOptions(LockMode.UPGRADE);public LockOptions(){}public LockOptions( LockMode lockMode) {this.lockMode = lockMode;
}
.....
public static final int NO_WAIT = 0;/*** Indicates that there is no timeout for the acquisition.* @see #getTimeOut*/
public static final int WAIT_FOREVER = -1;private int timeout = WAIT_FOREVER;private boolean scope=false;
...... LockOptions提供的加锁机制要比LockMode少很多，但是LockMode多出的加锁机制一般只是供Hibernate内部实现使用的
保证了操作的独占性，但严重影响数据库性能
二，乐观锁
乐观锁大多基于数据版本记录机制实现，既为数据增加一个版本标识
在数据库中增加version列，用来记录每行数据的版本
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC"-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" ><hibernate-mapping><class name="com.po.User" table="TEST_USER"><id name="id" column="id" type="java.lang.Integer"><generator class="assigned"/></id><version name="version"column="version"type="java.lang.Integer"/><property name="name"column="name"type="java.lang.String"not-null="true"unique="true"length="20"/><property name="age"column="age"type="java.lang.Integer"not-null="true"unique="false"length="0"/></class>
</hibernate-mapping>
public class Test {public static void main(String[] args) {Configuration conf = new Configuration(); SessionFactory sessionFactory = conf.configure().buildSessionFactory();Session sess1=sessionFactory.openSession();Session sess2=sessionFactory.openSession();try{User user1 = (User)sess1.get(User.class, 1);User user2 = (User)sess2.get(User.class, 1);System.out.println("v1="+user1.getVersion()+"--v2="+user2.getVersion());Transaction tx1 = sess1.beginTransaction();Transaction tx2 = sess2.beginTransaction();user1.setName("ll");tx1.commit();System.out.println("v1="+user1.getVersion()+"--v2="+user2.getVersion());user2.setName("LL");tx2.commit();}catch(Exception e){e.printStackTrace();}finally{sess1.close();sess2.close();}}
}
Hibernate: select user0_.id as id0_0_, user0_.version as version0_0_, user0_.name as name0_0_, user0_.age as age0_0_ from TEST_USER user0_ where user0_.id=?
Hibernate: select user0_.id as id0_0_, user0_.version as version0_0_, user0_.name as name0_0_, user0_.age as age0_0_ from TEST_USER user0_ where user0_.id=?
v1=0--v2=0
Hibernate: update TEST_USER set version=?, name=?, age=? where id=? and version=?
v1=1--v2=0
Hibernate: update TEST_USER set version=?, name=?, age=? where id=? and version=?
Exception in thread "main" org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [com.po.User#1]at org.hibernate.persister.entity.AbstractEntityPersister.check(AbstractEntityPersister.java:1932)at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2576)at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:2476)at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2803)at org.hibernate.action.EntityUpdateAction.execute(EntityUpdateAction.java:113)at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:273)at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:265)at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:185)at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:51)at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1216)at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:383)at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:133)at com.test.Test.main(Test.java:43)
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC"-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" ><hibernate-mapping><class name="com.po.User" table="TEST_USER"><id name="id" column="id" type="java.lang.Integer"><generator class="assigned"/></id><timestamp name="updatetime"column="updatetime"/><property name="name"column="name"type="java.lang.String"not-null="true"unique="true"length="20"/><property name="age"column="age"type="java.lang.Integer"not-null="true"unique="false"length="0"/></class>
</hibernate-mapping>在某些情况下，不允许修改数据库的表结构，此时Hibernate也有相应的处理手段：
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC"-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" ><hibernate-mapping><class name="com.po.User" table="TEST_USER"optimistic-lock="all"dynamic-update="true"dynamic-insert="true"><id name="id" column="id" type="java.lang.Integer"><generator class="assigned"/></id><property name="name"column="name"type="java.lang.String"not-null="true"unique="true"length="20"/><property name="age"column="age"type="java.lang.Integer"not-null="true"unique="false"length="0"/></class>
</hibernate-mapping>此时Hibernate将使用User类的所有字段作为版本控制信息
乐观锁相较悲观锁提高了不少性能，但是有一定的局限性，由于是在应用层加锁，如果此时在数据中直接修改数据（或其它应用程序修改数据库中的数据），应用层是无法感知到这种变化的，需要配合其它技术手段一起使用
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