Hibernate之悲观锁与乐观锁

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会在生成的SQL后面加上for update子句：

Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.age as age0_ from TEST_USER user0_ where user0_.id=1 for updatelonglong 

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>

每次更新User对象时时，对应行的version字段都在增加

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();}}}

运行结果如下，可以看到由于tx1提交时，version字段已经被修改，tx2提交时会抛出异常：

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=0Hibernate: update TEST_USER set version=?, name=?, age=? where id=? and version=?v1=1--v2=0Hibernate: 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)

除了使用version作为版本标识，还可以使用timestamp作为版本标识

<?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