Hibernate的悲观锁、乐观锁

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> 

HIbernateTest

public class HibernateTest {@Testpublic void testCreateDB(){Configuration cfg=new Configuration().configure();SchemaExport se=new SchemaExport(cfg);se.create(true, true);}@Testpublic 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();}@Testpublic 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();}@Testpublic 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();}}

上面代码中testGet和testGet1是两个测试悲观锁的方法，有两个session，如果同时启动，就相当于同时有两个线程启动，当第一个session执行了

User user=(User) session.get(User.class, 1, LockOptions.UPGRADE);

这句代码，后面的线程需要等待，直到这个session关闭之后，其他线程才能继续执行，以此类推。

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}

User.hbm.xml配置

<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 {@Testpublic void testCreateDB(){Configuration cfg=new Configuration().configure();SchemaExport se=new SchemaExport(cfg);se.create(true, true);}@Testpublic 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();}@Testpublic 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();      }  }@Testpublic 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 (?, ?, ?)

数据库表信息

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。