Hibernate悲观锁/乐观锁

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 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

Hibernate配置文件中，version节点需要在id节点之后并紧跟id节点

<?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=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)

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

timestamp节点没有type属性：

<?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