Hibernate4中使用getCurrentSession报Could not obtain transaction-synchronized Session for current thread
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架个spring4+hibernate4的demo,dao层直接注入的sessionFactory,然后用getCurrentSession方法获取session,然后问题来了,直接报错:
Could not obtain transaction-synchronized Session for current thread
提示无法获取当前线程的事务同步session,略微奇怪,这和事务有什么关系..然后百度一下有人说改成用openSession方法就好了,那我又百度了一下这2个方法的区别:
(1)openSession每次打开都是新的Session,所以多次获取的Session实例是不同的,并且需要人为的调用close方法进行Session关闭。(2)getCurrentSession是从当前上下文中获取Session并且会绑定到当前线程,第一次调用时会创建一个Session实例,如果该Session未关闭,后续多次获取的是同一个Session实例;事务提交或者回滚时会自动关闭Sesison,无需人工关闭。
看起来这个getCurrentSession方法的确和事务有点关系.然后我加上事务:
<bean id="transactionManager" class="org.springframework.orm.hibernate4.HibernateTransactionManager"> <property name="sessionFactory"> <ref bean="sessionFactory" /> </property> </bean> <tx:annotation-driven transaction-manager="transactionManager"/>
然后在dao层加上@Transaction注解,再运行ok了..不过好奇驱使吧,看了一下getCurrentSession的源码(我的demo中用的spring的实现类),关键点:
if (TransactionSynchronizationManager.isSynchronizationActive()) { Session session = this.sessionFactory.openSession(); if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) { session.setFlushMode(FlushMode.MANUAL); } SessionHolder sessionHolder = new SessionHolder(session); TransactionSynchronizationManager.registerSynchronization( new SpringSessionSynchronization(sessionHolder, this.sessionFactory, true)); TransactionSynchronizationManager.bindResource(this.sessionFactory, sessionHolder); sessionHolder.setSynchronizedWithTransaction(true); return session; } else { throw new HibernateException("Could not obtain transaction-synchronized Session for current thread"); }
然后点进去看了一下isSynchronizationActive()方法:
public static boolean isSynchronizationActive() { return (synchronizations.get() != null); }
get方法说明:
Returns the value in the current thread's copy of this thread-local variable. If the variable has no value for the current thread, it is first initialized to the value returned by an invocation of the initialValue method.
然后再看initialValue的说明:
This implementation simply returns null; if the programmer desires thread-local variables to have an initial value other than null, ThreadLocal must be subclassed, and this method overridden. Typically, an anonymous inner class will be used.
到此问题明了了,补充一点之前配置文件中配了事务,不过是原来那种在配置中根据方法名字来定义事务传播的方式,但是在dao中并没有继承它,故实际上是没有事务的,只有实现.而在spring的事务实现中需要判断当前线程中的事务是否同步,而没有事务的时候,那个判断是否同步的方法会因为get返回初始的null值而返回false,最终导致throw一个Could not obtain transaction-synchronized Session for current thread的异常.
综上:spring4+hibernate4,使用hibernate的api的时候需要配置事务的,如果不配置事务会导致获取当前session抛出异常.
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