Hibernate4中使用getCurrentSession报Could not obtain transaction-synchronized Session for current thread

org.hibernate.HibernateException: Could not obtain transaction-synchronized Session for current thread
at org.springframework.orm.hibernate4.SpringSessionContext.currentSession(SpringSessionContext.java:134)
at org.hibernate.internal.SessionFactoryImpl.getCurrentSession(SessionFactoryImpl.java:941)
at com.web.soulyogaadmin.member.dao.impl.MemberDaoImpl.showOneMember(MemberDaoImpl.java:77)
at com.web.soulyogaadmin.member.service.impl.MemberServiceImpl.showOneMember(MemberServiceImpl.java:47)
at com.web.webservice.member.action.MemberController.findMemberInfoById(MemberController.java:33)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)

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.