Spring4+Hibernate4+测试驱动开发时发现的问题：No session (下)
昨天发现的问题今天又看了一下，Hibernate的SessionFactory接口的实现类是SessionFactoryImpl。它在创建Session时候的工作原理如下：
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当调用SessionFactory.getCurrentSession的时候，它会调用CurrentSessionContext接口子类实例的currentSession()方法来获得Session，那CurrentSessionContext子类实例是怎么创建的呢？
SpringFactoryImpl会根据配置文件制定的"hibernate.current_session_context_class"这个配置项来决定实例化哪种CurrentSessionContext的子类实例。SessionFactoryImpl会通过先调用buildCurrentSessionContext()方法加载session管理的策略，然后生成对应策略的CurrentSessionContext子类实例以供SessionFactoryImpl使用。
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以下是buildCurrentSessionContext()方法的代码：
private CurrentSessionContext buildCurrentSessionContext() {String impl = properties.getProperty( Environment.CURRENT_SESSION_CONTEXT_CLASS );// for backward-compatibilityif ( impl == null ) {if ( canAccessTransactionManager() ) {impl = "jta";}else {return null;}}if ( "jta".equals( impl ) ) {if ( ! transactionFactory().compatibleWithJtaSynchronization() ) {LOG.autoFlushWillNotWork();}return new JTASessionContext( this );}else if ( "thread".equals( impl ) ) {return new ThreadLocalSessionContext( this );}else if ( "managed".equals( impl ) ) {return new ManagedSessionContext( this );}else {try {Class implClass = serviceRegistry.getService( ClassLoaderService.class ).classForName( impl );return ( CurrentSessionContext ) implClass.getConstructor( new Class[] { SessionFactoryImplementor.class } ).newInstance( this );}catch( Throwable t ) {LOG.unableToConstructCurrentSessionContext( impl, t );return null;}}}?在我上一篇博客提到，我没有设置任何hibernate.current_session_context_class的配置，这个时候为什么没报异常呢？通过Debug它的代码，发现Spring4在默认情况下会设org.springframework.orm.hibernate4.SpringSessionContext这个值作为hibernate.current_session_context_class属性的值，也就是说在默认情况下，SpringSessionContext作为Spring4-orm的Session生成策略类。
再看以下SpringSessionContext的currentSession()方法，代码如下：
public Session currentSession() throws HibernateException {Object value = TransactionSynchronizationManager.getResource(this.sessionFactory);if (value instanceof Session) {return (Session) value;}else if (value instanceof SessionHolder) {SessionHolder sessionHolder = (SessionHolder) value;Session session = sessionHolder.getSession();if (TransactionSynchronizationManager.isSynchronizationActive() &&!sessionHolder.isSynchronizedWithTransaction()) {TransactionSynchronizationManager.registerSynchronization(new SpringSessionSynchronization(sessionHolder, this.sessionFactory));sessionHolder.setSynchronizedWithTransaction(true);// Switch to FlushMode.AUTO, as we have to assume a thread-bound Session// with FlushMode.MANUAL, which needs to allow flushing within the transaction.FlushMode flushMode = session.getFlushMode();if (flushMode.equals(FlushMode.MANUAL) &&!TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {session.setFlushMode(FlushMode.AUTO);sessionHolder.setPreviousFlushMode(flushMode);}}return session;}else if (this.jtaSessionContext != null) {Session session = this.jtaSessionContext.currentSession();if (TransactionSynchronizationManager.isSynchronizationActive()) {TransactionSynchronizationManager.registerSynchronization(new SpringFlushSynchronization(session));}return session;}else {throw new HibernateException("No Session found for current thread");}}?如果我们没有配置事务，?
Object?value?=?TransactionSynchronizationManager.getResource(this.sessionFactory);????
这个值会是null，问题就出在这里。
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