openSession和getCurrentSession区别:
深入讨论：
在SessionFactory启动的时候，Hibernate会根据配置创建相应的CurrentSessionContext
在getCurrentSession()被调用的时候，实际上执行的方法是：CurrentSessionContext.currentSession()
在currentSession()执行时，如果当前Session为空，currentSession会调用SessionFactory的openSession
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* private static SessionFactory sessionFactory = null;
* /*使用线程局部模式*/
* private static ThreadLocal<Session> threadLocal = new ThreadLocal<Session>();
* static{
* sessionFactory = new Configuration().configure().buildSessionFactory();
* }
* /**
* * 获取和线程关联的session
* * @return
* */
* public static Session getCurrentSession(){
* Session session = threadLocal.get();
* /*判断是否得到*/
* if(session==null || !session.isOpen()){
* session = sessionFactory.openSession();
* //把session对象设置到threadLocal，相当于该session和线程绑定
* threadLocal.set(session);
* }
* return session;
* }
* /**
* * 获取全新的session
* * @return
* */
* public static Session openSession(){
* return sessionFactory.openSession();
* }
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