我将尝试使用一个更具体的示例进行解释。假设您有以下情况：
Session session = sessionFactory.openSession();Transaction transaction = session.beginTransaction();User userA = (User)session.get(User.class, 1101);transaction.commit();session.close();// Since session is closed, userA is detached.session = sessionFactory.openSession();transaction = session.beginTransaction();User userB = (User)session.get(User.class, 1101);//Now here, userB represents the same persistent row as userA.//When an attempt to reattach userA occurs, an exception is thrownsession.update(userA);transaction.commit();session.close();
尝试重新附加分离的对象时发生异常，userA。
Exception in thread "main" org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associatedwith the session:This is because Hibernate is enforcing that only a single instance of a Persistent object exists in memory.
为了解决上述问题，使用了merge（），如下所示：
Session session = sessionFactory.openSession();Transaction transaction = session.beginTransaction();User userA = (User)session.get(User.class, 1101);transaction.commit();session.close();//userA is now detached as session is closed.session = sessionFactory.openSession();transaction = session.beginTransaction();User userB = (User)session.get(User.class, 1101);User userC = (User)session.merge(userA);if (userB == userC) { System.out.println("Reattched user is equal");}transaction.commit();session.close();
