Hibernate三种状态：瞬时状态、持久化状态、离线状态

三种状态及相互转化

瞬时状态(transient)：刚new出来一个对象，还没有被保存到数据库中

持久化状态(persistent)：已经被保存到数据库中或刚从数据库中取得

离线状态(detached)：数据库中有，但是session中不存在该对象

方法的代码实例(下面代码1-8是连贯的，但由于需要分条说明，因此拆开)

(1) save()：把一个对象从[瞬时状态]转换成[持久化状态](要注意DB里没有ID重复的该条数据)。

        SessionFactory sessionFactory = SessionFactoryBuilder.getInstance();
        Session session = sessionFactory.openSession();
        session.beginTransaction();
        TestUser user1 = new TestUser("1", "name1"); // user变成瞬时状态
        session.save(user1); // user变成持久化状态
        session.getTransaction().commit(); // 发出insert语句
        session.close();
        // [输出LOG]
        // Hibernate: insert into test_user (user_name, user_id) values (?, ?)

(2) update()：把一个对象从[瞬时状态]转换成[持久化状态](要注意DB里要有ID重复的该条数据)。

         session = sessionFactory.openSession();
         session.beginTransaction();
         TestUser user2 = new TestUser("1", "name1-update(2)");
         session.update(user2);
         session.getTransaction().commit();
         session.close();
         // [输出LOG]
         // Hibernate: update test_user set user_name=? where user_id=?

(3) saveOrUpdate()：当(1)(2)中不确定DB里是否有ID重复的该条数据时可以用saveOrUpdate()。

         session = sessionFactory.openSession();
         session.beginTransaction();
         TestUser user3 = new TestUser("1", "name1-update(3)");
         TestUser user4 = new TestUser("2", "name2");
         session.saveOrUpdate(user3);
         session.saveOrUpdate(user4);
         session.getTransaction().commit();
         session.close();
         // [输出LOG]
         // Hibernate: select testuser_.user_id, testuser_.user_name as user_nam2_0_ from test_user testuser_ where testuser_.user_id=?
         // Hibernate: select testuser_.user_id, testuser_.user_name as user_nam2_0_ from test_user testuser_ where testuser_.user_id=?
         // Hibernate: insert into test_user (user_name, user_id) values (?, ?)
         // Hibernate: update test_user set user_name=? where user_id=?

(4) 一个commit内先save再多次update，commit时只会做一次insert和一次update。

         session = sessionFactory.openSession();
         session.beginTransaction();
         TestUser user5 = new TestUser("3", "name3");
         session.save(user5);
         user5.setUserName("name3-update(4)-1");
         session.update(user5); // 该条语句可省略
         user5.setUserName("name3-update(4)-2");
         session.update(user5); // 该条语句可省略
         user5.setUserName("name3-update(4)");
         // 在调用了save方法后，此时user已经是持久化对象了，被保存在了session缓存当中，
         // 这时user又重新修改了属性值，那么在commit时，此时hibernate对象就会拿当前这个user对象和保存在session缓存中的user对象进行比较，
         // 如果两个对象相同，则不会发送update语句，否则，如果两个对象不同，则会发出update语句。所以无论多少次update，最终只会发送一次update语句。
         session.getTransaction().commit();
         session.close();
         // [输出LOG]
         // Hibernate: insert into test_user (user_name, user_id) values (?, ?)
         // Hibernate: update test_user set user_name=? where user_id=?

(5) get();load();list();uniqueResult();等查询语句，如果数据库中有该对象，则该对象也变成了[持久化状态]，被session所托管。

         session = sessionFactory.openSession();
         session.beginTransaction();
         TestUser user6 = session.get(TestUser.class, "1");
         TestUser user7 = session.load(TestUser.class, "2");
         @SuppressWarnings("unchecked")
         List<TestUser> user8List = session.createQuery("from TestUser").list();
         user6.setUserName("name1-update(5)");
         user7.setUserName("name2-update(5)");
         user8List.get(2).setUserName("name3-update(5)");
         session.getTransaction().commit();
         session.close();
         // [输出LOG]
         // Hibernate: select testuser0_.user_id as user_id1_0_0_, testuser0_.user_name as user_nam2_0_0_ from test_user testuser0_ where testuser0_.user_id=?
         // Nov 17, 2017 3:14:20 PM org.hibernate.hql.internal.QueryTranslatorFactoryInitiator initiateService
         // INFO: HHH000397: Using ASTQueryTranslatorFactory
         // Hibernate: select testuser0_.user_id as user_id1_0_, testuser0_.user_name as user_nam2_0_ from test_user testuser0_
         // Hibernate: update test_user set user_name=? where user_id=?
         // Hibernate: update test_user set user_name=? where user_id=?
         // Hibernate: update test_user set user_name=? where user_id=?

(6) evict();clear();close();将session的缓存对象清空evict()清除指定对象；clear()&close()清除所有对象。它们使对象从[持久化状态]变成[离线状态]。

         session = sessionFactory.openSession();
         session.beginTransaction();
         TestUser user9 = session.get(TestUser.class, "1");
         TestUser user10 = session.get(TestUser.class, "2");
         user9.setUserName("name1-update(6)");
         user10.setUserName("name2-update(6)");
         session.evict(user9);
         session.clear();
         session.getTransaction().commit();
         session.close();
         // [输出LOG]
         // Hibernate: select testuser0_.user_id as user_id1_0_0_, testuser0_.user_name as user_nam2_0_0_ from test_user testuser0_ where testuser0_.user_id=?
         // Hibernate: select testuser0_.user_id as user_id1_0_0_, testuser0_.user_name as user_nam2_0_0_ from test_user testuser0_ where testuser0_.user_id=?

(7) update();saveOrUpdate();使对象从[离线状态]转成[持久化状态]。

         session = sessionFactory.openSession();
         session.beginTransaction();
         TestUser user11 = session.get(TestUser.class, "1"); // 持久化状态
         user11.setUserName("name1-update(7)");
         session.evict(user11); // 离线状态
         session.update(user11); // 持久化状态
         session.getTransaction().commit();
         session.close();
         // [输出LOG]
         // Hibernate: select testuser0_.user_id as user_id1_0_0_, testuser0_.user_name as user_nam2_0_0_ from test_user testuser0_ where testuser0_.user_id=?
         // Hibernate: update test_user set user_name=? where user_id=?

(8) delete();从[瞬时状态]和[持久化状态]都可以删除。

         session = sessionFactory.openSession();
         session.beginTransaction();
         TestUser user12 = new TestUser("1", "name1");
         session.delete(user12);
         TestUser user13 = session.get(TestUser.class, "2");
         session.delete(user13);
         session.getTransaction().commit();
         session.close();
         // [输出LOG]
         // Hibernate: select testuser_.user_id, testuser_.user_name as user_nam2_0_ from test_user testuser_ where testuser_.user_id=?
         // Hibernate: select testuser0_.user_id as user_id1_0_0_, testuser0_.user_name as user_nam2_0_0_ from test_user testuser0_ where testuser0_.user_id=?
         // Hibernate: delete from test_user where user_id=?
         // Hibernate: delete from test_user where user_id=?