数据的保存顺序与Hibernate对数据库操作的影响

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public void Map_Add() {
        Session session = null;
        Transaction tx = null;
        try {

            City city1 = new City();
            city1.setName("中国·唐山");
            city1.setPostcode("063009");

            City city2 = new City();
            city2.setName("中国·天津");
            city2.setPostcode("356148");

            Map<String, City> citys = new HashMap<String, City>();
            citys.put("唐山", city1);
            citys.put("天津", city2);

            Nation nation = new Nation();
            nation.setName("中国");

            nation.setCitys(citys);
//            当Nation.hbm.xml文件中map节点的inverse = "false"或不写时（即默认，false）
//            这行代码的作用是Hibernate可以根据nation中的citys去更新city表中的CITYNAME
//            如果没有setCitys则city表中的CITYNAME为null
//            当Nation.hbm.xml文件中map节点的inverse = "true"时不起作用
            
            
            
            city1.setNation(nation);
            city2.setNation(nation);

            session = HibernateUtil.getSession();
            tx = session.beginTransaction();
            session.save(nation);
            session.save(city1);
            session.save(city2);
            tx.commit();
        } catch (Exception ex) {
            Logger.getLogger(Many2OneAndOne2Many.class.getName()).log(Level.SEVERE, null, ex);
            if (tx != null) {
                tx.rollback();
            }
        } finally {
            if (session != null) {
                session.close();
            }
        }

Hibernate打印的sql语句：

Hibernate: insert into nation (NATION_NAME) values (?)

Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)

Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)

Hibernate: update city set NATION_ID=?, CITYNAME=? where CITY_ID=?

Hibernate: update city set NATION_ID=?, CITYNAME=? where CITY_ID=?

  public void Map_Add() {
        Session session = null;
        Transaction tx = null;
        try {

            City city1 = new City();
            city1.setName("中国·唐山");
            city1.setPostcode("063009");

            City city2 = new City();
            city2.setName("中国·天津");
            city2.setPostcode("356148");

            Map<String, City> citys = new HashMap<String, City>();
            citys.put("唐山", city1);
            citys.put("天津", city2);

            Nation nation = new Nation();
            nation.setName("中国");

            nation.setCitys(citys);
//            当Nation.hbm.xml文件中map节点的inverse = "false"或不写时（即默认，false）
//            这行代码的作用是Hibernate可以根据nation中的citys去更新city表中的CITYNAME
//            如果没有setCitys则city表中的CITYNAME为null
//            当Nation.hbm.xml文件中map节点的inverse = "true"时不起作用
            
            
            
            city1.setNation(nation);
            city2.setNation(nation);

            session = HibernateUtil.getSession();
            tx = session.beginTransaction();
            session.save(city1);
            session.save(city2);
            session.save(nation);            
            tx.commit();
        } catch (Exception ex) {
            Logger.getLogger(Many2OneAndOne2Many.class.getName()).log(Level.SEVERE, null, ex);
            if (tx != null) {
                tx.rollback();
            }
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }

Hibernate打印的sql语句：

Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)

Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)

Hibernate: insert into nation (NATION_NAME) values (?)

Hibernate: update city set CITY_NAME=?, POST_CODE=?, NATION_ID=? where CITY_ID=?

Hibernate: update city set CITY_NAME=?, POST_CODE=?, NATION_ID=? where CITY_ID=?

Hibernate: update city set NATION_ID=?, CITYNAME=? where CITY_ID=?

Hibernate: update city set NATION_ID=?, CITYNAME=? where CITY_ID=?

以下的两条更新语句是由于先保存的City而引起的：

Hibernate: update city set CITY_NAME=?, POST_CODE=?, NATION_ID=? where CITY_ID=?

Hibernate: update city set CITY_NAME=?, POST_CODE=?, NATION_ID=? where CITY_ID=?

原因是由于City依赖于Nation的NATION_ID，所以在保存City时如果没有保存Nation，则会在保存完Nation后更新City的NATION_ID字段。

所以建议将依赖的数据先保存（这里就是City依赖Nation），这样可以减少Hibernate对于数据库的操作，从而减少资源的消耗。