hibernate框架一对多之放弃外键的维护（十五）_hibernate取消外键约束-优快云博客

本文探讨了在Hibernate中一对多关系中外键的维护问题。内容包括双方维护外键导致的多余SQL语句及其原因，并详细介绍了如何通过设置`inverse="true"`来放弃外键的维护，以避免不必要的数据库操作。通过测试代码和运行结果，解释了为何在多对多关系中必须放弃一方维护外键，并以领导与下属的关系为例，阐述了选择放弃维护的一方的理由。

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在上一篇文章中，Customer中有private Set<Linkman> linkmans = new HashSet<Linkman>(); Linkman中有private Customer customer; 也就是两者都维护了外键。

可以放弃一方面的外键的维护。

一对多可以不用放弃。多对多必须放弃，不然程序会报错。

1. 先测试双方都维护外键的时候，会产生多余的SQL语句。
* 想修改客户和联系人的关系，进行双向关联，双方都会维护外键，会产生多余的SQL语句。
* 产生的原因：session的一级缓存中的快照机制，会让双方都更新数据库，产生了多余的SQL语句。
2. 如果不想产生多余的SQL语句，那么需要一方来放弃外键的维护！
* 在<set>标签上配置一个inverse=”true”.true:放弃.false:不放弃.默认值是false
* <inverse="true">

接着上一篇的项目。在customer表中添加一个记录xiaofeng。然后，让xiongda变成xiaofeng的联系人。

把customer和linkman的cascade都删除

一、双方维护外键

1.1 测试代码

	/**
	 * 放弃外键的维护 需求：让xiongda联系人属于xiaofeng客户
	 */
	@Test
	public void run11() {
		Session session = HibernateUtils.getCurrentSession();
		Transaction tx = session.beginTransaction();

		// 先获取客户
		Customer c2 = session.get(Customer.class, 2L);
		Linkman l1 = session.get(Linkman.class, 1L);

		// 做双向的关联
		c2.getLinkmans().add(l1);
		l1.setCustomer(c2);

		tx.commit();
	}

1.2 运行结果

看sql：

log4j:WARN No appenders could be found for logger (org.jboss.logging).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Hibernate: 
    select
        customer0_.cust_id as cust_id1_0_0_,
        customer0_.cust_name as cust_nam2_0_0_,
        customer0_.cust_user_id as cust_use3_0_0_,
        customer0_.cust_create_id as cust_cre4_0_0_,
        customer0_.cust_source as cust_sou5_0_0_,
        customer0_.cust_industry as cust_ind6_0_0_,
        customer0_.cust_level as cust_lev7_0_0_,
        customer0_.cust_linkman as cust_lin8_0_0_,
        customer0_.cust_phone as cust_pho9_0_0_,
        customer0_.cust_mobile as cust_mo10_0_0_ 
    from
        cst_customer customer0_ 
    where
        customer0_.cust_id=?
Hibernate: 
    select
        linkman0_.lkm_id as lkm_id1_1_0_,
        linkman0_.lkm_name as lkm_name2_1_0_,
        linkman0_.lkm_gender as lkm_gend3_1_0_,
        linkman0_.lkm_phone as lkm_phon4_1_0_,
        linkman0_.lkm_mobile as lkm_mobi5_1_0_,
        linkman0_.lkm_email as lkm_emai6_1_0_,
        linkman0_.lkm_qq as lkm_qq7_1_0_,
        linkman0_.lkm_position as lkm_posi8_1_0_,
        linkman0_.lkm_memo as lkm_memo9_1_0_,
        linkman0_.lkm_cust_id as lkm_cus10_1_0_ 
    from
        cst_linkman linkman0_ 
    where
        linkman0_.lkm_id=?
Hibernate: 
    select
        linkmans0_.lkm_cust_id as lkm_cus10_1_0_,
        linkmans0_.lkm_id as lkm_id1_1_0_,
        linkmans0_.lkm_id as lkm_id1_1_1_,
        linkmans0_.lkm_name as lkm_name2_1_1_,
        linkmans0_.lkm_gender as lkm_gend3_1_1_,
        linkmans0_.lkm_phone as lkm_phon4_1_1_,
        linkmans0_.lkm_mobile as lkm_mobi5_1_1_,
        linkmans0_.lkm_email as lkm_emai6_1_1_,
        linkmans0_.lkm_qq as lkm_qq7_1_1_,
        linkmans0_.lkm_position as lkm_posi8_1_1_,
        linkmans0_.lkm_memo as lkm_memo9_1_1_,
        linkmans0_.lkm_cust_id as lkm_cus10_1_1_ 
    from
        cst_linkman linkmans0_ 
    where
        linkmans0_.lkm_cust_id=?
Hibernate: 
    update
        cst_linkman 
    set
        lkm_name=?,
        lkm_gender=?,
        lkm_phone=?,
        lkm_mobile=?,
        lkm_email=?,
        lkm_qq=?,
        lkm_position=?,
        lkm_memo=?,
        lkm_cust_id=? 
    where
        lkm_id=?
Hibernate: 
    update
        cst_linkman 
    set
        lkm_cust_id=? 
    where
        lkm_id=?

可以看到有两条修改语句。因为在customer中有set集合维护外键，在linkman中有Customer维护了外键。

如何避免这种情况呢？只能是一方去放弃外键的维护。

为什么是一方放弃呢？比如，1个领导，n个下属。如果领导去维护外键的话，要维护n个，麻烦；如果，下属去维护的话，下属只要找到这个领导，简单。

二、放弃外键的维护

2.1 配置

2.2 测试代码

	@Test
	public void run11() {
		Session session = HibernateUtils.getCurrentSession();
		Transaction tx = session.beginTransaction();

		// 先获取客户
		Customer c2 = session.get(Customer.class, 2L);
		Linkman l1 = session.get(Linkman.class, 1L);

		// 做双向的关联
		c2.getLinkmans().add(l1);
		l1.setCustomer(c2);

		tx.commit();
	}

2.3 运行结果

看sql语句：

log4j:WARN No appenders could be found for logger (org.jboss.logging).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Hibernate: 
    select
        customer0_.cust_id as cust_id1_0_0_,
        customer0_.cust_name as cust_nam2_0_0_,
        customer0_.cust_user_id as cust_use3_0_0_,
        customer0_.cust_create_id as cust_cre4_0_0_,
        customer0_.cust_source as cust_sou5_0_0_,
        customer0_.cust_industry as cust_ind6_0_0_,
        customer0_.cust_level as cust_lev7_0_0_,
        customer0_.cust_linkman as cust_lin8_0_0_,
        customer0_.cust_phone as cust_pho9_0_0_,
        customer0_.cust_mobile as cust_mo10_0_0_ 
    from
        cst_customer customer0_ 
    where
        customer0_.cust_id=?
Hibernate: 
    select
        linkman0_.lkm_id as lkm_id1_1_0_,
        linkman0_.lkm_name as lkm_name2_1_0_,
        linkman0_.lkm_gender as lkm_gend3_1_0_,
        linkman0_.lkm_phone as lkm_phon4_1_0_,
        linkman0_.lkm_mobile as lkm_mobi5_1_0_,
        linkman0_.lkm_email as lkm_emai6_1_0_,
        linkman0_.lkm_qq as lkm_qq7_1_0_,
        linkman0_.lkm_position as lkm_posi8_1_0_,
        linkman0_.lkm_memo as lkm_memo9_1_0_,
        linkman0_.lkm_cust_id as lkm_cus10_1_0_ 
    from
        cst_linkman linkman0_ 
    where
        linkman0_.lkm_id=?
Hibernate: 
    select
        linkmans0_.lkm_cust_id as lkm_cus10_1_0_,
        linkmans0_.lkm_id as lkm_id1_1_0_,
        linkmans0_.lkm_id as lkm_id1_1_1_,
        linkmans0_.lkm_name as lkm_name2_1_1_,
        linkmans0_.lkm_gender as lkm_gend3_1_1_,
        linkmans0_.lkm_phone as lkm_phon4_1_1_,
        linkmans0_.lkm_mobile as lkm_mobi5_1_1_,
        linkmans0_.lkm_email as lkm_emai6_1_1_,
        linkmans0_.lkm_qq as lkm_qq7_1_1_,
        linkmans0_.lkm_position as lkm_posi8_1_1_,
        linkmans0_.lkm_memo as lkm_memo9_1_1_,
        linkmans0_.lkm_cust_id as lkm_cus10_1_1_ 
    from
        cst_linkman linkmans0_ 
    where
        linkmans0_.lkm_cust_id=?
Hibernate: 
    update
        cst_linkman 
    set
        lkm_name=?,
        lkm_gender=?,
        lkm_phone=?,
        lkm_mobile=?,
        lkm_email=?,
        lkm_qq=?,
        lkm_position=?,
        lkm_memo=?,
        lkm_cust_id=? 
    where
        lkm_id=?