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Hibernate 关联映射 之 多对多 关联（一）
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1、了解幾個知識點：
一、一般的設計中，多對多關聯映射，需要一個中間表
二、Hibernate會自動生成中間表
三、Hibernate使用many-to-many標簽來表示多對多的關聯
四、多對多的關聯映射，在實體類中，跟一對多一樣，也是用集合來表示的
2、示例代碼（用戶和角色之間的關系是多對多）
用戶的示例代碼：
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public?class?User?{????????private?int?id;??????private?String?name;??????private?String?password;??????private?Set<Role>?roles;??
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<hibernate-mapping>??????<class?name="com.shangxuetang.User"?table="t_user">??????????<id?name="id">??????????????<generator?class="native"/>??????????</id>??????????<property?name="name"></property>??????????<property?name="password"></property>??????????<set?name="roles"?table="t_user_group">?????????????<key?column="userId"></key>?????????????<many-to-many?class="com.shangxuetang.Role"?column="roleId"></many-to-many>??????????</set>??????</class>??</hibernate-mapping>??
需要注意的是：映射文件中屬性table是必須有的；many-to-many 中屬性column是必須有的（中間映射表中，關聯目標表的關聯字段），class是必須有的（關聯目標類）
[java] view plain
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public?class?Role?{??????private?int?id;??????private?String?name;??????private?Set<User>?users;??
[html] view plain
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<hibernate-mapping>??????<class?name="com.shangxuetang.Role"?table="t_role">??????????<id?name="id">??????????????<generator?class="native"/>??????????</id>??????????<property?name="name"></property>??????????<set?name="users"?table="t_user_group">??????????????<key?column="roleId"></key>??????????????<many-to-many?class="com.shangxuetang.User"?column="userId"></many-to-many>??????????</set>??????</class>??</hibernate-mapping>??
測試示例代碼：
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public?void??testManytoMany()?{??????????Session?session?=?HibernateUtil.getSession();??????????try?{??????????????session.beginTransaction();????????????????????????????User?u1?=?new?User("zhangsan");??????????????session.save(u1);??????????????User?u2?=?new?User("lisi");??????????????session.save(u2);????????????????????????????Role?r1?=?new?Role("管理員");??????????????session.save(r1);??????????????Role?r2?=?new?Role("普通用戶");??????????????session.save(r2);??????????????Role?r3?=?new?Role("VIP用戶");??????????????session.save(r3);????????????????????????????u1.addRole(r1).addRole(r3);??????????????u2.addRole(r2);????????????????????????????r1.addUser(u2);????????????????????????????session.getTransaction().commit();??????????}catch(Exception?e)?{??????????????e.printStackTrace();??????????????session.getTransaction().rollback();??????????}finally?{??????????????session.close();??????????}??????}??
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