员工与角色案例：
一个员工可以是多种角色（总监，经理），一种角色可以是多个员工（保洁）
这里发现无法使用外键表达关系，多对多总是创建第三张表来维护关系
这张表至少两列，都是外键，分别引用两张表的主键
员工（用户）实体类：
package domain;//用户实体import java.util.HashSet;import java.util.Set;public class User { ???private Long user_id; ???private String user_code; ???private String user_name; ???private String user_password; ???private Character user_state; ???//表达多对多 ???private Set<Role> roles = new HashSet<Role>(); ???public Long getUser_id() { ???????return user_id; ???} ???public void setUser_id(Long user_id) { ???????this.user_id = user_id; ???} ???public String getUser_code() { ???????return user_code; ???} ???public void setUser_code(String user_code) { ???????this.user_code = user_code; ???} ???public String getUser_name() { ???????return user_name; ???} ???public void setUser_name(String user_name) { ???????this.user_name = user_name; ???} ???public String getUser_password() { ???????return user_password; ???} ???public void setUser_password(String user_password) { ???????this.user_password = user_password; ???} ???public Character getUser_state() { ???????return user_state; ???} ???public void setUser_state(Character user_state) { ???????this.user_state = user_state; ???} ???public Set<Role> getRoles() { ???????return roles; ???} ???public void setRoles(Set<Role> roles) { ???????this.roles = roles; ???} ???????????}角色实体类：
package domain;//角色实体import java.util.HashSet;import java.util.Set;public class Role { ???private Long role_id; ???private String role_name; ???private String role_memo; ???//表达多对多 ???private Set<User> users = new HashSet<User>(); ???public Long getRole_id() { ???????return role_id; ???} ???public void setRole_id(Long role_id) { ???????this.role_id = role_id; ???} ???public String getRole_name() { ???????return role_name; ???} ???public void setRole_name(String role_name) { ???????this.role_name = role_name; ???} ???public String getRole_memo() { ???????return role_memo; ???} ???public void setRole_memo(String role_memo) { ???????this.role_memo = role_memo; ???} ???public Set<User> getUsers() { ???????return users; ???} ???public void setUsers(Set<User> users) { ???????this.users = users; ???} ???}User.hbm.xml:
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC ????"-//Hibernate/Hibernate Mapping DTD 3.0//EN" ???"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="domain" > ???<class name="User" table="sys_user" > ???????<id name="user_id" ?> ???????????<generator class="native"></generator> ???????</id> ???????<property name="user_code" ?></property> ???????<property name="user_name" ?></property> ???????<property name="user_password" ?></property> ???????<property name="user_state" ?></property> ???????????<!-- 多对多关系表达 --> ???????<!-- ????????????name: 集合属性名 ???????????table: 配置中间表名 ???????????key ????????????|-column:外键,别人引用"我"的外键列名 ????????????class: 我与哪个类是多对多关系 ????????????column:外键.我引用别人的外键列名 ????????--> ???????<set name="roles" table="sys_user_role"> ???????????<key column="user_id" ></key> ???????????<many-to-many class="Role" column="role_id" ></many-to-many> ???????</set> ???????</class></hibernate-mapping>
Role.hbm.xml:
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC ????"-//Hibernate/Hibernate Mapping DTD 3.0//EN" ???"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="domain" > ???<class name="Role" table="sys_role" > ???????<id name="role_id" ?> ???????????<generator class="native"></generator> ???????</id> ???????<property name="role_name" ?></property> ???????<property name="role_memo" ?></property> ???????<set name="users" table="sys_user_role"> ???????????<key column="role_id" ></key> ???????????<many-to-many class="User" column="user_id" ></many-to-many> ???????</set> ???</class></hibernate-mapping>
核心配置文件：hibernate.cfg.xml:
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-configuration PUBLIC ???"-//Hibernate/Hibernate Configuration DTD 3.0//EN" ???"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"><hibernate-configuration> ???<session-factory> ???????<!-- 数据库驱动 --> ???????<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property> ???????<!-- 数据库url --> ???????<property name="hibernate.connection.url">jdbc:mysql:///hibernate</property> ???????<!-- 数据库连接用户名 --> ???????<property name="hibernate.connection.username">root</property> ???????<!-- 数据库连接密码 --> ???????<property name="hibernate.connection.password">xuyiqing</property> ???????????????<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property> ???????<!-- 将hibernate生成的sql语句打印到控制台 --> ???????<property name="hibernate.show_sql">true</property> ???????<!-- 将hibernate生成的sql语句格式化(语法缩进) --> ???????<property name="hibernate.format_sql">true</property> ???????????????<property name="hibernate.hbm2ddl.auto">update</property> ???????<!-- 引入orm元数据 路径书写: 填写src下的路径 --> ???????????????<property name="hibernate.connection.isolation">4</property> ????????????????<!-- 指定session与当前线程绑定 --> ???????<property name="hibernate.current_session_context_class">thread</property> ???????<mapping resource="domain/User.hbm.xml" /> ???????<mapping resource="domain/Role.hbm.xml" /> ???</session-factory></hibernate-configuration>
自定义工具类：
package utils;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.cfg.Configuration;public class HibernateUtils { ???private static SessionFactory sf; ???????static{ ???????//1 创建,调用空参构造 ???????Configuration conf = new Configuration().configure(); ???????//2 根据配置信息,创建 SessionFactory对象 ????????sf = conf.buildSessionFactory(); ???} ???????//获得session => 获得全新session ???public static Session openSession(){ ???????????????//3 获得session ???????????????Session session = sf.openSession(); ???????????????????????????????return session; ???????????} ???//获得session => 获得与线程绑定的session ???public static Session getCurrentSession(){ ???????//3 获得session ???????Session session = sf.getCurrentSession(); ???????????????return session; ???} ???}这里先写一段测试代码：
public void fun1(){ ???????//1 获得session ???????Session session = HibernateUtils.openSession(); ???????//2 开启事务 ???????Transaction tx = session.beginTransaction(); ???????//------------------------------------------------- ???????//3操作 ???????//1> 创建两个 User ???????User u1 = new User(); ???????u1.setUser_name("张三"); ???????????????User u2 = new User(); ???????u2.setUser_name("李四"); ???????????????//2> 创建两个 Role ???????Role r1 = new Role(); ???????r1.setRole_name("保洁"); ???????????????Role r2 = new Role(); ???????r2.setRole_name("保安"); ???????//3> 用户表达关系 ???????u1.getRoles().add(r1); ???????u1.getRoles().add(r2); ???????????????u2.getRoles().add(r1); ???????u2.getRoles().add(r2); ???????????????//4> 角色表达关系 ???????r1.getUsers().add(u1); ???????r1.getUsers().add(u2); ???????????????r2.getUsers().add(u1); ???????r2.getUsers().add(u2); ???????????????//5> 调用Save方法一次保存 ???????session.save(u1); ???????session.save(u2); ???????session.save(r1); ???????session.save(r2); ???????//------------------------------------------------- ???????//4提交事务 ???????tx.commit(); ???????//5关闭资源 ???????session.close(); ???}看似很标准，不过运行会报错
原因：默认的inverse属性是维护的，两方关系都要维护，造成主键重复
解决办法：
1.改变代码：
???@Test ???//保存员工以及角色 ???public void fun1(){ ???????//1 获得session ???????Session session = HibernateUtils.openSession(); ???????//2 开启事务 ???????Transaction tx = session.beginTransaction(); ???????//------------------------------------------------- ???????//3操作 ???????//1> 创建两个 User ???????User u1 = new User(); ???????u1.setUser_name("张三"); ???????????????User u2 = new User(); ???????u2.setUser_name("李四"); ???????????????Role r1 = new Role(); ???????r1.setRole_name("保洁"); ???????????????Role r2 = new Role(); ???????r2.setRole_name("保安"); ???????u1.getRoles().add(r1); ???????u1.getRoles().add(r2); ???????u2.getRoles().add(r1); ???????u2.getRoles().add(r2); ???????session.save(u1); ???????session.save(u2); ???????session.save(r1); ???????session.save(r2); ???????//------------------------------------------------- ???????//4提交事务 ???????tx.commit(); ???????//5关闭资源 ???????session.close(); ???}2.配置文件修改：
Role的配置文件：
???????<set name="users" table="sys_user_role" inverse="true"> ???????????<key column="role_id" ></key> ???????????<many-to-many class="User" column="user_id" ></many-to-many> ???????</set>
这时候运行那一段代码就不会出错
结论: 将来在开发中,如果遇到多对多关系.一定要选择一方放弃维护关系.
一般谁来放弃要看业务方向. 例如录入员工时,需要为员工指定所属角色.
那么业务方向就是由员工维护角色. 角色不需要维护与员工关系.角色放弃维护
多对多同样有cascade属性：
cascade级联操作:
save-update: 级联保存更新
delete:级联删除
all:级联保存更新+级联删除
结论: cascade简化代码书写.该属性使不使用无所谓. 建议要用只用save-update.
?如果使用delete操作太过危险.尤其在多对多中.不建议使用.
这里是一对多的文章，对比后更有收获
http://www.cnblogs.com/xuyiqing/p/8453165.html
hibernate框架学习笔记9：多对多关系案例
原文地址：https://www.cnblogs.com/xuyiqing/p/8453230.html