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