ManyToMany双向外键关联（基于注解）的映射案例（简单版）

梦里梦外; 2022-07-20 15:23 106阅读 0赞

学生和老师就是多对多的关系，一个学生有多个学生，一个老师也有多个学生，这里的多对多映射采用中间表连接的映射策略，建立中间表的映射策略，建立中间表分别引入俩边的主键作为外键。通过中间表映射俩个表之间的关系。

下面就以学生类和老师类为例介绍多对多的映射关系的实例

Students类

package mtm_bfk;
    
    import java.io.Serializable;
    import java.util.Date;
    import java.util.Set;
    
    import javax.persistence.Entity;  //JPA注解
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.JoinColumn;
    import javax.persistence.JoinTable;
    import javax.persistence.ManyToMany;
    
    
    
    
    /** * 学生类 * @ClassName: Students * @Description: 学生类 * @author: Administrator * @date: 2016年3月28日 下午5:01:03 */
    //@Entity(name="t_students") //使用了这个注解以后 这个属性就可以映射成关系型数据库中的一张叫做t_students的表 没有name的话就默认生成students
    @Entity(name="t_students")
    public class Students implements Serializable{ 
    
        private static final long serialVersionUID = 1L;
    
            private int sid;//主键
            private String gender; //性别
            private Date brithday;//生日
            private Set<Teachers> teachers;//学生持有教师的集合
    
    
    
            public Students( String gender,
                    Date brithday) {
                //super();
    
                this.gender = gender;
                this.brithday = brithday;
    
            }
            @Id //主键
            @GeneratedValue  //数据库自增
            public int getSid() {
                return sid;
            }
            public void setSid(int sid) {
                this.sid = sid;
            }
    
    
            public String getGender() {
                return gender;
            }
            public void setGender(String gender) {
                this.gender = gender;
            }
            public Date getBrithday() {
                return brithday;
            }
            public void setBrithday(Date brithday) {
                this.brithday = brithday;
            }
    
            @ManyToMany //多对多外键关联的配置
            @JoinTable(name="teachers_students" ,//中间表的表名
                                joinColumns={
       @JoinColumn(name="sid")},//本表的主键
                                inverseJoinColumns={
       @JoinColumn(name="tid")})//所映射表的主键
            public Set<Teachers> getTeachers() {
                return teachers;
            }
            public void setTeachers(Set<Teachers> teachers) {
                this.teachers = teachers;
            }
    
    
    
    }

Teachers实体类

package mtm_bfk;
    
    import java.util.Set;
    
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.ManyToMany;
    
    
    /** * @ClassName: Teachers * @Description: 教师的实体类 * @author: Administrator * @date: 2016年6月12日 下午7:52:59 */
    @Entity
    public class Teachers { 
        @Id
        @GeneratedValue//(generator="tid")
    // @GenericGenerator(name="tid",strategy="assigned") //代表数据库非自增 
        @Column(length=8)
        private int tid;//教师的编号
        private String tname;//教师的名字
        @ManyToMany(mappedBy="teachers")//把主控方交给学生
        private  Set<Students>stus;//教师持有的学生的集合
    
        public Teachers() {
            //super();
        }
    
        public Teachers(String tname) {
        // super();
            this.tname = tname;
        }
    
    
        public int getTid() {
            return tid;
        }
    
    
        public void setTid(int tid) {
            this.tid = tid;
        }
    
    
        public String getTname() {
            return tname;
        }
        public void setTname(String tname) {
            this.tname = tname;
        }
    }

hibernate的配置文件

<!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>
        
            <property name="connection.username">用户名</property>
            <property name="connection.password">密码</property>
            <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
            <property name="connection.url">jdbc:mysql:///test?useUnicode=true&characterEncoding=UTF-8</property>
            <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
    
            
            <property name="show_sql">true</property>
            <property name="format_sql">true</property>
            <property name="hbm2ddl.auto">update</property>
            
            <property name="hibernate.current_session_context_class">thread</property>
    
        
            <mapping class="mtm_bfk.Students"/>
            <mapping class="mtm_bfk.Teachers"/>
    
        </session-factory>
    </hibernate-configuration>

TestStudents测试类

package mtm_bfk;
    
    
    
    
    import java.util.Date;
    import java.util.HashSet;
    import java.util.Set;
    
    import org.hibernate.Session;
    import org.hibernate.SessionFactory;
    import org.hibernate.Transaction;
    import org.hibernate.cfg.Configuration;
    import org.hibernate.service.ServiceRegistry;
    import org.hibernate.service.ServiceRegistryBuilder;
    import org.hibernate.tool.hbm2ddl.SchemaExport;
    import org.junit.Test;
    
    import mtm_bfk.Students;
    
    
    public class TestStudents { 
        @SuppressWarnings("unused")
        @Test
        //生成表结构
        public void textSchemaExport(){
            //创建配置对象
            Configuration config = new Configuration().configure();
            //创建服务对象
            ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry();
            //创建sessionFactory
            SessionFactory sessionFactory = config.buildSessionFactory(serviceRegistry);
            //创建session对象
            Session session = sessionFactory.getCurrentSession();
            //创建SchemaExport对象
            SchemaExport export  = new SchemaExport(config);
    
            export.create(true, true);//第一个true是生成表结构第二个是输出sql语句
        }
        @Test
        public void testSaveStudents(){
            //创建配置对象
            Configuration config = new Configuration().configure();
            //创建服务对象
            ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry();
            //创建sessionFactory
            SessionFactory sessionFactory = config.buildSessionFactory(serviceRegistry);
            //创建session对象
            Session session = sessionFactory.getCurrentSession();
            //创建事物对象
            Transaction tx = session.beginTransaction();
            //创建教师对象
            Teachers t1 = new Teachers("123");
            Teachers t2 = new Teachers("789");
            Teachers t3 = new Teachers("1235");
            Teachers t4 = new Teachers("784");
    
    
            Students s = new Students("nan",new Date());
            Students s1 = new Students("nv",new Date());
            Students s2 = new Students("na",new Date());
            //创建俩个集合对象
            Set<Teachers> set1 = new HashSet<Teachers>();
            set1.add(t1);
            set1.add(t2);
            Set<Teachers> set2 = new HashSet<Teachers>();
            set2.add(t3);
            set2.add(t4);
    
            s.setTeachers(set2);
            s1.setTeachers(set1);
            s2.setTeachers(set2);
            //先保存教师
            session.save(t1);
            session.save(t2);
            session.save(t3);
            session.save(t4);
    
            session.save(s);
            session.save(s1);
            session.save(s2);
    
            tx.commit();
            sessionFactory.close();
        }
    }

以上配置就实现了多对多的外键关联了  
这里面有多种映射关系的配置[http://download.csdn.net/detail/u013412790/9571110][http_download.csdn.net_detail_u013412790_9571110]

[http_download.csdn.net_detail_u013412790_9571110]: http://download.csdn.net/detail/u013412790/9571110