java框架篇---hibernate(多对多)映射关系

以学生和老师为例的来讲解多对多映射。

实体类：

Student

package cn.itcast.g_hbm_manyToMany;import java.util.HashSet;import java.util.Set;public class Student {    PRivate Long id;    private String name;    private Set<Teacher> teachers = new HashSet<Teacher>();    public Long getId() {        return id;    }    public void setId(Long id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Set<Teacher> getTeachers() {        return teachers;    }    public void setTeachers(Set<Teacher> teachers) {        this.teachers = teachers;    }    @Override    public String toString() {        return "[Student: id=" + id + ", name=" + name + "]";    }}

Teacher 

package cn.itcast.g_hbm_manyToMany;import java.util.HashSet;import java.util.Set;public class Teacher {    private Long id;    private String name;    private Set<Student> students = new HashSet<Student>();         public Long getId() {        return id;    }    public void setId(Long id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Set<Student> getStudents() {        return students;    }    public void setStudents(Set<Student> students) {        this.students = students;    }    @Override    public String toString() {        return "[Teacher: id=" + id + ", name=" + name + "]";    }}

映射文件：

Student.hbm.xml

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.itcast.g_hbm_manyToMany">        <class name="Student" table="student">        <id name="id">            <generator class="native"></generator>        </id>        <property name="name"/>                <!-- teachers属性，Set集合。            表达的是本类与Teacher的多对多。                        table属性：中间表（集合表）            key子元素：集合外键（引用当前表主键的那个外键）         -->        <set name="teachers" table="teacher_student" inverse="false">            <key column="studentId"></key>            <many-to-many class="Teacher" column="teacherId"></many-to-many>        </set>                </class>    </hibernate-mapping>

Teacher.hbm.xml

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.itcast.g_hbm_manyToMany">        <class name="Teacher" table="teacher">        <id name="id">            <generator class="native"></generator>        </id>        <property name="name" type="string" column="name"/>                <!-- students属性，Set集合。            表达的是本类与Student的多对多。         -->        <set name="students" table="teacher_student" inverse="true">            <key column="teacherId"></key>            <many-to-many class="Student" column="studentId"></many-to-many>        </set>            </class>    </hibernate-mapping>

测试

App.java

package cn.itcast.g_hbm_manyToMany;import org.hibernate.session;import org.hibernate.SessionFactory;import org.hibernate.cfg.Configuration;import org.junit.Test;import com.java1234.util.HibernateSessionFactory;public class App {    private static SessionFactory sessionFactory =HibernateSessionFactory.getSessionFactory();    // 保存，有关联关系    @Test    public void testSave() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // 新建对象        Student student1 = new Student();        student1.setName("王同学1");        Student student2 = new Student();        student2.setName("李同学2");        Teacher teacher1 = new Teacher();        teacher1.setName("赵老师3");        Teacher teacher2 = new Teacher();        teacher2.setName("蔡老师4");        // 关联起来        student1.getTeachers().add(teacher1);        student1.getTeachers().add(teacher2);        student2.getTeachers().add(teacher1);        student2.getTeachers().add(teacher2);        teacher1.getStudents().add(student1);        teacher1.getStudents().add(student2);        teacher2.getStudents().add(student1);        teacher2.getStudents().add(student2);        // 保存        session.save(student1);        session.save(student2);        session.save(teacher1);        session.save(teacher2);        // --------------------------------------------        session.getTransaction().commit();        session.close();    }    // 获取，可以获取到关联的对方    @Test    public void testGet() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // 获取一方，并显示另一方信息        Teacher teacher = (Teacher) session.get(Teacher.class, 3L);        System.out.println(teacher);        System.out.println(teacher.getStudents());        // --------------------------------------------        session.getTransaction().commit();        session.close();    }    // 解除关联关系    @Test    public void testRemoveRelation() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // 如果inverse=false就可以解除，如果为true就不可以解除        Teacher teacher = (Teacher) session.get(Teacher.class, 3L);        teacher.getStudents().clear();        // --------------------------------------------        session.getTransaction().commit();        session.close();    }    // 删除对象，对关联对象的影响    @Test    public void testDelete() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // a, 如果没有关联的对方：能删除。        // b, 如果有关联的对方且inverse=false，由于可以维护关联关系，他就会先删除关联关系，再删除自己。        // c, 如果有关联的对方且inverse=true，由于不能维护关联关系，所以会直接执行删除自己，就会有异常。        Teacher teacher = (Teacher) session.get(Teacher.class, 9L);        session.delete(teacher);        // --------------------------------------------        session.getTransaction().commit();        session.close();    }}

两张表的多对多关系，在数据库中通常是通过第三张中间表来实现的，第三张中间表放的是两张表各自的主键值，通过主键与主键的对应来体现表直接的关系。比如在权限系统中，一个用户可以拥有多种权限，而一种权限也可以授予多个用户。