Hibernate关系映射之many-to-many

1、建表

2、创建实体类及映射文件

Student.java类

public class Student implements java.io.Serializable {

    // Fields

    private Integer sid;
    private String sname;
    private Set<Teacher> teachers=new HashSet<Teacher>();

    // Constructors

    /** default constructor */
    public Student() {
    }

    /** full constructor */
    public Student(String sname) {
        this.sname = sname;
    }

    // Property accessors

    public Integer getSid() {
        return this.sid;
    }

    public void setSid(Integer sid) {
        this.sid = sid;
    }

    public String getSname() {
        return this.sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    public Set<Teacher> getTeachers() {
        return teachers;
    }

    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }
}

使用多对多注解应该是：

    @ManyToMany
    @JoinTable(name="teacher_student",joinColumns={@JoinColumn(name="sid")},inverseJoinColumns={@JoinColumn(name="tid")})
    public Set<Teacher> getTeachers() {
        return teachers;
    }

Teacher.java类

public class Teacher implements java.io.Serializable {

    // Fields

    private Integer tid;
    private String tname;
    private Set<Student> students=new HashSet<Student>();

    // Constructors

    /** default constructor */
    public Teacher() {
    }

    /** full constructor */
    public Teacher(String tname) {
        this.tname = tname;
    }

    // Property accessors

    public Integer getTid() {
        return this.tid;
    }

    public void setTid(Integer tid) {
        this.tid = tid;
    }

    public String getTname() {
        return this.tname;
    }

    public void setTname(String tname) {
        this.tname = tname;
    }

    public Set<Student> getStudents() {
        return students;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }
}

被控方注解是：

    @ManyToMany(mappedBy="teachers")
    public Set<Student> getStudents() {
        return students;
    }

Student.hbm.xml

<hibernate-mapping>
    <class name="com.db.Student" table="student" catalog="mydb">
        <id name="sid" type="java.lang.Integer">
            <column name="sid" />
            <generator class="native" />
        </id>
        <property name="sname" type="java.lang.String">
            <column name="sname" length="32" />
        </property>
        <!-- 通过table属性告诉hibernate中间表，cascade指明级联操作的类型，inverse属性说明Student实体类是主控方，负责维护关系表 --> 
        <set name="teachers" table="teacher_student" cascade="save-update,delete" inverse="false">
        <!-- 通过key属性告诉hibernate在中间表里面查询sid值相应的student记录 --> 
            <key>
                <column name="sid" not-null="true" />
            </key>
             <!-- 通过column项告诉hibernate对teacher表中查找tid值相应的teacher记录 --> 
            <many-to-many class="com.db.Teacher" column="tid"/>
        </set>
    </class>
</hibernate-mapping>

Teacher.hbm.xml

<hibernate-mapping>
    <class name="com.db.Teacher" table="teacher" catalog="mydb">
        <id name="tid" type="java.lang.Integer">
            <column name="tid" />
            <generator class="native" />
        </id>
        <property name="tname" type="java.lang.String">
            <column name="tname" length="32" />
        </property>
        <!-- 通过table属性告诉hibernate中间表，cascade指明级联操作的类型，inverse属性说明Teacher实体类是被控方，不负责维护关系表 ，不能触发对象和数据库的同步更新的。-->
        <set name="students" table="teacher_student" inverse="true"
            cascade="save-update,delete">
            <key>
                <column name="tid" not-null="true" />
            </key>
            <many-to-many class="com.db.Student" column="sid" />
        </set>
    </class>
</hibernate-mapping>

hibernate.cfg.xml

<hibernate-configuration>

    <session-factory>
        <property name="dialect">
            org.hibernate.dialect.MySQLInnoDBDialect
        </property>
        <property name="connection.url">
            jdbc:mysql://localhost:3306/mydb
        </property>
        <property name="connection.username">root</property>
        <property name="connection.password">123456</property>
        <property name="connection.driver_class">
            com.mysql.jdbc.Driver
        </property>
        <property name="myeclipse.connection.profile">
            MyDBAccount
        </property>
        <property name="show_sql">true</property>
        <mapping resource="com/db/Student.hbm.xml" />
        <mapping resource="com/db/Teacher.hbm.xml" />
    </session-factory>

</hibernate-configuration>

3、建立测试用例

测试用例一：

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Session session=HibernateSessionFactory.getSession();
        Student stu1=new Student();
        stu1.setSname("StudentAmy");
        Teacher t1=new Teacher();
        t1.setTname("TeacherSusan");
        Teacher t2=new Teacher();
        t2.setTname("TeacherLily");
        Teacher t3=new Teacher();
        t3.setTname("TeacherMaike");
        Set<Teacher> teachers=new HashSet<Teacher>();
        teachers.add(t1);
        teachers.add(t2);
        teachers.add(t3);
        stu1.setTeachers(teachers);
        session.save(stu1);
        session.getTransaction().commit();
    }

对应的SQL语句是：

Hibernate: insert into mydb.student (sname) values (?)
Hibernate: insert into mydb.teacher (tname) values (?)
Hibernate: insert into mydb.teacher (tname) values (?)
Hibernate: insert into mydb.teacher (tname) values (?)
Hibernate: insert into teacher_student (sid, tid) values (?, ?)
Hibernate: insert into teacher_student (sid, tid) values (?, ?)
Hibernate: insert into teacher_student (sid, tid) values (?, ?)

测试用例二：

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Session session=HibernateSessionFactory.getSession();
        Student stu1=new Student();
        stu1.setSname("StudentJhon1");    
        Student stu2=new Student();
        stu2.setSname("StudentLihua1");
        Teacher t1=new Teacher();
        t1.setTname("TeacherJay1");
        t1.getStudents().add(stu1);
        t1.getStudents().add(stu2);
        session.beginTransaction();
        session.save(t1);
        session.getTransaction().commit();
    }

}

对应的SQL语句是：

Hibernate: insert into mydb.teacher (tname) values (?)
Hibernate: insert into mydb.student (sname) values (?)
Hibernate: insert into mydb.student (sname) values (?)

测试用例三：

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Session session=HibernateSessionFactory.getSession();
        Student stu1=new Student();
        stu1.setSname("StudentJhon1");    
        Student stu2=new Student();
        stu2.setSname("StudentLihua1");
        Teacher t1=new Teacher();
        t1.setTname("TeacherJay1");
        t1.getStudents().add(stu1);
        t1.getStudents().add(stu2);
        stu2.getTeachers().add(t1);
        session.beginTransaction();
        session.save(t1);
        session.getTransaction().commit();
    }

}

对应的SQL语句是：

Hibernate: insert into mydb.teacher (tname) values (?)
Hibernate: insert into mydb.student (sname) values (?)
Hibernate: insert into mydb.student (sname) values (?)
Hibernate: insert into teacher_student (sid, tid) values (?, ?)

通过测试用例发现，两个实体类都进行了级联操作，但是只有将Teacher实体对象添加到Student的teachers属性集合中时才能更新维护中间表。因为Student中的teachers集合的inverse属性是false，使得Student类成为主控方，负责维护关联关系；而Teacher中的students集合的inverse属性为true，使得Teacher类成为被控方，不会维护关联关系。