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。