hibernate多对多关系维护

作者: just喔喔
来源: 51数据库
2021-01-07

直接以老师与学生的关系为例。在多对多关联关系中，其中一方都可通过Set保留另一方的所有信息，这样的关联是双向关联。在多对多关联关系中，也只能是双向关联。老师和学生分别对应一张表，通过一张有双方id的中间表来维护多对多的关联。

实体类
[java]?view plain?copy

package?test.hibernate.hbmManyToMany;??

import?java.util.HashSet;??

import?java.util.Set;??

public?class?Teacher?{??

private?Integer?id;??

private?String?name;??

private?Set<Student>?students?=?new?HashSet<Student>();??

public?Integer?getId()?{??

return?id;??

}??

public?void?setId(Integer?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()?{??

//?TODO?Auto-generated?method?stub??

return?"[Teacher:id="?+?id?+?",name="?+?name?+?"]";??

}??

}??
[java]?view plain?copy

package?test.hibernate.hbmManyToMany;??

import?java.util.HashSet;??

import?java.util.Set;??

public?class?Student?{??

private?Integer?id;??

private?String?name;??

private?Set<Teacher>?teachers?=?new?HashSet<Teacher>();??

public?Integer?getId()?{??

return?id;??

}??

public?void?setId(Integer?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()?{??

//?TODO?Auto-generated?method?stub??

return?"[Student:id="?+?id?+?",name="?+?name?+?"]";??

}??

Student.hbm.xml
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<?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="test.hibernate.hbmManyToMany">??

<class?name="Student"?table="student">??

<id?name="id"?type="integer"?column="id">??

<generator?class="native"?/>??

</id>??

<property?name="name"?/>??

??

<set?name="teachers"?table="teacher_student">??

<key?column="studentId"></key>??

<many-to-many?class="Teacher"?column="teacherId"></many-to-many>??

</set>??

</class>??

</hibernate-mapping>??
在多对多关系中，可在Set里通过inverse设置只有其中一方维护。

Teacher.hbm.xml
[html]?view plain?copy

<?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="test.hibernate.hbmManyToMany">??

<class?name="Teacher"?table="teacher">??

<id?name="id"?type="integer"?column="id">??

<generator?class="native"?/>??

</id>??

<property?name="name"?/>??

<set?name="students"?table="teacher_student">??

<key?column="teacherId"></key>??

<many-to-many?class="Student"?column="studentId"></many-to-many>??

</set>??

</class>??

</hibernate-mapping>??

测试类
[java]?view plain?copy

package?test.hibernate.hbmManyToMany;??

import?org.hibernate.Session;??

import?org.hibernate.SessionFactory;??

import?org.hibernate.cfg.Configuration;??

import?org.junit.Test;??

public?class?App?{??

private?static?SessionFactory?sessionFactory?=?new?Configuration()//??

.configure()//??

.addClass(Teacher.class)//?添加Hibernate实体类（加载对应的映射文件）??

.addClass(Student.class)//??

.buildSessionFactory();??

@Test??

public?void?testSave()?throws?Exception?{??

Session?session?=?sessionFactory.openSession();??

session.beginTransaction();??

//?--------------------------------------------??

//?构建对象??

Teacher?teacher1?=?new?Teacher();??

teacher1.setName("林老师");??

Teacher?teacher2?=?new?Teacher();??

teacher2.setName("张老师");??

Student?student1?=?new?Student();??

student1.setName("李明");??

Student?student2?=?new?Student();??

student2.setName("刘备");??

teacher1.getStudents().add(student1);??

teacher1.getStudents().add(student2);??

teacher2.getStudents().add(student1);??

teacher2.getStudents().add(student2);??

//?下面部分不能与上面的代码块同时进行,只能选择其中一部分,不然会抛出在中间表中重复插入相同记录的异常??

//?如果两边都要写，注意设置其中一方的inverse为true??

//?student1.getTeachers().add(teacher1);??

//?student1.getTeachers().add(teacher2);??

//?student2.getTeachers().add(teacher1);??

//?student2.getTeachers().add(teacher2);??

//?保存??

session.save(teacher1);??

session.save(teacher2);??

session.save(student1);??

session.save(student2);??

//?--------------------------------------------??

session.getTransaction().commit();??

session.close();??

}??

//?获取到部门关联的员工??

@Test??

public?void?testGet()?throws?Exception?{??

Session?session?=?sessionFactory.openSession();??

session.beginTransaction();??

//?获取数据??

Teacher?teacher?=?(Teacher)?session.get(Teacher.class,?4);??

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Teacher.hbm.xml中inverse=false就可以删除，否则不可以删除??

Teacher?teacher?=?(Teacher)?session.get(Teacher.class,?4);??

teacher.getStudents().clear();??

//?--------------------------------------------??

session.getTransaction().commit();??

session.close();??

}??

//?删除部门及对员工的影响??

@Test??

public?void?testDelete()?throws?Exception?{??

Session?session?=?sessionFactory.openSession();??

session.beginTransaction();??

//?--------------------------------------------??

//?会同时删除teacher_student和teacher中教师id=4的记录??

Teacher?teacher?=?(Teacher)?session.get(Teacher.class,?4);??

session.delete(teacher);??

//?--------------------------------------------??

session.getTransaction().commit();??

session.close();??

}??

} ?

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执行testGet方法时控制台内容

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