Hibernate 一对一、一对多、多对多注解mappedBy属性的总结
mappedBy:
所填内容必为本类在另一方的字段名。
表示:本类放弃控制关联关系,所有对关联关系的控制,如:建立、解除与另一方的关系,都由对方控制,本类不管。举个例子:
Teacher和Student之间是多对多关联关系,在Student端的teachers字段的@ManyToMany注解里面,配置属性mappedBy = "students"。表示:学生没有资格控制与老师的关联关系(如:建立关系、删除关系),只有老师才有资格控制关联关系。学生不能说:”我们两做朋友吧“。(即使说了,也无济于事,老师当学生没说。即Hibernate把学生说的当作耳边风,虽然Hibernate也不会抛出异常,程序照样能运行)。但是,老师可以说,“我们做朋友吧”。那么,学生只能无任何拒绝理由地接受。同样,如果学生说,”我们从此一刀两断吧“,那也是没用的。只有老师说:“滚,有多远,滚多远,为师今日将你逐出师门,再也不想见到你”,那么,这才能有效地解除关联关系(注意:这是一个对象(对应数据库里的一行记录)和一个对象之间的关系,而不是表和表之间的关联关系。一对师生关系的解除,不能影响整个社会的师生关系)。
测试代码:(这是完整类,后面只贴出test1的代码,其他部分不变)
public class MappedByTest {</span><span style="color: #0000ff;">private</span> <span style="color: #0000ff;">static</span><span style="color: #000000;"> SessionFactory sessionFactory; </span><span style="color: #0000ff;">private</span><span style="color: #000000;"> Session session; @BeforeClass </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> beforeTest(){ sessionFactory </span>=<span style="color: #000000;"> HibernateUtil.getSessionFactory(); } @AfterClass </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> afterTest(){ sessionFactory.close(); } @Before </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> beginTransaction(){ session </span>=<span style="color: #000000;"> sessionFactory.openSession(); session.beginTransaction(); } @After </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> commitTransaction(){ session.getTransaction().commit(); session.close(); } @Test </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> execute(){ test1(); } </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> test1(){ Student student1 </span>= <span style="color: #0000ff;">new</span> Student("学生张三",3<span style="color: #000000;">); Student student2 </span>= <span style="color: #0000ff;">new</span> Student("学生李四",4<span style="color: #000000;">); Teacher teacher1 </span>= <span style="color: #0000ff;">new</span> Teacher("老师王五",5<span style="color: #000000;">); Teacher teacher2 </span>= <span style="color: #0000ff;">new</span> Teacher("老师赵六",6<span style="color: #000000;">); List</span><Student> students = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); students.add(student1); students.add(student2); List</span><Teacher> teachers = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); teachers.add(teacher1); teachers.add(teacher2); session.save(teacher1); session.save(teacher2); student1.setTeachers(teachers); session.save(student1); }
}
产生的结果:
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然后,清空数据库,做第二次测试:
public void test1() { Student student1 = new Student("学生张三", 3); Student student2 = new Student("学生李四", 4); Teacher teacher1 = new Teacher("老师王五", 5); Teacher teacher2 = new Teacher("老师赵六", 6);List</span><Student> students = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); students.add(student1); students.add(student2); List</span><Teacher> teachers = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); teachers.add(teacher1); teachers.add(teacher2); session.save(student1); session.save(student2); teacher1.setStudents(students); session.save(teacher1); }</span></pre>
产生的结果:
可见:在Student类的teachers字段的@ManyToMany注解上配置属性mappedBy = "students",所有Student类产生的对象均不能建立和老师的关联关系,只有老师可以。
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然后,清空数据库,做第三次测试:
public void test1(){ Student student1 = new Student("学生张三",3); Student student2 = new Student("学生李四",4); Teacher teacher1 = new Teacher("老师王五",5); Teacher teacher2 = new Teacher("老师赵六",6);List</span><Student> students = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); students.add(student1); students.add(student2); List</span><Teacher> teachers = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); teachers.add(teacher1); teachers.add(teacher2); </span><span style="color: #0000ff;">int</span> stuId = 4<span style="color: #000000;">; </span><span style="color: #0000ff;">int</span> teacherId = 6<span style="color: #000000;">; Student persistentStudent </span>= (Student)session.get(Student.<span style="color: #0000ff;">class</span><span style="color: #000000;">, stuId); Teacher persistentTeacher </span>= (Teacher)session.get(Teacher.<span style="color: #0000ff;">class</span><span style="color: #000000;">, teacherId); persistentStudent.getTeachers().remove(persistentTeacher); session.update(persistentStudent); }</span></pre>
控制台打出的SQL语句:
Hibernate: select student0_.student_id as student_1_0_0_, student0_.age as age2_0_0_, student0_.name as name3_0_0_ from student student0_ where student0_.student_id=? Hibernate: select teacher0_.teacher_id as teacher_1_1_0_, teacher0_.age as age2_1_0_, teacher0_.name as name3_1_0_ from teacher teacher0_ where teacher0_.teacher_id=? Hibernate: select teachers0_.student_id as student_2_2_0_, teachers0_.teacher_id as teacher_1_2_0_, teacher1_.teacher_id as teacher_1_1_1_, teacher1_.age as age2_1_1_, teacher1_.name as name3_1_1_ from ts_relation teachers0_ inner join teacher teacher1_ on teachers0_.teacher_id=teacher1_.teacher_id where teachers0_.student_id=?
产生的结果:
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然后,清空数据库,做第四次测试:
public void test1(){ Student student1 = new Student("学生张三",3); Student student2 = new Student("学生李四",4); Teacher teacher1 = new Teacher("老师王五",5); Teacher teacher2 = new Teacher("老师赵六",6);List</span><Student> students = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); students.add(student1); students.add(student2); List</span><Teacher> teachers = <span style="color: #0000ff;">new</span> ArrayList<><span style="color: #000000;">(); teachers.add(teacher1); teachers.add(teacher2); </span><span style="color: #0000ff;">int</span> stuId = 4<span style="color: #000000;">; </span><span style="color: #0000ff;">int</span> teacherId = 6<span style="color: #000000;">; Student persistentStudent </span>= (Student)session.get(Student.<span style="color: #0000ff;">class</span><span style="color: #000000;">, stuId); Teacher persistentTeacher </span>= (Teacher)session.get(Teacher.<span style="color: #0000ff;">class</span><span style="color: #000000;">, teacherId); persistentTeacher.getStudents().remove(persistentStudent); session.update(persistentTeacher); }</span></pre>
控制台打出的SQL语句:
Hibernate: select student0_.student_id as student_1_0_0_, student0_.age as age2_0_0_, student0_.name as name3_0_0_ from student student0_ where student0_.student_id=? Hibernate: select teacher0_.teacher_id as teacher_1_1_0_, teacher0_.age as age2_1_0_, teacher0_.name as name3_1_0_ from teacher teacher0_ where teacher0_.teacher_id=? Hibernate: select students0_.teacher_id as teacher_1_2_0_, students0_.student_id as student_2_2_0_, student1_.student_id as student_1_0_1_, student1_.age as age2_0_1_, student1_.name as name3_0_1_ from ts_relation students0_ inner join student student1_ on students0_.student_id=student1_.student_id where students0_.teacher_id=? Hibernate: --------------------------注意这条 delete from ts_relation where teacher_id=? Hibernate: insert into ts_relation (teacher_id, student_id) values (?, ?)
产生的结果:
可见:在Student类的teachers字段的@ManyToMany注解上配置属性mappedBy = "students",所有Student类产生的对象均不能删除和老师的关联关系,只有老师可以。
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由此证明了之前所说的,mappedBy表示,本类防止控制和另一方的关联关系,所填内容必为本类在另一方的字段名。
注意:mappedBy和@JoinTable是互斥的,也就是说,@"关联关系注解"里面写了mappedBy属性,下面就不能再写@JoinTable。否则,Hibernate报异常。