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 {
private static SessionFactory sessionFactory;
private Session session;
@BeforeClass
public static void beforeTest(){
sessionFactory = HibernateUtil.getSessionFactory();
}
@AfterClass
public static void afterTest(){
sessionFactory.close();
}
@Before
public void beginTransaction(){
session = sessionFactory.openSession();
session.beginTransaction();
}
@After
public void commitTransaction(){
session.getTransaction().commit();
session.close();
}
@Test
public void execute(){
test1();
}
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<Student> students = new ArrayList<>();
students.add(student1);
students.add(student2);
List<Teacher> teachers = new ArrayList<>();
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<Student> students = new ArrayList<>();
students.add(student1);
students.add(student2);
List<Teacher> teachers = new ArrayList<>();
teachers.add(teacher1);
teachers.add(teacher2);
session.save(student1);
session.save(student2);
teacher1.setStudents(students);
session.save(teacher1);
}
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<Student> students = new ArrayList<>();
students.add(student1);
students.add(student2);
List<Teacher> teachers = new ArrayList<>();
teachers.add(teacher1);
teachers.add(teacher2);
int stuId = 4;
int teacherId = 6;
Student persistentStudent = (Student)session.get(Student.class, stuId);
Teacher persistentTeacher = (Teacher)session.get(Teacher.class, teacherId);
persistentStudent.getTeachers().remove(persistentTeacher);
session.update(persistentStudent);
}
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：
----------------------------------------------------------------------------------------
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<Student> students = new ArrayList<>();
students.add(student1);
students.add(student2);
List<Teacher> teachers = new ArrayList<>();
teachers.add(teacher1);
teachers.add(teacher2);
int stuId = 4;
int teacherId = 6;
Student persistentStudent = (Student)session.get(Student.class, stuId);
Teacher persistentTeacher = (Teacher)session.get(Teacher.class, teacherId);
persistentTeacher.getStudents().remove(persistentStudent);
session.update(persistentTeacher);
}
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。