【hibernate框架】多对多双向关联(Annotation实现)

多对多的双向关联：(非常少用)
通过老师可以知道他教了多少学生，这是单项多对多。而如果同时通过学生知道有多少个老师教他，那么就是多对多双向的关联。


Annotation实现：
Student.java:

package cn.edu.hpu.many2many;import java.util.HashSet; import java.util.Set;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToMany;import javax.persistence.Table;@Entity@Table(name="m_student")public class Student {private int id;private String name;private Set<Teacher> teachers=new HashSet<Teacher>();@ManyToMany(mappedBy="students")public Set<Teacher> getTeachers() {return teachers;}public void setTeachers(Set<Teacher> teachers) {this.teachers = teachers;}@Id@GeneratedValuepublic int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}}

Teacher.java:

package cn.edu.hpu.many2many;import java.util.HashSet;import java.util.Set;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.JoinTable;import javax.persistence.ManyToMany;import javax.persistence.Table;import cn.edu.hpu.many2many.Student;@Entity@Table(name="m_teacher")public class Teacher {private int id;private String name;private Set<Student> students=new HashSet<Student>();//选择set的原因是因为，set互相之间不会有重复的//跟数据库模型比较匹配@Id@GeneratedValuepublic int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@ManyToMany@JoinTable(name="t_s",joinColumns={@JoinColumn(name="teacher_id")},//指明的是当前这个实体类的表的名字(加s防止联合主键)inverseJoinColumns={@JoinColumn(name="student_id")}//逆转的、对方的的表名(所对应的另一张表))public Set<Student> getStudents() {return students;}public void setStudents(Set<Student> students) {this.students = students;}}

在hibernate.cfg.xml中配置：

<mapping class="cn.edu.hpu.many2many.Student"/><mapping class="cn.edu.hpu.many2many.Teacher"/>

生成的sql语句：
 create table m_student (
        id integer not null auto_increment,
        name varchar(255),
        primary key (id)
    )


    create table m_teacher (
        id integer not null auto_increment,
        name varchar(255),
        primary key (id)
    )


    create table t_s (
        teacher_id integer not null,
        student_id integer not null,
        primary key (teacher_id, student_id)
    )


    alter table t_s 
        add index FK1BF687B5893C2 (teacher_id), 
        add constraint FK1BF687B5893C2 
        foreign key (teacher_id) 
        references m_teacher (id)


    alter table t_s 
        add index FK1BF686ABD4922 (student_id), 
        add constraint FK1BF686ABD4922 
        foreign key (student_id) 
        references m_student (id)

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