【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
  @GeneratedValue
  public 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
  @GeneratedValue
  public 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)