一、多对多单向关联
假设一个老师教多个学生，一个学生被多个老师教，这就是典型的多对多关系
配置方式是在Teacher类的getStudents()方法上添加注解@ManyToMany
@JoinTable(name="t_s" ,
joinColumns = { @JoinColumn(name="teacher_id")},
inverseJoinColumns = { @JoinColumn(name="student_id")}
)
Teacher类:
package cn.orlion.hibernate.model;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;@Entitypublic class Teacher { private int id; private String name; private Set<Student> students = new HashSet<Student>(); @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" , // t_s是生成的中间表的表名 joinColumns = { @JoinColumn(name="teacher_id")},// teacher_id是生成中间表中参考自Teacher的外键 inverseJoinColumns = { @JoinColumn(name="student_id")}// inverseJoinColumns配置的是反转的对象（即Student）的idstudent_id是中间表中参考自Student的外键 ) public Set<Student> getStudents() { return students; } public void setStudents(Set<Student> students) { this.students = students; } }
Student类：
package cn.orlion.hibernate.model;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;@Entitypublic class Student { private int id; private String name; @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; }}
生成的sql语句：
create table Student ( id integer not null auto_increment, name varchar(255), primary key (id) )create table 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 constraint FK_3no0wm9bwtogr87t4gvd4t3x5 foreign key (student_id) references Student (id)alter table t_s add constraint FK_pj2fmd6skwfi7jm2avlo4n6mf foreign key (teacher_id) references Teacher (id)
二、多对多双向关联
单向关联可一理解为：老师知道教的学生的名字，但是学生不知道教的多个老师。
双向关联可以理解为：老师知道教的学生的名字，学生也知道教的多个老师。
配置方式：Teacher类与单向关联相同，Student类添加Set<Teacher> teachers属性，在getTeaches()方法上添加注解@ManyToMany(mappedBy="students")
Teacher类：
package cn.orlion.hibernate.model;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;@Entitypublic class Teacher { private int id; private String name; private Set<Student> students = new HashSet<Student>(); @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" , // t_s是生成的中间表的表名 joinColumns = { @JoinColumn(name="teacher_id")}, inverseJoinColumns = { @JoinColumn(name="student_id")} ) public Set<Student> getStudents() { return students; } public void setStudents(Set<Student> students) { this.students = students; } }
Student类：
package cn.orlion.hibernate.model;import java.util.HashSet;import java.util.Set;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToMany;@Entitypublic class Student { private int id; private String name; private Set<Teacher> teachers = new HashSet<Teacher>(); @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(mappedBy="students") public Set<Teacher> getTeachers() { return teachers; } public void setTeachers(Set<Teacher> teachers) { this.teachers = teachers; }}
生成的sql语句：
create table Student ( id integer not null auto_increment, name varchar(255), primary key (id) )create table 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 constraint FK_3no0wm9bwtogr87t4gvd4t3x5 foreign key (student_id) references Student (id)alter table t_s add constraint FK_pj2fmd6skwfi7jm2avlo4n6mf foreign key (teacher_id) references Teacher (id)