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Entity many-to-many association
In the course selection system of a university, a student can take multiple courses at the same time, and a course can be taken by several students. How should such a many-to-many relationship be realized?
Database many-to-many association
In order to express a many-to-many relationship, an intermediate table needs to be created.
xml format configuration
Project structure
Course.java
package com.hibernate.entity;
import java.util.HashSet;
import java.util.Set;
public class Course {
private Integer id;
private String courseName;
private int credit;//学分
//与Student之间的多对多
private Set students = new HashSet<>();
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getCourseName() {
return courseName;
}
public void setCourseName(String courseName) {
this.courseName = courseName;
}
public int getCredit() {
return credit;
}
public void setCredit(int credit) {
this.credit = credit;
}
public Set getStudents() {
return students;
}
public void setStudents(Set students) {
this.students = students;
}
@Override
public String toString() {
return "Course [id=" + id + ", courseName=" + courseName + ", credit=" + credit + ", students=" + students
+ "]";
}
}
Student.java
package com.hibernate.entity;
import java.util.HashSet;
import java.util.Set;
public class Student {
private Integer id;
private String stuName;
private String stuNo;
private Set courses = new HashSet<>();//直接初始化，防止空引用异常
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getStuName() {
return stuName;
}
public void setStuName(String stuName) {
this.stuName = stuName;
}
public String getStuNo() {
return stuNo;
}
public void setStuNo(String stuNo) {
this.stuNo = stuNo;
}
public Set getCourses() {
return courses;
}
public void setCourses(Set courses) {
this.courses = courses;
}
@Override
public String toString() {
return "Student [id=" + id + ", stuName=" + stuName + ", stuNo=" + stuNo + ", courses=" + courses + "]";
}
}
Course.hbm.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.hibernate.entity">
<class name="Course" table="hibernate_06_01_course">
<id name="id" type="int" column="id">
<generator class="identity"></generator>
</id>
<property name="courseName" column="COURSE_NAME" ></property>
<property name="credit"/>
<!-- 映射Course与Student之间的多对多关系
set用来映射Set类型的属性
table:指定中间表表名
key:指定外键字段名字（在中间表中参照当前实体类对应表）
many-to-many表示多对多关系 column="COURSE_ID"(指定中间表中参照关联类型对应表的外键)
inverse="true"放弃维护关联关系（Course作为被控方） -->
<set name="students" table="hibernate_06_01_student_course" inverse="true">
<key column="COURSE_ID"/>
<many-to-many class="Student" column="STU_ID"/>
</set>
</class>
</hibernate-mapping>
Student.hbm.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.hibernate.entity">
<class name="Student" table="hibernate_06_01_studnet">
<id name="id" type="int" column="id">
<generator class="identity"></generator>
</id>
<property name="stuName" column="STU_NAME" ></property>
<property name="stuNo" column="STU_NO"/>
<!-- 映射Student与Course之间的多对多关系
set用来映射Set类型的属性
table:指定中间表表名
key:指定外键字段名字（在中间表中参照当前实体类对应表）
many-to-many表示多对多关系 column="COURSE_ID"(指定中间表中参照关联类型对应表的外键) -->
<set name="courses" table="hibernate_06_01_student_course" >
<key column="STU_ID"/>
<!-- 配置另一个外键 -->
<many-to-many class="Course" column="COURSE_ID"/>
</set>
</class>
</hibernate-mapping>
Test.java
package com.hibernate.ui;
import org.hibernate.Session;
import org.hibernate.Transaction;
import com.hibernate.entity.Course;
import com.hibernate.entity.Student;
import com.hibernate.util.HibernateUtil;
public class Test {
public static void main(String[] args) {
save();
HibernateUtil.closeSessionFactory();
}
//保存学生和课程
private static void save() {
Session session = HibernateUtil.openSession();
Transaction tx = session.beginTransaction();
Student stu = new Student();
stu.setStuName("张三");
stu.setStuNo("2018011000");
Course cou = new Course();
cou.setCourseName("Android基础编程");
cou.setCredit(4);
//建立关联
stu.getCourses().add(cou);
//当Course作为被控方时，即便程序中修改了Course对象关联的学生，也不会去修改中间表
//修改包括插入与删除
cou.getStudents().add(stu);
session.save(stu);
session.save(cou);
tx.commit();
session.close();
}
}
Annotation format configuration
Project structure
Course.java
package com.hibernate.entity;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
@Entity
@Table(name="hibernate_06_01_course")
public class Course {
private Integer id;
private String courseName;
private int credit;//学分
//与Student之间的多对多
private Set students = new HashSet<>();
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)//identity室友数据库决定的
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name="course_name")
public String getCourseName() {
return courseName;
}
public void setCourseName(String courseName) {
this.courseName = courseName;
}
public int getCredit() {
return credit;
}
public void setCredit(int credit) {
this.credit = credit;
}
//为了判断集合中对象的类型
@ManyToMany(mappedBy="courses",targetEntity=Student.class)
public Set getStudents() {
return students;
}
public void setStudents(Set students) {
this.students = students;
}
@Override
public String toString() {
return "Course [id=" + id + ", courseName=" + courseName + ", credit=" + credit + ", students=" + students
+ "]";
}
}
Student.java
package com.hibernate.entity;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
@Entity
@Table(name="hibernate_06_01_studnet")
public class Student {
private Integer id;
private String stuName;
private String stuNo;
private Set<Course> courses = new HashSet<>();
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name="STU_NAME")
public String getStuName() {
return stuName;
}
public void setStuName(String stuName) {
this.stuName = stuName;
}
@Column(name="STU_NO")
public String getStuNo() {
return stuNo;
}
public void setStuNo(String stuNo) {
this.stuNo = stuNo;
}
@ManyToMany//映射多对多关系
//映射中间表
//name=中间表表明
//joinColumns=外键字段
//inverseJoinColumns
@JoinTable(name="hibernate_06_01_student_course",
joinColumns=@JoinColumn(name="STU_ID"),
inverseJoinColumns=@JoinColumn(name="COURSE_ID"))
public Set<Course> getCourses() {
return courses;
}
//这里和上面的定义时候加上课泛型，可以省略targerEntity标签
public void setCourses(Set<Course> courses) {
this.courses = courses;
}
@Override
public String toString() {
return "Student [id=" + id + ", stuName=" + stuName + ", stuNo=" + stuNo + ", courses=" + courses + "]";
}
}