hibernate学习六（关系映射多对多）

一、多对多

　　多对多：在学生与老师的情况下，一个老师可以教多个学生，一个学生可以被多个老师教；

　　对于类：某一方拥有另一方的一个集合类

二、修改student.java和teacher.java类（单向）

　　单向:老师知道自己教哪些学生，学生不知道教自己的老师

　　复制05工程，并更名为Hibernate_06_ManyToMany；

package com.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;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="teacher")
public class Teacher {
    private int id;
    private String teacherName;
    private int teacherAge;
    private String title;
    private Set<Student> students = new HashSet<Student>();
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getTeacherName() {
        return teacherName;
    }
    public void setTeacherName(String teacherName) {
        this.teacherName = teacherName;
    }
    public int getTeacherAge() {
        return teacherAge;
    }
    public void setTeacherAge(int teacherAge) {
        this.teacherAge = teacherAge;
    }
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    @ManyToMany
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }
}

package com.model;

import javax.persistence.Embedded;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToOne;
import javax.persistence.Table;

@Entity
@Table(name="student")
public class Student {
    private int id;
    private String name;
    private int age;
    
    @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;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}

三、运行测试

　　

　　出现一张中间表，两个id分别依赖teahcer的id和student的id；

四、修改student.java和teacher.java类（双向）

　　双向：老师知道自己教了哪些学生，学生知道自己被哪几个老师教；

package com.model;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Embedded;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.OneToOne;
import javax.persistence.Table;

@Entity
@Table(name="student")
public class Student {
    private int id;
    private String name;
    private int age;
    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;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @ManyToMany(mappedBy="students")
    public Set<Teacher> getTeachers() {
        return teachers;
    }
    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }
}

package com.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;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="teacher")
public class Teacher {
    private int id;
    private String teacherName;
    private int teacherAge;
    private String title;
    private Set<Student> students = new HashSet<Student>();
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getTeacherName() {
        return teacherName;
    }
    public void setTeacherName(String teacherName) {
        this.teacherName = teacherName;
    }
    public int getTeacherAge() {
        return teacherAge;
    }
    public void setTeacherAge(int teacherAge) {
        this.teacherAge = teacherAge;
    }
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    @ManyToMany
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }
}

五、测试运行

　　

PS：虽然两次结果一样，但是双向关联中，可以通过teacher的students找到老师所教的学生，通过student的teachers找到学生的老师，单向只能从teacher的students找到老师所教的学生，而学生找不到老师

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