hibernate学习五（关系映射多对一与一对多）

一、多对一

　　多对一（或者一对多）：在学生与老师的情况下，一个老师可以教多个学生，但一个学生只能被教一个老师教；

　　对于类：在多的那方拥有一的那方的一个实体

二、修改student.java和teacher.java类

　　复制03工程，并更名为Hibernate_04_ManyToOne；

　　在多的一方中添加@ManyToOne注解

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;
    private Teacher 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;
    }
    @ManyToOne
    public Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
}

package com.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="teacher")
public class Teacher {
    private int id;
    private String teacherName;
    private int teacherAge;
    private String title;
    
    @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;
    }
}

修改xml，添加使用teacher.java类的注解的配置；

        <mapping class="com.model.Teacher"/>

三、运行测试；

　　

　　

　　结果：

　　在student表中生成了一个外键，依赖teacher表，teacher表中没有特别的改变；

四、一对多

　　一对多：对于类，在一的那方存在多的那方的集合

五、修改student和teahcer类

　　复制04工程，并更名为Hibernate_05_OneToMany；

　　在一的那方添加集合类（推荐set），并添加@OneToMany注解

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;
    }
}

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.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;
    }
    @OneToMany
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }
}

六、测试运行

　　

　　结果生成了一张中间表teacher_student

　　

PS：关于双向关联，可以在studenjava中添加字段Teacher teacher，并使用注解@ManyToOne；

PSS：有双向关联的情况，加上mappingBy；

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