Hibernate注解配置一对一、一对多实例

使用Hibernate注解开发学生选课系统、其中共有4张表：
1、课程表Course：表明存在哪些课程。
2、学生表Student：记录学生信息。
3、选课表sc：此表为学生选课的记录，维护学生和课程的多对多关系。
4、教师表teacher：记录教师信息，一个老师只教一门课，但一门课程可以由多个老师教授，与课程表为一对多的关系。

开发流程：

1. 导入Hibernate相关jar包

image.png

2.配置hibernate的核心文件hibernate.cfg.xml

    
    
        
            
            com.mysql.jdbc.Driver
            jdbc:mysql://localhost:3306/spring?characterEncoding=utf8
            root
            1234
            
            
            10
            true
            true
            update
                
            org.hibernate.dialect.MySQL5InnoDBDialect
            
            
            
            
            
        
    
    

3.编写实体类
Course:

package com.lj.entity;

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

import javax.persistence.CascadeType;
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.OneToMany;

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;
    
    private String name;
    
    @ManyToMany
    @JoinTable(name="sc",
            joinColumns=@JoinColumn(name="cid"),
            inverseJoinColumns=@JoinColumn(name="sid")) 
    private Set students = new HashSet<>();
    
    //mappedBy表示此表为从表，关系由对方维护相当于inverse="true"
    @OneToMany(cascade=CascadeType.ALL,mappedBy="course")
    private Set teachers = new HashSet<>();
    
    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 Set getStudents() {
        return students;
    }
    public void setStudents(Set students) {
        this.students = students;
    }
    public Set getTeachers() {
        return teachers;
    }
    public void setTeachers(Set teachers) {
        this.teachers = teachers;
    }
}

Teacher:

package com.lj.entity;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;


@Entity
public class Teacher {
    
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;
    
    private String name;
    
    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="course_id")
    private Course course;
    
    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 Course getCourse () {
        return course;
    }
    public void setCourse (Course course) {
        this.course = course;
    }
}

Student:

package com.lj.entity;


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

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 org.hibernate.annotations.Cascade;
import org.hibernate.annotations.CascadeType;


@Entity
public class Student {
    
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;
    
    private String name;
    
    @ManyToMany
    //设置级联操作
    @Cascade(CascadeType.SAVE_UPDATE)
    //配置中间表sc，joinColumns为本表在sc的字段，inverseJoinColumns为对方表在sc的字段
    @JoinTable(name="sc",
        joinColumns=@JoinColumn(name="sid"),
        inverseJoinColumns=@JoinColumn(name="cid"))
    private Set courses = new HashSet<>();
    
    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 Set getCourses() {
        return courses;
    }
    public void setCourses(Set courses) {
        this.courses = courses;
    }
}

注意在使用注解的方法开发给entity类添加注解的时候一定注意注解要么全写在属性上要么全写在get方法上一定不要混着写否则会出现以下错误:
Caused by: org.hibernate.MappingException: Could not determine type for: java.util.List, at table...

编写测试类：

public class ManyTableTest {
    private Configuration cfg;
    private SessionFactory sessionFactory;
    private Session session;
    private Transaction tx;
    
    @Before
    public void common(){
        cfg = new Configuration().configure();
        sessionFactory = cfg.buildSessionFactory();
        session = sessionFactory.openSession();
        tx = session.beginTransaction();
    }
    
    @Test
    public void testManyToMany(){
        
        Teacher teacher1 =new Teacher();
        teacher1.setName("张老师");
        
        Teacher teacher2 = new Teacher();
        teacher2.setName("李老师");
        
        Course course1 = new Course();
        course1.setName("语文");
        teacher1.setCourse(course1);
        
        Course course2 = new Course();
        course2.setName("数学");
        
        Student student1 = new Student();
        student1.setName("小明");
        Student student2 = new Student();
        student2.setName("小刚");
        
        //小明选了语文和数学课
        student1.getCourses().add(course1);
        student1.getCourses().add(course2);
        //数学课添加了小刚
        course2.getStudents().add(student2);
        session.save(teacher1);
        session.save(teacher2);
        session.save(course1);
        session.save(course2);
        session.save(student1);
        session.save(student2);
    }
    
    @After
    public void after(){
        tx.commit();
        session.close();
        sessionFactory.close();
    }

}