Hibernate5.2之多对多关联关系(六)

                                             Hibernate5.2之多对多关联关系(六)

一.简介

　　Hibernate中多对多关联关系在工程中使用的频率也是非常高的，例如：大学中学生选课、用户订单等。在本博客中笔者一样通过实例代码为读者阐述Hibernate中的这种关联关系，笔者所采用的是学生选课的案例。

二.数据库的创建

create table courses (
   id number(10,0) not null, 
   cname varchar2(255 char), 
   primary key (id)
);

create table stu_course (
   student_id number(10,0) not null, 
   course_id number(10,0) not null, 
   primary key (course_id, student_id)
);

create table students (
   id number(10,0) not null, 
   name varchar2(255 char), 
   primary key (id)
);

三.hbm配置文件的方式

3.1 POJO类的创建

public class Student {
    private int id;
    private String name;
    private Set<Course> courses;
    
    //setter and getter
}

public class Course {
    private int id;
    private String name;
    private Set<Student> students;
    
    //setter and getter
}

3.2 hbm文件的创建

Student.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.demo.hibernate.many2many.Student" table="students">
        <id name="id" type="int" column="id">
            <generator class="increment"></generator>
        </id>
        
        <property name="name" type="string" column="name"></property>
        
        <set name="courses" table="stu_course">
            <key column="student_id"></key>
            <many-to-many class="com.demo.hibernate.many2many.Course" column="course_id"></many-to-many>
        </set>
    </class>
</hibernate-mapping>

Course.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.demo.hibernate.many2many.Course" table="courses">
        <id name="id" type="int" column="id">
            <generator class="increment"></generator>
        </id>
    
        <property name="name" type="string" column="cname"></property>
        
        <set name="students" table="stu_course" inverse="true">
            <key column="course_id"></key>
            <many-to-many class="com.demo.hibernate.many2many.Student" column="student_id"></many-to-many>
        </set>
    </class>
</hibernate-mapping>

四.注解的方式

Student.java

@Entity
@Table(name="students")
public class Student {
    
    @Id
    @Column(name="id")
    @GenericGenerator(name="incrementGenerator", strategy="increment")
    @GeneratedValue(generator="incrementGenerator")
    private int id;
    
    @Column(name="name")
    private String name;
    
    @ManyToMany
    @JoinTable(name="stu_course", 
        joinColumns={@JoinColumn(name="student_id")}, 
        inverseJoinColumns={@JoinColumn(name="course_id")})
    private Set<Course> courses;
   
    //setter and getter
}

 Course.java

@Entity
@Table(name="courses")
public class Course {
    
    @Id
    @Column(name="id")
    @GenericGenerator(name="incrementGenerator", strategy="increment")
    @GeneratedValue(generator="incrementGenerator")
    private int id;
    
    @Column(name="cname")
    private String name;
    
    @ManyToMany(mappedBy="courses")
    private Set<Student> students;
    
    //setter and getter
}

五.代码测试

A.保存

@Test
public void save(){
    Transaction tx = session.beginTransaction();
        
    Student s1 = new Student();
    s1.setName("BBB");
        
    Course c1 = new Course();
    c1.setName("音乐");
        
    Set<Student> students = new HashSet<Student>();
    Set<Course> courses = new HashSet<Course>();
        
    students.add(s1);
    courses.add(c1);
        
    s1.setCourses(courses);
    c1.setStudents(students);
        
    session.save(s1);
    session.save(c1);
        
    tx.commit();
}

B.更新1

//新开了一门课，然后学生选择了该课程
@Test
public void addCourse(){
    Transaction tx = session.beginTransaction();
    Student student = session.get(Student.class, 1);
    Course c1 = new Course();
    c1.setName("音乐");
        
    student.getCourses().add(c1);
    session.saveOrUpdate(c1);
    session.saveOrUpdate(student);
    tx.commit();
}

C.更新2

//新增学生，然后学生选择课程
@Test
public void  addStudent(){
    Transaction tx = session.beginTransaction();
    Student s = new Student();
    s.setName("HHH");
        
    Query<Course> query = session.createQuery("from Course where id in :ids", Course.class);
    List<Integer> list = new ArrayList<Integer>();
    list.add(1);
    list.add(2);
    query.setParameter("ids", list);
        
    Set<Course> courses = new HashSet<Course>(query.getResultList());
        
    s.setCourses(courses);
    session.save(s);
    tx.commit();
}

D.更新3

//学生改变课程
@Test
public void changeCourse(){
    Transaction tx = session.beginTransaction();
    Student student = session.get(Student.class, 1);
        
    Course course = session.get(Course.class, 2);
    Set<Course> courses = new HashSet<Course>();
    courses.add(course);
        
    student.setCourses(courses);
        
    session.update(student);
    tx.commit();
}

E.删除

//删除了学生
@Test
public void delete(){
    Transaction tx = session.beginTransaction();
    Student s = new Student();
    s.setId(1);
        
    session.delete(s);
    tx.commit();
}