《Hibernate上课笔记》-----class7----Hibernate实现多对多关联映射-阿里云开发者社区

《Hibernate上课笔记》-----class7----Hibernate实现多对多关联映射

2022-12-01 87

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简介： 《Hibernate上课笔记》-----class7----Hibernate实现多对多关联映射

一：多对多映射概念

1.实体多对多关联

多对多定义（1：n）：

如果对于实体集A中的每一个实体，实体集B中有n个实体（n>=0）与之联系，反之，对于实体集B中的每一个实体，实体集A中也有m个实体（m>=0）与之联系，则称为实体集A与实体集B具有多对多联系，记为m：n。

示例：一个人可以选修多门课程，一门课程可以被多名同学选修

示例：

2.数据库多对多关联

多增加一张表来表示学生和课程之间的关系。

二：Hibernate实现多对多关联映射

1.使用xml的方式实现

Course类：

public class Course {
  private int id;
  private String name;
  private String courseNo;
  private Set<Student> students = 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 String getCourseNo() {
    return courseNo;
  }
  public void setCourseNo(String courseNo) {
    this.courseNo = courseNo;
  }
  public Set<Student> getStudents() {
    return students;
  }
  public void setStudents(Set<Student> students) {
    this.students = students;
  }
  @Override
  public String toString() {
    return "Course [id=" + id + ", name=" + name + ", courseNo=" + courseNo + ", students=" + students + "]";
  }
}

Course类配置文件：

<?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="course">
    <id name="id" type="int">
      <column name="id"></column>
      <generator class="native"></generator>
    </id>
    <property name="name" type="java.lang.String">
    </property>
    <property name="courseNo" column="course_no"
      type="java.lang.String" />
    <set name="students" table="student_course" inverse="true"
      cascade="all">
      <key column="course_id"></key>
      <many-to-many class="Student" column="student_id">
      </many-to-many>
    </set>
  </class>
</hibernate-mapping>

Student类：

public class Student {
  private int id;
  private String name;
  private String studentNo;
  private Set<Course> 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 String getStudentNo() {
    return studentNo;
  }
  public void setStudentNo(String studentNo) {
    this.studentNo = studentNo;
  }
  public Set<Course> getCourses() {
    return courses;
  }
  public void setCourses(Set<Course> courses) {
    this.courses = courses;
  }
  @Override
  public String toString() {
    return "Student [id=" + id + ", name=" + name + ", studentNo=" + studentNo + ", courses=" + courses + "]";
  }
}

Student类配置文件：

<?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="student">
    <id name="id" type="int">
      <column name="id"></column>
      <generator class="native"></generator>
    </id>
    <property name="name" type="java.lang.String">
    </property>
    <property name="studentNo" column="stu_no"
      type="java.lang.String" />
    <set name="courses" table="student_course" inverse="false"
      cascade="all">
      <key column="student_id"></key>
      <many-to-many class="Course" column="course_id">
      </many-to-many>
    </set>
  </class>
</hibernate-mapping>

测试类：

public class Test {
  public static void main(String[] args) {
    saveStudent();
    HibernateUtil.closeFactory();
  }
  public static void saveStudent() {
    // 1.获取session对象
    Session session = HibernateUtil.openSession();
    // 2.得到用户对象
    Student student = new Student();
    student.setName("张三");
    student.setStudentNo("2018");
    Set<Course> courses = new HashSet<>();
    Course course = new Course();
    course.setName("Java程序设计");
    course.setCourseNo("2020");
    courses.add(course);
    student.setCourses(courses);
    Transaction transaction = session.beginTransaction();
    session.save(student);
    transaction.commit();
    // 4.关闭session对象
    session.close();
    // 5.关闭sessionFactory
  }
}

2.使用注解的方式

Course类：

@Entity
@Table(name = "course")
public class Course {
  private int id;
  private String name;
  private String courseNo;
  private Set<Student> students = new HashSet<>();
  @Id
  @GeneratedValue(generator = "nat")
  @GenericGenerator(name = "nat", strategy = "native")
  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;
  }
  @Column(name = "course_no")
  public String getCourseNo() {
    return courseNo;
  }
  public void setCourseNo(String courseNo) {
    this.courseNo = courseNo;
  }
  @ManyToMany(mappedBy = "courses", cascade = CascadeType.ALL)
  public Set<Student> getStudents() {
    return students;
  }
  public void setStudents(Set<Student> students) {
    this.students = students;
  }
  @Override
  public String toString() {
    return "Course [id=" + id + ", name=" + name + ", courseNo=" + courseNo + ", students=" + students + "]";
  }
}

Student类：

@Entity
@Table(name = "student")
public class Student {
  private int id;
  private String name;
  private String studentNo;
  private Set<Course> courses = new HashSet<>();
  @Id
  @GeneratedValue(generator = "nat")
  @GenericGenerator(name = "nat", strategy = "native")
  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;
  }
  @Column(name = "stu_no")
  public String getStudentNo() {
    return studentNo;
  }
  public void setStudentNo(String studentNo) {
    this.studentNo = studentNo;
  }
  @ManyToMany(cascade = CascadeType.ALL)
  @JoinTable(name = "student_course", joinColumns = @JoinColumn(name = "student_id"), inverseJoinColumns = @JoinColumn(name = "course_id"))
  public Set<Course> getCourses() {
    return courses;
  }
  public void setCourses(Set<Course> courses) {
    this.courses = courses;
  }
  @Override
  public String toString() {
    return "Student [id=" + id + ", name=" + name + ", studentNo=" + studentNo + ", courses=" + courses + "]";
  }
}