@ManyToMany mappedby-优快云博客

本文介绍使用Java Persistence API (JPA)实现学生与老师之间的多对多关系映射的方法。通过具体代码示例展示了如何定义实体类、设置关联关系及进行增删操作。

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学生和老师就是多对多的关系。一个学生有多个老师，一个老师教多个学生。多对多映射采取中间表连接的映射策略，建立的中间表将分别引入两边的主键作为外键。jpa 对于中间表的元数据提供了可配置的方式，用户可以自定义中间表的表名，列名。

下面就以学生和老师为例介绍多对多映射关系的实例开发

Student实体类

package com.ljq.entity;

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

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@SuppressWarnings("serial")
@Entity
public class Student implements java.io.Serializable {
    /** 学生ID **/
    private Integer studentid;
    /** 学生姓名 **/
    private String name;
    private Set<Teacher> teachers=new HashSet<Teacher>();
    
    public Student() {
        super();
    }
    
    public Student(String name) {
        super();
        this.name = name;
    }
    
    @Id
    @GeneratedValue
    public Integer getStudentid() {
        return studentid;
    }

    public void setStudentid(Integer studentid) {
        this.studentid = studentid;
    }

    @Column(nullable=false,length=32)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    
    //@ManyToMany注释表示Student是多对多关系的一边，mappedBy属性定义了Student为双向关系的维护端
    //Teacher表是关系的维护者，owner side，有主导权，它有个外键指向Student表。 
    @ManyToMany(mappedBy = "students")
    public Set<Teacher> getTeachers() {
        return teachers;
    }

    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }
    
}

Teacher实体类

package com.ljq.entity;

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

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;

@SuppressWarnings("serial")
@Entity
public class Teacher implements java.io.Serializable {
    /** 教师ID **/
    private Integer teacherid;
    /** 教师姓名 **/
    private String name;
    private Set<Student> students=new HashSet<Student>();
    
    public Teacher() {
        super();
    }
    
    public Teacher(String name) {
        super();
        this.name = name;
    }
    
    @Id
    @GeneratedValue
    public Integer getTeacherid() {
        return teacherid;
    }

    public void setTeacherid(Integer teacherid) {
        this.teacherid = teacherid;
    }

    @Column(nullable=false,length=32)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    //@ManyToMany注释表示Teacher是多对多关系的一端。
    //@JoinTable描述了多对多关系的数据表关系。name属性指定中间表名称，joinColumns定义中间表与Teacher表的外键关系。
    //中间表Teacher_Student的Teacher_ID列是Teacher表的主键列对应的外键列，inverseJoinColumns属性定义了中间表与另外一端(Student)的外键关系。
    @ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)
    @JoinTable(name = "Teacher_Student", 
            joinColumns = { @JoinColumn(name = "Teacher_ID", referencedColumnName = "teacherid") }, 
            inverseJoinColumns = { @JoinColumn(name = "Student_ID", referencedColumnName = "studentid") })
    public Set<Student> getStudents() {
        return students;
    }

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

    /**
     * 添加学生
     * 
     * @param student
     */
    public void addStudent(Student student) {
        if (!this.students.contains(student)) {
            this.students.add(student);
            student.setTeacher(this);
        }
    }

    /**
     * 删除学生
     * 
     * @param student
     */
    public void removeStudent(Student student) {
        if(this.students.contains(student)){
            student.setTeacher(null);
            this.students.remove(student);
        }
    }
}

ManyToManyTest测试类

package com.ljq.test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import org.junit.Test;

import com.ljq.entity.Student;
import com.ljq.entity.Teacher;


public class ManyToManyTest {

    @Test
    public void save() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em=factory.createEntityManager();
        em.getTransaction().begin();
        
        em.persist(new Teacher("张老师"));
        em.persist(new Student("小张"));
        
        em.getTransaction().commit();
        em.close();
        factory.close();
        
    }
    
    /**
     * 为老师添加一个学生
     *
     */
    @Test
    public void build() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em=factory.createEntityManager();
        em.getTransaction().begin();
        
        Teacher teacher = em.find(Teacher.class, 2);
        teacher.addStudent(em.getReference(Student.class, 2));
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    /**
     * 解除学生跟老师的关系
     *
     */
    @Test
    public void  remove() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em=factory.createEntityManager();
        em.getTransaction().begin();
        
        Teacher teacher = em.find(Teacher.class, 2);
        teacher.removeStudent(em.getReference(Student.class, 2));
        
        em.getTransaction().commit();
        em.close();
        factory.close();
        
    }
    
    /**
     * 删除学生，因为学生不是关系维护者，所以需要先手工解除老师与学生的关联，然后再删除学生
     *
     */
    @Test
    public void  deleteStudent() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em=factory.createEntityManager();
        em.getTransaction().begin();
        
        Teacher teacher = em.find(Teacher.class, 2);
        Student student = em.getReference(Student.class, 2);
        teacher.removeStudent(student); //手工解除老师与学生的关系
        em.remove(student); //删除学生
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    /**
     * 删除老师，因为老师是关系维护者，所以可以直接解除老师与学生的关系，不用我们手工解除
     *
     */
    @Test
    public void deleteTeacher() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em=factory.createEntityManager();
        em.getTransaction().begin();
        
        em.remove(em.getReference(Teacher.class, 3));
        
        em.getTransaction().commit();
        em.close();
        factory.close();

    }
    
    /**
     * 用来判断映射是否成功
     * 
     */
    @Test
    public void  test() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        factory.close();
    }

}