@ManyToMany mappedby

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")@Entitypublic 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")@Entitypublic 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();    }}