JPA 系列教程7-双向多对多

时间：2021-02-14 22:31:33

双向多对多的ddl语句

同单向多对多表的ddl语句一致

Student

package com.jege.jpa.many2many;

import java.util.HashSet;

import java.util.Set;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.ManyToMany;

import javax.persistence.Table;

/**

 * @author JE哥

 * @email 1272434821@qq.com

 * @description:双向：关系被维护端:不能操作中间表

 */

@Entity

@Table(name = "t_student")

public class Student {

  @Id

  @GeneratedValue

  private Long id;

  private String sname;

  @ManyToMany(mappedBy = "students")

  private Set<Teacher> teachers = new HashSet<Teacher>();

  public Student() {

  }

  public Student(String sname) {

    this.sname = sname;

  }

  public Long getId() {

    return id;

  }

  public void setId(Long id) {

    this.id = id;

  }

  public String getSname() {

    return sname;

  }

  public void setSname(String sname) {

    this.sname = sname;

  }

  public Set<Teacher> getTeachers() {

    return teachers;

  }

  public void setTeachers(Set<Teacher> teachers) {

    this.teachers = teachers;

  }

  @Override

  public String toString() {

    return "Student [id=" + id + ", sname=" + sname + "]";

  }

}

Teacher

package com.jege.jpa.many2many;

import java.util.HashSet;

import java.util.Set;

import javax.persistence.CascadeType;

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;

import javax.persistence.Table;

/**

 * @author JE哥

 * @email 1272434821@qq.com

 * @description:双向：关系维护端:可以操作中间表

 */

@Entity

@Table(name = "t_teacher")

public class Teacher {

  @Id

  @GeneratedValue

  private Long id;

  private String tname;

  // @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 = "t_teacher_student", joinColumns = { @JoinColumn(name = "teacher_id") }, inverseJoinColumns = {

      @JoinColumn(name = "student_id") })

  private Set<Student> students = new HashSet<Student>();

  public Teacher() {

  }

  public Teacher(String tname) {

    this.tname = tname;

  }

  public Long getId() {

    return id;

  }

  public void setId(Long id) {

    this.id = id;

  }

  public String getTname() {

    return tname;

  }

  public void setTname(String tname) {

    this.tname = tname;

  }

  public Set<Student> getStudents() {

    return students;

  }

  public void setStudents(Set<Student> students) {

    this.students = students;

  }

  @Override

  public String toString() {

    return "Teacher [id=" + id + ", tname=" + tname + "]";

  }

}

Many2ManyTest

package com.jege.jpa.many2many;

import javax.persistence.EntityManager;

import javax.persistence.EntityManagerFactory;

import javax.persistence.Persistence;

import org.junit.After;

import org.junit.AfterClass;

import org.junit.Before;

import org.junit.BeforeClass;

import org.junit.Test;

/**

 * @author JE哥

 * @email 1272434821@qq.com

 * @description:双向多对多Test

 */

public class Many2ManyTest {

  private static EntityManagerFactory entityManagerFactory = null;

  private EntityManager entityManager = null;

  @BeforeClass

  public static void setUpBeforeClass() throws Exception {

    entityManagerFactory = Persistence.createEntityManagerFactory("com.jege.jpa");

  }

  @Before

  public void setUp() throws Exception {

    entityManager = entityManagerFactory.createEntityManager();

  }

  // t1老师教2个学生s1,s2

  // t2老师教3个学生s1,s2,s3

  // 总共10条insert

  @Test

  public void persist() throws Exception {

    entityManager.getTransaction().begin();

    Teacher t1 = new Teacher("t1");

    Teacher t2 = new Teacher("t2");

    Student s1 = new Student("s1");

    Student s2 = new Student("s2");

    Student s3 = new Student("s3");

    entityManager.persist(t1);

    entityManager.persist(t2);

    entityManager.persist(s1);

    entityManager.persist(s2);

    entityManager.persist(s3);// 全部发出5条insert单表

    // 添加中间表

    // t1老师教2个学生s1,s2

    // t2老师教3个学生s1,s2,s3

    t1.getStudents().add(s1);

    t1.getStudents().add(s2);

    t2.getStudents().add(s1);

    t2.getStudents().add(s2);

    t2.getStudents().add(s3);

    entityManager.getTransaction().commit();// 发出5条insert中间表

    entityManager.close();

  }

  // t1老师教2个学生s1,s2

  // 修改为 教2个学生s1,s3:先删除在添加

  @Test

  public void update() throws Exception {

    persist();

    entityManager.getTransaction().begin();

    Teacher t1 = entityManager.find(Teacher.class, 1L);

    Student s2 = entityManager.find(Student.class, 2L);

    Student s3 = entityManager.find(Student.class, 3L);

    // 删除

    t1.getStudents().remove(s2);

    // 添加

    t1.getStudents().add(s3);

    entityManager.getTransaction().commit();

  }

  // 删除t1的教的所有学生

  @Test

  public void delete() throws Exception {

    persist();

    entityManager.getTransaction().begin();

    Teacher t1 = entityManager.find(Teacher.class, 1L);

    t1.getStudents().clear();

    entityManager.getTransaction().commit();

  }

  // insert 3条，单表插入,中间表无反应

  // 因为Student是关系被维护端:不能操作中间表

  @Test

  public void persist2() throws Exception {

    Teacher teacher = new Teacher("t1");

    Student student1 = new Student("s1");

    Student student2 = new Student("s2");

    student1.getTeachers().add(teacher);// 插入中间表

    student2.getTeachers().add(teacher);

    entityManager.getTransaction().begin();

    entityManager.persist(teacher);

    entityManager.persist(student1);

    entityManager.persist(student2);

    entityManager.getTransaction().commit();

  }

  @Test

  public void find() throws Exception {

    persist();

    entityManager = entityManagerFactory.createEntityManager();

    Teacher teacher = entityManager.find(Teacher.class, 1L);

    System.out.println(teacher.getTname());

    System.out.println("------------------");

    System.out.println(teacher.getStudents());

  }

  @Test

  public void find2() throws Exception {

    persist();

    entityManager = entityManagerFactory.createEntityManager();

    Student student = entityManager.find(Student.class, 1L);

    System.out.println(student.getSname());

    System.out.println("------------------");

    System.out.println(student.getTeachers());

  }

  @After

  public void tearDown() throws Exception {

    if (entityManager != null && entityManager.isOpen())

      entityManager.close();

  }

  @AfterClass

  public static void tearDownAfterClass() throws Exception {

    if (entityManagerFactory != null && entityManagerFactory.isOpen())

      entityManagerFactory.close();

  }

}

其他关联项目

源码地址

https://github.com/je-ge/jpa

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JPA 系列教程7-双向多对多

JPA 系列教程7-双向多对多

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