1、定义实体类，代码如下：
(1)、学生实体类：
package learn.jpa.entity; import java.util.HashSet;
import java.util.Set; import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.JoinColumn; /**
* @doc 多对多关系是对等的，认为决定谁是维护段，谁是被维护段
* 通过中间表(中间表)来关联它们之间的关系
*/
@Entity
public class Student { private int id;
private String name;
private Set<Teacher> teacher = new HashSet<Teacher>(); public Student() {
super();
} public Student(String name) {
this.name = name;
} @Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
@Column(length=10,nullable=false)
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
/**
* inverseJoinColumns 与被维护端关联外键
* joinColumns 与维护段关联外键
*/
@ManyToMany(cascade=CascadeType.REFRESH)
@JoinTable(name="student_teacher",inverseJoinColumns=@JoinColumn(name="teacher_id"),joinColumns=@JoinColumn(name="student_id"))
public Set<Teacher> getTeacher() {
return teacher;
}
public void setTeacher(Set<Teacher> teacher) {
this.teacher = teacher;
} // 建立老师和学生的关系
public void addTeacher(Teacher teachers){
this.teacher.add(teachers);
} // 解除老师和学生的关系
// 删除 teacher 是根据 id来删除的，所有在 Teacher 类中要重写 equals() 和 hashCode()
public void removeTeacher(Teacher teachers){
if(this.teacher.contains(teachers)){
this.teacher.remove(teachers);
}
}
}(2)、老师实体类：
package learn.jpa.entity; import java.util.HashSet;
import java.util.Set; import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany; @Entity
public class Teacher { private int id;
private String name;
private Set<Student> student = new HashSet<Student>(); // 如果声明了有参数的构造函数，那么一定要声明无参构造函数
public Teacher(){} public Teacher(String name){
this.name = name;
} @Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
@Column(length=10,nullable=false)
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@ManyToMany(cascade=CascadeType.REFRESH,mappedBy="teacher")
public Set<Student> getStudent() {
return student;
}
public void setStudent(Set<Student> student) {
this.student = student;
} @Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
} @Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Teacher other = (Teacher) obj;
if (id != other.id)
return false;
return true;
} }
2、相关操作类，代码如下：
package learn.jpa.test; import static org.junit.Assert.*; import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence; import learn.jpa.entity.Student;
import learn.jpa.entity.Teacher; import org.junit.Test; public class ManyToManyTest { /**
* 测试数据库是否可以生成表
*/
@Test
public void test() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
factory.close();
} @Test
public void save(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin(); // 开启事务
em.persist(new Student("xx"));
em.persist(new Teacher("xiuliu"));
em.getTransaction().commit();
em.close();
factory.close();
} /**
* 建立老师和学生的关系（将关联的字段数据保存到中间表中）
*/
@Test
public void buildTS(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin(); // 开启事务
// 处于托管状态，可以对数据进行更新
Student student = em.find(Student.class, 1);
student.addTeacher(em.getReference(Teacher.class, 1));
em.getTransaction().commit();
em.close();
factory.close();
} // 解除老师和学生的关系
@Test
public void deleteTS(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin(); // 开启事务
// 处于托管状态，可以对数据进行更新
Student student = em.find(Student.class, 1);
student.removeTeacher(em.getReference(Teacher.class, 1));
em.getTransaction().commit();
em.close();
factory.close();
} // 删除老师
@Test
public void deleteTeacher(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin(); // 开启事务
// 处于托管状态，可以对数据进行更新
// 先解除老师和学生的关系，在删除老师
Student student = em.find(Student.class, 1);
Teacher teacher = em.getReference(Teacher.class, 1);
student.removeTeacher(teacher);
em.remove(teacher);
em.getTransaction().commit();
em.close();
factory.close();
} // 删除学生
@Test
public void deleteStudent(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin(); // 开启事务
Student student = em.getReference(Student.class, 1);
em.remove(student);
em.getTransaction().commit();
em.close();
factory.close();
}
}