08讲、JPA中的多对多双向关联实体定义、注解设置与各项关系操作
一定要找准谁是关系维护端，谁是关系被维护端，所有操作都是从关系维护端进行的
2、
package cn.itcast.bean;
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.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
@Entity
public class Student {
private Long id;
private String name;
private Set<Teacher> teachers = new HashSet<Teacher>();
public Student(){}
public Student(String name) {
this.name = name;
}
@Id @GeneratedValue
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
@Column(length=10,nullable=false)
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
/**
* inverseJoinColumns关系被维护端外键的定义
* joinColumns关系维护端外键的定义
*/
@ManyToMany(cascade=CascadeType.REFRESH)
@JoinTable(name="student_teacher",inverseJoinColumns=@JoinColumn(name="teacher_id")
,joinColumns=@JoinColumn(name="student_id"))
public Set<Teacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}
public void addTeacher(Teacher teacher){
this.teachers.add(teacher);
}
public void removeTeacher(Teacher teacher){
if(this.teachers.contains(teacher)){
this.teachers.remove(teacher);
}
}
}
package cn.itcast.bean;
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 Long id;
private String name;
private Set<Student> students = new HashSet<Student>();
public Teacher(){}
public Teacher(String name) {
this.name = name;
}
@Id @GeneratedValue
public Long getId() {
return id;
}
public void setId(Long 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="teachers")
public Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((id == null) ? 0 : id.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
final Teacher other = (Teacher) obj;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
return true;
}
}
package junit.test;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import org.junit.BeforeClass;
import org.junit.Test;
import cn.itcast.bean.Student;
import cn.itcast.bean.Teacher;
public class ManyToManyTest {
@BeforeClass
public static void setUpBeforeClass() throws Exception {
}
@Test public void save(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
em.persist(new Student("小王"));
em.persist(new Teacher("李勇老师"));
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 建立学生跟老师的关系
*/
@Test public void buildTS(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Student student = em.find(Student.class, 45l);
student.addTeacher(em.getReference(Teacher.class, 48l));
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 删除学生跟老师的关系
*/
@Test public void deleteTS(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Student student = em.find(Student.class, 45l);
student.removeTeacher(em.getReference(Teacher.class, 46l));
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 删除老师
* 因为有外键的约束，必须把学生和老师的关系删除掉才可以删除学生
*/
@Test public void deleteT(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Student student = em.find(Student.class, 45l);
Teacher teacher = em.getReference(Teacher.class, 46l);
student.removeTeacher(teacher);
em.remove(teacher);
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 删除学生
* 可以直接删除
*/
@Test public void deleteS(){
EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Student student = em.find(Student.class, 45l);
em.remove(student);
em.getTransaction().commit();
em.close();
factory.close();
}
}