JPA--多对多关系

JPA中，多对多关系用@ManyToMany标示。

关系维护端：

package com.yl.demo1.bean.manyTomany;

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 Integer id;
    private String name;
    private Set<Teacher> teachers = new HashSet<Teacher>();
    
    
    public Student(){}
    
    public Student(String name) {
        super();
        this.name = name;
    }
    @Id @GeneratedValue
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    @Column(length=10, nullable=false)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    /**
     * Student是关系的维护端
     * inverseJoinColumns和joinColumns是定义关联表中的字段，其中inverseJoinColumns定义关系被维护端的字段
     * joinColumns定义关系维护端的字段
     */
    @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);
        }
    }
}

关系被维护端：

package com.yl.demo1.bean.manyTomany;

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.ManyToMany;

@Entity
public class Teacher {
    private Integer id;
    private String name;
    private Set<Student> students = new HashSet<Student>();
    
    public Teacher(){}
    
    public Teacher(String name) {
        super();
        this.name = name;
    }
    @Id @GeneratedValue
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    @Column(length=10, nullable=false)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    //teacher为关系的被维护端
    @ManyToMany(cascade=CascadeType.REFRESH, mappedBy="teachers", fetch=FetchType.LAZY)
    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;
        Teacher other = (Teacher) obj;
        if (id == null) {
            if (other.id != null)
                return false;
        } else if (!id.equals(other.id))
            return false;
        return true;
    }
    
    
    
    
}

常见操作：

@Test
    public void save() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();//事务开始
        
        Student student = new Student("yyyy");
        em.persist(student);
        
        Teacher teacher = new Teacher("jiawenhui");
        em.persist(teacher);
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }

    /**
     * 建立学生与老师之间的关系
     */
    @Test
    public void buildConnection() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();//事务开始
        
        Student student = em.find(Student.class, 1);
        
        Teacher teacher = em.getReference(Teacher.class, 1);
        
        student.addTeacher(teacher);
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }

    /**
     * 解除学生与老师之间的关系
     */
    @Test
    public void deleteConnection() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();//事务开始
        
        Student student = em.find(Student.class, 1);
        
        Teacher teacher = em.getReference(Teacher.class, 1);
        
        student.removeTeacher(teacher);
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    /**
     * 删除老师(删除关系被维护端)
     */
    @Test
    public void deleteTeacher() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();//事务开始
        
        Student student = em.find(Student.class, 1);
        Teacher teacher = em.getReference(Teacher.class, 1);
        //先解除Student和Teacher之间的关系
        student.removeTeacher(teacher);
        //在删除Teacher
        em.remove(teacher);
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    /**
     * 删除学生(删除关系维护端)
     */
    @Test
    public void deleteStudent() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();//事务开始
        
        Student student = em.getReference(Student.class, 1);
        em.remove(student);
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }