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();
     }