JPA中，多对多关系用@ManyToMany标示。
关系维护端：
1 package com.yl.demo1.bean.manyTomany; 2 3 import java.util.HashSet; 4 import java.util.Set; 5 6 import javax.persistence.CascadeType; 7 import javax.persistence.Column; 8 import javax.persistence.Entity; 9 import javax.persistence.GeneratedValue;10 import javax.persistence.Id;11 import javax.persistence.JoinColumn;12 import javax.persistence.JoinTable;13 import javax.persistence.ManyToMany;14 15 @Entity16 public class Student {17 private Integer id;18 private String name;19 private Set teachers = new HashSet ();20 21 22 public Student(){}23 24 public Student(String name) {25 super();26 this.name = name;27 }28 @Id @GeneratedValue29 public Integer getId() {30 return id;31 }32 public void setId(Integer id) {33 this.id = id;34 }35 @Column(length=10, nullable=false)36 public String getName() {37 return name;38 }39 public void setName(String name) {40 this.name = name;41 }42 /**43 * Student是关系的维护端44 * 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45 * joinColumns定义关系维护端的字段46 */47 @ManyToMany(cascade=CascadeType.REFRESH)48 @JoinTable(name="student_teacher", inverseJoinColumns=@JoinColumn(name="teacher_id"),49 joinColumns=@JoinColumn(name="student_id"))50 public Set getTeachers() {51 return teachers;52 }53 public void setTeachers(Set teachers) {54 this.teachers = teachers;55 }56 57 public void addTeacher(Teacher teacher) {58 this.teachers.add(teacher);59 }60 61 public void removeTeacher(Teacher teacher) {62 if (this.teachers.contains(teacher)) {63 this.teachers.remove(teacher);64 }65 }66 }
关系被维护端：
1 package com.yl.demo1.bean.manyTomany; 2 3 import java.util.HashSet; 4 import java.util.Set; 5 6 import javax.persistence.CascadeType; 7 import javax.persistence.Column; 8 import javax.persistence.Entity; 9 import javax.persistence.FetchType;10 import javax.persistence.GeneratedValue;11 import javax.persistence.Id;12 import javax.persistence.ManyToMany;13 14 @Entity15 public class Teacher {16 private Integer id;17 private String name;18 private Set students = new HashSet ();19 20 public Teacher(){}21 22 public Teacher(String name) {23 super();24 this.name = name;25 }26 @Id @GeneratedValue27 public Integer getId() {28 return id;29 }30 public void setId(Integer id) {31 this.id = id;32 }33 @Column(length=10, nullable=false)34 public String getName() {35 return name;36 }37 public void setName(String name) {38 this.name = name;39 }40 //teacher为关系的被维护端41 @ManyToMany(cascade=CascadeType.REFRESH, mappedBy="teachers", fetch=FetchType.LAZY)42 public Set getStudents() {43 return students;44 }45 public void setStudents(Set students) {46 this.students = students;47 }48 49 @Override50 public int hashCode() {51 final int prime = 31;52 int result = 1;53 result = prime * result + ((id == null) ? 0 : id.hashCode());54 return result;55 }56 57 @Override58 public boolean equals(Object obj) {59 if (this == obj)60 return true;61 if (obj == null)62 return false;63 if (getClass() != obj.getClass())64 return false;65 Teacher other = (Teacher) obj;66 if (id == null) {67 if (other.id != null)68 return false;69 } else if (!id.equals(other.id))70 return false;71 return true;72 }73 74 75 76 77 } 常见操作：
1 @Test 2 public void save() { 3 EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL"); 4 EntityManager em = factory.createEntityManager(); 5 em.getTransaction().begin();//事务开始 6 7 Student student = new Student("yyyy"); 8 em.persist(student); 9 10 Teacher teacher = new Teacher("jiawenhui");11 em.persist(teacher);12 13 em.getTransaction().commit();14 em.close();15 factory.close();16 }17 18 /**19 * 建立学生与老师之间的关系20 */21 @Test22 public void buildConnection() {23 EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");24 EntityManager em = factory.createEntityManager();25 em.getTransaction().begin();//事务开始26 27 Student student = em.find(Student.class, 1);28 29 Teacher teacher = em.getReference(Teacher.class, 1);30 31 student.addTeacher(teacher);32 33 em.getTransaction().commit();34 em.close();35 factory.close();36 }37 38 /**39 * 解除学生与老师之间的关系40 */41 @Test42 public void deleteConnection() {43 EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");44 EntityManager em = factory.createEntityManager();45 em.getTransaction().begin();//事务开始46 47 Student student = em.find(Student.class, 1);48 49 Teacher teacher = em.getReference(Teacher.class, 1);50 51 student.removeTeacher(teacher);52 53 em.getTransaction().commit();54 em.close();55 factory.close();56 }57 58 /**59 * 删除老师(删除关系被维护端)60 */61 @Test62 public void deleteTeacher() {63 EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");64 EntityManager em = factory.createEntityManager();65 em.getTransaction().begin();//事务开始66 67 Student student = em.find(Student.class, 1);68 Teacher teacher = em.getReference(Teacher.class, 1);69 //先解除Student和Teacher之间的关系70 student.removeTeacher(teacher);71 //在删除Teacher72 em.remove(teacher);73 74 em.getTransaction().commit();75 em.close();76 factory.close();77 }78 79 /**80 * 删除学生(删除关系维护端)81 */82 @Test83 public void deleteStudent() {84 EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");85 EntityManager em = factory.createEntityManager();86 em.getTransaction().begin();//事务开始87 88 Student student = em.getReference(Student.class, 1);89 em.remove(student);90 91 em.getTransaction().commit();92 em.close();93 factory.close();94 }95