实体关系之@ManyToMany

packagecom.entity;

importjava.io.Serializable;
importjava.util.HashSet;
importjava.util.Set;

importjavax.persistence.Column;
importjavax.persistence.Entity;
importjavax.persistence.GeneratedValue;
importjavax.persistence.Id;
importjavax.persistence.ManyToMany;
importjavax.persistence.Table;

@Entity
@Table(name="Student")
publicclassStudentimplementsSerializable{

privateIntegerstudentid;//学生ID
privateStringstudentName;//学生姓名
privateSet<Teacher>teachers=newHashSet<Teacher>();//对应的教师集合

publicStudent(){
}

publicStudent(StringstudentName){
this.studentName=studentName;
}

@Id
@GeneratedValue
publicIntegergetStudentid(){
returnstudentid;
}

publicvoidsetStudentid(Integerstudentid){
this.studentid=studentid;
}

@Column(nullable=false,length=32)
publicStringgetStudentName(){
returnstudentName;
}

publicvoidsetStudentName(StringstudentName){
this.studentName=studentName;
}

/*
*@ManyToMany注释表示Student是多对多关系的一边，mappedBy属性定义了Student为双向关系的维护端
*/
@ManyToMany(mappedBy="students")
publicSet<Teacher>getTeachers(){
returnteachers;
}

publicvoidsetTeachers(Set<Teacher>teachers){
this.teachers=teachers;
}
}

Teacher.java

packagecom.entity;

importjava.io.Serializable;
importjava.util.HashSet;
importjava.util.Set;

importjavax.persistence.CascadeType;
importjavax.persistence.Column;
importjavax.persistence.Entity;
importjavax.persistence.FetchType;
importjavax.persistence.GeneratedValue;
importjavax.persistence.Id;
importjavax.persistence.JoinTable;
importjavax.persistence.ManyToMany;
importjavax.persistence.Table;
importjavax.persistence.JoinColumn;

@Entity
@Table
publicclassTeacherimplementsSerializable{

privateIntegerteacherid;//教师ID
privateStringteacherName;//教师姓名
privateSet<Student>students=newHashSet<Student>();//对应的学生集合

publicTeacher(){

}

publicTeacher(StringteacherName){
this.teacherName=teacherName;
}

@Id
@GeneratedValue
publicIntegergetTeacherid(){
returnteacherid;
}

publicvoidsetTeacherid(Integerteacherid){
this.teacherid=teacherid;
}

@Column(nullable=false,length=32)
publicStringgetTeacherName(){
returnteacherName;
}

publicvoidsetTeacherName(StringteacherName){
this.teacherName=teacherName;
}
/*
*@ManyToMany注释表示Teacher是多对多关系的一端。
*@JoinTable描述了多对多关系的数据表关系，name属性指定中间表名称。
*joinColumns定义中间表与Teacher表的外键关系，中间表Teacher_Student的Teacher_ID列是Teacher表的主键列对应的外键列。
*inverseJoinColumns属性定义了中间表与另外一端(Student)的外键关系。
*/
@ManyToMany(cascade=CascadeType.PERSIST,fetch=FetchType.LAZY)
@JoinTable(name="Teacher_Student",
joinColumns={@JoinColumn(name="teacher_ID",referencedColumnName="teacherid")},
inverseJoinColumns={@JoinColumn(name="student_ID",referencedColumnName="studentid")
})
publicSet<Student>getStudents(){
returnstudents;
}

publicvoidsetStudents(Set<Student>students){
this.students=students;
}

publicvoidaddStudent(Studentstudent){
if(!this.students.contains(student)){//检测在该散列表中某些键是否映射到指定值,value查找的值。如果某些键映射到该散列表中的值为true，否则false
this.students.add(student);
}
}

publicvoidremoveStudent(Studentstudent){
this.students.remove(student);
}

}