实体关系之@ManyToMany

转自http://lym6520.iteye.com/blog/314065

Student.java

package com.entity;  
  
import java.io.Serializable;  
import java.util.HashSet;  
import java.util.Set;  
  
import javax.persistence.Column;  
import javax.persistence.Entity;  
import javax.persistence.GeneratedValue;  
import javax.persistence.Id;  
import javax.persistence.ManyToMany;  
import javax.persistence.Table;  
  
@Entity  
@Table(name = "Student")  
public class Student implements Serializable {  
      
    private Integer studentid;//学生ID  
    private String studentName;//学生姓名  
    private Set<Teacher> teachers = new HashSet<Teacher>();//对应的教师集合  
  
    public Student() {  
    }  
  
    public Student(String studentName) {  
        this.studentName = studentName;  
    }  
  
    @Id  
    @GeneratedValue  
    public Integer getStudentid() {  
        return studentid;  
    }  
  
    public void setStudentid(Integer studentid) {  
        this.studentid = studentid;  
    }  
  
    @Column(nullable = false, length = 32)  
    public String getStudentName() {  
        return studentName;  
    }  
  
    public void setStudentName(String studentName) {  
        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")  
    public Set<Teacher> getTeachers() {  
        return teachers;  
    }  
  
    public void setTeachers(Set<Teacher> teachers) {  
        this.teachers = teachers;  
    }  
}  

Teacher.java 

package com.entity;  
  
import java.io.Serializable;  
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.JoinTable;  
import javax.persistence.ManyToMany;  
import javax.persistence.Table;  
import javax.persistence.JoinColumn;  
  
@Entity  
@Table  
public class Teacher implements Serializable {  
  
    private Integer teacherid;// 教师ID  
    private String teacherName;// 教师姓名  
    private Set<Student> students = new HashSet<Student>();// 对应的学生集合  
  
    public Teacher() {  
  
    }  
  
    public Teacher(String teacherName) {  
        this.teacherName = teacherName;  
    }  
  
    @Id  
    @GeneratedValue  
    public Integer getTeacherid() {  
        return teacherid;  
    }  
  
    public void setTeacherid(Integer teacherid) {  
        this.teacherid = teacherid;  
    }  
  
    @Column(nullable = false, length = 32)  
    public String getTeacherName() {  
        return teacherName;  
    }  
  
    public void setTeacherName(String teacherName) {  
        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")   
    })  
    public Set<Student> getStudents() {  
        return students;  
    }  
  
    public void setStudents(Set<Student> students) {  
        this.students = students;  
    }  
      
    public void addStudent(Student student) {  
        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);  
        }  
    }  
  
    public void removeStudent(Student student) {  
        this.students.remove(student);  
    }  
  
}