实体关系之@ManyToMany

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);
	}

}

原文地址：http://lym6520.iteye.com/blog/314065