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IDprivate 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@GeneratedValuepublic 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;}}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@Tablepublic class Teacher implements Serializable {private Integer teacherid;// 教师IDprivate 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@GeneratedValuepublic 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falsethis.students.add(student);}}public void removeStudent(Student student) {this.students.remove(student);}}