EJB3实体映射之ManyToMany

本文解析了在Java中如何使用双向多对多关联映射实现学生与教师之间的关系管理,详细介绍了实体类的设计及Hibernate框架的具体应用。

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Student 类:


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;

@Entity
public class Student implements Serializable{
private static final long serialVersionUID = 4283862967633995348L;
private Integer studentid;
private String studentName;
private Set<Teacher> teachers = new HashSet<Teacher>();

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;
}

// Student类中有一个Teacher集合 对应Teacher类中students属性
@ManyToMany(mappedBy = "students")
public Set<Teacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}

/**
* 返回对象的散列代码值。该实现根据此对象
* 中 studentid 字段计算散列代码值。
* @return 此对象的散列代码值。
*/
@Override
public int hashCode() {
int hash = 0;
hash += (this.studentid != null ? this.studentid.hashCode() : super.hashCode());
return hash;
}

/**
* 确定其他对象是否等于此 Student。当且仅当
* 参数不为 null 且该参数是具有与此对象相同 studentid 字段值的 Student 对象时,
* 结果才为 <code>true</code>。
* @param 对象,要比较的引用对象
* 如果此对象与参数相同,则 @return <code>true</code>;
* 否则为 <code>false</code>。
*/
@Override
public boolean equals(Object object) {
if (!(object instanceof Student)) {
return false;
}
Student other = (Student)object;
if (this.studentid != other.studentid && (this.studentid == null || !this.studentid.equals(other.studentid))) return false;
return true;
}

/**
* 返回对象的字符串表示法。该实现根据 studentid 字段
* 构造此表示法。
* @return 对象的字符串表示法。
*/
@Override
public String toString() {
return this.getClass().getName()+ "[studentid=" + studentid + "]";
}
}



Teacher 类:


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.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;

@Entity
public class Teacher implements Serializable{
private static final long serialVersionUID = 3248995998128746336L;
private Integer teacherid;
private String teacherName;
private Set<Student> students = new HashSet<Student>();

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;
}

// ①Teacher中有一个Student集合
// ②用一张“Teacher_Student”表存两者的关系
// ③joinColumns 对应本类中的外键列 inverseJoinColumns对应另一方的外键列
// ④many 一方延迟加载
@ManyToMany(cascade = {CascadeType.PERSIST,CascadeType.REFRESH}, fetch = FetchType.LAZY)
@JoinTable(name = "Teacher_Student",
joinColumns = {@JoinColumn(name = "Teacher_ID")},
inverseJoinColumns = {@JoinColumn(name = "Student_ID")})
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)) {
this.students.add(student);
}
}

public void removeStudent(Student student) {
if (this.students.contains(student)) {
this.students.remove(student);
}
}

/**
* 返回对象的散列代码值。该实现根据此对象
* 中 teacherid 字段计算散列代码值。
* @return 此对象的散列代码值。
*/
@Override
public int hashCode() {
int hash = 0;
hash += (this.teacherid != null ? this.teacherid.hashCode() : 0);
return hash;
}

/**
* 确定其他对象是否等于此 Teacher。当且仅当
* 参数不为 null 且该参数是具有与此对象相同 teacherid 字段值的 Teacher 对象时,
* 结果才为 <code>true</code>。
* @param 对象,要比较的引用对象
* 如果此对象与参数相同,则 @return <code>true</code>;
* 否则为 <code>false</code>。
*/
@Override
public boolean equals(Object object) {
if (!(object instanceof Teacher)) {
return false;
}
Teacher other = (Teacher)object;
if (this.teacherid != other.teacherid && (this.teacherid == null || !this.teacherid.equals(other.teacherid))) return false;
return true;
}

/**
* 返回对象的字符串表示法。该实现根据 teacherid 字段
* 构造此表示法。
* @return 对象的字符串表示法。
*/
@Override
public String toString() {
return this.getClass().getName()+ "[teacherid=" + teacherid + "]";
}
}
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