EJB3实体映射之ManyToMany-优快云博客

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 + "]";
     } 
}