維護端注解

@ManyToMany (cascade?=?CascadeType.REFRESH)

@JoinTable (//關聯表

?????????????????? name?=? "student_teacher" , //關聯表名

?????????????????? inverseJoinColumns?=? @JoinColumn (name?=? "teacher_id" ),//被維護端外鍵

?????????????????? joinColumns?=? @JoinColumn (name?=? "student_id" ))//維護端外鍵

被維護端注解

@ManyToMany(

?????????????????? cascade = CascadeType.REFRESH,

?????????????????? mappedBy = "teachers",//通過維護端的屬性關聯

?????????????????? fetch = FetchType.LAZY)

關系維護端刪除時，如果中間表存在些紀錄的關聯信息，則會刪除該關聯信息;

關系被維護端刪除時，如果中間表存在些紀錄的關聯信息，則會刪除失敗 .

?

?

以學生和老師的對應關系為例。一個學生可以擁有多個老師，一個老師也可以擁有多個學生。

學生實體類

      package com.taoistwar.jpa.entity.manytomany;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;

@Entity
public class Student {
	private Integer id;
	private String name;
	private Set<Teacher> teachers = new HashSet<Teacher>();

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	@Column(nullable = false, length = 16)
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@ManyToMany(cascade = CascadeType.REFRESH)
	@JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "teacher_id"), joinColumns = @JoinColumn(name = "student_id"))
	public Set<Teacher> getTeachers() {
		return teachers;
	}

	public void setTeachers(Set<Teacher> teachers) {
		this.teachers = teachers;
	}

	public void addTeacher(Teacher teacher) {
		this.teachers.add(teacher);
	}

	public void removeTeachers(Teacher teacher) {
		this.teachers.remove(teacher);
	}

}

    

?重點在于：

      @ManyToMany(cascade = CascadeType.REFRESH)
	@JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "teacher_id"), joinColumns = @JoinColumn(name = "student_id"))
	public Set<Teacher> getTeachers() {
		return teachers;
	}


    

?老師實體類

      package com.taoistwar.jpa.entity.manytomany;

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.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Teacher {
	private Integer id;
	private String name;
	private Set<Student> students = new HashSet<Student>();

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	@Column(nullable = false, length = 16)
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers", fetch = FetchType.LAZY)
	public Set<Student> getStudents() {
		return students;
	}

	public void setStudents(Set<Student> students) {
		this.students = students;
	}

      @Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((id == null) ? 0 : id.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Teacher other = (Teacher) obj;
		if (id == null) {
			if (other.id != null)
				return false;
		} else if (!id.equals(other.id))
			return false;
		return true;
	}
}

    

?重點在于：

      @ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers", fetch = FetchType.LAZY)
	public Set<Student> getStudents() {
		return students;
	}
    

?擁有mappedBy注解的實體類為關系被維護端，另外的實體類為關系維護端的。顧名思意，關系的維護端對關系（在多對多為中間關聯表）的CRUD做操作。關系的被維護端沒有該操作，不能維護關系。

測試類

轉 http://taoistwar.iteye.com