多對多關係相比其他其中關聯關係，顯得稍微複雜了一點點，這個複雜度主要體現在對這種關聯關係的理解上。和其他關聯關係不同的是這種關聯多出來了一張中間表，操作上多了些許複雜，來隨便看下吧

1  實體的定義

Student表：

package org.lxh.info;

import java.util.*;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
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;

	@Id
	@GeneratedValue
	public Integer getId() {
		return id;
	}

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

	@Column(length = 50)
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
	@ManyToMany(cascade=CascadeType.REFRESH)
	@JoinTable(name="student_teacher",
 
[email protected](name="teacherId"),[email protected](name="stuId"))
	public Set<Teacher> getTeachers() {
		return teachers;
	}

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

@ManyToMany表示多對多關聯，對於這種關聯極少數情況會使用級聯刪除，我這裡設定的是級聯重新整理；因為有中間表的存在這裡使用@JoinTable來設定關聯表後面的name配置的是關聯表的名稱，inverseJoinColumn配置的是關係被維護一方主鍵對應的中間表字段，joinColumn配置的是關係維護方主鍵對應的中間表字段。這裡是最複雜的地方

Teacher表：

package org.lxh.info;

import java.util.*;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Teacher {
	private Integer id;
	private String name;
	private Set<Student> students;
       @Id @GeneratedValue
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}
        @Column(length=50)
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
        @ManyToMany(cascade=CascadeType.REFRESH,mappedBy="teachers")
	public Set<Student> getStudents() {
		return students;
	}

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

到這裡實體就定義完了，要注意的地方就是隻有關係維護的一方才能更新中間表裡的外來鍵記錄。如果反過來則會報錯

2  多對多資料儲存

/**
	 * 雙向多對多儲存
	 */
	@org.junit.Test
	public void testMany2ManyInsert() {
		EntityManager em=null;
		EntityTransaction tx=null;
		try{
			em=JpaUtil.getEntityManager();
			tx=em.getTransaction();
			tx.begin();
			
			Student s=new Student();
			s.setName("張小華");
			Student s2=new Student();
			s2.setName("陳小明");
			Set<Student> students=new HashSet<Student>();
			students.add(s);
			students.add(s2);
			
			Teacher t=new Teacher();
			t.setName("李老師");
			t.setStudents(students);
			
			Teacher t2=new Teacher();
			t2.setName("劉老師");
			t2.setStudents(students);
			
			Set<Teacher> teachers=new HashSet<Teacher>();
			teachers.add(t);
			teachers.add(t2);
			
			s.setTeachers(teachers);
			s2.setTeachers(teachers);
			
			
                        em.persist(s);
                        em.persist(s2);
                        em.persist(t);
                        em.persist(t2);
			
			
		    
			tx.commit();
		}catch(Exception e){
			e.printStackTrace();
		}finally{
			if(em!=null){
				em.close();
			}
		}
		
	}

儲存操作是比較簡單的，但是如果要刪除或解除關聯關係，就稍顯複雜了，來看這段程式碼

@org.junit.Test
	public void testMany2ManyDelete() {
		EntityManager em=null;
		EntityTransaction tx=null;
		try{
			em=JpaUtil.getEntityManager();
			tx=em.getTransaction();
			tx.begin();
			Student s=em.find(Student.class, 1);
			Student s2=em.find(Student.class, 2);
			Teacher t=em.find(Teacher.class, 2);
			//先解除關聯關係
			s.getTeachers().remove(t);
			s2.getTeachers().remove(t);
			//解除關聯關係後才能通過關係被維護端刪除資料
		    em.remove(t);
			tx.commit();
		}catch(Exception e){
			e.printStackTrace();
		}finally{
			if(em!=null){
				em.close();
			}
		}
		
	}

最後總結一下使用關聯關係的要點：分清楚關係維護端和關係被維護端、合理的使用註解、級聯的設定根據具體情況指定。