實體Player：玩家。

實體Game：游戲。

玩家和游戲是多對多的關系。一個玩家可以玩很多的游戲，一個游戲也可以被很多玩家玩。

JPA中使用@ManyToMany來注解多對多的關系，由一個關聯表來維護。這個關聯表的表名默認是：主表名+下劃線+從表名。(主表是指關系維護端對應的表,從表指關系被維護端對應的表)。這個關聯表只有兩個外鍵字段，分別指向主表ID和從表ID。字段的名稱默認為：主表名+下劃線+主表中的主鍵列名，從表名+下劃線+從表中的主鍵列名。

需要注意的：

1、多對多關系中一般不設置級聯保存、級聯刪除、級聯更新等操作。

2、可以隨意指定一方為關系維護端，在這個例子中，我指定Player為關系維護端，所以生成的關聯表名稱為： player_game，關聯表的字段為：player_id和game_id。

3、多對多關系的綁定由關系維護端來完成，即由Player.setGames(games)來綁定多對多的關系。關系被維護端不能綁定關系，即Game不能綁定關系。

4、多對多關系的解除由關系維護端來完成，即由Player.getGames().remove(game)來解除多對多的關系。關系被維護端不能解除關系，即Game不能解除關系。

5、如果Player和Game已經綁定了多對多的關系，那么不能直接刪除Game，需要由Player解除關系后，才能刪除Game。但是可以直接刪除Player，因為Player是關系維護端，刪除Player時，會先解除Player和Game的關系，再刪除Player。

Player.java如下：

package com.cndatacom.jpa.entity;
?
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.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
?
?
/**
?* 玩家
?* @author Luxh
?*/
@Entity
@Table(name="player")
public class Player {
?????
????@Id
????@GeneratedValue
????private Long id;
?????
????/**玩家姓名*/
????@Column(length=32)
????private String name;
?????
?????
????/**玩家玩的游戲*/
????@ManyToMany
????@JoinTable(name="player_game",joinColumns=@JoinColumn(name="player_id"),
????????????????????inverseJoinColumns=@JoinColumn(name="game_id"))
????//關系維護端，負責多對多關系的綁定和解除
????//@JoinTable注解的name屬性指定關聯表的名字，joinColumns指定外鍵的名字，關聯到關系維護端(Player)
????//inverseJoinColumns指定外鍵的名字，要關聯的關系被維護端(Game)
????//其實可以不使用@JoinTable注解，默認生成的關聯表名稱為主表表名+下劃線+從表表名，
????//即表名為player_game
????//關聯到主表的外鍵名：主表名+下劃線+主表中的主鍵列名,即player_id
????//關聯到從表的外鍵名：主表中用于關聯的屬性名+下劃線+從表的主鍵列名,即game_id
????//主表就是關系維護端對應的表，從表就是關系被維護端對應的表
????private Set<Game> games = new HashSet<Game>();
?
?
????public Long getId() {
????????return id;
????}
?
?
????public void setId(Long id) {
????????this.id = id;
????}
?
?
????public String getName() {
????????return name;
????}
?
?
????public void setName(String name) {
????????this.name = name;
????}
?
?
????public Set<Game> getGames() {
????????return games;
????}
?
?
????public void setGames(Set<Game> games) {
????????this.games = games;
????}
?????
}

Game.java如下：

package com.cndatacom.jpa.entity;
?
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;
?
?
/**
?* 游戲
?* @author Luxh
?*/
?
@Entity
@Table(name="game")
public class Game {
?????
????@Id
????@GeneratedValue
????private Long id;
?????
????/**游戲名稱*/
????@Column(length=32)
????private String name;
?????
????/**游戲擁有的玩家*/
????@ManyToMany(mappedBy="games")
????//只需要設置mappedBy="games"表明Game實體是關系被維護端就可以了
????//級聯保存、級聯刪除等之類的屬性在多對多關系中是不需要設置
????//不能說刪了游戲,把玩家也刪掉,玩家還可以玩其他的游戲
????private Set<Player> players = new HashSet<Player>();
?
????public Long getId() {
????????return id;
????}
?
????public void setId(Long id) {
????????this.id = id;
????}
?
????public String getName() {
????????return name;
????}
?
????public void setName(String name) {
????????this.name = name;
????}
?
????public Set<Player> getPlayers() {
????????return players;
????}
?
????public void setPlayers(Set<Player> players) {
????????this.players = players;
????}
?????
?????
}

簡單的測試如下：

package com.cndatacom.jpa.test;
?
import java.util.HashSet;
import java.util.Set;
?
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
?
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
?
import com.cndatacom.jpa.entity.Game;
import com.cndatacom.jpa.entity.Player;
?
public class TestManyToMany {
?????
EntityManagerFactory emf = null;
?????
????@Before
????public void before() {
????????//根據在persistence.xml中配置的persistence-unit name 創建EntityManagerFactory
????????emf = Persistence.createEntityManagerFactory("myJPA");
????}
?????
????@After
????public void after() {
????????//關閉EntityManagerFactory
????????if(null != emf) {
????????????emf.close();
????????}
????}
?????
????/**
?????* 創建玩家和游戲
?????*/
????@Test
????public void testSavePlayerAndGame() {
????????EntityManager em = emf.createEntityManager();
????????em.getTransaction().begin();
????????//因為Player和Game之間沒有級聯保存的關系，所以Playe和Game要分別保存
?????????
????????Player player = new Player();
????????player.setName("西門吹雪");
????????//保存Player
????????em.persist(player);
?????????
????????Game game = new Game();
????????game.setName("大菠蘿3");
????????//保存game
????????em.persist(game);
?????????
????????em.getTransaction().commit();
????????em.close();
????}
?????
????/**
?????* 給玩家添加游戲
?????*/
????@Test
????public void testAddGameToPlayer() {
????????EntityManager em = emf.createEntityManager();
????????em.getTransaction().begin();
????????//找出ID為1的玩家
????????Player player = em.find(Player.class, 1L);
????????//找出ID為1的游戲
????????Game game = em.find(Game.class, 1L);
?????????
????????Set<Game> games = new HashSet<Game>();
????????games.add(game);
?????????
????????//因為Player是關系的維護端，所以必須由Player來添加關系
????????player.setGames(games);
?????????
????????em.getTransaction().commit();
????????em.close();
????}
?????
????/**
?????* 玩家刪除游戲
?????*/
????@Test
????public void testRemoveGameFormPlayer() {
????????EntityManager em = emf.createEntityManager();
????????em.getTransaction().begin();
????????//找出ID為1的玩家
????????Player player = em.find(Player.class, 1L);
????????//找出ID為1的游戲
????????Game game = em.find(Game.class, 1L);
?????????
????????//因為Player是關系維護端，所以關系的解除由Player來完成
????????player.getGames().remove(game);
????????em.getTransaction().commit();
????????em.close();
????}
}

生成的關聯表結構如下：

以上就是動力節點小編介紹的"jpa多對多的映射實例"，希望對大家有幫助，如有疑問，請在線咨詢，有專業老師隨時為您務。