package 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;

@Table(name = "tb_items")

@Entity

public class Item {

    private Integer id;

    private String itemName;

    /**

     * 多对多的关系

     */

    private Set<Category> categories = new HashSet<>();

    @GeneratedValue

    @Id

    public Integer getId() {

        return id;

    }

    public void setId(Integer id) {

        this.id = id;

    }

    @Column(name = "ITEM_NAME")

    public String getItemName() {

        return itemName;

    }

    public void setItemName(String itemName) {

        this.itemName = itemName;

    }

    // joinColumns：指定自己的外键和中间表之关联的之列

    // @JoinColumn(name="ITEM_ID" 中间表的外键

    // referencedColumnName="ID"  自己的主键

    // ****inverseJoinColumns：指定与之相关表的外键和与之关联的之列

    //name="ITEM_CATEGORY":指定中间表的表名

    @JoinTable(name="ITEM_CATEGORY",

       joinColumns = { @JoinColumn(name = "ITEM_ID", referencedColumnName = "ID") },

       inverseJoinColumns = {@JoinColumn(name = "CATEGORY_ID", referencedColumnName = "ID") })

    @ManyToMany

    public Set<Category> getCategories() {

        return categories;

    }

    public void setCategories(Set<Category> categories) {

        this.categories = categories;

    }

    @Override

    public String toString() {

        return "Item [id=" + id + ", itemName=" + itemName + ", categories=" + categories + "]";

    }

}

package 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;

@Table(name="TB_CATEGORY")

@Entity

public class Category {

    private Integer id;

    private String categoryName;

    private Set<Item> items = new HashSet<>();

    @GeneratedValue

    @Id

    public Integer getId() {

        return id;

    }

    public void setId(Integer id) {

        this.id = id;

    }

    @Column(name="CATEGORY_NAME")

    public String getCategoryName() {

        return categoryName;

    }

    public void setCategoryName(String categoryName) {

        this.categoryName = categoryName;

    }

     //多对多的关系

    @ManyToMany(mappedBy="categories")  //放弃维护关系，有item维护

    public Set<Item> getItems() {

        return items;

    }

    public void setItems(Set<Item> items) {

        this.items = items;

    }

    @Override

    public String toString() {

        return "Category [id=" + id + ", categoryName=" + categoryName + ", items=" + items + "]";

    }

}

package test;

import java.util.Set;

import javax.persistence.EntityManager;

import javax.persistence.EntityManagerFactory;

import javax.persistence.EntityTransaction;

import javax.persistence.Persistence;

import org.junit.After;

import org.junit.Before;

import org.junit.Test;

import entity.Category;

import entity.Item;

public class ManyToManyTest {

    private EntityManagerFactory entityManagerFactory;

    private EntityManager entityManager;

    private EntityTransaction entityTransaction;

    @Before

    public void init() {

        entityManagerFactory = Persistence.createEntityManagerFactory("JPA");

        entityManager = entityManagerFactory.createEntityManager();

        entityTransaction = entityManager.getTransaction();

        entityTransaction.begin();

    }

    @After

    public void distory() {

        entityTransaction.commit();

        entityManager.close();

        entityManagerFactory.close();

    }

    @Test

    public void testFind(){

        //使用关系不维护的一方获取，

        Category category = entityManager.find(Category.class, 27);

        Set<Item> items = category.getItems();

        for (Item item : items) {

            System.out.println(item.getItemName());

        }

//        //对于关联的对象默认使用懒加载  ，使用关系维护的 一方获取还是使用关系不维护的一方获取，sql语句相同

//         Item item = entityManager.find(Item.class, 25);

//        Set<Category> categories = item.getCategories();

//        for (Category category : categories) {

//            System.out.println(category.getCategoryName());

//        }

    }

    @Test

    public void testmanyToManyPersistence() {

        Category category1 = new Category();

        category1.setCategoryName("c1");

        Category category2 = new Category();

        category2.setCategoryName("c2");

        Item item1 = new Item();

        item1.setItemName("i1");

        Item item2 = new Item();

        item1.setItemName("i2");

        //给予关系

        category1.getItems().add(item1);

        category1.getItems().add(item2);

        category2.getItems().add(item1);

        category2.getItems().add(item2);

        item1.getCategories().add(category1);

        item1.getCategories().add(category2);

        item2.getCategories().add(category1);

        item2.getCategories().add(category2);

        //持久化

        entityManager.persist(item1);

        entityManager.persist(item2);

        entityManager.persist(category1);

        entityManager.persist(category2);

    }

}