jpa 多对多

entity   Item

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 + "]";
    }

}

entity  Category

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 + "]";
    }

}

testManyToMany

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);

    }

}

原文地址：https://www.cnblogs.com/lshan/p/8647506.html