jpa一对多映射案例

订单和订单项就是一对多的关系。一个订单有多个订单项，一个订单项属于某个订单。

订单和订单项映射后的表结构如下：

订单表结构

订单项表结构

下面就以订单和订单项为例介绍多对多映射关系的实例开发

persistence.xml配置文件

<?xml version="1.0"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">
  <!--事务类型：本地事务(仅针对一个数据库)和全局事务(针对多个数据库)；RESOURCE_LOCAL: 本地事务 -->
  <persistence-unit name="ljq" transaction-type="RESOURCE_LOCAL">
      <properties>
         <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>
         <property name="hibernate.connection.driver_class" value="org.gjt.mm.mysql.Driver"/>
         <property name="hibernate.connection.username" value="root"/>
         <property name="hibernate.connection.password" value="mysql"/>
         <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/test?useUnicode=true&amp;characterEncoding=UTF-8"/>
         <property name="hibernate.hbm2ddl.auto" value="update"/>
      </properties>
  </persistence-unit>
</persistence>

Order订单类

package com.ljq.entity;

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.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

/**
 * one端
 *
 * 碰到many为末端的加载就是延迟加载，若one为末端则为立即加载，除了one-to-one。
 *
 * @author jiqinlin
 *
 */
@Entity
@Table(name = "tb_order")
@SuppressWarnings("serial")
public class Order implements java.io.Serializable {

    // String类型不能用uuid，需要人为指定
    @Id
    @Column(length = 12)
    private String orderid;
    @Column(nullable = false)
    private Float amount = 0f;
    // @OneToMany(cascade={CascadeType.PERSIST,CascadeType.MERGE,CascadeType.REMOVE})
    // mappedBy="order": 指明Order类为双向关系维护端，负责外键的更新
    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "order")
    private Set<OrderItem> items = new HashSet<OrderItem>();

    public Order() {
        super();
    }

    public Float getAmount() {
        return amount;
    }

    public void setAmount(Float amount) {
        this.amount = amount;
    }

    public String getOrderid() {
        return orderid;
    }

    public void setOrderid(String orderid) {
        this.orderid = orderid;
    }

    public Set<OrderItem> getItems() {
        return items;
    }

    public void setItems(Set<OrderItem> items) {
        this.items = items;
    }

    ////////////////////////////////////
    /**
     * 添加订单项
     *
     * @param student
     */
    public void addOrderItem(OrderItem item) {
        if (!this.items.contains(item)) {
            this.items.add(item);
            item.setOrder(this);
        }
    }

    /**
     * 删除订单项
     *
     * @param student
     */
    public void removeOrderItem(OrderItem item) {
        if (this.items.contains(item)) {
            item.setOrder(null);
            this.items.remove(item);
        }
    }

}

OrderItem订单项类

package com.ljq.entity;

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.ManyToOne;
import javax.persistence.Table;

/**
 * many端
 *
 * 在one-to-many双向关联中，多的一方为关系维护端，关系维护端负责外键记录的更新
 * 关系被维护端是没有权力更新外键记录的
 *
 * @author jiqinlin
 *
 */

@Entity
@Table(name = "tb_orderitem")
@SuppressWarnings("serial")
public class OrderItem implements java.io.Serializable {
    @Id
    @GeneratedValue
    private Integer id;
    @Column(length = 40, nullable = false)
    private String productName;
    @Column(nullable = false)
    private Float sellPrice = 0f;
    // optional=true：可选，表示此对象可以没有，可以为null；false表示必须存在
    @ManyToOne(cascade = { CascadeType.REFRESH, CascadeType.MERGE }, optional = true)
    @JoinColumn(name = "order_id")
    private Order order;

    public OrderItem() {
        super();
    }

    public OrderItem(String productName, Float sellPrice) {
        super();
        this.productName = productName;
        this.sellPrice = sellPrice;
    }

    public Integer getId() {
        return id;
    }

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

    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }

    public Float getSellPrice() {
        return sellPrice;
    }

    public void setSellPrice(Float sellPrice) {
        this.sellPrice = sellPrice;
    }

    public Order getOrder() {
        return order;
    }

    public void setOrder(Order order) {
        this.order = order;
    }

}

OrderTest测试类

package com.ljq.test;

import java.io.Serializable;
import java.util.List;
import java.util.Set;
import java.util.UUID;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import org.junit.Test;

import com.ljq.entity.Order;
import com.ljq.entity.OrderItem;
import com.sun.org.apache.xpath.internal.operations.Or;

public class OrderTest {

    /**
     * 添加订单Order时同时添加两个订单项OrderItem，因为订单Order为双向关系维护者，起着主导作用
     */
    @Test
    public void save() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();

        Order order = new Order();
        order.setAmount(34f);
        //UUID.randomUUID().toString()
        order.setOrderid("xxxx");
        order.addOrderItem(new OrderItem("足球",20f));
        order.addOrderItem(new OrderItem("篮球",25f));
        em.persist(order);

        em.getTransaction().commit();
        em.close();
        factory.close();

    }

    /**
     * 删除Order对象时，会自动删除OrderItem对象（即：父对象删除时子对象跟着删除）
     */
    @Test
    public void detele1(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();

        Order order=em.getReference(Order.class, "xxxx");
        em.remove(order);

        em.getTransaction().commit();
        em.close();
        factory.close();
    }

    /**
     * 删除子对象时，父对象没影响
     */
    @Test
    public void detele2(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();

        OrderItem item=em.getReference(OrderItem.);
        em.remove(item);

        em.getTransaction().commit();
        em.close();
        factory.close();

    }

    //运行结果为：
    //orderid:xxxx, amount:34.0
    //productName:足球price:20.0
    //productName:篮球price:25.0
    //===============
    @Test
    public void find1(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();

        List<Order> orders=em.createQuery("select o from Order o").getResultList();
        for(Order order:orders){
            System.out.println("orderid:"+order.getOrderid()+", "+"amount:"+order.getAmount());
            Set<OrderItem> orderItems=order.getItems();
            for(OrderItem item:orderItems){
                System.out.println("productName:"+item.getProductName()+"price:"+item.getSellPrice());
            }
            System.out.println("===============");
        }

        em.close();
        factory.close();
    }

    //运行结果为
    //productName:足球price:20.0
    //orderid:xxxx, amount:34.0
    //============
    //productName:篮球price:25.0
    //orderid:xxxx, amount:34.0
    //============
    @Test
    public void find2(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("ljq");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();

        List<OrderItem> items=em.createQuery("select o from OrderItem o").getResultList();
        for(OrderItem item:items){
            System.out.println("productName:"+item.getProductName()+"price:"+item.getSellPrice());
            Order order=item.getOrder();
            System.out.println("orderid:"+order.getOrderid()+", "+"amount:"+order.getAmount());
            System.out.println("============");
        }
        em.close();
        factory.close();
    }

    /**
     * 测试jpa环境是否搭建成功
     *
     */
    @Test
    public void test() {
        Persistence.createEntityManagerFactory("ljq");
    }

}