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&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. class , (Serializable) 3 ); 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 " ); } }
