JPA-一对多关系

 package com.yl.demo1.bean.oneTomany;

 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;

 @Entity

 public class OrderItem {

     private Integer id;

     private String productName;

     private Float sellPrice;

     private Order order;

     @Id @GeneratedValue

     public Integer getId() {

         return id;

     }

     public void setId(Integer id) {

         this.id = id;

     }

     @Column(length=40, nullable=false)

     public String getProductName() {

         return productName;

     }

     public void setProductName(String productName) {

         this.productName = productName;

     }

     @Column(nullable=false)

     public Float getSellPrice() {

         return sellPrice;

     }

     public void setSellPrice(Float sellPrice) {

         this.sellPrice = sellPrice;

     }

     /**

      * optional代表属性是否可选,

      * true--代表字段可为空

      * false--代表字段不允许为null

      *

      * @JoinColumn---指定外键的名称

      */

     @ManyToOne(cascade={CascadeType.MERGE, CascadeType.REFRESH}, optional=true)

     @JoinColumn(name="order_id")

     public Order getOrder() {

         return order;

     }

     public void setOrder(Order order) {

         this.order = order;

     }

 }

 package com.yl.demo1.bean.oneTomany;

 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;

 /**

  * JPA中，1-m的双向关系中，多的一方为关系维护端，关系维护端服务外键记录的更新，关系被维护端是没有权利更新外键记录的

  *

  *

  */

 @Entity

 @Table(name="orders")

 public class Order {

     private String orderId;

     private Float amount = 0f;

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

     @Id @Column(length=12)

     public String getOrderId() {

         return orderId;

     }

     public void setOrderId(String orderId) {

         this.orderId = orderId;

     }

     @Column(nullable=false)

     public Float getAmount() {

         return amount;

     }

     public void setAmount(Float amount) {

         this.amount = amount;

     }

     /**级联操作

      * CascadeType.REFRESH--select order, select OrderItem

      * CascadeType.PERSIST--insert into Order, insert into OrderItem

      * CascadeType.MERGE----当Order处于游离状态时，对Order更新，也会更新OrderItem--update Order, for(update OrderItem)

      * CascadeType.REMOVE---delete Order, delete OrderItem

      * 可以采用CascadeType.ALL包括上述四种.

      * 上述四种方法只有调用相应的方法时才会触发。即EntityManager.refresh(),EntityManager.persist(),EntityManager.merge(), EntityManager.remove()

      *

      *   *ToMany的fetch默认值为FetchType.LAZY

      *   *ToOne的fetch默认值为FetchType.EAGER

      *

      *

      *   mappedBy出现在关系的被维护端,它指定在关系维护端是由哪个属性进行维护关系

      */

     @OneToMany(cascade={CascadeType.REFRESH, CascadeType.PERSIST}

             , fetch=FetchType.LAZY

             , mappedBy="order")

     public Set<OrderItem> getItems() {

         return items;

     }

     public void setItems(Set<OrderItem> items) {

         this.items = items;

     }

     public void addOrderItem(OrderItem orderItem) {

         orderItem.setOrder(this);

         this.items.add(orderItem);

     }

 }

 @Test

     public void save() {

         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");

         EntityManager em = factory.createEntityManager();

         em.getTransaction().begin();//事务开始

         Order order = new Order();

         order.setAmount(34f);

         order.setOrderId("999");

         OrderItem orderItem1 = new OrderItem();

         orderItem1.setProductName("football");

         orderItem1.setSellPrice(90f);

         OrderItem orderItem2 = new OrderItem();

         orderItem2.setProductName("basketball");

         orderItem2.setSellPrice(100f);

         order.addOrderItem(orderItem1);

         order.addOrderItem(orderItem2);

         em.persist(order);

         em.getTransaction().commit();

         em.close();

         factory.close();

     }