摘要：
在JPA中，一对多关系使用@OneToMany标记关系维护结束：1packagecom.yl.demo1.bean.oneMany；23 importjavax.persistence.ScascadeType；4importjavax.persistence.Column；5importjavax.persistence.Entity；6导入javax.persistence.Generat
JPA中，一对多关系使用@OneToMany标示
关系维护端：
1 package com.yl.demo1.bean.oneTomany; 2 3 import javax.persistence.CascadeType; 4 import javax.persistence.Column; 5 import javax.persistence.Entity; 6 import javax.persistence.GeneratedValue; 7 import javax.persistence.Id; 8 import javax.persistence.JoinColumn; 9 import javax.persistence.ManyToOne; 10 11 12 @Entity 13 public class OrderItem { 14 15 private Integer id; 16 private String productName; 17 private Float sellPrice; 18 private Order order; 19 20 @Id @GeneratedValue 21 public Integer getId() { 22 return id; 23 } 24 public void setId(Integer id) { 25 this.id = id; 26 } 27 @Column(length=40, nullable=false) 28 public String getProductName() { 29 return productName; 30 } 31 public void setProductName(String productName) { 32 this.productName = productName; 33 } 34 @Column(nullable=false) 35 public Float getSellPrice() { 36 return sellPrice; 37 } 38 public void setSellPrice(Float sellPrice) { 39 this.sellPrice = sellPrice; 40 } 41 /** 42 * optional代表属性是否可选, 43 * true--代表字段可为空 44 * false--代表字段不允许为null 45 * 46 * @JoinColumn---指定外键的名称 47 */ 48 @ManyToOne(cascade={CascadeType.MERGE, CascadeType.REFRESH}, optional=true) 49 @JoinColumn(name="order_id") 50 public Order getOrder() { 51 return order; 52 } 53 public void setOrder(Order order) { 54 this.order = order; 55 } 56 57 58 }
关系被维护端：
1 package com.yl.demo1.bean.oneTomany; 2 3 import java.util.HashSet; 4 import java.util.Set; 5 6 import javax.persistence.CascadeType; 7 import javax.persistence.Column; 8 import javax.persistence.Entity; 9 import javax.persistence.FetchType; 10 import javax.persistence.Id; 11 import javax.persistence.OneToMany; 12 import javax.persistence.Table; 13 /** 14 * 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 15 * 16 * 17 */ 18 @Entity 19 @Table(name="orders") 20 public class Order { 21 private String orderId; 22 private Float amount = 0f; 23 private Set<OrderItem> items = new HashSet<OrderItem>(); 24 25 @Id @Column(length=12) 26 public String getOrderId() { 27 return orderId; 28 } 29 public void setOrderId(String orderId) { 30 this.orderId = orderId; 31 } 32 @Column(nullable=false) 33 public Float getAmount() { 34 return amount; 35 } 36 public void setAmount(Float amount) { 37 this.amount = amount; 38 } 39 /**级联操作 40 * CascadeType.REFRESH--select order, select OrderItem 41 * CascadeType.PERSIST--insert into Order, insert into OrderItem 42 * 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) 43 * CascadeType.REMOVE---delete Order, delete OrderItem 44 * 可以采用CascadeType.ALL包括上述四种. 45 * 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() 46 * 47 * *ToMany的fetch默认值为FetchType.LAZY 48 * *ToOne的fetch默认值为FetchType.EAGER 49 * 50 * 51 * 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 52 */ 53 @OneToMany(cascade={CascadeType.REFRESH, CascadeType.PERSIST} 54 , fetch=FetchType.LAZY 55 , mappedBy="order") 56 public Set<OrderItem> getItems() { 57 return items; 58 } 59 public void setItems(Set<OrderItem> items) { 60 this.items = items; 61 } 62 63 64 public void addOrderItem(OrderItem orderItem) { 65 orderItem.setOrder(this); 66 this.items.add(orderItem); 67 } 68 69 }
常见操作:
1 @Test 2 public void save() { 3 EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL"); 4 EntityManager em = factory.createEntityManager(); 5 em.getTransaction().begin();//事务开始 6 7 Order order = new Order(); 8 order.setAmount(34f); 9 order.setOrderId("999"); 10 11 OrderItem orderItem1 = new OrderItem(); 12 orderItem1.setProductName("football"); 13 orderItem1.setSellPrice(90f); 14 15 OrderItem orderItem2 = new OrderItem(); 16 orderItem2.setProductName("basketball"); 17 orderItem2.setSellPrice(100f); 18 19 order.addOrderItem(orderItem1); 20 order.addOrderItem(orderItem2); 21 22 em.persist(order); 23 24 em.getTransaction().commit(); 25 em.close(); 26 factory.close(); 27 }