JPA

JPA中，一对多关系使用@OneToMany标示

关系维护端：

1 1 package com.yl.demo1.bean.oneTomany;
2 2 
3 3 import javax.persistence.CascadeType;
4 4 import javax.persistence.Column;
5 5 import javax.persistence.Entity;
6 6 import javax.persistence.GeneratedValue;
7 7 import javax.persistence.Id;
8 8 import javax.persistence.JoinColumn;
9 9 import javax.persistence.ManyToOne;
1010 
1111 
1212 @Entity
1313 public class OrderItem {
1414     
1515     private Integer id;
1616     private String productName;
1717     private Float sellPrice;
1818     private Order order;
1919     
2020     @Id @GeneratedValue
2121     public Integer getId() {
2222         return id;
2323     }
2424     public void setId(Integer id) {
2525         this.id = id;
2626     }
2727     @Column(length=40, nullable=false)
2828     public String getProductName() {
2929         return productName;
3030     }
3131     public void setProductName(String productName) {
3232         this.productName = productName;
3333     }
3434     @Column(nullable=false)
3535     public Float getSellPrice() {
3636         return sellPrice;
3737     }
3838     public void setSellPrice(Float sellPrice) {
3939         this.sellPrice = sellPrice;
4040     }
4141     /**
4242      * optional代表属性是否可选,
4343      * true--代表字段可为空 
4444      * false--代表字段不允许为null
4545      * 
4646      * @JoinColumn---指定外键的名称
4747      */
4848     @ManyToOne(cascade={CascadeType.MERGE, CascadeType.REFRESH}, optional=true)
4949     @JoinColumn(name="order_id")
5050     public Order getOrder() {
5151         return order;
5252     }
5353     public void setOrder(Order order) {
5454         this.order = order;
5555     }
5656     
5757     
5858 }

关系被维护端：

1 1 package com.yl.demo1.bean.oneTomany;
2 2 
3 3 import java.util.HashSet;
4 4 import java.util.Set;
5 5 
6 6 import javax.persistence.CascadeType;
7 7 import javax.persistence.Column;
8 8 import javax.persistence.Entity;
9 9 import javax.persistence.FetchType;
1010 import javax.persistence.Id;
1111 import javax.persistence.OneToMany;
1212 import javax.persistence.Table;
1313 /**
1414  * 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
1515  * 
1616  * 
1717  */
1818 @Entity
1919 @Table(name="orders")
2020 public class Order {
2121     private String orderId;
2222     private Float amount = 0f;
2323     private Set<OrderItem> items = new HashSet<OrderItem>();
2424     
2525     @Id @Column(length=12)
2626     public String getOrderId() {
2727         return orderId;
2828     }
2929     public void setOrderId(String orderId) {
3030         this.orderId = orderId;
3131     }
3232     @Column(nullable=false)
3333     public Float getAmount() {
3434         return amount;
3535     }
3636     public void setAmount(Float amount) {
3737         this.amount = amount;
3838     }
3939     /**级联操作
4040      * CascadeType.REFRESH--select order, select OrderItem
4141      * CascadeType.PERSIST--insert into Order, insert into OrderItem
4242      * 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)
4343      * CascadeType.REMOVE---delete Order, delete OrderItem
4444      * 可以采用CascadeType.ALL包括上述四种.
4545      * 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()
4646      *
4747      *   *ToMany的fetch默认值为FetchType.LAZY
4848      *   *ToOne的fetch默认值为FetchType.EAGER
4949      *   
5050      *   
5151      *   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
5252      */
5353     @OneToMany(cascade={CascadeType.REFRESH, CascadeType.PERSIST}
5454             , fetch=FetchType.LAZY
5555             , mappedBy="order")
5656     public Set<OrderItem> getItems() {
5757         return items;
5858     }
5959     public void setItems(Set<OrderItem> items) {
6060         this.items = items;
6161     }
6262     
6363     
6464     public void addOrderItem(OrderItem orderItem) {
6565         orderItem.setOrder(this);
6666         this.items.add(orderItem);
6767     }
6868     
6969 }

 常见操作:

1 1 @Test
2 2     public void save() {
3 3         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
4 4         EntityManager em = factory.createEntityManager();
5 5         em.getTransaction().begin();//事务开始
6 6         
7 7         Order order = new Order();
8 8         order.setAmount(34f);
9 9         order.setOrderId("999");
1010         
1111         OrderItem orderItem1 = new OrderItem();
1212         orderItem1.setProductName("football");
1313         orderItem1.setSellPrice(90f);
1414         
1515         OrderItem orderItem2 = new OrderItem();
1616         orderItem2.setProductName("basketball");
1717         orderItem2.setSellPrice(100f);
1818         
1919         order.addOrderItem(orderItem1);
2020         order.addOrderItem(orderItem2);
2121 
2222         em.persist(order);
2323         
2424         em.getTransaction().commit();
2525         em.close();
2626         factory.close();
2727     }