关于JPA一对多关联这里使用Order与OrderItem来模拟。一个Order可以关联多个OrderItem，而一个OrderItem只能关联一个Order。Order与OrderItem是一对多的关联关系。Order类如下：

1 package com.yichun.bean; 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.Id;10 import javax.persistence.OneToMany;11 import javax.persistence.Table;12 13 @Entity14 @Table(name = "orders")15 public class Order {16 17     private String orderId;18     private Float amoutn = 0f;19     private Set
     
       items = new HashSet
      
       ();20 21     @Id22     @Column(length = 12, nullable = false)23     public String getOrderId() {24         return orderId;25     }26 27     public void setOrderId(String orderId) {28         this.orderId = orderId;29     }30 31     @Column(nullable = false)32     public Float getAmoutn() {33         return amoutn;34     }35 36     public void setAmoutn(Float amoutn) {37         this.amoutn = amoutn;38     }39 40     // CascadeType.REFRESH，级联刷新。orderItems更改的时候，就重新加载orderItems41 42     // CascadeType.PERSIST, 级联保存。insert order insert into orderitem 。调用manager.persist(new Order())的时候才会起作用43 44     // CascadeType.MERGE,级联更新 。order在游离状态的时候被修改了，order中的orderItem也被修改了，update order for (update orderItem)。 调用manager.merge(order) 的时候才会起作用45 46     // CascadeType.REMOVE级联删除。 删除order的时候，orderItems也会被删除，没有住外键约束的时候。调用manager.remove(order)的时候才会起作用。47     48     // 针对实体管理器的四种方法49     // fetch =FetchType.LAZY默认是延迟加载。××tomany由一方得到多的一方的时候，默认就是延迟加载，××toone的时候，默认就是立即加载50     // 出现mappedBy的类就是关系被维护端。关系由对方里面的order属性维护51     @OneToMany(cascade = { CascadeType.REFRESH, CascadeType.PERSIST,52             CascadeType.MERGE, CascadeType.REMOVE }, mappedBy = "order")53     public Set
       
         getItems() {54         return items;55     }56 57     public void setItems(Set
        
          items) {58         this.items = items;59     }60 61     // 建立两者之间的关系62     public void addOrderItem(OrderItem orderItem) {63         orderItem.setOrder(this);64         this.items.add(orderItem);65     }66 }
        
       
      
     

OrderItem类如下：

1 package com.yichun.bean; 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 @Entity12 public class OrderItem {13     private Integer id;14     private String productName;15     private Float sellPrice = 0f;16     private Order order;17 18     @Id19     @GeneratedValue20     public Integer getId() {21         return id;22     }23 24     public void setId(Integer id) {25         this.id = id;26     }27 28     @Column(length = 40, nullable = false)29     public String getProductName() {30         return productName;31     }32 33     public void setProductName(String productName) {34         this.productName = productName;35     }36 37     @Column(nullable = false)38     public Float getSellPrice() {39         return sellPrice;40     }41 42     public void setSellPrice(Float sellPrice) {43         this.sellPrice = sellPrice;44     }45 46     // optional=true，在数据库中允许为null47     @ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = true)// 级联更新，级联刷新48     @JoinColumn(name = "order_id")49     public Order getOrder() {50         return order;51     }52 53     public void setOrder(Order order) {54         this.order = order;55     }56 }

保存数据：

1     @Test 2     public void save() { 3         EntityManagerFactory factory = Persistence 4                 .createEntityManagerFactory("testjpa"); 5         EntityManager manager = factory.createEntityManager(); 6         manager.getTransaction().begin(); 7  8         Order order = new Order(); 9         order.setAmoutn(120f);10         order.setOrderId("999");// UUID.randomUUID().toString()11 12         OrderItem item = new OrderItem();13         item.setProductName("足球");14         item.setSellPrice(90f);15 16         OrderItem item2 = new OrderItem();17         item2.setProductName("排球");18         item2.setSellPrice(30f);19 20         // 设置两者之间的关联21         order.addOrderItem(item);22         order.addOrderItem(item2);23 24         // 保存Order，此时也会保持OrderItem25         manager.persist(order);26 27         manager.getTransaction().commit();28         manager.clear();29         factory.close();30     }