在实际开发过程中，JPA多对一单向关联是使用最多的关联方式。
下面是订单与订单项的配置关系。
订单（Order）：一的一方，不进行任何配置
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@Entity
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@Table(name=
"orders")
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public
class Order {
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@Id
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@Column(length=
40)
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private String orderId;
//订单ID
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@Column(nullable=
false)
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private Float amount =
0f;
//总价
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private Set<OrderItem> items;
//订单项（可有可无，查询订单时，无法查出此项）
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public String getOrderId() {
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return orderId;
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}
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public void setOrderId(String orderId) {
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this.orderId = orderId;
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}
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public Float getAmount() {
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return amount;
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}
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public void setAmount(Float amount) {
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this.amount = amount;
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}
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public Set<OrderItem> getItems() {
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return items;
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}
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public void setItems(Set<OrderItem> items) {
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this.items = items;
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}
订单项（OrderItem）：多的一方，进行ManyToOne关联配置。
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@Entity
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public
class OrderItem {
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@Id
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@GeneratedValue
//ID自增长
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private Integer id;
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@Column(length=
40,nullable=
false)
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private String productName;
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@Column(nullable=
false)
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private Float sellPrice;
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/*
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* @JoinColumn(name="order_id") 数据库中外键的名字为order_id
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*/
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@ManyToOne(cascade={CascadeType.MERGE,CascadeType.REFRESH},
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fetch=FetchType.EAGER,optional=
false)
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@JoinColumn(name=
"order_id")
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private Order order;
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public Integer getId() {
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return id;
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}
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public void setId(Integer id) {
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this.id = id;
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}
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public String getProductName() {
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return productName;
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}
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public void setProductName(String productName) {
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this.productName = productName;
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}
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public Float getSellPrice() {
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return sellPrice;
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}
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public void setSellPrice(Float sellPrice) {
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this.sellPrice = sellPrice;
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}
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public Order getOrder() {
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return order;
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}
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public void setOrder(Order order) {
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this.order = order;
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}
原文地址：https://blog.csdn.net/u011983531/article/details/48728761
