JPA:映射单向多对一关联关系

1.单向多对一是jpa中常用的关联关系,这里以订单和顾客举例子(多个订单可以对应同一顾客)

2.顾客实体类代码如下：

package com.wayne.helloworld;

import java.util.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
import javax.persistence.Transient;
/*
* 如果不添加@Table注解,则表名与实体类名相同
*/
@Table(name="JPA_CUSTOMERS")
@Entity
public class Customer {
private Integer id;
private String lastName;
private String email;
private int age;
private Date createdDate;
private Date birthDay;
@Column(name="ID")
@GeneratedValue(strategy=GenerationType.AUTO)
@Id
public Integer getId() {
   return id;
}
public void setId(Integer id) {
   this.id = id;
}
@Column(name="LAST_NAME",length=50,nullable=true)
public String getLastName() {
   return lastName;
}
public void setLastName(String lastName) {
   this.lastName = lastName;
}
//若属性名与字段名相同,则不需要填写注解
public String getEmail() {
   return email;
}
public void setEmail(String email) {
   this.email = email;
}
public int getAge() {
   return age;
}
public void setAge(int age) {
   this.age = age;
}
//精度时间戳
@Temporal(TemporalType.TIMESTAMP)
public Date getCreatedDate() {
   return createdDate;
}
public void setCreatedDate(Date createdDate) {
   this.createdDate = createdDate;
}
@Temporal(TemporalType.DATE)
public Date getBirthDay() {
   return birthDay;
}
public void setBirthDay(Date birthDay) {
   this.birthDay = birthDay;
}
//工具方法,不需要映射为某个数据库表的一列
@Transient
public String getInfo(){
   return "lastName:"+this.getLastName();
}
@Override
public String toString() {
   return "Customer [id=" + id + ", lastName=" + lastName + ", email=" + email + ", age=" + age + ", createdDate="
           + createdDate + ", birthDay=" + birthDay + "]";
}

}

3.订单代码如下:

package com.wayne.helloworld;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Table(name="JPA_ORDERS")
@Entity
public class Order {
private Integer id;
private String orderName;
private Customer customer;
@GeneratedValue
@Id
public Integer getId() {
   return id;
}
public void setId(Integer id) {
   this.id = id;
}
@Column(name="ORDER_NAME")
public String getOrderName() {
   return orderName;
}
public void setOrderName(String orderName) {
   this.orderName = orderName;
}
/*单向多对一的关联关系
使用@ManyToOne来映射多对一的关联关系
使用@JoinColumn来映射外键
name为关联表的列列名
可使用@ManyToOne的fetch属性来修改默认的关联属性的加载策略*/
@JoinColumn(name="CUSTOMER_ID")
@ManyToOne
public Customer getCustomer() {
   return customer;
}
public void setCustomer(Customer customer) {
   this.customer = customer;
}
}
4.首先测试插入数据: