如果要搭建JPA开发环境,请从JPA第一篇博客看起。
下面以坐飞机为例,讲解联合主键的使用。基本思想是一条航线由两个端点组成。
1、组合主键类
package compositepk;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Embeddable;
/**
* 组合主键类: 这里以坐飞机为例,一段航线由起始点和结束点组成 规则一:必须提供无参的构造函数 规则二:必须实现Serializable接口
* 规则三:必须重写hashCode()和equals()方法
*/
@Embeddable//该注解表示该类用在实体里面,直接使用该类定义的属性
public class AirLinePK implements Serializable {
private static final long serialVersionUID = 5913768452213218677L;
private String startCity;
private String endCity;
public AirLinePK() {
super();
}
public AirLinePK(String startCity, String endCity) {
super();
this.startCity = startCity;
this.endCity = endCity;
}
@Column(length=3)
public String getStartCity() {
return startCity;
}
public void setStartCity(String startCity) {
this.startCity = startCity;
}
@Column(length=3)
public String getEndCity() {
return endCity;
}
public void setEndCity(String endCity) {
this.endCity = endCity;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((endCity == null) ?
0 : endCity.hashCode());
result = prime * result
+ ((startCity == null) ? 0 : startCity.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
AirLinePK other = (AirLinePK) obj;
if (endCity == null) {
if (other.endCity != null)
return false;
} else if (!endCity.equals(other.endCity))
return false;
if (startCity == null) {
if (other.startCity != null)
return false;
} else if (!startCity.equals(other.startCity))
return false;
return true;
}
}
2、航线类
package compositepk;
import javax.persistence.Column;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
/**
* 航线类
*/
@Entity
public class AirLine {
private AirLinePK id;//可嵌入主键
private String name;
public AirLine() {
super();
}
public AirLine(AirLinePK id) {
super();
this.id = id;
}
public AirLine(String startCity, String endCity, String name){
this.id = new AirLinePK(startCity, endCity);
this.name = name;
}
//用于标识组合主键
@EmbeddedId
public AirLinePK getId() {
return id;
}
public void setId(AirLinePK id) {
this.id = id;
}
@Column(length=20)
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
3、测试类中的代码
package junit.test;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import org.junit.Test;
import compositepk.AirLine;
/**
* 测试组合主键
*/
public class CompositTest {
@Test
public void save() {
EntityManagerFactory factory = Persistence
.createEntityManagerFactory("sunft_first");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
em.persist(new AirLine("PEK", "SHA", "北京飞上海"));
em.getTransaction().commit();
em.close();
factory.close();
}
}
4、数据库中生成的记录如下