联合主键就是将几个字段都作为主键，或者说一个以上主键的都可作为联合主键或者复合主键，开发联合主键实体类对象必须做到三个要求，一是提供一个无参构造函数，二是必须实现序列化串行接口，三是必须重写HashCode和equals方法，参数是复合主键的属性。这里我们的实例用航班做，我们知道航班有起始地，也有终止点，他们有一个共同的航班名，所以可以用联合主键来做表设计，具体看代码。

首先定义航线实体：

/**
 * 航线实体
 */
@Entity
@Table(name="t_airline")
public class AirLine {

	@EmbeddedId
	private AirLinePK id;
	@Column(length=20,nullable=false)
	private String name;
	
	public AirLine(){}
	//默认构造函数中引入联合主键参数
	public AirLine(String startCity,String endCity,String name) {
		this.id=new AirLinePK(startCity, endCity);
		this.name = name;
	}
	
	
}<span style="font-family:Microsoft YaHei;font-size:14px;">
</span>

然后定义联合主键实体类：

/**
 * 复合主键类
 * 1.提供一个无参构造函数
 * 2.必须实现序列化接口
 * 3.必须重写HashCode和equals方法，参数是复合主键的属性
 * 
 * Embeddable表示它标识的类是用在别的实体里面
 */
@Embeddable
public class AirLinePK implements Serializable{
	@Column(length=3,nullable=false)
	private String startCity;
	@Column(length=3,nullable=false)
	private String endCity;
	
	public AirLinePK() {
	}
	public AirLinePK(String startCity, String endCity) {
		this.startCity = startCity;
		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;
	}
	
	<!--省略。。注意联合主键要求 -->
}<span style="font-family:Microsoft YaHei;font-size:14px;">
</span>

最后是测试类：

public class CompositePK {

	private static EntityManagerFactory factory;
	private static EntityManager em;
	
	@BeforeClass
	public static void setUpBeforeClass() throws Exception {
		
		factory = Persistence.createEntityManagerFactory("jpatest1");
		em = factory.createEntityManager();
	}
	
	@Test
	public void save() {
		
		em.getTransaction().begin(); //开启事务
		
		AirLine line = new AirLine("PEK", "SHA", "北京飞往上海");
		em.persist(line);
		
		em.getTransaction().commit(); //提交事务
		em.close(); //关闭
		factory.close(); //关闭
	}
}

JPA开发总结<六>--联合主键