JPA:联合主键

JPA中将联合主键看做一个对象，所以将联合主键放在一个类中

AirLine.java

import javax.persistence.Column;

import javax.persistence.EmbeddedId;

import javax.persistence.Entity;

@Entity

public class AirLine {

private AirLinePK id;

private String name;





public AirLine() {}

public AirLine(AirLinePK id) {

this.id = id;

}

public AirLine(String startCity, String endCity, String name) {

this.id = new AirLinePK(startCity, endCity);

this.name=name;

}

@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;

}





}

AirLinePK.java

import java.io.Serializable;

import javax.persistence.Column;

import javax.persistence.Embeddable;

/**

*

* 建立联合主键类有三个要求：

* 1.实现一个无参的构造函数

* 2.实现序列化接口，因为要保存在数据库中就必须实现序列化接口

* 3.重写equal和hashcode两个方法，要判断主键是否相等嘛

*

*/

@Embeddable//告诉JPA，我们只是使用联合主键类里面的属性作为实体类的持久化字段

public class AirLinePK implements Serializable {

private String startCity;

private String endCity;





public AirLinePK() {}

public AirLinePK(String startCity, String endCity) {

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;

}





}



操作

@Test

public void save(){

EntityManagerFactory factory=Persistence.createEntityManagerFactory("itcast");

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();

}

一般不会用，因为这属于业务主键，与业务相关，而实际上我们推荐使用自然主键，非业务相关的。