JPA学习---第十二节：JPA中的联合主键

1、定义实体类，代码如下：

(1)、将联合主键放到一个类中，代码如下：

package learn.jpa.entity;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Embeddable;

/**
 *
 * 1、必须要有无擦得构造函数
 * 2、必须要实现序列接口
 * 3、必须重写 equals() 和 hashCode() 方法
 * @Embeddable 告诉 jpa 只是使用实体类里面的属性
 */
@Embeddable
public class AirLinePK implements Serializable{

    /**
     *
     */
    private static final long serialVersionUID = 1L;
    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;
    }

}

(2)、定义 AirLine 实体类，代码如下：

package learn.jpa.entity;

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 用于标注为实体的标识符
    @EmbeddedId
    public AirLinePK getId() {
        return id;
    }
    public void setId(AirLinePK id) {
        this.id = id;
    }
    @Column(length=30)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

2、测试类，代码：

package learn.jpa.test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import learn.jpa.entity.AirLine;

import org.junit.Test;

public class AirLineTest {

    /**
     * 测试数据库是否可以生成表
     */
    @Test
    public void test() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        factory.close();
    }

    @Test
    public void save(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();   // 开启事务
        em.persist(new AirLine("PEK","SHA","北京飞上海"));
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
}

注意：

1、必须实现Serializable序列化

2、必须提示无参的构造方法

3、必须重写hashCode和equals方法

@Embeddable 表示该类中所有属性在应用该联合主键的类中作为它的属性（字段）