JPA联合主键

　　联合主键也就是说需要多个字段才能确定数据库记录中的唯一一行。这样就需要多个字段一起，组成主键，也叫联合主键。例如飞机航线，我们需要知道飞机起飞的地点以及飞机降落的地点。所以需要飞机起飞的地点和降落的地方才能确定一条航线。AirLine表示航线，AirLinePK表示主键类。AirLinePK代码如下：

package com.yichun.bean;

import java.io.Serializable;

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

/**
 * 联合主键。一般使用PK 只需要定义用作主键的字段
 * <p>
 * 联合主键类必须遵守的JPA规范：<br>
 * 1、必须要提供一个public的无参数的构造方法<br>
 * 2、必须要实现序列化接口<br>
 * 3、必须重写hashCode（）与equals（）方法
 */
// 用在实体里面,只是使用该类中的属性。（该类中的属性用在持久化的类中的字段）
@Embeddable
public class AirLinePK implements Serializable {
    private String startCity;// PEK，北京 CAN广州，SHA上海
    private String endCity;

    public AirLinePK() {
    }

    public AirLinePK(String startCity, String endCity) {
        this.startCity = startCity;
        this.endCity = endCity;
    }

    @Column(length = 3, nullable = false)
    public String getStartCity() {
        return startCity;
    }

    public void setStartCity(String startCity) {
        this.startCity = startCity;
    }

    @Column(length = 3, nullable = false)
    public String getEndCity() {
        return endCity;
    }

    public void setEndCity(String endCity) {
        this.endCity = endCity;
    }

    // 以下两个方法判断是否相等
    @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;
    }

}

　　AirLine 代码如下：

package com.yichun.bean;

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;
    }

    // 用于标识该属性为实体的标识符，专门用于复合主键类
    @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;
    }

}

保存数据如下：

    @Test
    public void save() {
        EntityManagerFactory factory = Persistence
                .createEntityManagerFactory("yichun");
        EntityManager manager = factory.createEntityManager();
        manager.getTransaction().begin();

        manager.persist(new AirLine("PEK", "SHA", "北京飞上海"));

        manager.getTransaction().commit();
        manager.close();
        factory.close();
    }

查找数据如下：

@Test
    public void find() {
        EntityManagerFactory factory = Persistence
                .createEntityManagerFactory("yichun");
        EntityManager manager = factory.createEntityManager();

        AirLine airLine = manager.find(AirLine.class, new AirLinePK("PEK",
                "SHA"));
        System.out.println(airLine.getName() + " : "
                + airLine.getId().getStartCity() + " --> "
                + airLine.getId().getEndCity());

        manager.close();
        factory.close();
    }