JPA聯合主鍵

　　聯合主鍵也就是說需要多個字段才能確定數據庫記錄中的唯一一行。這樣就需要多個字段一起，組成主鍵，也叫聯合主鍵。例如飛機航線，我們需要知道飛機起飛的地點以及飛機降落的地點。所以需要飛機起飛的地點和降落的地方才能確定一條航線。AirLine表示航線，AirLinePK表示主鍵類。AirLinePK代碼如下：

package com.yichun.bean;

import java.io.Serializable;

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

/**
 * 聯合主鍵。一般使用PK 只需要定義用作主鍵的字段
 * <p>
 * 聯合主鍵類必須遵守的JPA規范：<br>
 * 1、必須要提供一個public的無參數的構造方法<br>
 * 2、必須要實現序列化接口<br>
 * 3、必須重寫hashCode（）與equals（）方法
 */
// 用在實體里面,只是使用該類中的屬性。（該類中的屬性用在持久化的類中的字段）
@Embeddable
public class AirLinePK implements Serializable {
    private String startCity;// PEK，北京 CAN廣州，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;
    }

    // 以下兩個方法判斷是否相等
    @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 代碼如下：

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;
    }

    // 用於標識該屬性為實體的標識符，專門用於復合主鍵類
    @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;
    }

}

保存數據如下：

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

        manager.persist(new AirLine("PEK", "SHA", "北京飛上海"));

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

查找數據如下：

@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();
    }