JPA--联合主键

联合主键的一些知识：

使用@EmbeddedId标示联合主键；

在联合主键类中只是定义确定联合主键的字段即可；
* 联合主键类的规则
* 1.必须包含一个无参的构造函数
* 2.必须实现序列化接口
* 3.必须重写hashCode和equals方法，而且equals方法的参数必须包括确定联合主键的所有字段

联合主键类的定义：

package com.yl.demo1.bean.JointPK;

import java.io.Serializable;

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

/**
 * 在联合主键类中只是定义确定联合主键的字段即可
 * 联合主键类的规则
 *     1.必须包含一个无参的构造函数
 *     2.必须实现序列化接口
 *  3.必须重写hashCode和equals方法，而且equals方法的参数必须包括确定联合主键的所有字段
 * @author yul
 *
 *@Embeddable--告诉其它类，在使用AieLinePK时只是使用AieLinePK的属性作为实体的持久化属性
 */

@Embeddable //嵌入
public class AirLinePK implements Serializable {
    private String startCity;//航空中每个城市都有一个三位的字母标识符
    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;
    }
    
    
}

package com.yl.demo1.bean.JointPK;

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;
    }

}

常用操作:

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