Hibernate联合主键

     有时候我们在项目中可能需要联合主键来实现我们的业务，比如说 对于User类来说，我们可以把cardId和name它们两个联合起来作为User类的主键。

    官方API给我们解释的如下：

     You can use a component as an identifier of an entity class. Your component class must satisfy certain requirements:

      1.它必须实现java.io.Serializable接口

      2.It must re-implement equals() and hashCode() consistently with the database's notion of composite key equality.

      通过上面的我们可以知道，联合主键的实体必须实现Serializable接口以及重写equals()和hashCode()方法。

   为什么要这样做呢？

   重写这两个方法是因为hibernate要根据数据库的联合主键来判断某两行记录是否是一样的，如果一样那么就认为是同一个对象，如果不一样，那么认为是不同的对象，这反映到程序领域中就是根据hashCode与equals方法来判断某两个对象是否能够放到诸如Set这样的集合当中。
   联合主键的实体类实现Serializable接口的原因在于使用get或load方法的时候需要先构建出来该实体的对象，并且将查询依据(联合主键)设置进去，然后作为get或load方法第二个参数传进去即可。

   通过例子应该更好的帮助我们理解：

    User.java 用户实体类

 

package com.bawei.demo.vo;public class User {private UserPK id;public UserPK getId() {return id;}public void setId(UserPK id) {this.id = id;}private String sex;private String address;public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}@Overridepublic String toString() {return "User [id=" + id + ", sex=" + sex + ", address=" + address + "]";}}

    UserPK.java

package com.bawei.demo.vo;import java.io.Serializable;/** * @author shenyiya * 实体主键  * */public class UserPK implements Serializable {private String cardId;private String name;public String getCardId() {return cardId;}public void setCardId(String cardId) {this.cardId = cardId;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((cardId == null) ? 0 : cardId.hashCode());result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj) {return true;}if (obj == null) {return false;}if (getClass() != obj.getClass()) {return false;}UserPK other = (UserPK) obj;if (cardId == null) {if (other.cardId != null) {return false;}} else if (!cardId.equals(other.cardId)) {return false;}if (name == null) {if (other.name != null) {return false;} else if (!name.equals(other.name)) {return false;}}return true;}}

User.hbm.xml  实体映射文件

 

<?xml version="1.0" encoding="gb2312"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping><class name="com.bawei.demo.vo.User" table="user"><composite-id name="id" class="com.bawei.demo.vo.UserPK"><key-property name="cardId" type="string"></key-property><key-property name="name" type="string"></key-property></composite-id><property name="sex"></property><property name="address"></property></class></hibernate-mapping>

Hibernate.cfg.xml hibernate配置文件

<?xml version='1.0' encoding='UTF-8'?><!DOCTYPE hibernate-configuration PUBLIC          "-//Hibernate/Hibernate Configuration DTD 3.0//EN"          "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"><!-- Generated by MyEclipse Hibernate Tools.                   --><hibernate-configuration><session-factory><property name="dialect">org.hibernate.dialect.MySQLDialect</property><property name="connection.url">jdbc:mysql://localhost:3306/demo</property><property name="connection.username">root</property><property name="connection.password">123</property><property name="connection.driver_class">com.mysql.jdbc.Driver</property><property name="myeclipse.connection.profile">conn</property><property name="show_sql">false</property><property name="hbm2ddl.auto">update</property><mapping resource="com/bawei/demo/dao/Author.hbm.xml" /><mapping resource="com/bawei/demo/dao/Post.hbm.xml" /><mapping resource="com/bawei/demo/dao/User.hbm.xml" /></session-factory></hibernate-configuration>

 

Main 函数 ：

public static void main(String[] args) {// 1.初始化配置文件Configuration configuration = new Configuration().configure();// 2.建立session工厂SessionFactory sessionFactory = configuration.buildSessionFactory();// 3.从工厂中取得一个连接Session session = sessionFactory.openSession();// 4.开启事务Transaction tx = session.beginTransaction();//      保存//UserPK usersPK = new UserPK();//usersPK.setName("zhangsan");//usersPK.setCardId("110");//User user = new User();//user.setId(usersPK);//user.setAddress("beijing");//user.setSex("M");//session.save(user);//查询 UserPK usersPK = new UserPK();usersPK.setName("zhangsan");usersPK.setCardId("110");User user=(User) session.load(User.class, usersPK);System.out.println(user.toString());// 5.提交事务tx.commit();// 6.关闭连接session.close();}

效果如下图：