Hibernate学习笔记（5）联合主键

现在大家都不推荐使用联合主键，关键是因为其需要自己手工维护，比较麻烦。但是一个项目可能因为历史遗留原因，你不得不面对联合主键。 

Hibernate联合主键问题解决如下：
可以使用一个组件作为一个实体类的标识符。你的组件类必须满足以下要求： 
(1)它必须实现 java.io.Serializable 接口 
(2)它必须重新实现 equals() 和 hashCode() 方法，始终和组合关键字在数据库中的概念保持一致 
注意：在 Hibernate3 中，第二个要求并非是 Hibernate 强制必须的。但最好这样做。 
不能使用一个 IdentifierGenerator 产生组合关键字。一个应用程序必须分配它自己的标识符。

将主键所对应属性提取出一个类（称之为主键类），并且主键类需要实现Serializable接口，重写equals方法与hashCode方法，原因:

在于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，如果一样那么就认为是同一个对象，如果不一样，那么就认为是不同的对象。

具体做法如下步骤：

（1）将主键所对应属性提取出一个类（称之为主键类），并且主键类需要实现Serializable接口，重写equals方法与hashCode方法 

实例如下：

package com.model;

import java.io.Serializable;

//ScoreInfo主键类
//composite-id class must implement Serializable: com.model.ScorePK
public class ScorePK  implements Serializable{
	
	private static final long serialVersionUID = 1L;
	private Integer  studentID;
	private Integer  courseID;
	
	public int getStudentID() {
		return studentID;
	}
	public void setStudentID(int studentID) {
		this.studentID = studentID;
	}
	public int getCourseID() {
		return courseID;
	}
	public void setCourseID(int courseID) {
		this.courseID = courseID;
	}
	
	@Override
	public boolean equals(Object o){
		if(o instanceof ScorePK){
			ScorePK scorePK = (ScorePK)o;
			if(this.studentID == scorePK.studentID && this.courseID == scorePK.courseID){
				return true;
			}
		}
		return false;
	}
	@Override
	public int hashCode(){
		int result = 1;
		result = result + ((this.studentID == null) ? 0 : this.studentID.hashCode());
        result = result + ((this.courseID == null) ? 0 : this.courseID.hashCode());
        return result; 
	}
}

（2）实体类（这里不用包含主键所对应的属性）

package com.model;

public class ScoreInfo {
	//联合主键
	private ScorePK scorePK;
	private int score;
	
	public ScorePK getScorePK() {
		return scorePK;
	}
	public void setScorePK(ScorePK scorePK) {
		this.scorePK = scorePK;
	}
	
	public int getScore() {
		return score;
	}
	public void setScore(int score) {
		this.score = score;
	}
}

<?xml version="1.0"?>
<!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.model.ScoreInfo" table="ScoreInfo">
		<composite-id name = "ScorePK" class = "com.model.ScorePK">
			<key-property name="StudentID"></key-property>
			<key-property name="CourseID"></key-property>
		</composite-id>
		<property name="score" column = "Score"/>
    </class>
</hibernate-mapping>

<composite-id name = "ScorePK" class = "com.model.ScorePK">
			<key-property name="StudentID"></key-property>
			<key-property name="CourseID"></key-property>
		</composite-id>

（4）不要忘记在hibernate配置文件中引用映射文件。

<!-- XML配置方式 -->
        <mapping resource="com/model/ScoreInfo.hbm.xml"/>

package com.test;

import org.hibernate.Session;

import com.model.ScoreInfo;
import com.model.ScorePK;
import com.util.HibernateUtil;

public class Test {
	
	public static void main(String[] args) {	
		
		Test mgr = new Test();
		mgr.createAndStoreEvent();
		HibernateUtil.getSessionFactory().close();
    }

    private void createAndStoreEvent() {

        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();
        
        ScorePK scorePK = new ScorePK();
        scorePK.setStudentID(2);
        scorePK.setCourseID(1);
        
        ScoreInfo score = new ScoreInfo();
        score.setScorePK(scorePK);
        score.setScore(98);
        
        session.save(score);
        session.getTransaction().commit();
    }
}

package com.util;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {
	
	private static final SessionFactory sessionFactory;
    static 
    {
        try 
        {
            // Create the SessionFactory from hibernate.cfg.xml
            sessionFactory = new Configuration().configure().buildSessionFactory();
        }
        catch (Throwable ex) 
        {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
}