hibernate 映射组成关系

建立域模型和关系数据模型有着不同的出发点:

• 域模型: 由程序代码组成, 通过细化持久化类的的粒度可提高代码的可重用性, 简化编程
  
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• 在没有数据冗余的情况下, 应该尽可能减少表的数目, 简化表之间的参照关系, 以便提高数据的访问速度
  
  

Hibernate 把持久化类的属性分为两种:

• 值(value)类型: 没有 OID, 不能被单独持久化, 生命周期依赖于所属的持久化类的对象的生命周期
• 实体(entity)类型: 有 OID, 可以被单独持久化, 有独立的生命周期（如果实体类型包含值类型，这个值类型就是一个组件，虽然是由两个类，不同在数据库中用同一张数据库表表示）
  
  

显然无法直接用 property 映射 pay 属性

Hibernate 使用 <component> 元素来映射组成关系, 该元素表名 pay 属性是 Worker 类一个组成部分, 在 Hibernate 中称之为组件

component

Worker.java

package com.atguigu.hibernate.entities;public class Worker {private Integer id;private String name;private Pay pay;public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Pay getPay() {return pay;}public void setPay(Pay pay) {this.pay = pay;}}

Worker.hbm.xml

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!-- Generated 2014-1-2 16:14:33 by Hibernate Tools 3.4.0.CR1 --><hibernate-mapping package="com.atguigu.hibernate.entities">    <class name="Worker" table="WORKER">        <id name="id" type="java.lang.Integer">            <column name="ID" />            <generator class="native" />        </id>        <property name="name" type="java.lang.String">            <column name="NAME" />        </property>                <!-- 映射组成关系 -->        <component name="pay" class="Pay">        <!-- 指定组成关系的组件的属性 -->        <property name="monthlyPay" column="MONTHLY_PAY"></property>        <property name="yearPay" column="YEAR_PAY"></property>        <property name="vocationWithPay" column="VOCATION_WITH_PAY"></property>        </component>            </class></hibernate-mapping>

Pay.java

package com.atguigu.hibernate.entities;public class Pay {private int monthlyPay;private int yearPay;private int vocationWithPay;public int getMonthlyPay() {return monthlyPay;}public void setMonthlyPay(int monthlyPay) {this.monthlyPay = monthlyPay;}public int getYearPay() {return yearPay;}public void setYearPay(int yearPay) {this.yearPay = yearPay;}public int getVocationWithPay() {return vocationWithPay;}public void setVocationWithPay(int vocationWithPay) {this.vocationWithPay = vocationWithPay;}}

parent属性：

这里在组件中引用了父类private Worker worker;

Pay.java

package com.atguigu.hibernate.entities;public class Pay {private int monthlyPay;private int yearPay;private int vocationWithPay;private Worker worker;public Worker getWorker() {return worker;}public void setWorker(Worker worker) {this.worker = worker;}public int getMonthlyPay() {return monthlyPay;}public void setMonthlyPay(int monthlyPay) {this.monthlyPay = monthlyPay;}public int getYearPay() {return yearPay;}public void setYearPay(int yearPay) {this.yearPay = yearPay;}public int getVocationWithPay() {return vocationWithPay;}public void setVocationWithPay(int vocationWithPay) {this.vocationWithPay = vocationWithPay;}}

Worker.java

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!-- Generated 2014-1-2 16:14:33 by Hibernate Tools 3.4.0.CR1 --><hibernate-mapping package="com.atguigu.hibernate.entities">    <class name="Worker" table="WORKER">        <id name="id" type="java.lang.Integer">            <column name="ID" />            <generator class="native" />        </id>        <property name="name" type="java.lang.String">            <column name="NAME" />        </property>                <!-- 映射组成关系 -->        <component name="pay" class="Pay">        <parent name="worker"/>        <!-- 指定组成关系的组件的属性 -->        <property name="monthlyPay" column="MONTHLY_PAY"></property>        <property name="yearPay" column="YEAR_PAY"></property>        <property name="vocationWithPay" column="VOCATION_WITH_PAY"></property>        </component>            </class></hibernate-mapping>