精通hibernate学习笔记（5）[映射组成关系]

1、概念

1.1 聚集关系
  在域模型中，有些类由几个部分类组成，部分类的对象的生命周期依赖于整体类的对象的生命周期，当整体消失时，部分也就随之消失，这种整体与部分的关系被称为聚集关系。

1.2 组成关系
  如果部分只能属于特定的整体，这种聚集关系也叫做组成关系。
如：任何手足事整体与部分的关系，每双手只能属于特定的人。

2、粒度


建立关系数据模型的一个重要原则：在不会导致数据冗余的前提下，尽可能减少数据库表的数目以及表之间的外键参照关系，这样可以避免建立多表连接。

3. 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。域模型是由程序代码组成的，通过细化持久化类的粒度可提高代码可重用性，简化编程。关系数据模型是由关系数据组成的。在存在数据冗余的情况下，需要把粗粒度的表拆分成具有外键参照关系的几个细粒度的表，从而节省存储空间；另一方面，在没有冗余数据的前提下，应该尽可能减少表的数目，简化表之间的参照关系，以便提高数据库的访问速度。

例：
映射文件：
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping >

  <class name="mypack.Customer" table="CUSTOMERS" >
    <id name="id" type="long" column="ID">
      <generator class="increment"/>
    </id>

    <property name="name" type="string" >
        <column name="NAME" length="15" />
    </property>
   
    <component name="homeAddress" class="mypack.Address">
            <parent name="customer" />
            <property name="province" type="string" column="HOME_PROVINCE"/>
            <property name="city" type="string" column="HOME_CITY"/>
            <property name="street" type="string" column="HOME_STREET"/>
            <property name="zipcode" type="string" column="HOME_ZIPCODE"/>
    </component>
       
    <component name="comAddress" class="mypack.Address">
           <parent name="customer" />
           <property name="province" type="string" column="COM_PROVINCE"/>
           <property name="city" type="string" column="COM_CITY"/>
           <property name="street" type="string" column="COM_STREET"/>
           <property name="zipcode" type="string" column="COM_ZIPCODE"/>
    </component>
  </class>

</hibernate-mapping>

<component>元素表明homeAddress属性是Customer类的一个组成部分，在hibernate中称之为组件.
<parent>子元素指定Address类所属的整体类，应定义为Address中的一个属性，同时对应的getter、setter方法。
<property>子元素用来配置组件类的属性和表的字段的映射。

3.1 区分值(Value)类型和实体(Entity)类型

上面Address类的定义如下：
package mypack;

import java.io.Serializable;
import org.apache.commons.lang.builder.ToStringBuilder;


/** @author Hibernate CodeGenerator */
public class Address implements Serializable {

    /** nullable persistent field */
    private String province;

    /** nullable persistent field */
    private String city;

    /** nullable persistent field */
    private String street;

    /** nullable persistent field */
    private String zipcode;

    /** nullable persistent field */
    private mypack.Customer customer;

    /** full constructor */
    public Address(String province, String city, String street, String zipcode, mypack.Customer customer) {
        this.province = province;
        this.city = city;
        this.street = street;
        this.zipcode = zipcode;
        this.customer = customer;
    }

    /** default constructor */
    public Address() {
    }

    public String getProvince() {
        return this.province;
    }

    public void setProvince(String province) {
        this.province = province;
    }

    public String getCity() {
        return this.city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getStreet() {
        return this.street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    public String getZipcode() {
        return this.zipcode;
    }

    public void setZipcode(String zipcode) {
        this.zipcode = zipcode;
    }

    public mypack.Customer getCustomer() {
        return this.customer;
    }

    public void setCustomer(mypack.Customer customer) {
        this.customer = customer;
    }

    public String toString() {
        return new ToStringBuilder(this)
            .toString();
    }

}

可以发现Address类没有OID,这是Hibernate组件的一个重要特征，没有OID也就不能通过Session来单独管理，否则会抛出MappingException。
    持久化类分为两种，值类型和实体类型，最重要的区别是前者没有OID，不能被单独持久化，他的生命周期依赖于所属的持久化类对象的生命周期，组件类型就是一种值类型。

4、映射复合组成关系

配置：
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping >

  <class name="mypack.Computer" table="COMPUTERS" >
    <id name="id" type="long" column="ID">
      <generator class="increment"/>
    </id>

    <property name="type" type="string" >
        <column name="COMPUTER_TYPE" length="15" />
    </property>
   
    <component name="cpuBox" class="mypack.CpuBox">
            <parent name="computer" />
           
            <property name="type" type="string" >
                <column name="CPUBOX_TYPE" length="15" />
            </property>
           
            <component name="graphicsCard" class="mypack.GraphicsCard">
               <parent name="cpuBox" />
             
               <property name="type" type="string" >
                  <column name="GRAPHICSCARD_TYPE" length="15" />
               </property>

            </component>

            <many-to-one
                name="vendor"
                column="CPUBOX_VENDOR_ID"
                class="mypack.Vendor"
                not-null="true"
            />
    </component>
  </class>


</hibernate-mapping>

定义：
package mypack;

import java.io.Serializable;
import org.apache.commons.lang.builder.ToStringBuilder;


/** @author Hibernate CodeGenerator */
public class Computer implements Serializable {

    /** identifier field */
    private Long id;

    /** nullable persistent field */
    private String type;

    /** persistent field */
    private mypack.CpuBox cpuBox;

    /** full constructor */
    public Computer(String type, mypack.CpuBox cpuBox) {
        this.type = type;
        this.cpuBox = cpuBox;
    }

    /** default constructor */
    public Computer() {
    }

    /** minimal constructor */
    public Computer(mypack.CpuBox cpuBox) {
        this.cpuBox = cpuBox;
    }

    public Long getId() {
        return this.id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getType() {
        return this.type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public mypack.CpuBox getCpuBox() {
        return this.cpuBox;
    }

    public void setCpuBox(mypack.CpuBox cpuBox) {
        this.cpuBox = cpuBox;
    }

    public String toString() {
        return new ToStringBuilder(this)
            .append("id", getId())
            .toString();
    }

}
package mypack;

import java.io.Serializable;
import org.apache.commons.lang.builder.ToStringBuilder;


/** @author Hibernate CodeGenerator */
public class CpuBox implements Serializable {

    /** nullable persistent field */
    private String type;

    /** nullable persistent field */
    private mypack.Computer computer;

    /** persistent field */
    private mypack.Vendor vendor;

    /** persistent field */
    private mypack.GraphicsCard graphicsCard;

    /** full constructor */
    public CpuBox(String type, mypack.Computer computer, mypack.Vendor vendor, mypack.GraphicsCard graphicsCard) {
        this.type = type;
        this.computer = computer;
        this.vendor = vendor;
        this.graphicsCard = graphicsCard;
    }

    /** default constructor */
    public CpuBox() {
    }

    /** minimal constructor */
    public CpuBox(mypack.Vendor vendor, mypack.GraphicsCard graphicsCard) {
        this.vendor = vendor;
        this.graphicsCard = graphicsCard;
    }

    public String getType() {
        return this.type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public mypack.Computer getComputer() {
        return this.computer;
    }

    public void setComputer(mypack.Computer computer) {
        this.computer = computer;
    }

    public mypack.Vendor getVendor() {
        return this.vendor;
    }

    public void setVendor(mypack.Vendor vendor) {
        this.vendor = vendor;
    }

    public mypack.GraphicsCard getGraphicsCard() {
        return this.graphicsCard;
    }

    public void setGraphicsCard(mypack.GraphicsCard graphicsCard) {
        this.graphicsCard = graphicsCard;
    }

    public String toString() {
        return new ToStringBuilder(this)
            .toString();
    }

}