Hibernte继承映射

继承映射分为三种：

1. 继承结构共用一张表结构
2. 每个类一张表结构
3. 每个具体类一张表结构

下面使用Animal、Dog、Fish举例

Animal类如下：

package com.hml.domain.a;

public abstract class Animal {

    private Integer id;
    private String name;

    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;
    }

}

Dog类如下：

package com.hml.domain.a;

public class Dog extends Animal {

    private float runSpeed;

    public float getRunSpeed() {
        return runSpeed;
    }

    public void setRunSpeed(float runSpeed) {
        this.runSpeed = runSpeed;
    }

}

Fish类如下：

package com.hml.domain.a;

public class Fish extends Animal {

    private float swimSpeed;

    public float getSwimSpeed() {
        return swimSpeed;
    }

    public void setSwimSpeed(float swimSpeed) {
        this.swimSpeed = swimSpeed;
    }

}

1、继承结构共用一张表

Animal.hbm.xml映射文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.hml.domain.a">
    <class name="Animal" table="t_animal" discriminator-value="0">
       <id name="id">
           <generator class="native"></generator>
       </id>
       <discriminator column="type" type="int"></discriminator>
       <property name="name"></property>

       <subclass name="Dog" discriminator-value="1">
          <property name="runSpeed"></property>
       </subclass>

       <subclass name="Fish" discriminator-value="2">
           <property name="swimSpeed"></property>
       </subclass>

    </class>

</hibernate-mapping>

生成的表结构：

整个继承结构一张表，在映射文件建中通过subclass节点，对子类进行映射。由于整个继承结构一张表，所以在表中需要区分具体是哪个类，所以需要一个鉴别字段，这里通过 discriminator指定，然后在映射类的节点上添加鉴别值，使用这种映射的限制是，子类属性不能有非空约束。

2、每个类一张表

Animal.hbm.xml映射文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.hml.domain.a">
    <class name="Animal" table="t_animal" >
       <id name="id">
           <generator class="native"></generator>
       </id>
       <property name="name"></property>

       <joined-subclass name="Dog" table="t_dog">
           <key column="id"></key>
           <property name="runSpeed"></property>
       </joined-subclass>

       <joined-subclass name="Fish" table="t_fish">
           <key column="id"></key>
           <property name="swimSpeed"></property>
       </joined-subclass>

    </class>

</hibernate-mapping>

生成的表结构如下：

t_animal:

t_dog

t_fish

每个类一张表是通过joined-subclass映射子类，joined-subclass中的key是个外键，在基类中存储基类中的信息，然后每个子类生成的表存储本类中的数据。

3、每个具体类一张表

映射文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.hml.domain.a">
    <class name="Animal" table="t_animal"  abstract="true">
       <id name="id">
           <generator class="hilo">
                <param name="table">hi_value</param>
                <param name="column">next_value</param>
                <param name="max_lo">100</param>
            </generator>
       </id>
       <property name="name"></property>

       <union-subclass name="Dog" table="t_dog">
          <property name="runSpeed"></property>
       </union-subclass>

       <union-subclass name="Fish" table="t_fish">
          <property name="swimSpeed"></property>
       </union-subclass>

    </class>

</hibernate-mapping>

每个具体类一张表需要在抽象类上添加abstract="true"，不然，抽象了也会生成表的。每个具体类一张表时，主键不能使用自增长，如果使用自增长会报错的。

值得注意的是，继承结构表结构的映射一般不会使用，原因是太复杂，效率低。