Hibernate繼承映射多態的詳解

• 一.使用註解，繼承映射寫法

     繼承映射在Annotation中使用@Inheritance註解，並且需要使用strategy屬性指定繼承策略，繼承策略有SINGLE_TABLE,TABLE_PER_CLASS和JOINED三種。

     在這三種方法中查詢速度：第二種方案 > 第一種方案 > 第三種方案。解耦程度：第三種方案 > 第一種方案 > 第二種方案

---

      SINGLE_TABLE:將父類和及其所有子類集合在一塊，存在一張表中，並創建一個新的字段來判斷對象類型。

person.java
package com.xiaoxiao.model;

import javax.persistence.*;

/**
*
* @ClassName: Person
* @Description: 人類
*
*/
@Entity
@Table(name = "SALES_PERSON")
@Inheritance(strategy =InheritanceType.SINGLE_TABLE)//指定繼承關係的生成策略
@DiscriminatorColumn(name = "discriminator", discriminatorType = DiscriminatorType.STRING)//指定生成新的判斷對象類型的字段的名稱和類型
@DiscriminatorValue("person")//DiscriminatorColumn的值
public class Person {

private Integer id;
private String name;

@Id
@Column(name = "id")
@SequenceGenerator(name = "SALES_PERSON_ID", sequenceName = "SALES_PERSON_ID", allocationSize = 1, initialValue = 1)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SALES_PERSON_ID")
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;
}

}


Student.java
package com.xiaoxiao.model;

import javax.persistence.*;

@Entity
@DiscriminatorValue("student")
public class Student extends Person {

private Integer score;

public Integer getScore() {

return score;
}

public void setScore(Integer score) {

this.score = score;
}

}


Teacher.java
package com.xiaoxiao.model;

import javax.persistence.*;

@Entity
@DiscriminatorValue("teacher")
public class Teacher extends Person {

private String title;

public String getTitle() {

return title;
}

public void setTitle(String title) {

this.title = title;
}

}

    生成的數據庫表如下                                                    存入數據後如下

                   

                                   

---

     TABLE_PER_CLASS：是爲每一個類創建一個表，這些表示相互獨立的。

       

---

     JOINED：是將父類和子類分別存放在不同表中，並且建立相應的外鍵，以確定相互之間的關係。

        

---

• 二.使用配置，繼承映射寫法

SINGLE_TABLE

<?xml version="1.0" encoding="utf-8"?>
<!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.xiaoxiao.pojo.People" table="PEOPLE">
        <id name="id" type="string">
            <column name="ID"></column>
            <generator class="uuid"></generator>
        </id>

        <discriminator column="PEOPLETYPE" type="string"></discriminator>

        <property name="name" column="NAME" type="string"></property>
        <property name="sex" column="SEX" type="string"></property>
        <property name="age" column="AGE" type="string"></property>
        <property name="birthday" column="BIRTHDAY" type="timestamp"></property>

        <subclass name="com.xiaoxiao.pojo.Student"discriminator-value="student">
            <property name="cardId" column="CARDID" type="string"></property>
        </subclass>

        <subclass name="com.xiaoxiao.pojo.Teacher" discriminator-value="teacher">
            <property name="salary" column="SALARY" type="string"></property>
        </subclass>
    </class>
</hibernate-mapping>

TABLE_PER_CLASS

<?xml version="1.0" encoding="utf-8"?>
<!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.xiaoxiao.pojo.People" abstract="true">//<class>標籤中的"abstract"屬性如果值爲true則，不會生成表結構。如果值爲false則會生成表結構，但是不會插入數據
        <id name="id" type="string">
            <column name="ID"></column>
            <generator class="uuid"></generator>
        </id>

        <property name="name" column="NAME" type="string"></property>
        <property name="sex" column="SEX" type="string"></property>
        <property name="age" column="AGE" type="string"></property>
        <property name="birthday" column="BIRTHDAY" type="timestamp"></property>

        <!-- 
        <union-subclass name="com.xiaoxiao.pojo.Student" table="STUDENT"> 
            <property name="cardId" column="CARDID" type="string"></property> 
        </union-subclass> 
        <union-subclass name="com.xiaoxiao.pojo.Teacher" table="TEACHER"> 
            <property name="salary" column="SALARY" type="integer"></property> 
        </union-subclass> 
        -->
    </class>

        <!-- 可單獨寫在Student.hbm.xml裏 -->
    <union-subclass name="com.xiaoxiao.pojo.Student"
        table="STUDENT" extends="com.xiaoxiao.pojo.People">
        <property name="cardId" column="CARDID" type="string"></property>
    </union-subclass>

        <!-- 可單獨寫在Teacher.hbm.xml裏 -->
    <union-subclass name="com.xiaoxiao.pojo.Teacher"
        table="TEACHER" extends="com.xiaoxiao.pojo.People">
        <property name="salary" column="SALARY" type="integer"></property>
    </union-subclass>
</hibernate-mapping>

JOINED

<?xml version="1.0" encoding="utf-8"?>
<!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.xiaoxiao.pojo.People" table="PEOPLE">
        <id name="id" type="string">
            <column name="ID"></column>
            <generator class="uuid"></generator>
        </id>

        <property name="name" column="NAME" type="string"></property>
        <property name="sex" column="SEX" type="string"></property>
        <property name="age" column="AGE" type="string"></property>
        <property name="birthday" column="BIRTHDAY" type="timestamp"></property>

        <joined-subclass name="com.xiaoxiao.pojo.Student" table="STUDENT">
            <key column="ID"></key>
            <property name="cardId" column="CARDID" type="string"></property>
        </joined-subclass>

        <joined-subclass name="com.xiaoxiao.pojo.Teacher" table="TEACHER">
            <key column="ID"></key>
            <property name="salary" column="SALARY" type="integer"></property>
        </joined-subclass>
    </class>
</hibernate-mapping>