Hibernate 继承

 

在域模型中，类与类之间除了关联关系和聚集关系，还可以存在继承关系，在下图所示的域模型中，Deparment类和Employee类之间为一对多的 双向关联关系，Employee类有两个子 类：Skiller类和Sales类。由于Java只允许一个类最多有一个直接的父类，因此Employee类、 Skiller类和Sales类构成了一棵继承关系树。

 在 面向对象的范畴中，还存在多态的概念，多态建立在继承关系的基础上。简单地理解，多态是指当一个Java应用变量被声明为Employee类时，这个变量 实际上既可以引用Employee类自己的实例，Skiller类的实例，也可以引用Sales类的实例。Department类的getEmps()方 法通过Hibernate API从数据库中检索出所有Employee对象。getEmps()方法返回的集合既可以包含Employee类自己的实例，Skiller类的实例， 也可以引用Sales类的实例。，这种查询被称为多态查询。数据库表之间并不存在继承关系，那么如何把域模型的继承关系映射到关系数据模型中 呢？hibernate有以下三种映射方式：

继承关系树的根类对应一个表：对关系数据模型进行非常规设计，在数据库表中加入额外的区分子类型的字段。通过这种方式，可以使关系数据模型支持继承关系和多态。

继承关系树的每个类对应一个表(子类与父类通过外键关联)：在关系数据模型中用外键参照关系来表示继承关系。

继承关系树的每个具体类对应一个表：关系数据模型完全不支持域模型中的继承关系和多态。

1.继承关系树的根类对应一个表employee（整个继承树一张表）：

employee的表结构如下所示：

mysql> desc employee;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int(11)      | NO   | PRI | NULL    | auto_increment |
| type       | int(11)      | NO   |     | NULL    |                |
| name       | varchar(255) | YES  | UNI | NULL    |                |
| depart_id  | int(11)      | YES  | MUL | NULL    |                |
| skill       | varchar(255) | YES  |     | NULL    |                |
| saleAmount | int(11)      | YES  |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

实体类Department和Employee请参看我前面的文章，Skiller和Sales分别如下所示：

package com.reiyen.hibernate.domain;

public class Skiller extends Employee {

    private String skill;
//setter和getter方法
}

 

package com.reiyen.hibernate.domain;
public class Sales extends Employee {

    private int saleAmount;
//setter和getter方法
}

 Employee.hbm.xml映射文件如下：

xml version="1.0"?>

    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.reiyen.hibernate.domain">
    <class name="Employee" discriminator-value="0">
        <id name="id">
            <generator class="native" />
        id>

        <discriminator column="type" type="int">discriminator>
        <property name="name" unique="true"/>

        <many-to-one name="department" column="depart_id" />
        <subclass name="Skiller" discriminator-value="1">
            <property name="skill" />
        subclass>
        <subclass name="Sales" discriminator-value="2">
            <property name="saleAmount" />
        subclass>
    class>
hibernate-mapping>

 

 测试类如下：

public class Many2One {

    public static void main(String[] args) {
        add();
         query(1);//1
    }

    static void query(int empId) {
        Session s = null;
        Transaction tx = null;
        try {
            s = HibernateUtil.getSession();
            tx = s.beginTransaction();
            Employee emp = (Employee) s.get(Employee.class, empId);//2
            System.out.println(emp.getClass());
            tx.commit();
        } finally {
            if (s != null)
                s.close();
        }
    }



    static void add() {
        Session s = null;
        Transaction tx = null;
        try {
            Department depart = new Department();
            depart.setName("department name");

            Employee employee1 = new Employee();
            employee1.setDepartment(depart); //1 对象模型：建立两个对象的关联
            employee1.setName("employee1 name1");

            Skiller employee2 = new Skiller();
            employee2.setDepartment(depart); //2 对象模型：建立两个对象的关联
            employee2.setName("employee2 name2");
            employee2.setSkill("j2se");

            Sales employee3 = new Sales();
            employee3.setDepartment(depart); //2 对象模型：建立两个对象的关联
            employee3.setName("employee3 name3");
            employee3.setSaleAmount(1000);

            s = HibernateUtil.getSession();
            tx = s.beginTransaction();
            s.save(depart);
            s.save(employee1);
            s.save(employee2);
            s.save(employee3);
            tx.commit();
        } finally {
            if (s != null)
                s.close();
        }
    }
}

 程序运行后，控制台打印信息如下所示：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id, type) values (?, ?, 0)
Hibernate: insert into Employee (name, depart_id, skill, type) values (?, ?, ?, 1)
Hibernate: insert into Employee (name, depart_id, saleAmount, type) values (?, ?, ?, 2)
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.depart_id as depart4_1_0_, employee0_.skill as skill1_0_, employee0_.saleAmount as saleAmount1_0_, employee0_.type as type1_0_ from Employee employee0_ where employee0_.id=? 
class com.reiyen.hibernate.domain.Employee

employee表中记录如下所示：

mysql> select * from employee;
+----+------+-----------------+-----------+-------+------------+
| id | type | name            | depart_id | skill | saleAmount |
+----+------+-----------------+-----------+-------+------------+
|  1 |    0 | employee1 name1 |         1 | NULL  |       NULL |
|  2 |    1 | employee2 name2 |         1 | j2se  |       NULL |
|  3 |    2 | employee3 name3 |         1 | NULL  |       1000 |
+----+------+-----------------+-----------+-------+------------+
3 rows in set (0.00 sec)

将测试代码中注释为1的语句改成:

query(2);

再运行，控制台打印的class如下所示(因为hibernate支持多态查询)： class com.reiyen.hibernate.domain.Skiller

打印的查询语句还是如上面所示的没有改变。

在上面修改的基础上，再将测试代码中注释为2的语句改成:

Employee emp = (Employee) s.get(Skiller.class, empId);

 再运行，则控制台打印的查询语句为：

Hibernate: select skiller0_.id as id1_0_, skiller0_.name as name1_0_, skiller0_.depart_id as depart4_1_0_, skiller0_.skill as skill1_0_ from Employee skiller0_ where skiller0_.id=? and skiller0_.type=1 
class com.reiyen.hibernate.domain.Skiller

优点：操作效率高

缺点：如果说给employee增加子类的话,必须修改表结构,给表结构增加一个字段；同时表中对应子类的字段不能有非空约束.

2.继承关系树的每子类对应一个表(joined-subclass),表结构如下所示：

 修改Employee.hbm.xml映射文件如下所示：

xml version="1.0"?>

    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.reiyen.hibernate.domain">
    <class name="Employee">
        <id name="id">
            <generator class="native" />
        id>
        <property name="name" unique="true"/>

        <many-to-one name="department" column="depart_id" />
        <joined-subclass name="Skiller" table="skiller">
         <key column="employee_id" />
         <property name="skill" />
        joined-subclass>
        <joined-subclass name="Sales" table="sales">
         <key column="employee_id" />
         <property name="saleAmount" column="sale_amount" />
        joined-subclass>
    class>
hibernate-mapping>

 测试类不变，只是将测试代码中注释为1的语句改成:

query(2);

则控制台打印的信息如下所示：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: insert into skiller (skill, employee_id) values (?, ?) 
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: insert into sales (sale_amount, employee_id) values (?, ?)
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.depart_id as depart3_1_0_, employee0_1_.skill as skill2_0_, employee0_2_.sale_amount as sale2_3_0_, case when employee0_1_.employee_id is not null then 1 when employee0_2_.employee_id is not null then 2 when employee0_.id is not null then 0 end as clazz_0_ from Employee employee0_ left outer join skiller employee0_1_ on employee0_.id=employee0_1_.employee_id left outer join sales employee0_2_ on employee0_.id=employee0_2_.employee_id where employee0_.id=?
class com.reiyen.hibernate.domain.Skiller
 从打印的SQL语句可以看出，此时，如果保存的是Employee对象的子类的实例的话，则要在两张表中保存记录；如果查询的是子类对象的话，是三张表关联在一起进行查询。

 

在上面修改的基础上，再将测试代码中注释为2的语句改成:

Employee emp = (Employee) s.get(Skiller.class, empId);

 再运行，则控制台打印的查询语句为：

Hibernate: select skiller0_.employee_id as id1_0_, skiller0_1_.name as name1_0_, skiller0_1_.depart_id as depart3_1_0_, skiller0_.skill as skill2_0_ from skiller skiller0_ inner join Employee skiller0_1_ on skiller0_.employee_id=skiller0_1_.id where skiller0_.employee_id=?
此时只关联两张表查询。

数据库中表记录如下所示：

mysql> select * from employee;
+----+-----------------+-----------+
| id | name            | depart_id |
+----+-----------------+-----------+
|  1 | employee1 name1 |         1 |
|  2 | employee2 name2 |         1 |
|  3 | employee3 name3 |         1 |
+----+-----------------+-----------+
3 rows in set (0.00 sec)

mysql> select * from skiller;
+-------------+-------+
| employee_id | skill |
+-------------+-------+
|           2 | j2se  |
+-------------+-------+
1 row in set (0.00 sec)

mysql> select * from sales;
+-------------+-------------+
| employee_id | sale_amount |
+-------------+-------------+
|           3 |        1000 |
+-------------+-------------+
1 row in set (0.00 sec)

 

3.混合使用，假设如果Sales的属性很多，而Skiller的属性很少，这时可以混使用“一个类继承体系一张表”和“每个子类一张表”，表结构如下所示：

 Employee.hbm.xml映射文件如下所示：

xml version="1.0"?>

    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.reiyen.hibernate.domain">
    <class name="Employee" discriminator-value="0">
        <id name="id">
            <generator class="native" />
        id>
        <discriminator column="type" type="int" />
        <property name="name" unique="true" />

        <many-to-one name="department" column="depart_id" />

<subclass name="Skiller" discriminator-value="1">
            <property name="skill" />
        subclass>
        <subclass name="Sales" discriminator-value="2">
            <join table="sales">
                <key column="employee_id" />
                <property name="saleAmount" column="sale_amount" />
            join>
        subclass>
    class>
hibernate-mapping>

 此时测试类不变，只是将测试代码中注释为1的语句改成:

query(2);

然后在上面的基础上运行原程序，则控制台会打印出如下异常信息：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id, type) values (?, ?, 0)
Exception in thread "main" org.hibernate.exception.SQLGrammarException: could not insert: [com.reiyen.hibernate.domain.Employee]

Caused by: com.mysql.jdbc.exceptions.MySQLSyntaxErrorException: Unknown column 'type' in 'field list'

这是因为我在hibernate.cfg.xml配置文件中配置了此项：

<property name="hbm2ddl.auto">createproperty>

 所以在此次程序运行时，会删除数据库中的employee表，sales表，而employee表中有skiller表的外键关联，所以不能删除employee数据表了，所以抛出了上面的异常。此时你再查看数据库表，如下所示 ：

mysql> select * from employee;
+----+-----------------+-----------+
| id | name            | depart_id |
+----+-----------------+-----------+
|  1 | employee1 name1 |         1 |
|  2 | employee2 name2 |         1 |
|  3 | employee3 name3 |         1 |
+----+-----------------+-----------+
3 rows in set (0.00 sec)

mysql> select * from skiller;
+-------------+-------+
| employee_id | skill |
+-------------+-------+
|           2 | j2se  |
+-------------+-------+
1 row in set (0.00 sec)

mysql> select * from sales;
Empty set (0.00 sec)

所以得先手动删除skiller数据表，然后再来运行程序：

 

如果Employee.hbm.xml配置文件中

<subclass name="Skiller" >

不配置discriminator-value="1"，则会抛出如下异常：

java.lang.ExceptionInInitializerError

Caused by: org.hibernate.MappingException: Could not format discriminator value to SQL string

因为如果discriminator-value没有显式的给定值的话，则与name属性的值保持一致，即为Skiller ,所以会抛出如上异常！

 

4.继承关系树的每个具体类对应一个表（union-subclass）

表结构如下所示：

Employee.hbm.xml映射文件如下：

xml version="1.0"?>

    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.reiyen.hibernate.domain">
    <class name="Employee">
        <id name="id">
            <generator class="hilo" />
        id>
        <property name="name" unique="true" />
        <many-to-one name="department" column="depart_id" />
        <union-subclass name="Skiller" table="skiller">
         <property name="skill" />
        union-subclass>
        <union-subclass name="Sales" table="sales">
         <property name="saleAmount" column="sale_amount" />
        union-subclass>
    class>
hibernate-mapping>

 此时主键增长不能再是:

<generator class="native" />

因为如果使用native的话三张表会产生相同的id值，这样当根据id查询Employee时就会出错了。所以如果你配置成native时会抛出如下异常(因为Employee实体类中id对应 的是int了，所以在此使用hilo（高低位）主键生成方式)：

org.hibernate.MappingException: Cannot use identity column key generation with mapping for: com.reiyen.hibernate.domain.Skiller

 

运行测试程序后，此时控制台打印信息如下所示：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id, id) values (?, ?, ?)
Hibernate: insert into skiller (name, depart_id, skill, id) values (?, ?, ?, ?)
Hibernate: insert into sales (name, depart_id, sale_amount, id) values (?, ?, ?, ?)
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.depart_id as depart3_1_0_, employee0_.skill as skill2_0_, employee0_.sale_amount as sale1_3_0_, employee0_.clazz_ as clazz_0_ from ( select id, null as sale_amount, depart_id, null as skill, name, 0 as clazz_ from Employee unionselect id, null as sale_amount, depart_id, skill, name, 1 as clazz_ from skiller union select id, sale_amount, depart_id, null as skill, name, 2 as clazz_ from sales ) employee0_ where employee0_.id=? 
class com.reiyen.hibernate.domain.Skiller

执行查询时，首先使用子查询，在子查询中使用union将三张表的结果全成一张表，然后再在合成的结果集中进行查询。

如果Employee是一个抽象类， 你不想 在数据表中对应相应的数据表，则可以设置abstract="true" .如下所示：

<class name="Employee" abstract="true" >

  此外，如果继承关系中有接口，可以把它当作抽象类对待。

三种映射方式的比较和选择
为了方便说明为三种方式按顺序标号为[1]整个继承树一张表;[2]每子类对应一个表(joined-subclass);[4]每个具体类对应一个表（union-subclass）。
1、复杂度：

    [1]简单；
    [2]表较多且之间有外键约束；

    [4]包含重复字段；
2、查询性能：

    [1]效率高；
    [2]需要表内连接或左外连接；

    [4]若查询父类需查所有子类表；
3、可维护性：

    [1]只需修改一个表；
    [2]若某个类属性变化只修改这个类对应的表；

    [4]若父类属性变化需要修改所有子类对应的表；
综上，选择时，可以参考以下原则：
1、子类属性不是非常多时，优先考虑[1]，因为其性能最佳。
2、子类属性非常多，且对性能要求不是很严格时，优先考虑[2]