jpa继承关系详解

7 Inheritance
对象使用引用以便关联到其它对象；关系型数据库表之间采用外键来描述表的关系。在关系型数据库中通常没有自然且有效的方法来描述类的继承关系。JPA通过Inheritance annotation提供了几种继承策略，它有以下属性：

InheritanceType strategy:用来声明继承策略。可选值是InheritanceType.SINGLE_TABLE、InheritanceType.JOINED和InheritanceType .TABLE_PER_CLASS。缺省值是InheritanceType.SINGLE_TABLE。
关于Inheritance的更多内容，可以参考Hibernate实战by Christian Bauer, Gavin King。



7．1 Single Table
InheritanceType.SINGLE_TABLE 策略为类的继承体系采用同一个表。表名是基类的名称。例如：

Java代码
@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
public class Base {
@Id
private int id;

@Basic
private String baseName;
}

@Entity
public class Derived1 extends Base {
@Basic
private String derived1Name;
}

@Entity
public class Derived2 extends Base {
@Basic
private String derived2Name;
}

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
public class Base {
@Id
private int id;

@Basic
private String baseName;
}

@Entity
public class Derived1 extends Base {
@Basic
private String derived1Name;
}

@Entity
public class Derived2 extends Base {
@Basic
private String derived2Name;
}

使用MappingTool建立的表结构如下：

Sql代码
mysql> describe base;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
| DTYPE | varchar(255) | YES | MUL | NULL | |
| derived1Name | varchar(255) | YES | | NULL | |
| derived2Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+

mysql> describe base;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
| DTYPE | varchar(255) | YES | MUL | NULL | |
| derived1Name | varchar(255) | YES | | NULL | |
| derived2Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+
Java代码
EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();
Base base = new Base();
base.setBaseName("base");
em.persist(base);
Derived1 d1 = new Derived1();
d1.setBaseName("derived1's base");
d1.setDerived1Name("derived1");
em.persist(d1);
Derived2 d2 = new Derived2();
d2.setBaseName("derived2's base");
d2.setDerived2Name("derived2");
em.persist(d2);
em.getTransaction().commit();
em.close();

EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();
Base base = new Base();
base.setBaseName("base");
em.persist(base);
Derived1 d1 = new Derived1();
d1.setBaseName("derived1's base");
d1.setDerived1Name("derived1");
em.persist(d1);
Derived2 d2 = new Derived2();
d2.setBaseName("derived2's base");
d2.setDerived2Name("derived2");
em.persist(d2);
em.getTransaction().commit();
em.close(); 以上代码执行后，数据库中base表的数据如下（其中DTYPE列由OpenJPA自动插入，用于区分不同的class，关于Discriminator的详细用法请参考OpenJPA User's Guide）：Sql代码
mysql> select * from base;
+----+-----------------+----------+--------------+--------------+
| id | baseName | DTYPE | derived1Name | derived2Name |
+----+-----------------+----------+--------------+--------------+
| 1 | base | Base | NULL | NULL |
| 2 | derived1's base | Derived1 | derived1 | NULL |
| 3 | derived2's base | Derived2 | NULL | derived2 |
+----+-----------------+----------+--------------+--------------+

mysql> select * from base;
+----+-----------------+----------+--------------+--------------+
| id | baseName | DTYPE | derived1Name | derived2Name |
+----+-----------------+----------+--------------+--------------+
| 1 | base | Base | NULL | NULL |
| 2 | derived1's base | Derived1 | derived1 | NULL |
| 3 | derived2's base | Derived2 | NULL | derived2 |
+----+-----------------+----------+--------------+--------------+
7．1．1 Advantages
InheritanceType.SINGLE_TABLE 策略的优势在于简单且性能高（因为不需要使用连接查询等）。如果类的继承体系中，子类和父类间的差异主要在于行为，同时子类之间以及子类和父类之间的属性差异不大（例如子类不增加属性或者增加的属性数目比较少），那么适用于这个策略。

7．1．2 Disadvantages
这个策略导致规范化级别降低。由于类继承体系中的每个类的属性都要映射到表的一列，因此当类的继承体系变的复杂的时候，表也随之变大。子类中属性对应的列必须声明为nullable。



7．2 Joined
InheritanceType.JOINED策略为类继承体系中的每个类创建不同的表。每个表只包含类中定义的列，因此在load一个子类的时候，JPA实现需要同时查询子类映射的表，以及通过关联查询所有的父类映射的表。PrimaryKeyJoinColumn annotation用来指定子类映射的表如何关联到父类映射的表。它有以下属性：

String name: 子类映射表中的列名。如果只有一个identity filed，那么缺省使用这个field对应的列名。
String referencedColumnName: 父类映射表中用来关联的列名。如果只有一个identity filed，那么缺省使用这个field对应的列名。
String columnDefinition: 数据库中列的数据类型。只有当JPA vendor支持通过metadata创建表的时候，这个属性才被使用。
以下是个简单的例子：

Java代码
@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class Base {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;

@Basic
private String baseName;
}

@Entity
@PrimaryKeyJoinColumn(name="id", referencedColumnName="id")
public class Derived1 extends Base {
@Basic
private String derived1Name;
}

@Entity
@PrimaryKeyJoinColumn(name="id", referencedColumnName="id")
public class Derived2 extends Base {
@Basic
private String derived2Name;
}

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class Base {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;

@Basic
private String baseName;
}

@Entity
@PrimaryKeyJoinColumn(name="id", referencedColumnName="id")
public class Derived1 extends Base {
@Basic
private String derived1Name;
}

@Entity
@PrimaryKeyJoinColumn(name="id", referencedColumnName="id")
public class Derived2 extends Base {
@Basic
private String derived2Name;
}
使用MappingTool建立的表结构如下：

Sql代码
mysql> describe base;
+----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
+----------+--------------+------+-----+---------+----------------+

mysql> describe derived1;
+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | | |
| derived1Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+-------+

mysql> describe derived2;
+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | | |
| derived2Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+-------+

mysql> describe base;
+----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
+----------+--------------+------+-----+---------+----------------+

mysql> describe derived1;
+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | | |
| derived1Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+-------+

mysql> describe derived2;
+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | | |
| derived2Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+-------+

Java代码
EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();
Base base = new Base();
base.setBaseName("base");
em.persist(base);
Derived1 d1 = new Derived1();
d1.setBaseName("derived1's base");
d1.setDerived1Name("derived1");
em.persist(d1);
Derived2 d2 = new Derived2();
d2.setBaseName("derived2's base");
d2.setDerived2Name("derived2");
em.persist(d2);
em.getTransaction().commit();
em.close();

EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();
Base base = new Base();
base.setBaseName("base");
em.persist(base);
Derived1 d1 = new Derived1();
d1.setBaseName("derived1's base");
d1.setDerived1Name("derived1");
em.persist(d1);
Derived2 d2 = new Derived2();
d2.setBaseName("derived2's base");
d2.setDerived2Name("derived2");
em.persist(d2);
em.getTransaction().commit();
em.close(); 以上代码执行后，数据库中base表的数据如下：Sql代码
mysql> select * from base;
+----+-----------------+
| id | baseName |
+----+-----------------+
| 1 | derived2's base |
| 2 | derived1's base |
| 3 | base |
+----+-----------------+

mysql> select * from derived1;
+----+--------------+
| id | derived1Name |
+----+--------------+
| 2 | derived1 |
+----+--------------+

mysql> select * from derived2;
+----+--------------+
| id | derived2Name |
+----+--------------+
| 1 | derived2 |
+----+--------------+

mysql> select * from base;
+----+-----------------+
| id | baseName |
+----+-----------------+
| 1 | derived2's base |
| 2 | derived1's base |
| 3 | base |
+----+-----------------+

mysql> select * from derived1;
+----+--------------+
| id | derived1Name |
+----+--------------+
| 2 | derived1 |
+----+--------------+

mysql> select * from derived2;
+----+--------------+
| id | derived2Name |
+----+--------------+
| 1 | derived2 |
+----+--------------+

7．2．1 Advantages
InheritanceType. JOINED策略的优势在于数据库表中没有冗余字段，因此规范化级别比较高；当有新的子类加入到类的继承体系中时，已有表的schema无须修改。如果类的继承体系中，子类和父类间的差异不在于行为，同时子类间的属性差异比较大，那么适用于这个策略。

7．2．2 Disadvantages
由于在查询的时候需要进行关联，那么查询的速度会比其它方式慢。此外可能需要多个插入和更新语句来处理多个表。



7．3 Table Per Class
InheritanceType.TABLE_PER_CLASS策略为类继承体系中的每个类创建不同的表。和InheritanceType.JOINED策略不同的是，每个表中包含所有的子类和父类中定义的所有列。因此在load一个子类的时候，JPA实现只需要同时查询子类映射的表。
以下是个简单的例子：

Java代码
@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public class Base {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;

@Basic
private String baseName;
}

@Entity
public class Derived1 extends Base {
@Basic
private String derived1Name;
}

@Entity
public class Derived2 extends Base {
@Basic
private String derived2Name;
}

@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public class Base {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;

@Basic
private String baseName;
}

@Entity
public class Derived1 extends Base {
@Basic
private String derived1Name;
}

@Entity
public class Derived2 extends Base {
@Basic
private String derived2Name;
}

使用MappingTool建立的表结构如下：

Sql代码
mysql> describe base;
+----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
+----------+--------------+------+-----+---------+----------------+

mysql> describe derived1;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
| derived1Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+

mysql> describe derived2;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
| derived2Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+

mysql> describe base;
+----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
+----------+--------------+------+-----+---------+----------------+

mysql> describe derived1;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
| derived1Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+

mysql> describe derived2;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| baseName | varchar(255) | YES | | NULL | |
| derived2Name | varchar(255) | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+ Java代码
EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();
Base base = new Base();
base.setBaseName("base");
em.persist(base);
Derived1 d1 = new Derived1();
d1.setBaseName("derived1's base");
d1.setDerived1Name("derived1");
em.persist(d1);
Derived2 d2 = new Derived2();
d2.setBaseName("derived2's base");
d2.setDerived2Name("derived2");
em.persist(d2);
em.getTransaction().commit();
em.close();

EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();
Base base = new Base();
base.setBaseName("base");
em.persist(base);
Derived1 d1 = new Derived1();
d1.setBaseName("derived1's base");
d1.setDerived1Name("derived1");
em.persist(d1);
Derived2 d2 = new Derived2();
d2.setBaseName("derived2's base");
d2.setDerived2Name("derived2");
em.persist(d2);
em.getTransaction().commit();
em.close(); 以上代码执行后，数据库中base表的数据如下：

Sql代码
mysql> select * from base;
+----+----------+
| id | baseName |
+----+----------+
| 1 | base |
+----+----------+

mysql> select * from derived1;
+----+-----------------+--------------+
| id | baseName | derived1Name |
+----+-----------------+--------------+
| 1 | derived1's base | derived1 |
+----+-----------------+--------------+

mysql> select * from derived2;
+----+-----------------+--------------+
| id | baseName | derived2Name |
+----+-----------------+--------------+
| 1 | derived2's base | derived2 |
+----+-----------------+--------------+

mysql> select * from base;
+----+----------+
| id | baseName |
+----+----------+
| 1 | base |
+----+----------+

mysql> select * from derived1;
+----+-----------------+--------------+
| id | baseName | derived1Name |
+----+-----------------+--------------+
| 1 | derived1's base | derived1 |
+----+-----------------+--------------+

mysql> select * from derived2;
+----+-----------------+--------------+
| id | baseName | derived2Name |
+----+-----------------+--------------+
| 1 | derived2's base | derived2 |
+----+-----------------+--------------+

7．3．1 Advantages
对于已知class类型的实例来说，这个策略十分有效。跟InheritanceType.JOINED策略类似，当有新的子类加入到类的继承体系中时，已有表的schema无须修改。

7．3．2 Disadvantages
这个策略在处理多态关系的时候会存在很多限制，此时某个引用（或者集合中的引用）可能指向任何子类的实例。由于无法使用关联查询，因此在查询的时候可能需要使用多个SQL语句或者使用UNION。