java - Mapping Sub-Classes by Type Table (Many-To-One Hibernate)

Question

I have to make the domain-layer of a collage project. We have few standards, like we have to use Hibernate and the database also is fix.

The relevant part of the database looks nearly like that:

BusEntitys (Table 1)

• BusId
• Bus specific information

BusType (Table 2)

• BusTypeId
• Seats
• ...
• SubClass Discriminator

The problem I have is that there are 2 types of buses in the domain-layer distinguish by the discriminator in the BusType-table:

@Entity
class Bus
{
   @Id
   int _id;
   ...
}

@Entity
class BusSubClass1 extends Bus
{
   ...
}

@Entity
class BusSubClass2 extends Bus
{
   ...
}

Is there a way to map something like this with Hibernate and JPA? Thanks for every answer.

Jochen Morent

Answer 1

Yes, there are few ways to map such scenario. JPA provides support for three different data representations:

• single-table strategy
• joined strategy
• table-per-class strategy

In other words depending on inheritance type used you will get different database model. Various JPA providers may support additional inheritance strategies.

Consider the following example:

@Entity
@Inheritance //by default SINGLE_TABLE strategy
@DiscriminatorColumn( //not supported for TABLE_PER_CLASS strategy
    name = "BUS_TYPE", 
    discriminatorType = DiscriminatorType.INTEGER
)
public abstract class Bus {
    @Id
    protected int id;
    protected int seats;

    public Bus() {
    }
}

@Entity
@DiscriminatorValue(value = "1") //not supported for TABLE_PER_CLASS strategy
public class BusSubClass1 extends Bus {
    private String specific1;

    public BusSubClass1() {
    }
}

@Entity
@DiscriminatorValue(value = "2") //not supported for TABLE_PER_CLASS strategy
public class BusSubClass2 extends Bus {
    @Temporal
    private Data specific2;

    public BusSubClass2() {
    }
}

---

Using InheritanceType.SINGLE_TABLE strategy leads to a single database table containing all concrete entity types:

 Bus

 ID BUS_TYPE SEATS SPECIFIC1 SPECIFIC2
 -- -------- ----- --------- ---------
 1  1        50    qwerty
 2  1        55    asdfgh
 3  2        30              2014-01-01

• every concrete entity instance in the inheritance hierarchy is distinguished by discriminator value (here indicated by BUS_TYPE column)
• all database columns must be declared as nullable because not all of them may contain values (thus may be problematic for columns that cannot be set to null)
• wide or deep hierarchy may affect performance (a lot of redundant columns or rows respectively in a table)
• lack of normalization can waste space in database table
• offers good performance for read and write operations (polymorphic queries don't require joins, only discriminator value)
• adding/removing entity fields may be problematic (from database administration point of view)

---

Using InheritanceType.JOINED strategy leads to multiple database tables per entity type (all shared fields from Bus are stored in the corresponding table):

 Bus                        BusSubClass1          BusSubClass2

 ID BUS_TYPE SEATS          ID SPECIFIC1          ID SPECIFIC2
 -- -------- -----          -- ---------          -- ---------
 1  1        50             1  qwerty             3  2014-01-01
 2  1        55             2  asdfgh
 3  2        30

• every concrete entity class in the inheritance hierarchy is distinguished by the discriminator value (here indicated by BUS_TYPE column)
• normalization improves data storage (less unused space in comparison to SINGLE_TABLE strategy)
• non-polymorphic queries (for a single concrete entity) require a join
• polymorphic queries (for wide or deep hierarchy of entity classes) require multiple joins and may be expensive
• adding/removing entity fields is rather straightforward (from database administration point of view)

---

Using InheritanceType.TABLE_PER_CLASS strategy leads to exqactly one database table per entity type (all shared fields from Bus are redefined in the concrete subclasses):

 BusSubClass1                BusSubClass2

 ID SEATS SPECIFIC1          ID SEATS SPECIFIC2
 -- ----- ---------          -- ----- ---------
 1  50    qwerty             3  30    2014-01-01
 2  55    asdfgh                     

• every concrete entity class in the inheritance hierarchy is distinguished only by the shared identifier (the discriminator is not used)
• normalization improves data storage (less unused space in comparison to SINGLE_TABLE strategy, but more than in case of JOINED strategy)
• non-polymorphic queries (for a single concrete entity) are efficient as join is not required
• polymorphic queries (for wide or deep hierarchy of entity classes) require multiple joins and may be expensive
• adding/removing entity fields is straightforward (from database administration point of view)