Component polymorphism (<subclass> within <componen

melquiades · **Posted:** Mon Mar 29, 2004 7:02 pm

I want to map a component with subtypes. The DTD seems not to support a <subclass> tag nested within <component>. Am I missing something? Is this just not supported? Is there a workaround?

Or am I stuck with writing a user type?

________________________________________________

In case you need more detail, here's my situation, boiled down and made humorous:

Code:

class Critter {
    BreathingStrategy getBreathingStrategy();
     ...
}
interface BreathingStrategy {
    void breathe();
}
class Lungs implements BreathingStrategy {
    Set<Alveolus> getAlveoli();
    ...
}
class Gills implements BreathingStrategy {
    List<Gill> getGills();
    ...
}

CRITTER:
    BREATHING_STRATEGY
     ...
ALVEOLI:
     CRITTER_ID
     ...
GILL:
     CRITTER_ID
     ...

I'd want to map this roughly as follows:

Code:

<class name="Critter">
    <component name="breathingStrategy" class="BreathingStrategy">
        
        <discriminator column="BREATHING_STRATEGY" type="integer" />

        <subclass name="Lungs" discriminator-value="0" >
            <set name="alveoli">
                <key column="CRITTER_ID" />
                <one-to-many class="Alveolus" />
            </set>
        </subclass>
        
        <subclass name="Gills" discriminator-value="1" >
            <set name="alveoli">
                <key column="CRITTER_ID" />
                <one-to-many class="Gill" />
            </set>
        </subclass>
    </component>
</class>

For the sake of this example, the different breathing strategies aren't supposed to know about each other, and the implementation Critter should be completely decoupled from all BreathingStrategy implementations. So I can't cheat by putting both Lungs and Gills on Critter internally, and then just exposing one or the other.

Thoughts?

emmanuel · **Posted:** Wed Mar 31, 2004 9:34 am

A component is just a wrapper for several properties, It cannot be subclassed.

melquiades · **Posted:** Wed Mar 31, 2004 11:13 am

emmanuel wrote:

A component is just a wrapper for several properties, It cannot be subclassed.

Well yes, that is clearly the case now. But why does this need to be so? There's no reason of logical consistency that I can see -- allowing component subclassing would make perfect sense, and seems like the only reasonable way of handling the case I laid out above.

If you can come up with a good alternative, I'm all ears!

gavin · **Posted:** Wed Mar 31, 2004 7:20 pm

You can easily write a UserType that does this. Or you might like to create a patch to allow <component-subclass>es (this is not very hard).

cyaconi · **Joined:** Mon Mar 08, 2004 6:09 pm **Posts:** 20

I'm interested on this topic, can you give me some tips or readings on how to do it??

Thank you!

letmein2 · **Joined:** Fri Feb 09, 2007 3:47 pm **Posts:** 56

I also wanted this ability and couldn't do it. As a workaround, I tried to create a one-to-one relationship with a shared key, but couldn't figure out how to get a shared key without a back reference. I'd be really nice if component sub-classing just worked.

edbras · **Posted:** Thu Oct 01, 2009 6:18 am

Any news on this ?
I am looking for examples on how to realize this with a UserType.