Let's suppose you have an interface with a selection of classes implementing the interface something like this

You then have a class that contains an object of this interface type

public class ShapeHolder
{
    protected Shape shape=null;
    ...
}
                

To allow persistence of this interface field with JPOX you have 2 levels of control. The first level is global control. Since all of our Square, Circle, Rectangle classes implements Shape then we just define them in the MetaData as we would normally.

<package name="org.jpox.samples.shape">
    <class name="Square">
        ...
    </class>
    <class name="Circle">
        ...
    </class>
    <class name="Rectangle">
        ...
    </class>
</package>

The global way means that when mapping that field JPOX will look at all PersistenceCapable classes it knows about that implement the specified interface.

JDO2 also allows users to specify a list of classes implementing the interface on a field-by-field basis, defining which of these implementations are accepted for a particular interface field. To do this you define the Meta-Data like this

<package name="org.jpox.samples.shape">
    <class name="ShapeHolder">
        <field name="shape" persistence-modifier="persistent"
               field-type="org.jpox.samples.shape.Circle,
                           org.jpox.samples.shape.Rectangle,
                           org.jpox.samples.shape.Square"/>
</class>

That is, for any interface object in a class to be persisted, you define the possible implementation classes that can be stored there. JPOX interprets this information and will map the above example classes to the following in the database

So JPOX adds foreign keys from the containers table to all of the possible implementation tables for the shape field.

You can have a Collection/Map containing elements of an interface type. You specify this in the same way as you would any Collection/Map. You can have a Collection of interfaces as long as you use a join table relation and it is unidirectional. You need to use a JPOX extension tag "implementation-classes" if you want to restrict the collection to only contain particular implementations of an interface.