Typical usage of JDOQL is to query for instances of a class. DataNucleus also allows you to query for instances of implementations of an interface . Let's take an example. We have an interface

public interface ComputerPeripheral
{
    long getId();
    void setId(long val);

    String getManufacturer();
    void setManufacturer(String name);

    String getModel();
    void setModel(String name);
}

and we have the following implementations

public class Mouse implements ComputerPeripheral
{
    ...
}
public class Keyboard implements ComputerPeripheral
{
    ...
}

So to make it possible to query over ComputerPeripheral we need to make it persistable, hence defining metadata like this (or alternatively using annotations)

        <interface name="ComputerPeripheral">
            <property name="id" primary-key="true"/>
            <property name="manufacturer"/>
            <property name="model"/>
        </interface>

        <class name="Mouse" table="COMPUTER_MOUSE">
            <property name="id" primary-key="true"/>
            <property name="manufacturer"/>
            <property name="model"/>
        </class>

        <class name="Keyboard" table="COMPUTER_KEYBOARD">
            <property name="id" primary-key="true"/>
            <property name="manufacturer"/>
            <property name="model"/>
        </class>

so we have the interface defining the identity (and consistent in the implementations), and now we simply do

Query q = pm.newQuery(ComputerPeripheral.class);
List<ComputerPeripheral> results = (List<ComputerPeripheral>)q.execute();

The key rules are

• You must define the interface as persistent
• The interface must define the identity/primary key member(s)
• The implementations must have the same definition of identity and primary key
• This is only possible in version 2.2 M3 and later