public class ContainingEntity
{
    private Long oid; //pk
    private Map<MapKeyEntity, MapValueEntity> valuesByKey;
}

public class MapKeyEntity
{
    private Long oid; //pk
}

public class MapValueEntity
{
    private Long oid; //pk
}

<one-to-many name="valuesByKey">
    <map-key-join-column name="MAP_KEY_ID" />
    <join-table name="MAP_VALUES_BY_KEY">
        <join-column name="CONTAINER_ID" />
        <inverse-join-column name="MAP_VALUE_ID" />
    </join-table>
    <cascade><cascade-persist /></cascade>
</one-to-many>