public interface InstanceManager {
   public ManagedInstance getManagedInstance(String key);
   public void addManagedInstance(String key, ManagedInstance instance);
   public void removeManagedInstance(String key);
}

public interface ManagedInstance {
   public InstanceManager getOwner();
   // ... other methods of the managed instance
}

@Entity
public class InstanceManagerImpl implements InstanceManager {
   private long id;
   private Map<String, ManagedInstance> instances;
   
   @Id
   @GeneratedValue(strategy=GenerationType.AUTO)
   public long getId() {
      return this.id;
   }
   public void setId(long id) {
      this.id = id;
   }
   
   @CollectionOfElements
   public Map<String, ManagedInstance> getInstances() {
      return this.instances;
   }
   public void setInstances(Map<String, ManagedInstance> instances) {
      this.instances = instances;
   }
   
   // other methods not important here
}

@MappedSuperclass
public abstract class AbstractManagedInstance implements ManagedInstance {
   private InstanceManager owner;
   @Target(InstanceManagerImpl.class)
   public InstanceManager getOwner() {
      return this.owner;
   }
   public void setOwner(InstanceManager owner) {
      this.owner = owner;
   }
}

@Entity
public class FirstManagedInstance extends AbstractManagedInstance {
   // ...
}
@Entity
public class SecondManagedInstance extends AbstractManagedInstance {
   // ...
}
@Entity
public class JustAnotherManagedInstance extends AbstractManagedInstance {
   // ...
}