import java.io.Serializable;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.MappedSuperclass;

@MappedSuperclass
public abstract class AbstractEntity
        implements Serializable
{

    @ManyToOne(optional = false)
    @JoinColumn(name = "ref_id", referencedColumnName = "id", nullable = false)
    private ReferencedEntity realm;

    protected AbstractEntity()
    {
    }

    public Realm getRef()
    {
        return ref;
    }

    public void setRef(final ReferencedEntity ref)
    {
        this.ref = ref;
    }
}

import java.io.Serializable;
import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public abstract class ReferencedEntity
        implements Serializable
{
    @Id
    private int id;

    public ReferencedEntity()
    {
        super();
    }

    public int getId() { return id; }
    public int setId(int id) { this.id = id; }
}

import java.io.Serializable;
import javax.persistence.AssociationOverride;
import javax.persistence.AssociationOverrides;
import javax.persistence.Entity;
import javax.persistence.ForeignKey;
import javax.persistence.Id;
import javax.persistence.JoinColumn;

@Entity
@AssociationOverrides({
    @AssociationOverride(name = "realm", 
                         joinColumns = @JoinColumn(name = "realm_id"),
                         foreignKey = @ForeignKey(name = "FK_CONCRETEENT_REF"))
})
public abstract class ConcreteEntity
        implements Serializable
{
    @Id
    private int id;

    public ConcreteEntity()
    {
        super();
    }

    public int getId() { return id; }
    public int setId(int id) { this.id = id; }
}