TagPojo
         *
         |         |-------- EmailPojo
         *         V
ObjectPojo <-- MessagePojo <--- DocumentPojo
       ^
       |____ ImagePojo
       |
       |____ MusicPojo

@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS

   @ManyToMany(targetEntity = ObjectPojo.class, fetch = FetchType.EAGER, cascade = {
         CascadeType.PERSIST, CascadeType.MERGE })
   @JoinTable(name = "TAG_OBJECT_MAPPING", joinColumns = { @JoinColumn(name = "TAG_ID") }, inverseJoinColumns = { @JoinColumn(name = "OBJECT_ID") })
class TagPojo { ... }

@Entity
@Table(name="SHB_PRINCIPAL")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="TYPE",discriminatorType=DiscriminatorType.INTEGER)
@Cache(usage=CacheConcurrencyStrategy.TRANSACTIONAL)
public class Principal implements Serializable {
....

@Entity
@DiscriminatorValue(PrincipalType.USER_NR)
@Cache(usage=CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class ApplicationUser extends Principal {

   private static final long serialVersionUID = 1L;
         
   @ManyToMany(fetch=FetchType.LAZY, cascade={CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH},
         targetEntity=Role.class)
   @JoinTable(name="SHB_PRINCIPAL_TO_PRINCIPAL",
      joinColumns=@JoinColumn(name="FROM_PRINCIPAL_ID"),
      inverseJoinColumns=@JoinColumn(name="TO_PRINCIPAL_ID"))
   private Set<Role> roles;
   
   @ManyToMany(fetch=FetchType.LAZY, cascade={CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH},
         targetEntity=ApplicationGroup.class)
   @JoinTable(name="SHB_PRINCIPAL_TO_PRINCIPAL",
      joinColumns=@JoinColumn(name="FROM_PRINCIPAL_ID"),
      inverseJoinColumns=@JoinColumn(name="TO_PRINCIPAL_ID"))      
   private Set<ApplicationGroup> groups;

@Entity
@DiscriminatorValue(PrincipalType.GROUP_NR)
@Cache(usage=CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class ApplicationGroup extends Principal {

   private static final long serialVersionUID = 1L;   

   @ManyToMany(fetch=FetchType.LAZY, cascade={CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH})
   @JoinTable(name="SHB_PRINCIPAL_TO_PRINCIPAL",
      joinColumns=@JoinColumn(name="TO_PRINCIPAL_ID"),
      inverseJoinColumns=@JoinColumn(name="FROM_PRINCIPAL_ID"))      
   private Set<ApplicationUser> users;
   
   public ApplicationGroup() {
      super();
      this.type = PrincipalType.GROUP;
   }

   public Set<ApplicationUser> getUsers() {
      return users;
   }
   public void setUsers(Set<ApplicationUser> users) {
      this.users = users;
   }
}

@Entity
@Table(name="SHB_PRINCIPAL_TO_PRINCIPAL")
@Cache(usage=CacheConcurrencyStrategy.TRANSACTIONAL)
public class PrincipalPrincipal implements Serializable {
   
   private static final long serialVersionUID = 1L;
   
   @Id
   @GeneratedValue(strategy=GenerationType.IDENTITY)
   @Column(name="ID")
   private Integer id;

   @ManyToOne
   @JoinColumn(name="FROM_PRINCIPAL_ID")
   private Principal fromPrincipal;
   
   @Required
   @ManyToOne
   @JoinColumn(name="TO_PRINCIPAL_ID")
   private Principal toPrincipal;
   
   public PrincipalPrincipal() {
      super();
   }

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

   public Principal getFromPrincipal() {
      return fromPrincipal;
   }

   public void setFromPrincipal(Principal fromPrincipal) {
      this.fromPrincipal = fromPrincipal;
   }

   public Principal getToPrincipal() {
      return toPrincipal;
   }

   public void setToPrincipal(Principal toPrincipal) {
      this.toPrincipal = toPrincipal;
   }
}