hibernate - NamedQuery with self-referencing ManyToMany relation -

i've user entity maintaining self-referencing manytomany relation represents user's friends. have entity, attribute, holding manytoone relation user entity.

now want retrieve, through namedquery, , given user u, user's friends whit exact attribute value.

what query?

i tried like:

"select a.user "+ "from attribute "+ "where a.attribute= :attribute , "+       "c.user in ( :user.friends )" 

this give me null pointer exception during sessionfactory creation while initializing application.

here entities:

@entity public class user implements serializable {      private static final long serialversionuid = 1l;     @id     private string id;     private string nickname;     private string thumbnailurl;       @manytomany      @jointable(          name = "user_friends",           joincolumns = @joincolumn(name = "user_id"),               inversejoincolumns = @joincolumn(name = "friend_id")     )     protected list<user> friends = new arraylist<user>();     @manytomany(mappedby = "friends")     protected list<user> befriended = new arraylist<user>();      /*     *   getters , setters...     */ }   public class attribute implements serializable {      /*       * other fields...     */      @manytoone         @joincolumn(name="userid")     private user user;      /*     *   getters , setters     */  }