Who is blocking sessions in Oracle

Who is blocking  sessions   in Oracle

set echo off
column blocker format a10;
column blockee format a10;
alter session set optimizer_mode=rule;

select
           (select username from v$session where sid=a.sid) blocker,
         a.sid,
       ' is blocking ' "IS BLOCKING",
         (select username from v$session where sid=b.sid) blockee,
             b.sid
    from v$lock a, v$lock b
   where a.block = 1
     and b.request > 0
     and a.id1 = b.id1
     and a.id2 = b.id2
/

alter session set optimizer_mode=choose;


set lines 200
select s1.USERNAME || '@' || s1.MACHINE
  || ' ( SID=' || s1.SID || ' ) is blocking '
  || s2.USERNAME || '@' || s2.MACHINE || ' ( SID=' || s2.SID || ' ) ' as BLOCKING_STATUS
  from v$lock l1, v$session s1, v$lock l2, v$session s2
  where s1.SID = l1.SID and s2.SID = l2.SID
  and l1.BLOCK = 1 and l2.REQUEST > 0
  and l1.ID1 = l2.ID1
  and l2.ID2 = l2.ID2
/