Check the blocking sessions in database

In Oracle how to check what are the blocking sessions in a database:

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 ;