How to find out the user who locked the row?

n some particular situations, we need to know the user-id who has acquired a lock on a table’s row and not yet released it. The following steps can give you an opportunity to find out this information.
1] Connect to SYS user.
2] Make use of the views: v$lock and v$session
v$lock gives you the SID which acquire a TX or TM type of lock if the table is locked by this session and v$session gives you the information of username who is running that session. Remember SID 2 & 3 in v$lock are for oracle background process, which you should not be concerned about.
3] Run the Query:

SELECT B.SID,

         C.USERNAME,

         C.OSUSER,

         C.TERMINAL,

         DECODE (B.ID2, 0, A.OBJECT_NAME, 'Trans-' || TO_CHAR (B.ID1))

            OBJECT_NAME,

         B.TYPE,

         DECODE (B.LMODE,

                 0, '--Waiting--',

                 1, 'Null',

                 2, 'Row Share',

                 3, 'Row Excl',

                 4, 'Share',

                 5, 'Sha Row Exc',

                 6, 'Exclusive',

                 'Other')

            "Lock Mode",

         DECODE (B.REQUEST,

                 0, ' ',

                 1, 'Null',

                 2, 'Row Share',

                 3, 'Row Excl',

                 4, 'Share',

                 5, 'Sha Row Exc',

                 6, 'Exclusive',

                 'Other')

            "Req Mode"

    FROM DBA_OBJECTS A, V$LOCK B, V$SESSION C

   WHERE A.OBJECT_ID(+) = B.ID1

       AND B.SID = C.SID

       AND C.USERNAME IS NOT NULL

ORDER BY B.SID, B.ID2;