Oracle Database- Who is using my undo Tablespace??

It Happened once that i came in morning and logged onto the database. I was just checking tablespaces when i observed undo tablespace is totally full....so what was eatin up my undo..in such case oracle provides you a view called V$transaction that gives you information about transactions using Oracle db undo tablespace. This view can be joined with V$session and with help of this you can easily come to know which user is using your undo tablespace. In my case some developer has left running an update query which was to update a major table at night and had not committed..I called up the developer..and told him to either commit or rollback..he committed his transaction..and undo got freed..