Question

I am learning Oracle Lock.

In the one session

lock table countries in ROW Exclusive mode nowait;

In the another session, I update counties table.

SQL> update countries set country_name = 'ddd';
25 rows updated.

what I expected is the session will hang. do you know why ?

And I check EM, I do see the lock.

Answer 1

ROW EXCLUSIVE

doesn’t actually lock the table completely.

ROW EXCLUSIVE and ROW SHARE allows concurrent access but prevents anyone else locking the entire table for exclusive access.
Exclusive also prohibits other actions from locking the table in SHARE mode.

If you want to lock the table from anything but Reads; you need to omit ROW so

LOCK TABLE countries 
    IN EXCLUSIVE MODE 
    NOWAIT;

There are more examples here: http://docs.oracle.com/cd/B14117_01/server.101/b10759/statements_9015.htm