create sequence A

insert into T1(a,b) values(nextval for A, "data");
insert into T2(a,b) values(prevval for A, "data);

I am running a multiple threads java application.

The first field of table T1 and T2 should be the same if nobody issue a NEXTVAL for A in between.

My question is how can I ensure nobody(other threads, or other DB2 user) can issue a 'nextval for A'...