Problem while setting new value to entity object attribute in doDML meathod

Question

Hi all,
I am overriding the entity objects doDML method for generating the value of Sequence Number just before insert .
For this puropose i am using doDML method. I am fetching the maximum value for the sequence number feild from the table by a prepared statement
and then incrementing that value by one.
    protected void doDML(int operation, TransactionEvent e) {
        if (operation == DML_INSERT) {
            // code for getting the max+1 of sequence number before insert
            //command executes.
            try {
                System.out.println("Inside doDML Method");
                String sql =
                    "select nvl(max(seq_no),0)+1  from WF_LEAVE_HDR where org_unit_code = " +
                    this.getOrgUnitCode();
                PreparedStatement pstmt =
                    getDBTransaction().createPreparedStatement(sql, 0);
                ResultSet rs = pstmt.executeQuery();
                rs.next();
                Integer newSeqNo =rs.getInt(1); //(Number)rs.getString(0);
                //this.setSeqNo(new Number(newSeqNo));
                setAttributeInternal("SeqNo",new Number(newSeqNo));
                System.out.println("Value of new seq no is -->>"+getSeqNo());
            } catch (Exception excpt) {
                System.out.println("Inside catch block ");
                excpt.printStackTrace();
        super.doDML(operation, e);
    }i am getting the value correct by using the sql statement but while i am using setAttributeInternal("SeqNo",new Number(newSeqNo));
it is giving an error and value for new sequence no is not passed to the seq no feild of the entity object. I have tried this.setSeqNo(new Number(newSeqNo))
but it is also not wotrking .
Any one please help , I am using Jdeveloper 10.1.3
Thanks all in advance.

Answer 1

iloveoracle,
Sigh... in addition to doing this in doDML (which Dimitar points out is the wrong place to do this)... you are making a huge huge mistake.
select nvl(max(seq_no),0)+1  from WF_LEAVE_HDRWhat happens when two people do this at around the same time? You don't do any locking, so you will get two rows with the same SeqNo. This is absolutely the wrong way of doing sequence numbers. The best way of doing this would be to use real sequence numbers (an Oracle sequence) and ignore the fact that there will be gaps. If you insist on using your approach, you MUST LOCK THE ENTIRE TABLE before you try to do your little max() + 1 trick, otherwise you run the very real risk of getting duplicate SeqNos. OK, I see that you are trying to do sequences by org_unit_code, so you don't have to lock the whole table, but you do have to have some way of holding a lock. You must also write some code to be able to handle an "unable to get the lock because someone else already holds it" type of situation.
<rant>
I have seen so many people try to do this little max() + 1 trick. It DOES NOT, WILL NOT work until you handle locking properly. One question that I often ask when I interview database developers is about generating "gapless" sequences; unless the job is for a brand-new-with-absolutely-no-experience trainee, answering "select max() + 1" without any mention of concurrency issues would be grounds for an immediate rejection of the candidate. Seriously. Have a run over to http://asktom.oracle.com and search for "gapless" if you'd like to see a more strongly worded rant.
</rant>
Bottom line, just use an Oracle sequence if it's at all possible; otherwise, be prepared to write some bunches of code to deal with locking.
John