lock during update

I have a Java class that wants to read a file to get the last used
log number for the order file on an AS/400, increment it, then update
it.  The code I was using was:

       ControlFile.open(KeyedFile.READ_WRITE,
                        0,  // blocking factor
                        KeyedFile.COMMIT_LOCK_LEVEL_NONE);
       ControlFile.lock(KeyedFile.WRITE_ALLOW_SHARED_READ_LOCK);

       key = new Object[2];
       key[0] = "LE"; key[1] = "          ";
       rec = ControlFile.read(key);
       ConfirmationNumber = (BigDecimal)rec.getField("CNVA2");
       ConfirmationNumber = ConfirmationNumber.add(new BigDecimal("1"));
       rec.setField("CNVA2", ConfirmationNumber);
       ControlFile.update(key, rec);
     } catch (Exception e) {
       return (Object)e.getMessage();
     } finally {
       try {
         ControlFile.releaseExplicitLocks();
         ControlFile.close();

But this fails when a background job kicks in that processes the orders
that this same program writes, so it seems like "ControlFile.lock()" is
excessive.  An RPG programmer in our shop tells me that when you update
a record, that *record* is automatically locked.  But how do I ensure
that no one changes the CNVA2 field out from under me between the time
I read that record and the time I update it?  What the RPG programmer
explained to me is that when you do a CHAIN, you either update the record
or release it, but meantime it's yours.  What's the equivalent in Java?
(Sorry if this isn't clear to AS/400 programmers, I'm mainly a PC guy.)

-- 
____________________________________________________________
Glenn Holmer                          gholmer@weycogroup.com
Programmer/Analyst                       phone: 414.908.1809
Weyco Group, Inc.                          fax: 414.908.1601

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