Need advice to deal with concurrency

Question

Hi,
I just need you're advice on how I should handle concurrency. Iam doing ordering assignment, and I need to ensure concurrency. In my assignment concurrency comes into play if/when many customers try to order the same product but there is not enough product quantity.
At the moment I have made all my preparedstatement objects synchronized. Like following:
      synchronized(updateProductQuantity){
        updateProductQuantity.clearParameters();
        updateProductQuantity.setInt(1, prod_qty);
        updateProductQuantity.setInt(2, prod_no);
        updateProductQuantity.executeUpdate();
      }Iam wondering if this is enough. Do you think I should lock the database table?
Thanks for you're help!

Answer 1

What you have done there only ensures that one thread can execute that particular update at a time. You should do this regardless, since you wouldn't want two threads both getting into that code -- you might get prod_qty from one thread and prod_no from another thread.
However what you have done there doesn't prevent two customers from both ordering your last widget. And I don't think locking the table will help. What you need is a method that does this:
- Get the product record
- see if there is anything on hand
- if there isn't then set on the out-of-stock flag (or whatever)
- if there is then decrement the on-hand quantity and update the product record
And this entire method needs to be synchronized on some object.