This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Just like all servlets, the rule is you must use only local variables to be Thread safe. The amount of time the transaction takes doesn't matter. I would worry more about access to the file by other Threads. Bill