This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
It sounds like you're asking how to create a critical section of code. In Java, that's very easy. Just use the key word "synchronized". It's typically used to synchronize an entire method, by you can also synchronize a block of code if you want.