Wednesday, August 25, 2010

Java Initialization Barrier Pattern With AtomicBoolean

I've found myself starting to use this little mini-pattern. You might find it useful.

private final AtomicBoolean _hasBeenInitialized = new AtomicBoolean(false);
public void expensiveInitialization() {
  if (_hasBeenInitialized.getAndSet(true)) {
    // Someone else has already done the initialization,
    // or is currently doing it.
    return;
  }
  // Do the initialization that I want to be done only once.
}

Much simpler than any of the other mutual exclusion patterns that I've found myself using.

The caveat here is that in the case of multiple threads, it's possible that one thread (that got to the party late) may return to the caller before the initialization is done (if another thread is currently doing the initialization). Therefore, this pattern isn't suitable where the caller has to guarantee that the initialization is done before continuing in a multi-threaded environment.

I primarily use it where there's an initialization method that many parts of the code are going to call as a defensive measure before continuing. Think of it as a simple solution to the initializeIfYouHaventBeenButDoNothingOtherwise problem.

Of course, it well could be that everybody else in the world is already doing single-initialization this way, and I'm too addled from being outside the day-to-day coding world to have caught up.

UPDATE 2010-08-25 : I changed the name of the control AtomicBoolean to make it clearer that this is a multiple-execution-over-time pattern, and not a general purpose synchronization barrier.

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