Question

I’d like to implement a simple caching of heavyweight objects in a web java application. But I can’t figure out how to do it properly.

Am I missing something or ConcurrentHashMap methods (putIfAbsent, …) are not enough and additional synchronization is needed ?

Is there a better simple API (In memory storage, no external config) to do this ?

P.

Answer 1

If it is safe to temporarily have more than one instance for the thing you’re trying to cache, you can do a “lock-free” cache like this:

public Heavy instance(Object key) {
  Heavy info = infoMap.get(key);
  if ( info == null ) {
    // It's OK to construct a Heavy that ends up not being used
    info = new Heavy(key);
    Heavy putByOtherThreadJustNow = infoMap.putIfAbsent(key, info);
    if ( putByOtherThreadJustNow != null ) {
      // Some other thread "won"
      info = putByOtherThreadJustNow;
    }
    else {
      // This thread was the winner
    }
  }
  return info;
}

Multiple threads can “race” to create and add an item for the key, but only one should “win”.