Implementing a cache using a java ConcurrentHashMap

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

Further to Ken's answer, if creating a heavyweight object which later gets thrown away is NOT acceptable (you want to guarantee that only one object gets created for each key, for some reason), then you can do this by.... actually, don't. Don't do it yourself. Use the google-collections (now guava) MapMaker class:

Map<KeyType, HeavyData> cache = new MapMaker<KeyType, HeavyData>()
  .makeComputingMap(new Function<KeyType, HeavyData>() {
      public HeavyData apply(KeyType key) {
          return new HeavyData(key); // Guaranteed to be called ONCE for each key
      }
  });

Then a simple cache.get(key) just works and completely removes you from having to worry about tricky aspects of concurrency and syncrhonization.

Note that if you want to add some fancier features, like expiry, it's just

Map<....> cache = new MapMaker<....>()
  .expiration(30, TimeUnit.MINUTES)
  .makeComputingMap(.....)

and you can also easily use soft or weak values for either keys or data if required (see the Javadoc for more details)

Answer 2

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".