Question

I am looking for a concurrent Set with expiration functionality for a Java 1.5 application. It would be used as a simple way to store / cache names (i.e. String values) that expire after a certain time.

The problem I’m trying to solve is that two threads should not be able to use the same name value within a certain time (so this is sort of a blacklist ensuring the same “name”, which is something like a message reference, can’t be reused by another thread until a certain time period has passed). I do not control name generation myself, so there’s nothing I can do about the actual names / strings to enforce uniqueness, it should rather be seen as a throttling / limiting mechanism to prevent the same name to be used more than once per second.

Example:
Thread #1 does cache.add("unique_string, 1) which stores the name “unique_string” for 1 second.
If any thread is looking for “unique_string” by doing e.g. cache.get("unique_string") within 1 second it will get a positive response (item exists), but after that the item should be expired and removed from the set.

The container would at times handle 50-100 inserts / reads per second.

I have really been looking around at different solutions but am not finding anything that I feel really suites my needs. It feels like an easy problem, but all solutions I find are way too complex or overkill.

A simple idea would be to have a ConcurrentHashMap object with key set to “name” and value to the expiration time then a thread running every second and removing all elements whose value (expiration time) has passed, but I’m not sure how efficient that would be? Is there not a simpler solution I’m missing?

Answer 1

How about creating a Map where the item expires using a thread executor

//Declare your Map and executor service
final Map<String, ScheduledFuture<String>> cacheNames = new HashMap<String, ScheduledFuture<String>>();
ScheduledExecutorService executorService = Executors.newSingleThreadScheduledExecutor();

You can then have a method that adds the cache name to your collection which will remove it after it has expired, in this example its one second. I know it seems like quite a bit of code but it can be quite an elegant solution in just a couple of methods.

ScheduledFuture<String> task = executorService.schedule(new Callable<String>() {
  @Override
  public String call() {
    cacheNames.remove("unique_string");
    return "unique_string";
  }
}, 1, TimeUnit.SECONDS);
cacheNames.put("unique_string", task);