Using Guava Cache As Map with Time Based Eviction

Question

I basically need a map where entries would expire after a specific known time period and then are removed. The way it's being used in my app is not really a cache but it seems Guava cache can serve the purpose. Would this be the right choice? One thing is I'm going to need to query if the map is empty and I saw that Guava has only a size function which its documentation says is only an approximation.

Answer 1

You can use Guava for this purpose. Note the caveat about cleanup, as noted in the documentation here (reproduced below).

Caches built with CacheBuilder do not perform cleanup and evict values "automatically," or instantly after a value expires, or anything of the sort. Instead, it performs small amounts of maintenance during write operations, or during occasional read operations if writes are rare.

The reason for this is as follows: if we wanted to perform Cache maintenance continuously, we would need to create a thread, and its operations would be competing with user operations for shared locks. Additionally, some environments restrict the creation of threads, which would make CacheBuilder unusable in that environment.

Instead, we put the choice in your hands. If your cache is high-throughput, then you don't have to worry about performing cache maintenance to clean up expired entries and the like. If your cache does writes only rarely and you don't want cleanup to block cache reads, you may wish to create your own maintenance thread that calls Cache.cleanUp() at regular intervals.

If you want to schedule regular cache maintenance for a cache which only rarely has writes, just schedule the maintenance using ScheduledExecutorService.

As for the checking size point, you are correct that the size() is approximate. If you need to perform some action whenever an entry is invalidated, you should use the removalListener functionality. Relevant sample code from the documentation reproduced now.

CacheLoader<Key, DatabaseConnection> loader = new CacheLoader<Key, DatabaseConnection> () {
  public DatabaseConnection load(Key key) throws Exception {
    return openConnection(key);
  }
};
RemovalListener<Key, DatabaseConnection> removalListener = new RemovalListener<Key, DatabaseConnection>() {
  public void onRemoval(RemovalNotification<Key, DatabaseConnection> removal) {
    DatabaseConnection conn = removal.getValue();
    conn.close(); // tear down properly
  }
};

return CacheBuilder.newBuilder()
  .expireAfterWrite(2, TimeUnit.MINUTES)
  .removalListener(removalListener)
  .build(loader);