public abstract class LazyLoadingMapping<DataKey, DataType>
{
  private Map<DataKey, DataType> store;
  private Set<DataKey> toFetch;
  private Thread dataFetcher;

  public LazyLoadingMap()
  {
    dataFetcher = new Thread(new DataFetcher(1000));
  }

  /**
   * Actually fetch a single item.
   */
  protected abstract DataItem doFetch(DataKey id);

  /**
   * Public API for resolving an id to data.
   */
  public DataType fetch(DataKey id)
  {
    // not convinced that the synchronization here is perfect
    synchronized(store)
    {
      DataType dt = store.get(id);
      if(dt == null) {
        synchronized(toFetch)
        {
          dt = doFetch(id);
          toFetch.remove(id);
        }
      }

      return dt;
    }
  }

  /**
   * Public API for indicating that we may wish to resolve an ID to data soon.
   */
  public void interestedIn(DataKey id)
  {
    synchronized(toFetch)
    {
      toFetch.add(id);
    }
  }

  private class DataFetcher
          extends Runnable
  {
    private final long sleepTime;

    DataFetcher(long sleepTime)
    {
      this.sleepTime = sleepTime;
    }

    public void run()
    {
      while(true) {
        synchronized(toFetch)
        {
          for(DataKey dk: toFetch)
          {
            DataType dt = doFetch(dk);
            synchronzied(store) {
              store.put(dk, dt);
            }
          }
        }

        synchronized(this)
        {
          // todo: add exception handlers
          sleep(sleepTime);
        }
      }
    }
  }
}