code: public AlbumArt requestArtworkFrom(final DataReference artReference, final CdjStatus.TrackType trackType) {
          ensureRunning();
          AlbumArt artwork = findArtInMemoryCaches(artReference);  // First check the in-memory artwork caches.
          if (artwork == null) {
              artwork = requestArtworkInternal(artReference, trackType, false);
          }
          return artwork;
      }
docs: Ask the specified player for the specified artwork from the specified media slot, first checking if we have a
  cached copy.
  
  @param artReference uniquely identifies the desired artwork
  @param trackType the kind of track that owns the artwork
  
  @return the artwork, if it was found, or {@code null}
  
  @throws IllegalStateException if the ArtFinder is not running