@Override
public CompletableFuture<File> processDownload( final FileDownload fileDownload ) {
    Objects.requireNonNull( fileDownload );
    fileDownload.setDirectory( getTmpDirectoryPath() );
    CompletableFuture<File> future = CompletableFuture.supplyAsync( fileDownload, executorService );
    future... throwable -> {
        if ( throwable != null ) {
            logError( throwable );
        }
        ...
        return null; // client won't receive file.
    } );
    return future;

}

future = future.whenComplete((t, ex) -> {
  if (ex != null) {
    logException(ex);
  }
});

future = future.handle((t, ex) -> {
  if (ex != null) {
    logException(ex);
    return null;
  } else {
    return t;
  }
});

future = future.exceptionally(ex -> {
  logException(ex);
  return null;
});