code: @Override
      public Promise<Buffer> readFile(String path) {
          return adapter.toPromise(handler -> vertx.fileSystem().readFile(path, handler));
      }
docs: Reads the entire file as represented by the path {@code path} as a {@link io.vertx.core.buffer.Buffer}, asynchronously.
  <p>
  Do not user this method to read very large files or you risk running out of available RAM.
  
  @param path path to the file
  @return a promise for the file data