public KeysDownloader build() {
      if (url == null) {
        throw new IllegalArgumentException("must provide a url with {#setUrl}");
      }
      return new KeysDownloader(executor, httpTransport, url);
    }
Sets the HTTP transport.

<p>You generally should not need to set a custom transport as the default transport {@link
KeysDownloader#DEFAULT_HTTP_TRANSPORT} should be suited for most use cases.
java_src/src/main/java/com/google/crypto/tink/util/KeysDownloader.java