The InputStream obtained from uploadRequest.getStreamProvider().get() is opened before the main upload loop but is never closed when the method exits (either normally by returning a response or abruptly by throwing an exception). This can lead to resource leaks (e.g., open file descriptors or network connections). Wrap the stream usage in a try-finally block to ensure it is always closed properly.

The header values in requestHeaders.getHeaders() are stored as Object and can be of type List<String> (especially for multi-valued headers or extra headers from the call context). Casting them directly to (String) will throw a ClassCastException at runtime. Instead, handle both List and single String values safely.

    for (Map.Entry<String, Object> entry : requestHeaders.getHeaders().entrySet()) {
      String key = entry.getKey();
      Object valueObj = entry.getValue();
      if (valueObj instanceof List) {
        for (Object val : (List<?>) valueObj) {
          if (val != null) {
            String value = val.toString();
            if (isMetadataHeaderDenylisted(key)) {
              httpRequest.getHeaders().set("X-Goog-Upload-Header-" + key, value);
            } else {
              httpRequest.getHeaders().set(key, value);
            }
          }
        }
      } else if (valueObj != null) {
        String value = valueObj.toString();
        if (isMetadataHeaderDenylisted(key)) {
          httpRequest.getHeaders().set("X-Goog-Upload-Header-" + key, value);
        } else {
          httpRequest.getHeaders().set(key, value);
        }
      }
    }

In Java, do not swallow InterruptedException. When catching it, restore the thread's interrupted status by calling Thread.currentThread().interrupt() and handle the interruption appropriately, such as by throwing a relevant exception to signal that the operation cannot proceed.

If the clientContext passed to this callable is configured with a gRPC transport channel (which is the case when called from GrpcEchoStub if the background HTTP/JSON channel was not properly resolved), clientContext.getTransportChannel() will return a GrpcTransportChannel. Casting it directly to HttpJsonTransportChannel will throw a ClassCastException at runtime. Consider adding an explicit type check or ensuring that the background HTTP/JSON channel is fully resolved and wrapped before invoking the callable.

Parsing the already-serialized requestBody JSON string into a GenericData object only to re-serialize it using JsonHttpContent is highly inefficient. Since requestBody is already a valid JSON string, you can use ByteArrayContent.fromString directly to avoid this overhead.

    if (!Strings.isNullOrEmpty(requestBody)) {
      initialContent = 
          ByteArrayContent.fromString("application/json; charset=utf-8", requestBody);
    } else {
      initialContent = new EmptyContent();
    }

Creating a new HttpRequestFactory for every single request/chunk is inefficient because it sets up the credentials adapter and initializer repeatedly. Since HttpRequestFactory is thread-safe and reusable, it should be initialized once in the constructor and stored as a final field.

If the configured deadline is very far in the future (e.g., more than 24 days), remainingMs can exceed Integer.MAX_VALUE. Casting it directly to (int) will cause an integer overflow, resulting in a negative timeout value which will throw an IllegalArgumentException or cause unexpected behavior. Clamp the value to Integer.MAX_VALUE to prevent this.