In order to read the content of the requests my HttpHandler use receivePartialBytes as follows:

public class AsyncReaderHttpHandler implements HttpHandler {

  @Override
  public void handleRequest(HttpServerExchange exchange) throws Exception {

    if (exchange.isInIoThread()) {
      exchange.dispatch(this);
      return;
    }

    readRequest(exchange);
  }

  private void readRequest(HttpServerExchange exchange) {
    exchange.getRequestReceiver().receivePartialBytes(new MyCallback());
  }

  public class MyCallback implements PartialBytesCallback {

    @Override
    public void handle(HttpServerExchange exchange, byte[] message, boolean last) {
      if (exchange.isInIoThread()) {
        exchange.dispatch(() -> handleData(exchange, message, last));
      } else {
        handleData(exchange, message, last);
      }
    }

    private void handleData(HttpServerExchange exchange, byte[] message, boolean last) {
      // do some blocking operation with the data
    }
  }
}

My handler dispatching to worker thread and than calls receivePartialBytes() with a callback.

At first, I thought that the handle() method of the callback will always be called from a worker thread context.

I was surprised to discover that in some cases the callback will be called from I/O thread (why?), so I had to add the second call to dispatch() inside the callback.

After adding the second call to dispatch() my code started to fail with:

What am I missing here? how can I use the asynchronous receiver and not risking in blocking I/O thread?

I'm using Undertow 1.4.19.Final.

Thanks!