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
    }
  }
}