Test.java

public class FrancescoTest {

  private static final HttpClient client = null;

  public static void main(String[] args) {

    MessageFactory factory = new MessageFactory(builder -> {
      builder.writeBinary(new CloudEvent() {
      });
    });
    
    client.request(new RequestOptions(), factory);

  }
  
  interface CloudEvent {
  }

  interface MessageBuilder {
    void writeBinary(CloudEvent event);
  }

  static class MessageFactory implements Handler<AsyncResult<HttpClientRequest>> {

    final Handler<MessageBuilder> handler;

    public MessageFactory(Handler<MessageBuilder> handler) {
      this.handler = handler;
    }

    @Override
    public void handle(AsyncResult<HttpClientRequest> ar) {
      if (ar.succeeded()) {
        HttpClientRequest request = ar.result();
        MessageBuilder builder = new MessageBuilder() {
          @Override
          public void writeBinary(CloudEvent event) {
            // Write to request
          }
        };
        handler.handle(builder);
      }
    }
  }
}