I'm using simple-java-mail 6.4.3 with batch-module and try to understand how is it guaranteed for onSuccess and onException handlers that their setting will be performed before their usage?

AsyncOperationHelper contains comment

But if I correctly understand it's still possible (if inner NamedRunnable will be executed fast) that

will be executed before the user will set corresponding callback in main thread

E.g. I tried to perform simple test based on example from site:

AsyncResponse asyncResponse = mailer.sendMail(email, true);
asyncResponse.onSuccess(() -> System.out.println("Success"));
asyncResponse.onException((e) -> System.err.println("error"));

In this case the callback was executed successfully.
But if the main thread will sleep for some time, e.g.

AsyncResponse asyncResponse = mailer.sendMail(email, true);
Thread.sleep(10000);
asyncResponse.onSuccess(() -> System.out.println("Success"));
asyncResponse.onException((e) -> System.err.println("error"));

The callback was not executed at all because

was called when onSuccess method was not called yet and successHandler was still null.

Do I understand something wrong or is it a bug?

This can probably be solved if callbacks will be passed directly into the sendMail method and then directly into the AsyncResponseImpl constructor