Using SpringFrameWork @Async for methods that return void

Simply return

return new AsyncResult<>(null);

Try the following:

@Async
public void saveUser(String userid) {
    User user = new User();
    user.setUserId(userid);
    mongoTemplate.save(user);
}

Future needs to be used only when there is return type other than void.

The only value a Void can have is null. So all you need is

User user = new User();
user.setUserId(userid);
mongoTemplate.save(user)
return new AsyncResult<Void>(null);

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