@Override
  protected void checkOpen() {
    if (!initialized.get()) {
      throw new IllegalStateException("AccountService is closed.");
    }
  }
Initializes this instance. Does all work which requires handing out references to this instance itself which can't
be done during normal instance construction without letting a reference to an incomplete constructed instance escape.
ambry-account/src/main/java/com/github/ambry/account/InMemoryAccountService.java