code: protected void callback(Callback... callbacks) throws IOException,
        UnsupportedCallbackException, LoginException {
      if (callbackHandler != null) {
        callbackHandler.handle(callbacks);
      } else if (requireCallbackHandler) {
        throw new LoginException("Login module requires a callback handler");
      }
    }
docs: Issues the specified callbacks to acquire login information.
  @param callbacks The collection of callbacks to issue.
  @throws IOException If an I/O exception was thrown by the callback
  handler.
  @throws UnsupportedCallbackException If a requested callback is not
  supported by the callback handler.
  @throws LoginException If a callback handler is required (see
  {@link #AbstractLoginModule(boolean)}) but was not provided.