code_exchange.java

    private void startCodeExchange() {
        Log.i(TAG, "Starting code exchange");

        loginListener.onEvent(AuthRepo.this, AUTH_CODE_EXCHANGE_START);

        AuthorizationResponse resp = authState.getLastAuthorizationResponse();
        authService.performTokenRequest(
                resp.createTokenExchangeRequest(), this::onTokenRequestCompleted);
    }

    private void onTokenRequestCompleted(TokenResponse resp, AuthorizationException ex) {
        if (resp == null) {
            failLogin(new AuthException(ex.getMessage()));
            return;
        }

        authState.update(resp, ex);
        finishCodeExchange();
    }