如何获取AWS Access令牌？

Question

在进行POST方法调用时，我需要访问令牌作为标头传递。我正在尝试获取如下访问令牌。

@Override
protected Void doInBackground(Void... voids) {
    AmazonCognitoIdentityProviderClient identityProviderClient = new AmazonCognitoIdentityProviderClient(new AnonymousAWSCredentials(), new ClientConfiguration());
    identityProviderClient.setRegion(Region.getRegion(Regions.SOMETHING));

    final CognitoUserPool userPool = new CognitoUserPool(context,
            poolId,
            clientId, null, identityProviderClient);
    final CognitoUser cognitoUser = userPool.getCurrentUser();
    System.out.println(cognitoUser.getUserId());
    final AuthenticationHandler authenticationHandler = new AuthenticationHandler() {

        @Override
        public void onSuccess(CognitoUserSession userSession, CognitoDevice newDevice) {
            Log.i("ACCESS_TOKEN", userSession.getIdToken().getJWTToken());
        }
    }

    cognitoUser.getSession(authenticationHandler);
}

我总是将访问令牌设为null。我的代码有什么错误？