private T getRefreshedValue() throws IllegalStateException {
        T refreshableValue = refreshableValueHolder.get();
        if (refreshableValue != null) {
            return refreshableValue;
        } else {
            throw new IllegalStateException("Refreshed value should never be null.");
        }
    }
@return The refreshed value.
@throws IllegalStateException If the refreshed value is still invalid.
aws-java-sdk-sts/src/main/java/com/amazonaws/auth/RefreshableTask.java