如何检查API级别17之前的activity.isDestroyed()？

private boolean isDestroyed = false;

@Override
protected void onDestroy() {
    super.onDestroy();
    isDestroyed = true;
}

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
    if (isDestroyed()) {
        // activity已被销毁
    } else {
        // activity未被销毁
    }
} else {
    if (isDestroyed) {
        // activity已被销毁
    } else {
        // activity未被销毁
    }
}