为什么我无法在LockScreen中获取Settings.System.DATE_FORMAT？

mDateFormatString = getContext().getString(R.string.full_wday_month_day_no_year);

mDateFormatString =Settings.System.getString(getContext().getContentResolver(), Settings.System.DATE_FORMAT) ;

private void resetStatusInfo(KeyguardUpdateMonitor updateMonitor) {
    mShowingBatteryInfo = updateMonitor.shouldShowBatteryInfo();
    mPluggedIn = updateMonitor.isDevicePluggedIn();
    mBatteryLevel = updateMonitor.getBatteryLevel();

    /* for dual sim, by jing.wang */
    mStatus[SimCardID.ID_ZERO.toInt()] = getCurrentStatus(updateMonitor.getSimState());
    if (isDualMode) {
        mStatus[SimCardID.ID_ONE.toInt()] = getCurrentStatus(updateMonitor.getSimState (SimCardID.ID_ONE));
        updateLayout(mStatus[SimCardID.ID_ZERO.toInt()], SimCardID.ID_ZERO, mStatus[SimCardID.ID_ONE.toInt()]);
        updateLayout(mStatus[SimCardID.ID_ONE.toInt()], SimCardID.ID_ONE, mStatus[SimCardID.ID_ZERO.toInt()]);
    } else {
        updateLayout(mStatus[SimCardID.ID_ZERO.toInt()]);
    }

    refreshBatteryStringAndIcon();
    refreshAlarmDisplay();

    mTimeFormat = DateFormat.getTimeFormat(getContext());
    mDateFormatString = getContext().getString(R.string.full_wday_month_day_no_year);

    refreshTimeAndDateDisplay();
    updateStatusLines();
}

mDateFormatString = getContext().getString(R.string.full_wday_month_day_no_year); 

private void refreshTimeAndDateDisplay() {
    mDate.setText(DateFormat.format(mDateFormatString, new Date()));
}