How to get current cell signal strength?

CellSignalStrengthGsm() is added Added in API level 17

CellSignalStrengthGsm().getDbm() will give you the signal strength as dBm

 private static String getSignalStrength(Context context) throws SecurityException {
    TelephonyManager telephonyManager = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
    String strength = null;
    List<CellInfo> cellInfos = telephonyManager.getAllCellInfo();   //This will give info of all sims present inside your mobile
    if(cellInfos != null) {
        for (int i = 0 ; i < cellInfos.size() ; i++) {
            if (cellInfos.get(i).isRegistered()) {
                if (cellInfos.get(i) instanceof CellInfoWcdma) {
                    CellInfoWcdma cellInfoWcdma = (CellInfoWcdma) cellInfos.get(i);
                    CellSignalStrengthWcdma cellSignalStrengthWcdma = cellInfoWcdma.getCellSignalStrength();
                    strength = String.valueOf(cellSignalStrengthWcdma.getDbm());
                } else if (cellInfos.get(i) instanceof CellInfoGsm) {
                    CellInfoGsm cellInfogsm = (CellInfoGsm) cellInfos.get(i);
                    CellSignalStrengthGsm cellSignalStrengthGsm = cellInfogsm.getCellSignalStrength();
                    strength = String.valueOf(cellSignalStrengthGsm.getDbm());
                } else if (cellInfos.get(i) instanceof CellInfoLte) {
                    CellInfoLte cellInfoLte = (CellInfoLte) cellInfos.get(i);
                    CellSignalStrengthLte cellSignalStrengthLte = cellInfoLte.getCellSignalStrength();
                    strength = String.valueOf(cellSignalStrengthLte.getDbm());
                } else if (cellInfos.get(i) instanceof CellInfoCdma) {
                    CellInfoCdma cellInfoCdma = (CellInfoCdma) cellInfos.get(i);
                    CellSignalStrengthCdma cellSignalStrengthCdma = cellInfoCdma.getCellSignalStrength();
                    strength = String.valueOf(cellSignalStrengthCdma.getDbm());
                }
            }
        }
    }
    return strength;
}

Please note that above code will return strength of the last cell in the list.

You can learn more from: https://developer.android.com/reference/android/telephony/CellInfo.html

CellInfoCdma, CellInfoGsm, CellInfoLte, CellInfoWcdma are the subclasses of CellInfo. Which gives all information related to your mobile network.

If you are using a SDK version > 28, you can use getSignalStrength() function like so:

telephonyManager.getSignalStrength().getLevel();

Here is a fully implemented function:

TelephonyManager telephonyManager;

...

int getCellularSignalStrength()  {
    if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {

        if(telephonyManager == null)
            telephonyManager = (TelephonyManager) common.appContext.getSystemService(Context.TELEPHONY_SERVICE);

        return telephonyManager.getSignalStrength().getLevel();
    }
    else
    {
        logRecorder.addWarningLog("Cannot read signal lvl on API lvl " + android.os.Build.VERSION.SDK_INT);
        return -1;
    }
}

There is getAllCellInfo() method in TelephonyManager added in API 17 that could be good solution. Example of use:

TelephonyManager telephonyManager = (TelephonyManager)this.getSystemService(Context.TELEPHONY_SERVICE);
// for example value of first element
CellInfoGsm cellInfoGsm = (CellInfoGsm)telephonyManager.getAllCellInfo().get(0);
CellSignalStrengthGsm cellSignalStrengthGsm = cellInfoGsm.getCellSignalStrength();
cellSignalStrengthGsm.getDbm();