Android: Bluetooth Low Energy scanner receives null data

Question

This is the advertiser (notice data passed as AdvertiseData type)

  private void advertise() {
    BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
    BluetoothLeAdvertiser advertiser = bluetoothAdapter.getBluetoothLeAdvertiser();
    AdvertiseSettings settings = new AdvertiseSettings.Builder()
            .setAdvertiseMode(AdvertiseSettings.ADVERTISE_MODE_BALANCED)
            .setTxPowerLevel(AdvertiseSettings.ADVERTISE_TX_POWER_MEDIUM)
            .setConnectable(false)
            .build();
    ParcelUuid pUuid = new ParcelUuid(UUID.fromString("cf2c82b6-6a06-403d-b7e6-13934e602664"));
    AdvertiseData data = new AdvertiseData.Builder()
            //.setIncludeDeviceName(true)
            .addServiceUuid(pUuid)
            .addServiceData(pUuid, "123456".getBytes(Charset.forName("UTF-8")))
            .build();
    AdvertiseCallback advertiseCallback = new AdvertiseCallback() {
        @Override
        public void onStartSuccess(AdvertiseSettings settingsInEffect) {
            Log.i(tag, "Advertising onStartSuccess");
            super.onStartSuccess(settingsInEffect);
        }

        @Override
        public void onStartFailure(int errorCode) {
            Log.e(tag, "Advertising onStartFailure: " + errorCode);
            super.onStartFailure(errorCode);
        }
    };
    advertiser.startAdvertising(settings, data, advertiseCallback);
}

It starts succesfully.

This is the scanner

 private void discover() {
    ScanSettings settings = new ScanSettings.Builder()
            .setScanMode(ScanSettings.SCAN_MODE_BALANCED)
            .build();
    mBluetoothLeScanner.startScan(null, settings, mScanCallback);
}

private ScanCallback mScanCallback = new ScanCallback() {
    @Override
    public void onScanResult(int callbackType, ScanResult result) {
        super.onScanResult(callbackType, result);
        Log.i(tag, "Discovery onScanResult");
        if (result == null) {
            Log.i(tag, "no result");
            return;
        }
        ScanRecord scanRecord = result.getScanRecord();
        List<ParcelUuid> list = scanRecord != null ? scanRecord.getServiceUuids() : null;
        if (list != null) {
            Log.i(tag, scanRecord.toString());
            for (ParcelUuid uuid : list) {
                byte[] data = scanRecord.getServiceData(uuid);
            }
    }

    @Override
    public void onBatchScanResults(List<ScanResult> results) {
        super.onBatchScanResults(results);
        Log.i(tag, "Discovery onBatchScanResults");
    }

    @Override
    public void onScanFailed(int errorCode) {
        super.onScanFailed(errorCode);
        Log.e(tag, "Discovery onScanFailed: " + errorCode);
    }
};

In the callback onScarnResult I log the scan record toString() that produces this output

 ScanRecord [mAdvertiseFlags=2, 
         mServiceUuids=[cf2c82b6-6a06-403d-b7e6-13934e602664],
         mManufacturerSpecificData={}, 
         mServiceData={000082b6-0000-1000-8000-00805f9b34fb=[49, 50, 51, 52, 53, 54]}, 
         mTxPowerLevel=-2147483648, mDeviceName=null]

The uuid matches, unfortunately the result of

  byte[] data = scanRecord.getServiceData(uuid) 

is null. I noticed that the toString output had the ASCII codes of the advertised data characters "123456", that are 49,50,51,52,53,54

 mServiceData={000082b6-0000-1000-8000-00805f9b34fb=[49, 50, 51, 52, 53, 54]}

I'd like to receive the right advertised data, am I doing something wrong?

EDIT: the manifest has permissions for bluetooth, bt admin and location. The third one launches a request at runtime in Android 6

EDIT: by printing the whole scanRecord you get this output

ScanRecord [mAdvertiseFlags=-1, mServiceUuids=[cf2c82b6-6a06-403d-b7e6-13934e602664], mManufacturerSpecificData={}, mServiceData={000082b6-0000-1000-8000-00805f9b34fb=[49, 50, 51, 52, 53, 54]}, mTxPowerLevel=-2147483648, mDeviceName=null]

Basically you can't use the uuid decided by the advertiser, which is in mServiceUuids array, because the key associated to mServiceData is another one. So I changed the code in this way, to navigate the data map and get the value (please, see the two if-blocks)

   public void onBatchScanResults(List<ScanResult> results) {
        super.onBatchScanResults(results);
        for (ScanResult result : results) {
            ScanRecord scanRecord = result.getScanRecord();
            List<ParcelUuid> uuids = scanRecord.getServiceUuids();
            Map<ParcelUuid, byte[]> map = scanRecord.getServiceData();
            if (uuids != null) {
                for (ParcelUuid uuid : uuids) {
                    byte[] data = scanRecord.getServiceData(uuid);
                    Log.i(tag, uuid + " -> " + data + " contain " + map.containsKey(uuid));
                }
            }

            if (map != null) {
                Set<Map.Entry<ParcelUuid, byte[]>> set = map.entrySet();
                Iterator<Map.Entry<ParcelUuid, byte[]>> iterator = set.iterator();
                while (iterator.hasNext()) {
                    Log.i(tag, new String(iterator.next().getValue()));
                }
            }
        }
    }

In fact, the line

 map.containsKey(uuid)

returns false because the uuid of the advertiser is not used by the data map.

I had to navigate the map to find the value (2nd if-block), but I don't have any means to know if that's the value I'm interested in. Either way I can't get the value if the system put another key that I can't know while running the scanner's code on the receiver app.

How can I handle this problem on the receiver? I'd like to use the data field, but the string key to get them is not known a priori and decided by the system.

Splutter

Answer 1

I know it's an old thread, but since I had the same issue and found a solution...

UUIDs to be used with .addServiceUuid() and .addServiceData() in advertiser are different objects. The first one identifies the service, and is a 128-bits UUID. The second one identifies the serviceData within that service, and is expected to be a 16-bits UUID.

This is why the scanner receives

0000**82b6**-0000-1000-8000-00805f9b34fb

Note that 16 bits 0x82b6 are common with the UUID passed to .addServiceData:

cf2c**82b6**-6a06-403d-b7e6-13934e602664

A 16-bits UUID is converted to a 128 bits by left-shifting of 96 bits and adding a Bluetooth constant UUID.

The solution is just to use an UUID of this form [0000xxxx-0000-1000-8000-00805f9b34fb ] to identify the serviceData in both advertiser and scanner. You can keep your orignal 128-bits UUID to identify the service.

Collarbone

This should be the accepted answer. Such a small detail that I think it's easy to be missed (and no validation or error). Also, I noticed that setting the wrong 128-bit UUID as the Service Data UUID, completely screw up the advertising data. – Cromlech

Answer 2

I found the error. Change this line:

.addServiceData(pUuid, "123456".getBytes(Charset.forName("UTF-8")))

to:

.addServiceData(pUuid, "123456".getBytes()

That's it. I used the same example code as you did.

Find a better example here:
https://github.com/googlesamples/android-BluetoothAdvertisements

Jeremy

Answer 3

Try adding the ScanFilter and retreive the results

 private void discover() 
{
    List<ScanFilter> filters = new ArrayList<ScanFilter>();

    ScanFilter filter = new ScanFilter.Builder()
            .setServiceUuid( new ParcelUuid(UUID.fromString( 
   getString(R.string.ble_uuid ) ) ) )
            .build();
    filters.add( filter );

Link for source code:https://github.com/tutsplus/Android-BluetoohLEAdvertising/blob/master/app/src/main/java/com/tutsplus/bleadvertising/MainActivity.java

Phytology

Answer 4

@guiv gave the right answer https://mcmap.net/q/626716/-android-bluetooth-low-energy-scanner-receives-null-data here

Let me add by putting it in a more concise way.

addServiceData expects 16-bit UUIDS while addServiceUUIDs can accept either 16-bit or 128. Rather than raise an exception addServiceData automatically adjusts 128-bit UUIDs to 16-bit, hence the difference in input parameter of addServiceData (in Advertiser) and keys of the return value of getServiceData() (in Scanner).

Solution - Ensure you use 16-bit UUIDs in your advertiser

Leblanc