How to read Bluetooth Low Energy GAP characteristic values as a string?

Hello I am new to developing an app and even newer to the concept of BLE. I am developing an application on android using Java that is capable of scanning BLE devices nearby. The main purpose is to get device model / manufacturer and send it over to a server in a readable format. The app is able to discover them and connect on click. Once connected I’m able to read and view all the services UUID from said device. The UUID for GAP profiles is x180A which I understand is supposed to be the UUID for GAP. The device is capable of reading the characteristics which is highlighted as gray in the screenshot below. The question is the value that I am getting is in a hex string format. Is there any way to convert them into a readable string.
Below is a snippet of the code im using to display the characteristics of the service.

@Override
    public boolean onChildClick(ExpandableListView parent, View v, int groupPosition, int childPosition, long id) {

        BluetoothGattCharacteristic characteristic = Objects.requireNonNull(characteristics_HashMapList.get(
                        services_ArrayList.get(groupPosition).getUuid().toString())).get(childPosition);

        if (Utils.hasWriteProperty(characteristic.getProperties()) != 0) {
            String uuid = characteristic.getUuid().toString();

            Dialog_BTLE_Characteristic dialog_btle_characteristic = new Dialog_BTLE_Characteristic();

            dialog_btle_characteristic.setTitle(uuid);
            dialog_btle_characteristic.setService(mBTLE_Service);
            dialog_btle_characteristic.setCharacteristic(characteristic);

            dialog_btle_characteristic.show(getFragmentManager(), "Dialog_BTLE_Characteristic");
        } else if (Utils.hasReadProperty(characteristic.getProperties()) != 0) {
            if (mBTLE_Service != null) {
                mBTLE_Service.readCharacteristic(characteristic);
            }
        } else if (Utils.hasNotifyProperty(characteristic.getProperties()) != 0) {
            if (mBTLE_Service != null) {
                mBTLE_Service.setCharacteristicNotification(characteristic, true);
            }
        }

        return false;
    }

Here is the readcharacteristic method.

 public void readCharacteristic(BluetoothGattCharacteristic characteristic) {
        if (mBluetoothAdapter == null || mBluetoothGatt == null) {
            Log.w(TAG, "BluetoothAdapter not initialized");
            return;
        }

        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.BLUETOOTH_CONNECT) != PackageManager.PERMISSION_GRANTED) {
            // TODO: Consider calling
            //    ActivityCompat#requestPermissions
            // here to request the missing permissions, and then overriding
            //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
            //                                          int[] grantResults)
            // to handle the case where the user grants the permission. See the documentation
            // for ActivityCompat#requestPermissions for more details.
            return;
        }
        mBluetoothGatt.readCharacteristic(characteristic);
    }

I have tried reading the hex value but have no idea how to transform it in a readable string format.

The grey entries show two values in hex: the UUID and the read values.

Characteristics read values

There is no general format which allows you to interpret the values of all the different characteristic. For each official Bluetooth SIG standardized characteristic there is a specification you need to read an implement for parsing the read values.

Characteristics and services UUID

You need to maintain your own UUID to name lookup database.

If the service or characteristic UUID is an official one (has the format xxxxxxxx-0000-1000-8000-00805F9B34FB) you can get the 16bit respectively 32bit identifier from it. See How can I convert a Bluetooth 16 bit service UUID into a 128 bit UUID? for details.

Using this 16 or 32 bit identifier you can look it up in the official specification: https://www.bluetooth.com/specifications/assigned-numbers/

At the moment there is a PDF document where you can search for section Services by UUID and Characteristics by UUID where you can find mapping tables from 16bit UUID to it’s name.

If you want to generate a mapping database programmatically there is now also a BitBucket repo containing all the values in YAML format.

Some libraries contain BLE GATT lookup tables like RxAndroidBle: https://github.com/dariuszseweryn/RxAndroidBle/blob/master/rxandroidble/src/main/java/com/polidea/rxandroidble2/utils/StandardUUIDsParser.java

But I don’t know if these mapping tables are up-to-date. The Bluetooth SIG adds new standardized services and characteristics several times a year.

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Filed under: Kiến thức lập trình - @ 04:39

Thẻ: javaandroidbluetooth-lowenergyandroid-bluetoothbluetooth-gatt

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Danh mục

How to read Bluetooth Low Energy GAP characteristic values as a string?

Characteristics read values

Characteristics and services UUID