Things I have done in my apps.

• Configure the NFC:

        private void configureNfc(){
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        mPendingIntent = PendingIntent.getActivity(this, 0, new Intent(this,
                getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0);

        Log.v(LOG_TAG, "NFC enabled = " + mNfcAdapter.isEnabled());
        IntentFilter intnfcv = new IntentFilter(
                NfcAdapter.ACTION_TECH_DISCOVERED);
        mFilters = new IntentFilter[] {
                intnfcv};
        mTechLists = new String[][] { new String[] {
                NfcV.class.getName() }};
    }

• Enable NFC for this app:

NfcAdapter.enableForegroundDispatch(PatchDevicesActivity.this, mPendingIntent, mFilters, mTechLists);

• When device gets near enough you get an intent:

        protected void onNewIntent(Intent intent) {
        super.onNewIntent(intent);

        final String action = intent.getAction();
        Log.v(LOG_TAG, "onNewIntent:" + action);

        setIntent(intent);
        if (mNfcEnabled == true && 
            NfcAdapter.ACTION_TECH_DISCOVERED.equals(getIntent().getAction())) {
              Tag tag = getIntent().getParcelableExtra(NfcAdapter.EXTRA_TAG);
              tranceive() to send and get response
        }

• Then disable NFC for this app:

            NfcAdapter nfcAdapter = NfcAdapter.getDefaultAdapter(this);
            nfcAdapter.disableForegroundDispatch(PatchDevicesActivity.this);

        final BluetoothManager bluetoothManager =
                (BluetoothManager)getSystemService(Context.BLUETOOTH_SERVICE);
        mBluetoothAdapter = bluetoothManager.getAdapter();
        
        mLEScanner = mBluetoothAdapter.getBluetoothLeScanner();
        settings = new ScanSettings.Builder()
                .setScanMode(ScanSettings.SCAN_MODE_LOW_LATENCY)
                .build();
                
        filters = new ArrayList<ScanFilter>();
        ScanFilter scanFilter = new ScanFilter.Builder()
                .setServiceUuid(ParcelUuid.fromString(FILTER_UUID))
                .build();
        filters.add(scanFilter);
        
        IntentFilter filter = new IntentFilter(BluetoothAdapter.ACTION_STATE_CHANGED);
        registerReceiver(mBLEStateChangedReceiver, filter);

• Then start scanning and get the scan callbacks:

        private ScanCallback mScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {

• Start the service from the main Activity. This will bind to the service if it exists. Otherwise it will start the service.

        mServiceIntent = new Intent(this, PatchService.class);

        bindService(mServiceIntent, mConnection, BIND_AUTO_CREATE);
        startService(mServiceIntent);

• In the Service class instance call startForeground() with a notification to display to the user.
• This will keep the service active in the foreground and never die.

• To start the service when the device is powered on do this in the manifest:

        <receiver android:name=".Autostart">
            <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED" />
            </intent-filter>
        </receiver>

• Autostart is a BroadcastReceiver:

public class Autostart extends BroadcastReceiver
{
    private static final String LOG_TAG = "PATCH_SERVICE_AUTOSTART";

    public void onReceive(Context context, Intent arg1)
    {
        Intent intent = new Intent(context, PatchService.class);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            context.startForegroundService(intent);
        } else {
            context.startService(intent);
        }
        Log.v(LOG_TAG, "started");
    }
}

FTSLink by Redpark

In the app the manifest enables USB attach and detach intents to be delivered.

* Asked for permission:

IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);			
this.parent.getParentContext().getApplicationContext().registerReceiver(mUsbReceiver, filter);

• ACTION_USB_ACCESSORY_ATTACHED re-starts the activity
• ACTION_USB_ACCESSORY_DETACHED is received in a BroadcastReceiver:

private final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {

		public void onReceive(Context context, Intent intent) {
			String action = intent.getAction();
			Log.v(debugTag, "ACTION: " + action);
			if (action.equals(UsbManager.ACTION_USB_ACCESSORY_DETACHED)) {
				Log.v(debugTag, "Closing Accessory");
				closeAccessory();
			}

		}

	};

• Upon attach, connect to the Device

UsbAccessory[] accessories = mUSBManager.getAccessoryList();

            // There is only one USB, this must be the first one
            UsbAccessory accessory = (accessories == null ? null : accessories[0]);

            if (accessory != null) {
                Log.v(FTSLinkServer.debugTag, "Accessory found, try to connect....");
                RPUsbConnection connection = new RPUsbConnection(mUSBManager, accessory);
                ParcelFileDescriptor mFileDescriptor = usbManager.openAccessory(accessory);
                FileDescriptor fd = mFileDescriptor.getFileDescriptor();
                mInputStream = new FileInputStream(fd);
                mOutputStream = new FileOutputStream(fd);
                
                ... Create threads 
            }

Then create threads

private static class ReadThread implements Runnable {
  @Override
        public void run() {
        }
}

parentDevice.readThread = new ReadThread(parentDevice);
Thread rt = new Thread(parentDevice.readThread);
rt.setPriority(Thread.MAX_PRIORITY);
rt.start();