首次单击扫描按钮后，应用程序崩溃，但重新打开后该应用程序可以工作

Question

I'm trying to create an indoor location services app in android studio.There is a scan button which start the discovery of BLE devices. When i click on the scan button,the app crashes. But when i reopen the app and click on the scan button again,it works.

i tried this taken from one of the projects from stackoverflow.

Class variable:

private BluetoothAdapter mBtAdapter = null;

final BluetoothManager btManager = (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
mBtAdapter = btManager.getAdapter();

if (mBtAdapter == null || !mBtAdapter.isEnabled()) {
    Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
    startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
}
}

public void onScanButton(){
    if (mBtAdapter.isEnabled()){
        scanLeDevice(true);
    }
}

this is my code

    BluetoothManager btManager; //field 'btManager' is never used
    private BluetoothAdapter btAdapter = null;
    BluetoothLeScanner btScanner;
    Button startScanningButton;
    Button stopScanningButton;
    TextView peripheralTextView;
    private final static int REQUEST_ENABLE_BT = 1;
    private static final int PERMISSION_REQUEST_COARSE_LOCATION = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        peripheralTextView = (TextView) findViewById(R.id.peripheralTextView);
        peripheralTextView.setMovementMethod(new ScrollingMovementMethod());

        startScanningButton = (Button) findViewById(R.id.StartScanButton);
        startScanningButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                startScanning();
            }
        });

        stopScanningButton = (Button) findViewById(R.id.StopScanButton);
        stopScanningButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                stopScanning();
            }
        });
        stopScanningButton.setVisibility(View.INVISIBLE);

        final BluetoothManager btManager = (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
        btAdapter = btManager.getAdapter();
        btScanner = btAdapter.getBluetoothLeScanner();

        if (btAdapter != null && !btAdapter.isEnabled()) {
            Intent enableIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableIntent,REQUEST_ENABLE_BT);
        }

        // Make sure we have access coarse location enabled, if not, prompt the user to enable it
        if (this.checkSelfPermission(Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            final AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setTitle("This app needs location access");
            builder.setMessage("Please grant location access so this app can detect peripherals.");
            builder.setPositiveButton(android.R.string.ok, null);
            builder.setOnDismissListener(new DialogInterface.OnDismissListener() {
                @Override
                public void onDismiss(DialogInterface dialog) {
                    requestPermissions(new String[]{Manifest.permission.ACCESS_COARSE_LOCATION}, PERMISSION_REQUEST_COARSE_LOCATION);
                }
            });
            builder.show();
        }
    }

    // Device scan callback.
    private ScanCallback leScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            peripheralTextView.append("MAC address: " + result.getDevice().getAddress() + " rssi: " + result.getRssi() + "TxPower:" + result.getTxPower() + "\n");

            // auto scroll for text view
            final int scrollAmount = peripheralTextView.getLayout().getLineTop(peripheralTextView.getLineCount()) - peripheralTextView.getHeight();
            // if there is no need to scroll, scrollAmount will be <=0
            if (scrollAmount > 0)
                peripheralTextView.scrollTo(0, scrollAmount);
        }
    };

    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           String permissions[], int[] grantResults) {
        switch (requestCode) {
            case PERMISSION_REQUEST_COARSE_LOCATION: {
                if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    System.out.println("coarse location permission granted");
                } else {
                    final AlertDialog.Builder builder = new AlertDialog.Builder(this);
                    builder.setTitle("Functionality limited");
                    builder.setMessage("Since location access has not been granted, this app will not be able to discover beacons when in the background.");
                    builder.setPositiveButton(android.R.string.ok, null);
                    builder.setOnDismissListener(new DialogInterface.OnDismissListener() {

                        @Override
                        public void onDismiss(DialogInterface dialog) {
                        }

                    });
                    builder.show();
                }
            }
        }
    }

    public void startScanning() {
        System.out.println("start scanning");
        peripheralTextView.setText("");
        startScanningButton.setVisibility(View.INVISIBLE);
        stopScanningButton.setVisibility(View.VISIBLE);
        AsyncTask.execute(new Runnable() {
            @Override
            public void run() {
                btScanner.startScan(leScanCallback);
            }
        });
    }

    public void stopScanning() {
        System.out.println("stopping scanning");
        peripheralTextView.append("Stopped Scanning");
        startScanningButton.setVisibility(View.VISIBLE);
        stopScanningButton.setVisibility(View.INVISIBLE);
        AsyncTask.execute(new Runnable() {
            @Override
            public void run() {
                btScanner.stopScan(leScanCallback);
            }
        });
    }
}

The logcat shows java.lang.NullPointerException: Attempt to invoke virtual method 'void android.bluetooth.le.BluetoothLeScanner.startScan(android.bluetooth.le.ScanCallback)' on a null object reference

at com.example.myapplication.MainActivity$6.run(MainActivity.java:137)

at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:245)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)

at java.lang.Thread.run(Thread.java:764)

Answer 1

Before i click the Scan button,a prompt will be display asking to turn on the bluetooth.So bluetooth will be turned on

You are wrong about that part. You ask the user to enable it, but it might not have happened yet. At least you need to get the Scanner later on.

Currently you set the Scanner reference before the permission requesting has been initiated.

This also explains why it works after your App has crashed for the first time, because the 2nd time you come here the Permission has been enabled.

---

From the Javadoc of BluetoothAdapter#getBluetoothLeScanner() :

Will return null if Bluetooth is turned off or if Bluetooth LE Advertising is not supported on this device.

---

You can change your code to:

    public void startScanning() {

        btScanner = btAdapter.getBluetoothLeScanner();
        if (btScanner == null) {
            // not enabled yet or not supported
            return;
        }

        System.out.println("start scanning");
        peripheralTextView.setText("");
        startScanningButton.setVisibility(View.INVISIBLE);
        stopScanningButton.setVisibility(View.VISIBLE);
        AsyncTask.execute(new Runnable() {
            @Override
            public void run() {
                btScanner.startScan(leScanCallback);
            }
        });
    }