Null pointer exception when calling getSystemService on Service class

Question

In my main activity I start a service. In that service, I try to get the sensor manager but I get the following exception

    Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference.

The error doesn't occur if I try to get the sensor manager in an activity class, it only happens in service class.

This is my main activity class:

    public class MainActivity extends AppCompatActivity
    {
        GridLayout mainGrid;

        @Override
        protected void onCreate(Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            Intent intent = new Intent(MainActivity.this, accelerometerBackgroundService.class);
            startService(intent);

            mainGrid = (GridLayout) findViewById(R.id.mainGrid);


            //Set Event
            setSingleEvent(mainGrid);
    //        setToggleEvent(mainGrid);
        }
    ...
     }

This is my service class:

    public class accelerometerBackgroundService extends Service implements SensorEventListener
    {
        //We need to log our activity to check what happens
        private static final String TAG = "Accelerometer_Activity";
        private SensorManager mSensorManager; //defining a sensor manager and the accelerometer
        private Sensor accelerometer;


        public accelerometerBackgroundService()
        {
            Log.i(TAG, "GOT HERE 100");
            //getting the sensor manager services
            mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);

            //getting the accelerometer sensor
            accelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
            if(accelerometer != null)
            {
                //If accelerometer sensor is available, create a listener
                mSensorManager.registerListener(this,mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER), SensorManager.SENSOR_DELAY_NORMAL);
            }
            else
            {
                String noAccelError = "This device does not support accelerometer sensor!";
            }
        }
        ...
        }

I imagine it has to do with the context. I've searched similar questions but didn't help me.

Thank you.

Answer 1

You should move your initialization code inside Service#onCreate() not in its constructor.

So, override the method onCreate() of Service and you can access its Context.

Furthermore, since the Android framework requires an empty constructor on the Service you should NOT pass the parameters in the Service's constructor otherwise a java.lang.InstantiationException can be thrown.