Question

I’m trying to track down a new null pointer exception which is appearing in my ACRA logs and which I can’t reproduce. Here’s the relevant code:

public class MyApplication extends Application {
   public void onCreate() {
       DataManager.instance().initializeData(this);
   }
}

public class DataManager {
    private static DataManger instance = new DataManger();
    private List<DataModel> dataModels;
    private List<I_Callback> callbacks = new ArrayList<I_Callback>();
    private boolean isInitialized = false;

    private DataManager(){}

    public static DataManager instance() {
       return instance;
    }

    public void initializeData(Context context) {
        new DataManagerInitializer().execute(context);
    }

    public void setDataModels(List<DataModel> models) {
        dataModels = models;
    }

    public void synchronized registerInitializeCallbacks(I_Callback callback) {
        if (isInitialized) {
             callback.executeCallback();
        } else {
             callbacks.add(callback);
        }
    }

    public void synchronized setInitialized() {
        isInitialized = true;
        for (I_Callback callback:callbacks) {
             callback.executeCallback();
        }
        callbacks.clear();
    }
}

public class DataManagerInitializer extends AsyncTask<Context, Void, Void>{
    protected Void doInBackground(Context... contexts){
        List<DataModel> dataModels = new ArrayList<DataModel>();
        /*various code to create DataModel objects and add to dataModels list*/
        DataManager.instance().setDataModels(dataModels);
        return null;
    }

    protected void onPostExecute(Void result) {
        DataManager.instance().setInitialized();
    }
}

public class ActivityA extends Activity implements I_Callback{
     public void onCreate(Bundle savedInstanceState) {
        setContentView(R.layout.graphical_layout);
        DataManager.instance().registerInitializeCallbacks(this);
     }

     public void executeCallback() {
        /* wire up button to call Activity B */
     }
}

public class ActivityB extends Activity {
     public void onCreate(Bundle savedInstanceState) {
         List<DataModel> dataModels = DataManager.instance().getDataModels();

         /* The following line of code throws a null pointer exception 
            in the stack trace*/
         for (int i=0; i < dataModels.size(); i++){
              /* do something with the data model */
         }
     }
}

To break down the above more simply, the application is launched which kicks off the initializion of the data manager singleton. ActivityA, the main activity, launches and waits for the data manager to complete initialization before allowing any actions, wiring up any events, etc. From ActivityA, its not possible to get to ActivityB without the call back method executing and ActivityB is only reachable from ActivityA. The only way for the list of data models to be null in the DataManager is for it to not have been initialized, but I’m struggling to see how this is possible. Any suggestions on how my null pointer may have occurred?

Answer 1

Let’s assume that:

you are interacting with the Activity B

press the home button:

start playing with other apps (consuming memory)

at some point the so needs memory and it’s gonna start garbage collecting objects, included your “instance”.

If that happens when you launch your app the framework will resume the activity B and the npe will happen.

You need to re-create the instance (in the activity B) if it is null.